DESIGN SYSTEMS OF FLUID HOOK-UPS - Bay Port Valvebayportvalve.com/pdffiles/spirax...

DESIGNOF FLUID

SYSTEMS

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OK

-UP

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First Printing January, 1968Second Edition – First Printing October, 1968

Third Edition – First Printing May, 1970Fourth Edition – First Printing September, 1974

Fifth Edition – First Printing August, 1975Sixth Edition – First Printing May, 1978

Seventh Edition – First Printing September, 1981Eighth Edition – First Printing January, 1987Ninth Edition – First Printing April, 1990Tenth Edition – First Printing January, 1991

Eleventh Edition – First Printing April, 1997Twelfth Edition – First Printing June, 2000

Second Printing June, 2001

Copyright © 2004by Spirax Sarco, Inc.

All Rights ReservedNo part of this publication may be reproduced, stored in a

retrieval system or transmitted, in any form or by any means,electronic, mechanical, photocopying, recording, or otherwise,

without the prior written permission of the publisher.

Spirax Sarco, Inc.1150 Northpoint Blvd. Blythewood, SC 29016

Phone: (803) 714-2000Fax: (803) 714-2222

www.spiraxsarco.com/us

II

Spirax Sarco

III

Spirax Sarco is the recognized industry standard forknowledge and products and for over 85 years hasbeen committed to servicing the steam users world-wide. The existing and potential applications for steam,water and air are virtually unlimited. Beginning withsteam generation, through distribution and utilizationand ultimately returning condensate to the boiler,Spirax Sarco has the solutions to optimize steam sys-tem performance and increase productivity to savevaluable time and money.

In today’s economy, corporations are looking for reli-able products and services to expedite processes andalleviate workers of problems which may arise withtheir steam systems. As support to industries aroundthe globe, Spirax Sarco offers decades of experience,knowledge, and expert advice to steam users world-wide on the proper control and conditioning of steamsystems.

Spirax Sarco draws upon its worldwide resources ofover 3500 people to bring complete and thorough ser-vice to steam users. This service is built into ourproducts as a performance guarantee. From initial con-sultation to effective solutions, our goal is tomanufacture safe, reliable products that improve pro-ductivity. With a quick, responsive team of salesengineers and a dedicated network of local authorizeddistributors Spirax Sarco provides quality service andsupport with fast, efficient delivery.

Reliable steam system components are at the heart ofSpirax Sarco’s commitment. Controls and regulatorsfor ideal temperature, pressure and flow control; steamtraps for efficient drainage of condensate for maximumheat transfer; flowmeters for precise measurement ofliquids; liquid drain traps for automatic and continuousdrain trap operation to boost system efficiency; rotaryfilters for increased productivity through proper filteringof fluids; condensate recovery pumps for effective con-densate management to save water and sewage costs;stainless steel specialty products for maintaining qual-ity and purity of steam; and a full range of pipelineauxiliaries, all work together to produce a productivesteam system. Spirax Sarco’s new line of engineeredequipment reduces installation costs with prefabricatedassemblies and fabricated modules for system integri-ty and turnkey advantages.

From large oil refineries and chemical plants to locallaundries, from horticulture to shipping, for hospitals,universities, offices and hotels, in business and gov-ernment, wherever steam, hot water and compressedair is generated and handled effectively and efficiently,Spirax Sarco is there with knowledge and experience.

For assistance with the installation or operation of anySpirax Sarco product or application, call toll free:

1-800-883-4411

How to Use This Book

IV

Selection of the most appropriate type and size ofcontrol valves, steam traps and other fluid controlvalves, steam traps and other fluid control equip-ment, and installation in a hook up enabling thesecomponents of a system to operate in an optimalmanner, all bear directly on the efficiency and econ-omy obtainable in any plant or system.

To help make the best choice, we have assembledinto this book the accumulation of over 85 years ofexperience with energy services in industrial andcommercial use. The hook ups illustrated have allbeen proven in practice, and the reference informa-tion included is that which we use ourselves whenassisting customers choose and use our products.

The Case in Action stories dispersed throughout thisbook are actual applications put to the test by steamusers throughout the country. Their stories are testi-monials to the products and services Spirax Sarcooffers and the benefits they have received from uti-lizing our knowledge and services.

The Hook Up Book is divided into three sections:

Section I is a compilation of engineering data andinformation to assist in estimating loads and flowrates, the basic parameters which enable the bestchoice when selecting sizes.

Section II illustrates how the services and controlequipment can be assembled into hook ups tobest meet the particular needs of each application.

Section III is a summary of the range of SpiraxSarco equipment utilized in the hook ups. Althoughit is not a complete catalog of the entire range, itdoes describe generically the capabilities and limi-tations which must be remembered when makingproper product choices.

Most application problems will be approached in thesame order. Section I will enable the load informa-tion to be collected and the calculations made sothat sizing can be carried out; Section II will makesure that the essentials of the hook up, or combina-tion of hook ups, are not overlooked; and Section IIIwill serve as a guide to the complete equipment cat-alog so that the most suitable equipment can readilybe selected.

The Hook Up Book is intended to serve as a refer-ence for those actively engaged in the design,operation and maintenance of steam, air and liquidsystems. It is also intended as a learning tool toteach engineers how to design productive steamsystems, efficiently and cost effectively.

We gratefully acknowledge the valuable contribu-tions made by our field engineers, representatives,application engineers, and customers to the bodyof accumulated experience contained in this text.

V

Section 1: System Design Information .........................................................1The Working Pressure in the Boiler and the Mains ............................................................2Sizing Steam Lines on Velocity...........................................................................................3Steam Pipe Sizing for Pressure Drop.................................................................................5Sizing Superheated Mains..................................................................................................6Properties of Saturated Steam ...........................................................................................7Draining Steam Mains ........................................................................................................8Steam Tracing ...................................................................................................................12Pressure Reducing Stations .............................................................................................19Parallel and Series Operation of Reducing Valves ...........................................................21How to Size Temperature and Pressure Control Valves ...................................................23Temperature Control Valves for Steam Service ................................................................26Temperature Control Valves for Liquid Service.................................................................28Makeup Air Heating Coils .................................................................................................31Draining Temperature Controlled Steam Equipment ........................................................33Multi-Coil Heaters .............................................................................................................36Steam Trap Selection........................................................................................................38Flash Steam......................................................................................................................41Condensate Recovery Systems .......................................................................................45Condensate Pumping .......................................................................................................48Clean Steam .....................................................................................................................50Testing Steam Traps..........................................................................................................55Spira-tec Trap Leak Detector Systems for Checking Steam Traps ...................................58Steam Meters....................................................................................................................59Compressed Air Systems .................................................................................................62Reference Charts and Tables ...........................................................................................66

Section 2: Hook-up Application Diagrams................................................83For Diagram Content, please refer to Subject Index on page 149.

Section 3: Product Information .....................................................................143An overview of the Spirax Sarco Product Line

Subject Index ...................................................................................149

Table of Contents

SYSTEMDESIGN

INFORMATION

Section 1

The Working Pressure in the Boiler and the Mains

2

Steam should be generated at apressure as close as possible tothat at which the boiler isdesigned to run, even if this ishigher than is needed in theplant. The reasoning behind thisis clear when consideration isgiven to what happens in thewater and steam space within theboiler. Energy flows into the boilerwater through the outer surface ofthe tubes, and if the water isalready at saturation tempera-ture, bubbles of steam areproduced. These bubbles thenrise to the surface and break, torelease steam into the steamspace.

The volume of a given weightof steam contained in the bubblesdepends directly on the pressureat which the boiler is operating. Ifthis pressure is lower than the

design pressure, the volume inthe bubbles is greater. It followsthat as this volume increases, theapparent water level is raised.The volume of the steam spaceabove the water level is therebyreduced. There is increased tur-bulence as the greater volume ofbubbles break the surface, andless room for separation of waterdroplets above the surface.Further, the steam movingtowards the crown or steam take-off valve must move at greatervelocity with a higher volumemoving across a smaller space.All these factors tend to encour-age carryover of water dropletswith the steam.

There is much to be said infavor of carrying the steam closeto the points of use at a high pres-sure, near to that of the boiler.

The use of such pressure meansthat the size of the distributionmains is reduced. The smallermains have smaller heat losses,and better quality steam at thesteam users is likely to result.

Pressure reduction to the val-ues needed by the steam usingequipment can then take placethrough pressure reducing sta-tions close to the steam usersthemselves. The individual reduc-ing valves will be smaller in size,will tend to give tighter control ofreduced pressures, and emit lessnoise. Problems of having awhole plant dependent on a sin-gle reducing station are avoided,and the effects on the steamusers of pressure drops throughthe pipework, which change withvarying loads, disappear.

SYSTEMDESIGN

Table 1: Steam Pipe Sizing for Steam Velocity Capacity of Sch. 80 Pipe in lb/hr steam

Pressure Velocitypsi ft/sec 1/2" 3/4" 1" 11/4" 11/2" 2" 21/2" 3" 4" 5" 6" 8" 10" 12"

50 12 26 45 70 100 190 280 410 760 1250 1770 3100 5000 71005 80 19 45 75 115 170 300 490 710 1250 1800 2700 5200 7600 11000

120 29 60 110 175 245 460 700 1000 1800 2900 4000 7500 12000 1650050 15 35 55 88 130 240 365 550 950 1500 2200 3770 6160 8500

10 80 24 52 95 150 210 380 600 900 1500 2400 3300 5900 9700 13000120 35 72 135 210 330 590 850 1250 2200 3400 4800 9000 14400 2050050 21 47 82 123 185 320 520 740 1340 1980 2900 5300 8000 11500

20 80 32 70 120 190 260 520 810 1100 1900 3100 4500 8400 13200 18300120 50 105 190 300 440 840 1250 1720 3100 4850 6750 13000 19800 2800050 26 56 100 160 230 420 650 950 1650 2600 3650 6500 10500 14500

30 80 42 94 155 250 360 655 950 1460 2700 3900 5600 10700 16500 23500120 62 130 240 370 570 990 1550 2100 3950 6100 8700 16000 25000 3500050 32 75 120 190 260 505 790 1100 1900 3100 4200 8200 12800 18000

40 80 51 110 195 300 445 840 1250 1800 3120 4900 6800 13400 20300 28300120 75 160 290 460 660 1100 1900 2700 4700 7500 11000 19400 30500 4250050 43 95 160 250 360 650 1000 1470 2700 3900 5700 10700 16500 24000

60 80 65 140 250 400 600 1000 1650 2400 4400 6500 9400 17500 27200 38500120 102 240 410 610 950 1660 2600 3800 6500 10300 14700 26400 41000 5800050 53 120 215 315 460 870 1300 1900 3200 5200 7000 13700 21200 29500

80 80 85 190 320 500 730 1300 2100 3000 5000 8400 12200 21000 33800 47500120 130 290 500 750 1100 1900 3000 4200 7800 12000 17500 30600 51600 7170050 63 130 240 360 570 980 1550 2100 4000 6100 8800 16300 26500 35500

100 80 102 240 400 610 950 1660 2550 3700 6400 10200 14600 26000 41000 57300120 150 350 600 900 1370 2400 3700 5000 9100 15000 21600 38000 61500 8630050 74 160 290 440 660 1100 1850 2600 4600 7000 10500 18600 29200 41000

120 80 120 270 450 710 1030 1800 2800 4150 7200 11600 16500 29200 48000 73800120 175 400 680 1060 1520 2850 4300 6500 10700 17500 26000 44300 70200 9770050 90 208 340 550 820 1380 2230 3220 5500 8800 12900 22000 35600 50000

150 80 145 320 570 900 1250 2200 3400 4900 8500 14000 20000 35500 57500 79800120 215 450 850 1280 1890 3400 5300 7500 13400 20600 30000 55500 85500 12000050 110 265 450 680 1020 1780 2800 4120 7100 11500 16300 28500 45300 64000

200 80 180 410 700 1100 1560 2910 4400 6600 11000 18000 26600 46000 72300 100000120 250 600 1100 1630 2400 4350 6800 9400 16900 25900 37000 70600 109000 152000

Sizing Steam Lines On Velocity

3

The appropriate size of pipe tocarry the required amount ofsteam at the local pressure mustbe chosen, since an undersizedpipe means high pressure dropsand velocities, noise and erosion,while a generously sized pipe isunnecessarily expensive to installand heat losses from it will alsobe greater than they need be.

Steam pipes may be sizedeither so that the pressure dropalong them is below an accept-able limit, or so that velocitiesalong them are not too high. It isconvenient and quick to sizeshort mains and branches onvelocity, but longer runs of pipeshould also be checked to seethat pressure drops are not toohigh.

Steam Line VelocitiesIn saturated steam lines, reason-able maximum for velocities areoften taken at 80/120 ft. per sec-ond or 4800/7200 fpm. In thepast, many process plants haveused higher velocities up to 200ft. per second or 12,000 fpm, onthe basis that the increased pipenoise is not a problem within aprocess plant. This ignores theother problems which accompanyhigh velocities, and especially theerosion of the pipework and fit-tings by water droplets moving athigh speed. Only where apprecia-ble superheat is present, with thepipes carrying only a dry gas,should the velocities mentionedbe exceeded. Velocity of saturat-ed steam in any pipe may beobtained from either Table 1, Fig.1 or calculated in ft. per minuteusing the formula:

Formula For Velocity OfSteam In Pipes

V = 2.4Q VsA

Where:V - Velocity in feet per minuteQ - Flow lbs./hr. steamVs - Sp. Vol. in cu. ft./lb. at the

flowing pressureA - Internal area of the pipe—

sq. in.

Steam Piping For PRV’s andFlash VentsVelocity in piping other thansteam distribution lines must becorrectly chosen, including pres-sure reducing valve and flashsteam vent applications.

A look at Steam Properties(Table 3) illustrates how the spe-cific volume of steam increasesas pressure is reduced. To keepreducing valve high and low pres-sure pipe velocity constant, thedownstream piping cross-section-al area must be larger by thesame ratio as the change in vol-ume. When downstream pipe sizeis not increased, low pressuresteam velocity increases propor-tionally. For best PRV operation,without excessive noise, longstraight pipe runs must be provid-ed on both sides, with pipingreduced to the valve thenexpanded downstream graduallyto limit approach and exit steamvelocities to 4000/ 6000 fpm. Asizing example is given in Fig. 1.

Line velocity is also important

in discharge piping from steamtraps where two-phase steam/condensate mixtures must beslowed to allow some gravity sep-aration and reduce carryover ofcondensate from flash vent lines.Here line velocities of the flashsteam should not exceed 50/66 ft.per second. A much lower veloci-ty must be provided forseparation inside the flash vesselby expanding its size. The flashload is the total released by hotcondensate from all traps drain-ing into the receiver. Forcondensate line sizing example,see page 46 and see page 43 forvent line sizing example.

SYSTEMDESIGN

Sizing Steam Lines On Velocity

4

Fig. 1 lists steam capacities ofpipes under various pressure andvelocity conditions.EXAMPLE: Given a steam heat-ing system with a 100 psig inletpressure ahead of the pressurereducing valve and a capacity of1,000 pounds of steam per hourat 25 psig, find the smallest sizesof upstream and downstream pip-ing for reasonable quiet steamvelocities.

Upstream Piping SizingEnter the velocity chart at A for1,000 pounds per hour. Go overto point B where the 100 psigdiagonal line intersects. Follow upvertically to C where an inter-section with a diagonal line fallsinside the 4,000-6,000 foot-per-minute velocity band. Actualvelocity at D is about 4,800 feetper minute for 1-1/2 inchupstream piping.

Downstream Piping SizingEnter the velocity chart at A for1,000 pounds per hour. Go overto point E where the 25 psig diag-onal line intersects. Follow upvertically to F where an intersec-tion with a diagonal line fallsinside the 4,000-6,000 foot-per-minute velocity band. Actualvelocity at G is 5,500 feet perminute for 2-1/2 inch downstreampiping.

Pressure Drop in Steam LinesAlways check that pressure dropis within allowable limits beforeselecting pipe size in long steammains and whenever it is critical.Fig. 2 and Fig. 3 provide drops inSch. 40 and Sch. 80 pipe. Use ofthe charts is illustrated in the twoexamples.

EXAMPLE 1What will be the smallest sched-ule 40 pipe that can be used ifdrop per 100 feet shall notexceed 3 psi when flow rate is10,000 pounds per hour, andsteam pressure is 60 psig?Solution:1. Find factor for steam pres-

sure in main, in this case 60psig. Factor from chart = 1.5.

2. Divide allowable pressuredrop by factor 3 .–. 1.5 = 2 psi.

3. Enter pressure drop chart at2 psi and proceed horizontal-ly to flow rate of 10,000pounds per hour. Select pipesize on or to the right of thispoint. In this case a 4" main.

EXAMPLE 2What will be the pressure dropper 100 feet in an 8" schedule 40steam main when flow is 20,000pounds per hour, and steampressure is 15 psig?

Solution:Enter schedule 40 chart at20,000 pounds per hour, proceedvertically upward to 8" pipe curve,then horizontally to pressure dropscale, read 0.23 psi per 100 feet.This would be the drop if thesteam pressure were 100 psig.Since pressure is 15 psig, a cor-rection factor must be used.Correction factor for 15 psig = 3.60.23 x 3.6 = 0.828 psi drop per100 feet for 15 psig

SYSTEMDESIGN

Figure 1: Steam Velocity Chart

Steam Pipe Sizing For Pressure Drop

5

SYSTEMDESIGN

Figure 2: Pressure Drop in Schedule 40 Pipe

Figure 3: Pressure Drop in Schedule 80 Pipe

100 300200 400 500 1,000 2 3 4 5 10,000 2 3 4 5 6 7 8 100,000 2 3 4 5 1,000,000 2.1

.2

.3

.4

.5

.6

.7

.8

.91.0

2.0

3.0

4.05.06.07.08.09.0

10.0

15.03/4" 1" 1-1/4" 1-1/2" 2" 2-1/2" 3" 4" 5" 6" 8" 20"10" 12" 14" 16" 18"

24"

psi 0 2 5 10 15 20 30 40 60 75 90 100 110 125 150 175 200 225 250 300factor 6.9 6.0 5.2 4.3 3.6 3.1 2.4 2.0 1.5 1.3 1.1 1.0 0.92 0.83 0.70 0.62 0.55 0.49 0.45 0.38

350 400 500 6000.33 0.29 0.23 0.19

Steam Flow lbs/hr

100 psig Saturated SteamFor other pressures use correction factors

Pre

ssu

re D

rop

psi

/100

ft

psi 0 2 5 10 15 20 30 40 60 75 90 100 110 125 150 175 200 225 250 300factor 6.9 6.0 5.2 4.3 3.6 3.1 2.4 2.0 1.5 1.3 1.1 1.0 0.92 0.83 0.70 0.62 0.55 0.49 0.45 0.38

350 400 500 6000.33 0.29 0.23 0.19

Steam Flow lbs/hr

100 psig Saturated SteamFor other pressures use correction factors

Pre

ssu

re D

rop

psi

/100

ft

100 300200 400 500 1,000 2 3 4 5 10,000 2 3 4 5 6 7 8100,000 2 3 4 5 1,000,000 2.1

.2

.3

.4

.5

.6

.7

.8

.91.0

2.0

3.0

4.05.06.07.08.09.0

10.0

15.03/4" 1" 1-1/4" 1-1/2" 2" 2-1/2" 3" 4" 5" 6" 8" 20"10" 12" 14" 16" 18" 24"

6

Sizing Superheated Mains

6

Sizing Superheated MainsWhen sizing steam mains forsuperheated service, the follow-ing procedure should be used.Divide the required flow rate bythe factor in Table 2. This will givean equivalent saturated steamflow. Enter Fig. 1, Steam VelocityChart on page 4 to select appro-priate pipe size. If unable, thenuse the formula on page 3 to cal-culate cross sectional area of thepipe and then Tables 38 and 39,page 81, to select the pipe sizewhich closely matches calculatedinternal transverse area.

Example:Size a steam main to carry34,000 lb/h of 300 psig steam at atemperature of 500° F.From Table 2 the correction factoris .96. The equivalent capacity is34,000

.96 = 35,417 lb/h.

Since 300 psig is not found onFig. 1, the pipe size will have tobe calculated. From the formulaon page 3:

V =2.4 x Q x Vs

ASolving for area the formulabecomes:

A =2.4 x Q x Vs

V

Select a velocity of 10,000 ft/min.(which is within the processvelocity range of 8,000 - 12,000ft/min.) and determine Vs (specif-ic volume) of 1.47 ft3/lb (from theSteam Table on page 7). The for-mula is now:

A =2.4 x 35,417 x 1.47

= 12.5 in210,000

From Tables 38 and 39 (page 81)the pipe closest to this area is 4"schedule 40 or 5" schedule 80.

SYSTEMDESIGN

Table 2: Superheated Steam Correction Factor Gauge Saturated

Pressure Temp. TOTAL STEAM TEMPERATURE IN DEGREES FARENHEITPSI ˚F 340 360 380 400 420 440 460 480 500 520 540 560 580 600 620 640 660 680

15 250 .99 .99 .98 .98 .97 .96 .95 .94 .93 .92 .91 .90 .89 .88 .87 .86 .86 .8520 259 .99 .99 .98 .98 .97 .96 .95 .94 .93 .92 .91 .90 .89 .88 .87 .86 .86 .8540 287 1.00 .99 .99 .98 .97 .96 .95 .94 .93 .92 .91 .90 .89 .88 .87 .86 .86 .8560 308 1.00 .99 .99 .98 .97 .96 .95 .94 .93 .92 .91 .90 .89 .88 .87 .86 .86 .8580 324 1.00 1.00 .99 .99 .98 .97 .96 .94 .93 .92 .91 .90 .89 .88 .87 .86 .86 .85

100 338 – 1.00 1.00 .99 .98 .97 .96 .95 .94 .93 .92 .91 .90 .89 .88 .87 .86 .85120 350 – 1.00 1.00 .99 .98 .97 .96 .95 .94 .93 .92 .91 .90 .89 .88 .87 .86 .85140 361 – – 1.00 1.00 .99 .97 .96 .95 .94 .93 .92 .91 .90 .89 .88 .87 .86 .85160 371 – – – 1.00 .99 .98 .97 .96 .95 .94 .93 .92 .91 .90 .89 .88 .87 .86180 380 – – – 1.00 .99 .98 .97 .96 .95 .94 .93 .92 .91 .90 .89 .88 .87 .86

200 388 – – – 1.00 .99 .99 .97 .96 .95 .94 .93 .92 .91 .90 .89 .88 .87 .86220 395 – – – 1.00 1.00 .99 .98 .96 .95 .94 .93 .92 .91 .90 .89 .88 .87 .86240 403 – – – – 1.00 .99 .98 .97 .95 .94 .93 .92 .91 .90 .89 .88 .87 .86260 409 – – – – 1.00 .99 .98 .97 .96 .94 .93 .92 .91 .90 .89 .88 .87 .86280 416 – – – – 1.00 1.00 .99 .97 .96 .95 .93 .92 .91 .90 .89 .88 .87 .86

300 422 – – – – – 1.00 .99 .98 .96 .95 .93 .92 .91 .90 .89 .88 .87 .86350 436 – – – – – 1.00 1.00 .99 .97 .96 .94 .93 .92 .91 .90 .89 .88 .87400 448 – – – – – – 1.00 .99 .98 .96 .95 .93 .92 .91 .90 .89 .88 .87450 460 – – – – – – – 1.00 .99 .97 .96 .94 .93 .92 .91 .89 .88 .87500 470 – – – – – – – 1.00 .99 .98 .96 .94 .93 .92 .91 .90 .89 .88

550 480 – – – – – – – – 1.00 .99 .97 .95 .94 .92 .91 .90 .89 .88600 489 – – – – – – – – 1.00 .99 .98 .96 .94 .93 .92 .90 .89 .88650 497 – – – – – – – – – 1.00 .99 .97 .95 .94 .92 .91 .90 .89700 506 – – – – – – – – – 1.00 .99 .97 .96 .94 .93 .91 .90 .89750 513 – – – – – – – – – 1.00 1.00 .98 .96 .95 .93 .92 .90 .89

800 520 – – – – – – – – – – 1.00 .99 .97 .95 .94 .92 .91 .90850 527 – – – – – – – – – – 1.00 .99 .98 .96 .94 .93 .92 .90900 533 – – – – – – – – – – 1.00 1.00 .99 .97 .95 .93 .92 .90950 540 – – – – – – – – – – – 1.00 .99 .97 .95 .94 .92 .91

1000 546 – – – – – – – – – – – 1.00 .99 .98 .96 .94 .93 .91

700 720 740 760

.84 .83 .83 .82

.84 .83 .83 .82

.84 .84 .83 .82

.84 .84 .83 .82

.84 .84 .83 .82

.85 .84 .83 .82

.85 .84 .83 .82

.85 .84 .83 .82

.85 .84 .83 .82

.85 .84 .83 .83

.85 .84 .83 .83

.85 .84 .84 .83

.85 .84 .84 .83

.85 .85 .84 .83

.85 .85 .84 .83

.86 .85 .84 .83

.86 .85 .84 .83

.86 .85 .84 .84

.86 .86 .84 .84

.87 .86 .85 .84

.87 .86 .85 .84

.87 .86 .85 .84

.87 .86 .86 .85

.88 .87 .86 .85

.88 .87 .86 .85

.88 .87 .86 .85

.89 .88 .87 .86

.89 .88 .87 .86

.89 .88 .87 .86

.90 .89 .87 .86

Properties Of Saturated Steam

7

SYSTEMDESIGN

Table 3: Properties of Saturated SteamSpecific Specific

Gauge Temper- Heat in Btu/lb. Volume Gauge Temper- Heat in Btu/lb. VolumePressure ature Cu. ft. Pressure ature Cu. ft.

PSIG °F Sensible Latent Total per lb. PSIG °F Sensible Latent Total per lb.

25 134 102 1017 1119 142.0 185 382 355 843 1198 2.2920 162 129 1001 1130 73.9 190 384 358 841 1199 2.2415 179 147 990 1137 51.3 195 386 360 839 1199 2.1910 192 160 982 1142 39.4 200 388 362 837 1199 2.145 203 171 976 1147 31.8 205 390 364 836 1200 2.090 212 180 970 1150 26.8 210 392 366 834 1200 2.051 215 183 968 1151 25.2 215 394 368 832 1200 2.002 219 187 966 1153 23.5 220 396 370 830 1200 1.963 222 190 964 1154 22.3 225 397 372 828 1200 1.924 224 192 962 1154 21.4 230 399 374 827 1201 1.895 227 195 960 1155 20.1 235 401 376 825 1201 1.856 230 198 959 1157 19.4 240 403 378 823 1201 1.817 232 200 957 1157 18.7 245 404 380 822 1202 1.788 233 201 956 1157 18.4 250 406 382 820 1202 1.759 237 205 954 1159 17.1 255 408 383 819 1202 1.72

10 239 207 953 1160 16.5 260 409 385 817 1202 1.6912 244 212 949 1161 15.3 265 411 387 815 1202 1.6614 248 216 947 1163 14.3 270 413 389 814 1203 1.6316 252 220 944 1164 13.4 275 414 391 812 1203 1.6018 256 224 941 1165 12.6 280 416 392 811 1203 1.5720 259 227 939 1166 11.9 285 417 394 809 1203 1.5522 262 230 937 1167 11.3 290 418 395 808 1203 1.5324 265 233 934 1167 10.8 295 420 397 806 1203 1.4926 268 236 933 1169 10.3 300 421 398 805 1203 1.4728 271 239 930 1169 9.85 305 423 400 803 1203 1.4530 274 243 929 1172 9.46 310 425 402 802 1204 1.4332 277 246 927 1173 9.10 315 426 404 800 1204 1.4134 279 248 925 1173 8.75 320 427 405 799 1204 1.3836 282 251 923 1174 8.42 325 429 407 797 1204 1.3638 284 253 922 1175 8.08 330 430 408 796 1204 1.3440 286 256 920 1176 7.82 335 432 410 794 1204 1.3342 289 258 918 1176 7.57 340 433 411 793 1204 1.3144 291 260 917 1177 7.31 345 434 413 791 1204 1.2946 293 262 915 1177 7.14 350 435 414 790 1204 1.2848 295 264 914 1178 6.94 355 437 416 789 1205 1.2650 298 267 912 1179 6.68 360 438 417 788 1205 1.2455 300 271 909 1180 6.27 365 440 419 786 1205 1.2260 307 277 906 1183 5.84 370 441 420 785 1205 1.2065 312 282 901 1183 5.49 375 442 421 784 1205 1.1970 316 286 898 1184 5.18 380 443 422 783 1205 1.1875 320 290 895 1185 4.91 385 445 424 781 1205 1.1680 324 294 891 1185 4.67 390 446 425 780 1205 1.1485 328 298 889 1187 4.44 395 447 427 778 1205 1.1390 331 302 886 1188 4.24 400 448 428 777 1205 1.1295 335 305 883 1188 4.05 450 460 439 766 1205 1.00

100 338 309 880 1189 3.89 500 470 453 751 1204 .89105 341 312 878 1190 3.74 550 479 464 740 1204 .82110 344 316 875 1191 3.59 600 489 473 730 1203 .75115 347 319 873 1192 3.46 650 497 483 719 1202 .69120 350 322 871 1193 3.34 700 505 491 710 1201 .64125 353 325 868 1193 3.23 750 513 504 696 1200 .60130 356 328 866 1194 3.12 800 520 512 686 1198 .56135 358 330 864 1194 3.02 900 534 529 666 1195 .49140 361 333 861 1194 2.92 1000 546 544 647 1191 .44145 363 336 859 1195 2.84 1250 574 580 600 1180 .34150 366 339 857 1196 2.74 1500 597 610 557 1167 .23155 368 341 855 1196 2.68 1750 618 642 509 1151 .22160 371 344 853 1197 2.60 2000 636 672 462 1134 .19165 373 346 851 1197 2.54 2250 654 701 413 1114 .16170 375 348 849 1197 2.47 2500 669 733 358 1091 .13175 377 351 847 1198 2.41 2750 683 764 295 1059 .11180 380 353 845 1198 2.34 3000 696 804 213 1017 .08

IN V

AC

.

Draining Steam Mains

8

Steam main drainage is one of themost common applications forsteam traps. It is important thatwater is removed from steammains as quickly as possible, forreasons of safety and to permitgreater plant efficiency. A build-upof water can lead to waterham-mer, capable of fracturing pipesand fittings. When carried into thesteam spaces of heat exchang-ers, it simply adds to the thicknessof the condensate film andreduces heat transfer. Inadequatedrainage leads to leaking joints,and is a potential cause of wire-drawing of control valve seats.

WaterhammerWaterhammer occurs when a

slug of water, pushed by steampressure along a pipe instead ofdraining away at the low points, issuddenly stopped by impact on avalve or fitting such as a pipebend or tee. The velocities whichsuch slugs of water can achieveare not often appreciated. Theycan be much higher than the nor-mal steam velocity in the pipe,especially when the waterham-mer is occurring at startup.

When these velocities aredestroyed, the kinetic energy in thewater is converted into pressureenergy and a pressure shock isapplied to the obstruction. In mildcases, there is noise and perhapsmovement of the pipe. More severecases lead to fracture of the pipe orfittings with almost explosive effect,and consequent escape of livesteam at the fracture.

Waterhammer is avoided com-pletely if steps are taken to ensurethat water is drained away before itaccumulates in sufficient quantityto be picked up by the steam.

Careful consideration ofsteam main drainage can avoiddamage to the steam main andpossible injury or even loss of life.It offers a better alternative thanan acceptance of waterhammerand an attempt to contain it bychoice of materials, or pressurerating of equipment.

Efficient Steam MainDrainageProper drainage of lines, andsome care in start up methods,not only prevent damage bywaterhammer, but help improvesteam quality, so that equipmentoutput can be maximized andmaintenance of control valvesreduced.

The use of oversized steamtraps giving very generous “safe-ty factors” does not necessarilyensure safe and effective steammain drainage. A number ofpoints must be kept in mind, for asatisfactory installation.1) The heat up method

employed.2) Provision of suitable collect-

ing legs or reservoirs for thecondensate.

3) Provision of a minimum pres-sure differential across thesteam trap.

4) Choice of steam trap typeand size.

5) Proper trap installation.

Heat Up MethodThe choice of steam trap dependson the heat up method adopted tobring the steam main up to fullpressure and temperature. Thetwo most usual methods are:

(a) supervised start up and(b) automatic start up.

A) Supervised Start UpIn this case, at each drain point inthe steam system, a manual drainvalve is fitted, bypassing thesteam trap and discharging toatmosphere.

These drain valves areopened fully before any steam isadmitted to the system. When the“heat up” condensate has beendischarged and as the pressurein the main begins to rise, thevalves are closed. The conden-sate formed under operatingconditions is then dischargedthrough the traps. Clearly, thetraps need only be sized to han-dle the losses from the linesunder operating conditions, givenin Table 5 (page 10).

This heat up procedure ismost often used in large installa-tions where start up of the systemis an infrequent, perhaps even anannual, occurrence. Large heat-ing systems and chemicalprocessing plants are typicalexamples.

SYSTEMDESIGN

Figure 4Trap Boiler header or takeoff separator

and size for maximum carryover. On heavydemand this could be 10% of generating capacity

Separator

Trap Set

SteamSupply


9

B) Automatic Start UpOne traditional method of achiev-ing automatic start up is simply toallow the steam boiler to be firedand brought up to pressure withthe steam take off valve (crownvalve) wide open. Thus the steammain and branch lines come up topressure and temperature with-out supervision, and the steamtraps are relied on to automatical-ly discharge the condensate as itis formed.

This method is generally con-fined to small installations thatare regularly and frequently shutdown and started up again. Forexample, the boilers in manylaundry and drycleaning plantsare often shut down at night andrestarted the next morning.

In anything but the smallestplants, the flow of steam from theboiler into the cold pipes at startup, while the boiler pressure isstill only a few psi, will lead toexcessive carryover of boilerwater with the steam. Such carry-over can be enough to overloadseparators in the steam takeoff,where these are fitted. Greatcare, and even good fortune, areneeded if waterhammer is to beavoided.

For these reasons, modernpractice calls for an automaticvalve to be fitted in the steamsupply line, arranged so that thevalve stays closed until a reason-able pressure is attained in theboiler. The valve can then bemade to open over a timed periodso that steam is admitted onlyslowly into the distributionpipework. The pressure with theboiler may be climbing at a fastrate, of course, but the slow open-ing valve protects the pipework.

Where these valves areused, the time available to warmup the pipework will be known, asit is set on the valve control. Inother cases it is necessary toknow the details of the boiler startup procedure so that the time canbe estimated. Boilers started fromcold are often fired for a short

time and then shut off while tem-peratures equalize. The boilersare protected from undue stressby these short bursts of firing,which extend the warmup timeand reduce the rate at which con-densation in the mains is to bedischarged at the traps.

Determining Condensate LoadsAs previously discussed there aretwo methods for bringing a steammain “on line”. The supervisedstart up bypasses the traps thusavoiding the large warm up loads.The traps are then sized basedon the running conditions found inTable 5 (page 10). A safety factorof 2:1 and a differential pressureof inlet minus condensate returnpressure.

Systems employing automat-ic start up procedures requiresestimation of the amount of con-densate produced in bringing upthe main to working temperatureand pressure within the timeavailable. The amount of conden-sate being formed and thepressure available to discharge itare both varying continually andat any given moment are indeter-minate due to many unknownvariables. Table 4 (page 10) indi-cates the warm up loads per 100feet of steam main during a one

hour start up. If the start up timeis different, the new load can becalculated as follows:

lbs. of Condensate (Table 4) x 60Warm up time in minutes

= Actual warm-up load.

Apply a safety factor of 2:1and size the trap at a differentialpressure of working steam pres-sure minus condensate returnline presure. Since most driptraps see running loads muchmore often than start up loads,care must be taken when sizingthem for start up conditions. If thestart up load forces the selectionof a trap exceeding the capabilityof the “running load trap,” then thewarm up time needs to beincreased and/or the length ofpipe decreased.

SYSTEMDESIGN

Warm Up Load ExampleConsider a length of 8" main which is to carry steam at 125 psig. Drippoints are to be 150 ft. apart and outside ambient conditions can be aslow as 0°F. Warm-up time is to be 30 minutes.

From Table 4, Warm Up Load is 107 lb./100 ft.For a 150 ft run, load is 107 x 1.5 = 160.5 lb/150 ft.

Correction Factor for 0°F (see Table 4) 1.25 x 160.5 = 200.6 lb/150 ft.A 30 minute warm up time increases the load by

200.6 x 6030

= 401 lb/htotal load

Applying a safety factor of 2:1, the trap sizing load is 802 lb/h. If the backpressure in the condensate return is 0 psig, the trap would be sized fora 125 psi differential pressure.This would result in an oversized trap dur-ing running conditions, calculated at 94 lb/h using Tabe 5 (page 10).Either increase the warm up time to one hour or decrease the distancebetween drip traps.


10

SYSTEMDESIGN

Table 4: Warm-Up Load in Pounds of Steam per 100 Ft of Steam Main Ambient Temperature 70°F. Based on Sch. 40 pipe to 250 psi, Sch. 80 above 250 except Sch. 120 5" and larger above 800 psi

Steam O°FPressure Main Size Correction

psi 2" 21/2" 3" 4" 5" 6" 8" 10" 12" 14" 16" 18" 20" 24" Factor†0 6•2 9•7 12•8 18•2 24•6 31•9 48 68 90 107 140 176 207 308 1•505 6•9 11•0 14•4 20•4 27•7 35•9 48 77 101 120 157 198 233 324 1•44

10 7•5 11•8 15•5 22•0 29•9 38•8 58 83 109 130 169 213 251 350 1•4120 8•4 13•4 17•5 24•9 33•8 44 66 93 124 146 191 241 284 396 1•3740 9•9 15•8 20•6 90•3 39•7 52 78 110 145 172 225 284 334 465 1•3260 11•0 17•5 22•9 32•6 44 57 86 122 162 192 250 316 372 518 1•2980 12•0 19•0 24•9 35•3 48 62 93 132 175 208 271 342 403 561 1•27

100 12•8 20•3 26•6 37•8 51 67 100 142 188 222 290 366 431 600 1•26125 13•7 21•7 28•4 40 55 71 107 152 200 238 310 391 461 642 1•25150 14•5 23•0 30•0 43 58 75 113 160 212 251 328 414 487 679 1•24175 15•3 24•2 31•7 45 61 79 119 169 224 265 347 437 514 716 1•23200 16•0 25•3 33•1 47 64 83 125 177 234 277 362 456 537 748 1•22250 17•2 27•3 35•8 51 69 89 134 191 252 299 390 492 579 807 1•21300 25•0 38•3 51 75 104 143 217 322 443 531 682 854 1045 1182 1•20400 27•8 43 57 83 116 159 241 358 493 590 759 971 1163 1650 1•18500 30•2 46 62 91 126 173 262 389 535 642 825 1033 1263 1793 1•17600 32•7 50 67 98 136 187 284 421 579 694 893 1118 1367 1939 1•16800 38 58 77 113 203 274 455 670 943 1132 1445 1835 2227 3227 1•156

1000 45 64 86 126 227 305 508 748 1052 1263 1612 2047 2485 3601 1•1471200 52 72 96 140 253 340 566 833 1172 1407 1796 2280 2767 4010 1•1401400 62 79 106 155 280 376 626 922 1297 1558 1988 2524 3064 4440 1•1351600 71 87 117 171 309 415 692 1018 1432 1720 2194 2786 3382 4901 1•1301750 78 94 126 184 333 448 746 1098 1544 1855 2367 3006 3648 5285 1•1281800 80 97 129 189 341 459 764 1125 1584 1902 2427 3082 3741 5420 1•127

†For outdoor temperature of 0°F, multiply load value in table for each main size by correction factor shown.

Table 5: Running Load in Pounds per Hour per 100 Ft of Insulated Steam Main Ambient Temperature 70°F. Insulation 80% efficient. Load due to radiation and convection for saturated steam.

Steam 0°FPressure Main Size Correction

psi 2" 21/2" 3" 4" 5" 6" 8" 10" 12" 14" 16" 18" 20" 24" Factor†10 6 7 9 11 13 16 20 24 29 32 36 39 44 53 1•5830 8 9 11 14 17 20 26 32 38 42 48 51 57 68 1•5060 10 12 14 18 24 27 33 41 49 54 62 67 74 89 1•45

100 12 15 18 22 28 33 41 51 61 67 77 83 93 111 1•41125 13 16 20 24 30 36 45 56 66 73 84 90 101 121 1•39175 16 19 23 26 33 38 53 66 78 86 98 107 119 142 1•38250 18 22 27 34 42 50 62 77 92 101 116 126 140 168 1•36300 20 25 30 37 46 54 68 85 101 111 126 138 154 184 1•35400 23 28 34 43 53 63 80 99 118 130 148 162 180 216 1•33500 27 33 39 49 61 73 91 114 135 148 170 185 206 246 1•32600 30 37 44 55 68 82 103 128 152 167 191 208 232 277 1•31800 36 44 53 69 85 101 131 164 194 214 244 274 305 365 1•30

1000 43 52 63 82 101 120 156 195 231 254 290 326 363 435 1•271200 51 62 75 97 119 142 185 230 274 301 343 386 430 515 1•261400 60 73 89 114 141 168 219 273 324 356 407 457 509 610 1•251600 69 85 103 132 163 195 253 315 375 412 470 528 588 704 1•221750 76 93 113 145 179 213 278 346 411 452 516 580 645 773 1•221800 79 96 117 150 185 221 288 358 425 467 534 600 667 800 1•21

†For outdoor temperature of 0°F, multiply load value in table for each main size by correction factor shown.


11

Draining Steam MainsNote from the example that inmost cases, other than large dis-tribution mains, 1/2" Thermo-Dynamic® traps have amplecapacity. For shorter lengthsbetween drip points, and for smalldiameter pipes, the 1/2" lowcapacity TD trap more than meetseven start up loads, but on largermains it may be worth fitting par-allel 1/2" traps as in Fig. II-6 (page86). Low pressure mains are bestdrained using float and thermo-static traps, and these traps canalso be used at higher pressures.

The design of drip stationsare fairly simple. The most com-mon rules to follow for sizing thedrip pockets are:1. The diameter of the drip pock-

ets shall be the same size asthe distribution line up to 6inches in diameter.The diame-ter shall be half the size of thedistribution line over 6 inchesbut never less than 6 inches.

2. The length of the drip pocketshall be 1-1/2 times the diam-eter of the distribution line butnot less than 18 inches.

Drip Leg SpacingThe spacing between thedrainage points is often greaterthan is desirable. On a long hori-zontal run (or rather one with afall in the direction of the flow ofabout 1/2" in 10 feet or 1/250)drain points should be provided atintervals of 100 to 200 feet.Longer lengths should be split upby additional drain points. Anynatural collecting points in thesystems, such as at the foot ofany riser, should also be drained.

A very long run laid with a fallin this way may become so lowthat at intervals it must be elevatedwith a riser. The foot of each ofthese “relay points” also requires acollecting pocket and steam trap.

Sometimes the ground con-tours are such that the steammain can only be run uphill. Thiswill mean the drain points shouldbe at closer intervals, say 50 ft.apart, and the size of the mainincreased. The lower steamvelocity then allows the conden-sate to drain in the oppositedirection to the steam flow.

Air venting of steam mains isof paramount importance and isfar too often overlooked. Steamentering the pipes tends to pushthe air already there in front of itas would a piston. Automatic airvents, fitted on top of tees at theterminal points of the main andthe larger branches, will allow dis-charge of this air. Absence of airvents means that the air will passthrough the steam traps (where itmay well slow down the dis-charge of condensate) or throughthe steam using equipment itself.

SYSTEMDESIGN

Figure 5Draining and Relaying Steam Main

Fall 1/2" in 10 FtSteam

Steam Trap

Steam Trap

Steam TrapSteam TrapSteam Trap

Condensate

The majority of steam traps in refineries are installed onsteam main and steam tracing systems. Thoroughdrainage of steam mains/branch lines is essential for effec-tive heat transfer around the refinery and for waterhammerprevention. This holds true for condensate drainage fromsteam tracing lines/jackets, though some degree of back-up (or sub-cooling) is permissible in some applications.

The predominant steam trap installed is a non-repairable type that incorporates a permanent pipelineconnector. Scattered throughout the system are a numberof iron and steel body repairable types.

Most notable failure of steam traps are precipitate for-mation on bucket weep-holes and discharge orifices thateventually plugs the trap shut. A common culprit is valvesealing compound injected into leaking valves which formssmall pellets that settle in low points, such as driplegs/steam traps and on strainer screens making blowdown difficult. This problem also occurs during occasional“system upset” when hydrocarbon contaminants are mis-takenly introduced to the steam system.

A noise detector and/or a temperature-indicatingdevice is required to detect trap failure. Especially costly is

the fact that operators are not allowed to remove traps forrepair when threading from the line is required.Maintenance personnel must be involved.

SolutionUniversal connector steam traps were installed for trial inone of the dirtiest drip stations at the refinery. The trapsheld up under adverse operating conditions requiring onlyperiodic cleaning. Since the time of installation, all failedinverted bucket traps in this service were replaced with uni-versal connector traps. Strainers were installed upstreamof each.

Benefits• The addition of Thermo-Dynamic® traps allowed for eas-

ier field trap testing.• The addition of universal connectors significantly

reduced steam trap installation and repair time.• 33% reduction in steam trap inventory due to standard

trap for all sizes.• Reduced energy loss is significantly reduced using Thermo-

Dynamic® steam traps versus original inverted bucket traps.

Case in Action: Steam Main and Steam Tracing System Drainage

Steam Tracing

12

The temperature of process liquidsbeing transferred through pipelinesoften must be maintained to meetthe requirements of a process, toprevent thickening and solidifica-tion, or simply to protect againstfreezeup. This is achieved by theuse of jacketed pipes, or by attach-ing to the product line one or moreseparate tracer lines carrying aheating medium such as steam orhot water.

The steam usage may be rel-atively small but the tracingsystem is often a major part ofthe steam installation, and thesource of many problems.

Many large users and plantcontractors have their owninhouse rules for tracer lines, butthe following guidelines may beuseful in other cases. We havedealt only with external tracing,this being the area likely to causedifficulties where no existingexperience is available. Externaltracing is simple and thereforecheap to install, and fulfills theneeds of most processes.

External Tracer LinesOne or more heat carrying lines, ofsizes usually from 3/8" up to 1"nominal bore are attached to themain product pipe as in Fig. 6.Transfer of heat to the product linemay be three ways—by conductionthrough direct contact, by convec-tion currents in the air pocketformed inside the insulating jacket,and by radiation. The tracer linesmay be of carbon steel or copper,or sometimes stainless steel.

Where the product line is of aparticular material to suit the fluidit is carrying, the material for thetracer line must be chosen toavoid electrolytic corrosion at anycontact points.

For short runs of tracer, suchas around short vertical pipes, orvalves and fittings, small bore cop-per pipes, perhaps 1/4" bore maybe wound around the product linesas at Fig. 7. The layout should bearranged to give a continuous fallalong the tracers as Fig. 9a rather

than Fig. 9b, and the use of wraparound tracers should be avoidedon long horizontal lines.

A run of even 100 ft. of 6 inchproduct line will have a total ofabout 500 to 600 ft. of wraparound tracer. The pressure dropalong the tracer would be veryhigh and the temperature at theend remote from the supply wouldbe very low. Indeed, this end ofthe tracer would probably containonly condensate and the temper-ature of this water would fall as itgives up heat. Where steam ispresent in the tracer, lifting thecondensate from the multiplicity oflow points increases the problemsassociated with this arrangement.

SYSTEMDESIGN

Figure 6Tracer Attached To Product Line

LaggingProduct

AluminumFoil

Air Space

Tracer

Figure 7Small Bore TracingWraped AroundVertical Product Line

Figure 8Clipping Tracer Around Bends

Figure 9 Continuous Fall On Wrap Around Tracer

Figure 10 Attaching Tracer To Line

Figure 10a Short Run Welds

Figure 10b Continuous Weld

Figure 10c Heat Conducting Paste

Product

Lagging

Tracer

HeatConducting

Paste

9a 9b

Steam Tracing

13

Clip On TracersThe simplest form of tracer is onethat is clipped or wired on to themain product line. Maximum heatflow is achieved when the traceris in tight contact with the productline. The securing clips should beno further apart than 12" to 18"on 3/8" tracers, 18" to 24 on 1/2",and 24" to 36" on 3/4" and larger.

The tracer pipes can be liter-ally wired on, but to maintainclose contact it is better to useeither galvanized or stainlesssteel bands, about 1/2" wide and18 to 20 gauge thickness. Onevery practical method is to use apacking case banding machine.Where tracers are carried aroundbends particular care should betaken to ensure that good contactis maintained by using three ormore bands as in Fig. 8.

Where it is not possible to usebands as at valve bodies, softannealed stainless steel wire 18gauge thick is a useful alternative.Once again, any special needs toavoid external corrosion or elec-trolytic action may lead to thesesuggestions being varied.

Welded TracersWhere the temperature differencebetween the tracer and the prod-uct is low, the tracer may bewelded to the product line. Thiscan be done either by short runwelds as Fig. 10a or by a continu-ous weld as Fig. 10b formaximum heat transfer.

In these cases the tracer issometimes laid along the top ofthe pipe rather than at the bot-tom, which greatly simplifies thewelding procedure. Advocates ofthis method claim that this loca-tion does not adversely affect theheat transfer rates.

Heat Conducting PasteFor maximum heat transfer, it canbe an advantage to use a heatconducting paste to fill the normalhot air gap as in Fig. 10c. Thepaste can be used to improveheat transfer with any of the clip-ping methods described, but it isessential that the surfaces arewirebrushed clean before apply-ing the paste.

Spacer TracingThe product being carried in theline can be sensitive to tempera-ture in some cases and it is thenimportant to avoid any local hotspots on the pipe such as couldoccur with direct contact betweenthe tracer and the line.

This is done by introducing astrip of insulating materialbetween the tracer and the prod-uct pipe such as fiberglass,mineral wool, or packing blocks ofan inert material.

InsulationThe insulation must cover boththe product line and the tracer butit is important that the air spaceremains clear. This can beachieved in more than one way.1. The product line and tracer

can first be wrapped with alu-minum foil, or by galvanizedsteel sheet, held on by wiringand the insulation is thenapplied outside this sheet.Alternatively, small mesh gal-vanized wire netting can beused in the same way asmetal sheet Fig. 11a.

2. Sectional insulation, pre-formed to one or two sizeslarger than the product main,can be used.This has the dis-advantage that it can easilybe crushed Fig. 11b.

3. Preformed sectional insula-tion designed to cover bothproduct line and tracer canbe used, as Fig. 11c.Preformed sectional insula-

tion is usually preferred to plasticmaterial, because being rigid itretains better thickness and effi-ciency. In all cases, the insulationshould be properly finished withwaterproof covering. Most insula-tion is porous and becomesuseless as heat conserving mate-rial if it is allowed to absorb water.Adequate steps may also beneeded to protect the insulationfrom mechanical damage.

SYSTEMDESIGN

Figure 11Insulating Tracer and Product Lines

ProductLagging

TracerAluminum

Foil

Wire Netting

ProductLagging

Tracer

Product

Lagging

Tracer

11b 11c11a

Steam Tracing

14

Sizing of External TracersThe tracing or jacketing of any linenormally aims at maintaining thecontents of the line at a satisfacto-ry working temperature under allconditions of low ambient temper-ature with adequate reserve tomeet extreme conditions.

Remember that on someexposed sites, with an ambientstill air temperature of say 0°F, theeffect of a 15 mph wind will be tolower the temperature to anequivalent of -36°F.

Even 32°F in still air can belowered to an effective 4°F with a20 mph wind—circumstanceswhich must be taken into full con-sideration when studying thetracer line requirements.

Details of prevailing condi-tions can usually be obtainedfrom the local meteorologicaloffice or civil air authority.

Most of the sizing of externaltracers is done by rule of thumb,but the problem which arises hereis what rule and whose thumb?

Rules of thumb are generallybased on the experiences of a cer-tain company on a particularprocess and do not necessarilyapply elsewhere. There are alsowidely differing opinions on the lay-out: some say that multiple tracersshould all be below the center lineof the product line while others say,

with equal conviction, that it is per-fectly satisfactory to space thetracers equally around the line.

Then there are those who willendeavor to size their tracers from3/8", 1/2", 3/4" or 1" and evenlarger pipe: while another schoolof thought says that as tracershave only minute contact with theproduct line it will give much moreeven distribution of heat if all trac-ers are from 1/2" pipe in multiplesto meet the requirements. Thisdoes have the added advantageof needing to hold a stock of onlyone size of pipe and fittings ratherthan a variety of sizes.

For those who like to followthis idea, Table 6 will be useful formost average requirements.

Type A would suffice for mostfuel oil requirements and wouldalso meet the requirement ofthose lines carrying acid, phenol,water and some other chemicals,but in some cases spacersbetween the product line andsteam line would be employed.

The steam pressure is impor-tant and must be chosenaccording to the product temper-ature required.

For noncritical tracing TypesA & B (Table 6) a steam pressureof 50 psi would generally be suit-able. For Type C, a higherpressure and a trap with a hotdischarge may be required.

Jacketed LinesIdeally jacketed lines should beconstructed in no more than 20 ft.lengths and the condensateremoved from each section.Steam should enter at the highestend so that there is a natural fall tothe condensate outlet as Fig. 12a.

When it is considered imprac-tical to trap each length, anumber of lengths up a total of80-100 ft. approx. may be joinedtogether in moderate climates,but in extremely cold parts of theworld 40 ft. should be the maxi-mum. See Fig. 12b.

Always avoid connectingsolely through the bottom loop.This can only handle the conden-sate and impedes the free flow ofsteam as Fig. 12c. As a generalguide, see Table 7.

Although in most cases 1/2"condensate outlet will be ade-quate, it is usual to make this thesame size as the steam connec-tion as it simplifies installation.

External TracersIn horizontal runs, the steam willgenerally flow parallel to the prod-uct line, but as far as possible,steam should enter from the highend to allow free flow of the con-densate to the low end, i.e. itshould always be self-draining.

It is generally consideredpreferable to fit one tracer on thebottom of the line as Fig. 13a, twotracers at 30° as Fig. 13b, threetracers at 45° as Fig. 13c.

Where multiple 1/2" tracersare used, they should be arrangedin loop fashion on either side of theproduct line, as Fig. 14. In verticallines, the tracers would be spaceduniformly, as Fig. 15a & b.

The maximum permissiblelength of tracer will depend to someextent on the size and initial steampressure, but as a general guide3/8” tracers should not exceed 60ft. in length and the limit for all othersizes should be about 150 ft.

Bends and low points in thetracer, as Fig. 16a should alwaysbe avoided. For example, if it isnecessary to carry a tracer lineround a pipe support or flange,

SYSTEMDESIGN

Table 6: Number of 1/2" (15mm) Tracers Usedwith Different Sizes of Product Lines

Type A Type B Type CNoncritical Noncritical Critical

General frost protection or Where solidification may When solidification maywhere solidification may occur at temps between occur at temps between

occur at temps below 75°F 75-150°F 150-300°F

Product Number of 1/2" Number of 1/2" Number of 1/2"Line Size Tracers Tracers Tracers1" 1 1 111/2" 1 1 22" 1 1 23" 1 1 34" 1 2 36" 2 2 38" 2 2 310"-12" 2 3 614"-16" 2 3 818"-20" 2 3 10

Steam Tracing

15

this should be done in the hori-zontal plane, Fig. 16b.

Where it is essential to main-tain the flow of heat to theproduct, the tracer should betaken up to the back of the flangeFig. 17, and the coupling shouldalways be on the center line ofthe flanged joint.

The same applies to an in-line run where the tracer has tobe jointed. This can be done intwo ways, Fig. 18 or Fig. 19.

Each of these is preferable toFig. 20 which could produce acold spot. Where two tracers areused it can be better to doubleback at a union or flange as Fig.21, rather than jump over it.

ExpansionExpansion in tracer lines is oftenoverlooked. Naturally the steamheated tracer will tend to expandmore than the product line.Wherethe tracer has to pass aroundflanges, the bends are quite ade-quate to take care of theexpansion, Fig. 22.

But where this does not occurand there is a long run of uninter-rupted tracer, it is essential toprovide for expansion which canbe done by forming a completeloop, Fig. 23.

SYSTEMDESIGN

Table 7: Steam Connection Size for Jacketed Lines Product Jacket Steam

Line Diameter Connection2-1/2" 65mm 4" 100mm 1/2" 15mm

3" 80mm 6" 150mm 3/4" 20mm4" 100mm 6" 150mm 3/4" 20mm6" 150mm 8" 200mm 3/4" 20mm8" 200mm 10" 250mm 1" 25mm10" 250mm 12" 300mm 1" 25mm

Figure 13Single and Multiple Tracing

Figure 12aJacketed Lines, Drained Separately

Figure 12bJacketed Lines, Connected

Figure 12cIncorrect Arrangement of Jacketed Lines

13a 13b 13c

Figure 14Multiple Tracing

Figure 15 Vertical Tracing

15a 15b

Figure 16a Incorrect Arrangement

Figure 16b Correct Arrangement

Figure 17

Figure 18

Figure 22Correct Arrangement

Figure 19 for Tracer-line Joints

Figure 20 Incorrect Arrangement

Figure 21Dual Tracer Double Back

Figure 23Expansion Arrangementson Long Tracers

Steam

SteamFall

Fall

Steam TrapSteam Trap

SteamTrap

Steam Trap

Steam

General Installation

Steam Tracing

16

Tracer Steam DistributionIt is important that the steam sup-ply should always be taken from asource which is continuouslyavailable, even during a normalshut down period.

Tracer lines and jacketed pipemay have to work at any steampressure (usually in the rangebetween 10 and 250 psi, butalways choose the lowest pres-sure to give the required producttemperature. Excessively highpressures cause much waste andshould only be used where a highproduct temperature is essential).

To suit product temperaturerequirements, it may be necessaryto use steam at different pres-sures. It should be distributed atthe highest pressure and reduceddown to meet the lower pressurerequirements. A Reducing Valvecan be used for this purpose, Fig.24. Note: it may be necessary tosteam trace the valve body to pre-vent damage due to freezing..

A number of tracers can besupplied from one local distribu-tion header. This header shouldbe adequately sized to meet themaximum load and drained at itslow point by a steam trap as Fig.25. All branches should be takenoff the top of this header, onebranch to each tracer line. Thesebranches should be fitted withisolating valves.

Don’t undersize these branchconnections (1/2" supply to even a3/8" tracer will avoid undue pres-sure drop) and serve only tracers

local to the header, otherwise highpressure drop may result.

The size of the header will, ofcourse, depend upon the steampressure and the total load on thetracers but as a general guide,see Table 8:

Tracer Trap SizingSubcooled discharge traps areusually a good choice for tracerservice.Tracing loads are approx-imately 10 to 50 lb./hr., and eachtracer requires its own low capaci-ty trap.

No two tracers can haveexactly the same duty, so grouptrapping two or more tracers toone trap can considerably impairthe efficiency of heat transfer, seeFig. 26 and Fig. 27.

Even with multiple tracers ona single product line, each tracer

should be separately trapped—Fig. 28.

When branched tracers aretaken to serve valves, then eachshould be separately trapped,Figs. 29, 30, 31 and 32.

SYSTEMDESIGN

Table 8 Recommended header size

for supplying steam tracer lines

Header Size Number of 1/2" Tracers3/4" 21" 3-5

11/2" 6-152" 16-30

Recommended header sizefor condensate lines

Header Size Number of 1/2" Tracers1" Up to 5

11/2" 6-102" 11-25 Figure 28

Header Steam Trap

Tracers

Figure 27Correct Arrangement

Figure 26Incorrect Arrangement

Figure 25

Steam Trap

Steam Trap

Steam Trap

Steam Trap

Steam TrapSteamTrap

Steam Trap

Steam Trap

Steam

Steam

Steam

3/8" (10mm) OD1/4" (6mm) Bore

3/8" (10mm) OD, 1/4" (6mm) Bore

Figure 31Tracer Lines Around Pump Casing

Figure 29

Figure 30

Figure 24Spirax SarcoReducingValve

Steam

Steam

Steam

Steam Tracing

17

Important—Getting Rid of the MuckPipes delivered to the site maycontain mill scale, paint, preserv-ing oils, etc. and during storageand erection will collect dirt, sand,weld splatter and other debris, sothat on completion, the averagetracer line contains a consider-able amount of “muck.”

Hydraulic testing will convertthis “muck” into a mobile sludgewhich is not adequately washedout by simply draining down aftertesting.

It is most important that thelines are properly cleaned byblowing through with steam to anopen end before diverting to thesteam traps.

Unless this is done, the trapswill almost certainly fail to operatecorrectly and more time will bespent cleaning them out when theplant is commissioned.

Steam Traps For Tracer LinesAlmost any type of steam trapcould be used to drain tracerlines, but some lend themselvesto this application better than oth-ers. The traps should bephysically small and light inweight, and as they are often fit-ted in exposed positions, theyshould be resistant to frost. Thetemperature at which the conden-sate is discharged by the trap isperhaps the most important con-sideration when selecting thetype of trap.

Thermo-Dynamic® traps arethe simplest and most robust ofall traps, they meet all of theabove criteria and they dischargecondensate at a temperatureclose to that of steam. Thus theyare especially suitable on thosetracing applications where theholding back of condensate in thetracer line until it has subcooledwould be unacceptable. Tracersor jackets on lines carrying sul-phur or asphalt typify theseapplications where the tracermust be at steam temperaturealong its whole length.

It must be remembered thatevery time a Thermo-Dynamic®

trap opens, it discharges conden-sate at the maximum ratecorresponding to the differentialpressure applied. The instanta-neous release rates of the steamflashing off the condensate canbe appreciable, and care is need-ed to ensure that condensatereturn lines are adequately sized

if high back pressures are to beavoided. Thus, the use of sweptback or “y” connections from trapdischarges into common headersof generous size will help avoidproblems.

Where the traps are exposedto wind, rain or snow, or low ambi-ent temperatures, the steambubbles in the top cap of the trapcan condense more quickly, lead-ing to more rapid wear. Specialinsulating caps are available forfitting to the top caps to avoid this,Fig. 33.

In other non-critical applica-tions, it can be convenient andenergy efficient to allow the con-densate to sub-cool within thetracer before being discharged.This enables use to be made ofsome of the sensible heat in thecondensate, and reduces or eveneliminates the release of flashsteam. Temperature sensitivetraps are then selected, usingeither balanced pressure orbimetallic elements.

The bimetallic traps usuallydischarge condensate at somefairly constant differential such as50°F below condensing tempera-tures, and tend to give acontinuous dribble of condensatewhen handling tracer loads, help-ing minimize the size ofcondensate line needed. Theyare available either in maintain-able versions, with a replaceableelement set which includes thevalve and seat as well as thebimetallic stack, or as sealednon-maintainable units asrequired.

Balanced pressure traps nor-mally operate just below steamtemperature, for critical tracingapplications, see Fig. 34.

The trap is especially suitablewhere small quantities ofcondensate are produced, onapplications where sub-cooling isdesirable, and where the conden-sate is not to be returned to therecovery system.

SYSTEMDESIGN

Figure 32Typical Instrument Tracing

Steam Trap

Steam

3/8" (10mm) OD1/4" (6mm) Bore

1/2" (15mm) OD

Figure 34Balanced Pressure Tracer Trap

Figure 33Insulating Cap forThermo-Dynamic®

Trap

Steam Tracing

18

A similar but maintainabletype intended for use on instru-ment tracer lines, where thephysical size of the trap is impor-tant as well as its operatingcharacteristics is shown in Fig. 35.

Just as the distribution ofsteam is from a common header,it often is convenient to connect anumber of traps to a commoncondensate header and this sim-plifies maintenance. As noted, thedischarge should preferably enterthe header through swept con-nections and the headers beadequately sized as suggested inTable 8 (page 16).

SYSTEMDESIGN

During steam tracing project design, it was found that fivethousand feet of 2" product piping was to be traced with150 psig steam. Product temperature was to be maintainedat 100°F, with maximum allowable temperature of 150°Fand a minimum allowable temperature of 50°F.

Of particular concern was the fact that the pipelinewould always be full of the product, but flow would beintermittent. Overheating could be a real problem. In addi-tion, the tracing system had to be protected from freezing.

SolutionThe 5,000 feet of product piping was divided into 30 sep-arate traced sections including: a cast steel temperatureregulator, a bronze temperature control valve used as ahigh limit safety cutout, a sealed balanced pressure ther-mostatic steam trap, a vacuum breaker, and pressureregulators supplying steam to all 30 tracing sections. Eachsection operates effectively at the desired temperature,regardless of flow rate or ambient temperature.

Benefits• The chance of product damage from overheating is min-

imized and steam consumption is reduced throughsteam pressure reduction (150 psig to 50 psig) with thepressure regulator.

• The product temperature is maintained at a consistentset temperature, maximizing process control under allflow conditions with the temperature regulator.

• Product damage from overheating is prevented throughuse of the high limit safety cutout. The system will shutdown completely, should the temperature regulator over-shoot its set point.

• The tracing system is protected from freezing with thesealed balanced pressure thermostatic steam trap dis-charging to drain. Thorough drainage is also facilitatedby the vacuum breaker.

Case in Action: Product Steam Tracing with Temperature Control and Overheat Protection

These may be increasedwhere high pressures and trapsdischarging condensate at nearsteam temperature are used, ordecreased with low pressuresand traps discharging cooler con-densate.

Temperature Controlof TracerWhere it is essential to preventoverheating of the product, orwhere constant viscosity isrequired for instrumentation,automatic temperature control isfrequently used.

On many systems, the sim-plest way to achieve control is touse a reducing valve on thesteam supply to the tracer lines orjacket. This can be adjusted in thelight of experience to give the cor-rect steam pressure to producethe required product temperature.

Clearly this is an approximateway to control product tempera-ture and can only be used wherethe product flow is fairly constant.Where closer control is required,the simple direct acting tempera-

Figure 35Maintainable Balanced PressureTracer Trap.

ture control often provides aneconomic solution. This will giveclose control and since it is notnecessary to provide either elec-tric power or compressed air, thefirst cost and indeed the runningcosts are low.

Pressure Reducing Stations

19

Pressure Reducing StationsIt is a mistake to install even thebest of pressure reducing valvesin a pipeline without giving somethought to how best it can behelped to give optimal perfor-mance.

The valve selected should beof such a size that it can handlethe necessary load, but oversiz-ing should be avoided.The weightof steam to be handled in a giventime must be calculated or esti-mated, and a valve capable ofpassing this weight from thegiven upstream pressure to therequired downstream pressure ischosen. The valve size is usuallysmaller than the steam pipeseither upstream or downstream,because of the high velocitieswhich accompany the pressuredrop within the valve.

Types of Pressure ReducingValves are also important andcan be divided into three groupsof operation as follows:

Direct Operated ValvesThe direct acting valve shown inFig. II-17 (page 91) is the sim-plest design of reducing valve.

This type of valve has twodrawbacks in that it allows greaterfluctuation of the downstreampressure under unstable loaddemands, and these valves haverelatively low capacity for theirsize. It is nevertheless perfectlyadequate for a whole range ofsimple applications where accu-rate control is not essential andwhere the steam flow is fairlysmall and constant.

Pilot Operated ValvesWhere accurate control of pres-sure or large capacity is required,a pilot operated reducing valveshould be used. Such a valve isshown in Fig. II-12 (page 89).

The pilot operated designoffers a number of advantages overthe direct acting valve. Only a verysmall amount of steam has to flowthrough the pilot valve to pressurizethe main diaphragm chamber and

fully open the main valve. Thus,only very small changes in down-stream pressure are necessary toproduce large changes in flow. The“droop” of pilot operated valves istherefore small. Although any risein upstream pressure will apply anincreased closing force on the mainvalve, this is offset by the force ofthe upstream pressure acting onthe main diaphragm.The result is avalve which gives close control ofdownstream pressure regardlessof variations on the upstream side.

Pneumatically OperatedValvesPneumatically operated controlvalves, Fig. II-20 (page 93), withactuators and positioners beingpiloted by controllers, will providepressure reduction with evenmore accurate control.

Controllers sense down-stream pressure fluctuations,interpolate the signals and regu-late an air supply signal to apneumatic positioner which in turnsupplies air to a disphragm open-ing a valve. Springs are utilized asan opposing force causing thevalves to close upon a loss orreduction of air pressure appliedon the diaphragm. Industrysophistication and control needsare demanding closer and moreaccurate control of steam pres-sures, making pneumatic controlvalves much more popular today.

Piping And Noise Considera-tionThe piping around a steam pres-sure reducing valve must beproperly sized and fitted for bestoperation. Noise level of a reduc-ing station is lowest when thevalve is installed as follows:1. Avoid abrupt changes in

direction of flow. Use longradius bends and “Y” pipinginstead of “T” connections.

2. Limit approach and exitsteam velocity to 4000 to6000 FPM.

3. Change piping gradually

before and after the valve withtapered expanders, or changepipe only 1 or 2 sizes at a time.

4. Provide long, straight, full-sizeruns of heavy wall pipe onboth sides of the valve, andbetween two-stage reductionsto stabilize the flow.

5. Use low pressure turndownratios (non-critical.)

6. Install vibration absorbingpipe hangers and acousticalinsulation.Most noise is generated by a

reducing valve that operates atcritical pressure drop, especiallywith high flow requirements.Fitting a noise diffuser directly tothe valve outlet will reduce thenoise level by approx. 15 dBA.

It must also be rememberedthat a valve designed to operate onsteam should not be expected towork at its best when supplied witha mixture of steam, water and dirt.

A separator, drained with asteam trap, will remove almost allthe water from the steam enteringthe pressure reducing set. Thebaffle type separator illustrated inFig. 36 has been found to be veryeffective over a broad range offlows.

SYSTEMDESIGN

Figure 36Moisture Separator for Steam or Air

Pressure Reducing Stations

20

PRV Station ComponentsA stop valve is usually needed sothat the steam supply can be shutoff when necessary, and thisshould be followed by a line sizestrainer. A fine mesh stainlesssteel screen in the strainer willcatch the finer particles of dirtwhich pass freely through stan-dard strainers. The strainer shouldbe installed in the pipe on its side,rather than in the conventionalway with the screen hangingbelow the pipe. This is to avoid thescreen space acting as a collect-ing pocket for condensate, sincewhen installed horizontally thestrainer can be self-draining

Remember that water whichcollects in the conventionally pipedstrainer at times when the reducingvalve has closed, will be carriedinto the valve when it begins toopen. This water, when forcedbetween the valve disc and seat ofthe just-opening valve, can leadrapidly to wire-drawing, and theneed for expensive replacements.

Pressure gauges at each sideof the reducing valve allow its per-formance to be monitored. At thereduced pressure side of the valve,a relief or safety valve may berequired. If all the equipment con-nected on the low pressure side iscapable of safely withstanding theupstream pressure in the event ofreducing valve failure, the reliefvalve may not be needed. It maybe called for if it is sought to protectmaterial in process from overlyhigh temperatures, and it is essen-tial if any downstream equipmentis designed for a pressure lowerthan the supply pressure.

Steam Safety Valve SizingWhen selecting a safety valve, thepressure at which it is to openmust be decided. Opening pres-sure must be below the limitationsof the downstream equipment yetfar enough above the normalreduced pressure that minor fluc-tuations do not cause opening ordribbling. Type “UV” Safety Valvesfor unfired pressure vessels aretested to ASME Pressure VesselCode, Section VIII and achieverated capacity at an accumulatedpressure 10% above the set-to-

open pressure. Safety valves foruse on boilers carry a “V” stampand achieve rated capacity at only3% overpressure as required bySection I of the Code.

The capacity of the safetyvalve must then equal or exceed

SYSTEMDESIGN

Figure 37Typical Installation of Single Reducing Valve with Noise Diffuser

the capacity of the pressurereducing valve, if it should failopen when discharging steamfrom the upstream pressure to theaccumulated pressure at the safe-ty valve. Any bypass line leakagemust also be accounted for.

Figure 38Typical Installationof Two ReducingValves in Parallel

Separator

PressureSensing Line

ReducingValve

Figure 39Two-Stage Pressure Reducing Valve Stationwith Bypass Arrangement to Operate EitherValve Independently on Emergency Basis

Safety Valve

Diffuser

Trap Set

Downstream Isolating Valve isneeded only with an alternativesteam supply into the L.P. System

Bypasses may be prohibitedby local regulation or byinsurance requirements

Parallel and Series Operation of Reducing Valves

21

Parallel OperationIn steam systems where loaddemands fluctuate through a widerange, multiple pressure controlvalves with combined capacitiesmeeting the maximum load per-form better than a single, largevalve. Maintenance needs, down-time and overall lifetime cost canall be minimized with this arrange-ment, Fig. 38 (page 20).

Any reducing valve must becapable of both meeting its maxi-mum load and also modulatingdown towards zero loads whenrequired. The amount of loadturndown which a given valve cansatisfactorily cover is limited, andwhile there are no rules whichapply without exception, if the lowload condition represents 10% orless of the maximum load, twovalves should always be pre-ferred. Consider a valve whichmoves away from the seat by 0.1inches when a downstream pres-sure 1 psi below the set pressureis detected, and which then pass-es 1,000 pounds per hour ofsteam. A rise of 0.1 psi in thedetected pressure then movesthe valve 0.01 inches toward the

seat and reduces the flow byapproximately 100 pph, or 10%.

The same valve might laterbe on a light load of 100 pph totalwhen it will be only 0.01 inchesaway from the seat. A similar risein the downstream pressure of0.1 psi would then close the valvecompletely and the change inflow through the valve which was10% at the high load, is now100% at low load. The figureschosen are arbitrary, but the prin-ciple remains true that instabilityor “hunting” is much more likelyon a valve asked to cope with ahigh turndown in load.

A single valve, when used inthis way, tends to open and close,or at least move further open andfurther closed, on light loads. Thisaction leads to wear on both theseating and guiding surfaces andreduces the life of thediaphragms which operate thevalve. The situation is worsenedwith those valves which use pis-tons sliding within cylinders toposition the valve head. Frictionand sticking between the slidingsurfaces mean that the valvehead can only be moved in a

series of discreet steps.Especially at light loads, suchmovements are likely to result inchanges in flow rate which aregrossly in excess of the loadchanges which initiate them.Load turndown ratios with piston-operated valves are almostinevitably smaller than wherediaphragm-operated valves arechosen.

Pressure Settingsfor Parallel ValvesAutomatic selection of the valveor valves needed to meet givenload conditions is readilyachieved by setting the valves tocontrol at pressures separated byone or two psi. At full load, orloads not too much below fullload, both valves are in use. Asthe load is reduced, the controlledpressure begins to increase andthe valve set at the lower pres-sure modulates toward the closedposition. When the load can besupplied completely by the valveset at the higher pressure, theother valve closes and with anyfurther load reduction, the valvestill in use modulates through itsown proportional band.

SYSTEMDESIGN

As part of a broad scope strategy to reduce operatingcosts throughout the refinery, a plan was established toeliminate all possible steam waste. The focus of the planwas piping leaks, steam trap failures and steam pressureoptimization.

Programs having been previously established todetect/repair steam trap failures and fix piping leaks, par-ticular emphasis was placed on steam pressureoptimization. Results from a system audit showed that aconsiderable amount of non-critical, low temperature trac-ing was being done with 190 psi (medium pressure)steam, an expensive overkill. It appeared that the mediumpressure header had been tapped for numerous smalltracing projects over the years.

SolutionRefinery engineers looked for ways to reduce pres-

sure to the tracer lines. Being part of a cost-cuttingexercise, it had to be done without spending large sums ofcapital money on expensive control valves. The self-con-

tained cast steel pressure regulators and bronze reducingvalves were chosen for the job. In 1-1/2 years, approxi-mately 40 pressure regulators and hundreds of bronzereducing valves have been installed at a cost of $250K.Annualized steam energy savings are $1.2M/year. Morespecifically, in the Blending and Shipping Division,$62,640 was saved during the winter of 1995, compared tothe same period in 1994.

Benefits• Low installed cost. The Spirax Sarco regulators and

bronze reducing valves are completely self-contained,requiring no auxiliary controllers, positioners, convert-ers, etc.

• Energy savings worth an estimated $1.2M/year.• The utilities supervisor who worked closely with Spirax

Sarco and drove the project through to successful com-pletion received company wide recognition and apromotion in grade.

Case in Action: Elimination of Steam Energy Waste

Parallel and Series Operation of Reducing Valves

22

This can be clarified by anexample. Suppose that a maxi-mum load of 5,000 lb/h at 30 psican be supplied through onevalve capable of passing 4,000lb/h and a parallel valve capableof 1,300 lb/h. One valve is set at29 psi and the other at 31 psi. Ifthe smaller valve is the one set at31 psi, this valve is used to meetloads from zero up to 1,300 lb/hwith a controlled pressure atapproximately 31 psi. At greaterloads, the controlled pressuredrops to 29 psi and the largervalve opens, until eventually it ispassing 3,700 lb/h to add to the1,300 lb/h coming through thesmaller valve for a total of 5,000lb/h.

There may be applicationswhere the load does not normallyfall below the minimum capacityof the larger valve. It would thenbe quite normal to set the 4,000lb/h valve at 31 psi and to supple-ment the flow through the 1,300lb/h valve at 29 psi in those fewoccasions when the extra capaci-ty was required.

Sometimes the split betweenthe loads is effectively unknown.It is usual then to simply selectvalves with capacities of 1/3 and2/3 of the maximum with thesmaller valve at the slightly high-er pressure and the larger one atthe slightly lower pressure.

Two-Stageor Series OperationWhere the total reduction in pres-sure is through a ratio of more than10 to 1, consideration should begiven to using two valves in series,Fig.39 (page 20).Much will dependon the valves being used, on thetotal pressure reduction neededand the variations in the load. PilotOperated controls have been usedsuccessfully with a pressure turn-down ratio as great as 20 to 1, andcould perhaps be used on a fairlysteady load from 100 psig to 5 psi.The same valve would probably beunstable on a variable load, reduc-ing from 40 to 2 psi.

There is no hard and fastrule, but two valves in series willusually provide more accuratecontrol. The second, or LowPressure valve, should give the“fine control” with a modest turn-down, with due considerationbeing given to valve sizes andcapacities. A practical approachwhen selecting the turndown ofeach valve, that results in small-est most economical valves, is toavoid having a non-critical drop inthe final valve, and stay close tothe recommended 10 to 1 turn-down.

Series InstallationsFor correct operation of thevalves, some volume betweenthem is needed if stability is to beachieved. A length of 50 pipediameters of the appropriatelysized pipe for the intermediatepressure, or the equivalent vol-ume of larger diameter pipe isoften recommended.

It is important that the down-stream pressure sensing pipesare connected to a straight sec-tion of pipe 10 diametersdownstream from the nearestelbow, tee, valve or other obstruc-tion. This sensing line should bepitched to drain away from thepressure pilot. If it is not possibleto arrange for this and to still con-nect into the top of thedownstream pipe, the sensingline can often be connected to theside of the pipe instead.

Equally, the pipe between thetwo reducing valves shouldalways be drained through astream trap, just as any riserdownstream of the pressurereducing station should bedrained. The same applies wherea pressure reducing valve sup-plies a control valve, and it isessential that the connecting pipeis drained upstream of the controlvalve.

SYSTEMDESIGN

BypassesThe use of bypass lines andvalves should usually be avoided.Where they are fitted, the capaci-ty through the bypass should beadded to that through the wideopen reducing valve when sizingrelief valves. Bypass valves areoften found to be leaking steambecause of wiredrawing of theseating faces when valves havenot been closed tightly.

If a genuine need exists for abypass because it is essential tomaintain the supply of steam,even when a reducing valve hasdeveloped some fault or is under-going maintenance, considera-tion should be given to fitting areducing valve in the bypass line.Sometimes the use of a parallelreducing station of itself avoidsthe need for bypasses.

Back Pressure ControlsA Back Pressure regulator or sur-plussing valve is a derivative of apressure reducing valve, incorpo-rating a reverse acting pilot valve.The pressure sensing pipe is con-nected to the inlet piping so that thepilot valve responds to upstreampressure. Any increase in upstreampressure then opens the reverseacting pilot valve, causing the mainvalve to open, while a fall below theset pressure causes the main valveto close down, Fig. II-18 (page 92).

These controls are useful inflash steam recovery applicationswhen the supply of flash steammay at times exceed the demandfor it.The BP control can then sur-plus to atmosphere any excesssteam tending to increase thepressure within the flash steamrecovery system, and maintainsthe recovery pressure at therequired level.

The control is also useful ineliminating non-essential loads inany system that suffers under-capacity at peak load times,leaving essential loads on line.

Back Pressure Controls arenot Safety Valves and must neverbe used to replace them.

How to Size Temperature and Pressure Control Valves

23

control may or may not be fullyopen. For three-port valves, it isthe difference in pressurebetween the two open ports.Working Pressure. The pressureexerted on the interior of a valveunder normal working conditions.In water systems, it is the algebra-ic sum of the static pressure andthe pressure created by pumps.Set Point. Pressure or tempera-ture at which controller is set.Accuracy of Regulation or“Droop”. Pressure reducingvalve drop in set point pressurenecessary to obtain the publishedcapacity. Usually stated for pilot-operated PRV’s in psi, and as a %of set pressure for direct-actingtypes.Hunting or Cycling. Persistentperiodic change in the controlledpressure or temperature.Control Point. Actual value ofthe controlled variable (e.g. airtemperature) which the sensor istrying to maintain.Deviation. The differencebetween the set point and themeasured value of the controlled

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