Reprinted from the September issue of:

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THE MAGAZINE FOR PUMP USERS September 1997

Rotary Pump InletPressure RequirementsShedding light on one of the least understood yet most importantaspects of a successful pump installation.

By James R. Brennan

COVER STORY

I Iill

IShedding light on one of the least understood yet

most important aspects of a successful pump installation

ByJamesR. Brennan, ImoI~dustries

Incorrectly specifying the re-quired inlet pressure for a pump

will result ~n either poor perfor-mance, nOIse, premature wear,

high operating and maintenanceexpenses and failures, or a seeminglyexcellent installation that costs agood deal more than it should.

Rotary pumps handle the broad-est range of liquids of any genericpump classification - from moltenmetal, food, liquefied petroleum gasand sewage to asphalt, fuels, chemi-cal slurries and plastics, polymersand pharmaceuticals. Capabilitiesand user expectations for rotarypumps are significantly differentfrom those of other pump classifica-tions.

In the United States, theHydraulic Institute is the major con-trolling organization for pumpingdefinitions, and each of the centrifu-gal, rotary and reciprocating pumpmanufacturers have its own set of

similar but not necessarily identicalstandards. Inlet pressure require-ments for rotary, positive displace-ment pumps are similar to NPSHr(Net Positive Suction Head Required)for centrifugal pumps. For rotarypumps, pressure units are normallyin force per unit area (psi, bar, MPa)rather than elevation difference (feetor meters).

The variety of labels used forthis parameter, as well as an aston-ishingly long list of units of measureand reference scales, perpetuatesmisunderstanding of required inletpressure. The purpose of this articleis to provide a basic physical under-standing such that the various scales

and units do not, at least initially,intrude on our ability to grasp theprinciples.

Every pump has a minimumrequired inlet pressure. What thatminimum pressure is depends onpump type, size, speed and the vis-cosity of the fluid pumped. If theminimum required inlet pressure isnot made available to the inlet side ofthe pump, cavitation will result. Cav-

itation is the incomplete filling orfeeding of the pumping elementswith liquid. This results in a reduc-tion of flow and, if the condition issevere, noise, vibration, instability,internal erosion and catastrophic fail-ure can result. Cavitation must there-fore be avoided. Pure cavitation is

the partial vaporization of thepumped liquid caused by allowingthe fluid pressure to fall below itsvapor pressure at the pumping tem-

PUMPFLOWORHEAD

t5% FLOW REDUCTION (ROTARY)

3% HEAD REDUCTION (CENTRIFUGAL)

,.:::?OW REDUCTION (RECIPROCATING)

: MINIMUM REQUIRED

VINLET PRESSURE

INLET PRESSURE

Figure 1. Hydraulic Institute cavitationdefinitions

perature. Pseudo-cavitation canoccur if the liquid contains dissolvedgas or air - a not uncommon condi-tion. The dissolved gas will expandas the fluid pressure is reduced andcause exactly the same symptoms aspure cavitation. Entrained gas or airin the fluid, such as can be found insome restricted or poorly designedlubrication systems, will also causepumps to exhibit cavitation symp-toms, as will an air leak in a pumpinlet line below atmospheric pres-sure.

The Hydraulic Institute definesminimum inlet pressure as that pres-sure, for a specified pump and set ofoperating conditions, that will resultin a flow loss of 3% for reciprocatingpumps, and 5% for rotary pumps anda 3% head loss for centrifugal pumpswhile all other operating conditionsare held constant. Most pump manu-facturers accept these fairly arbitrarydefinitions as a condition that theirpumps will tolerate indefinitely (Fig-ure 1). It is, however, operation in avery mildly cavitated condition. Fig-ure 2 illustrates what is happening tothe pump above and below this min-

RotaryPump InletPressure Requirements

ATMOSPHERICPRESSUREON SITE

ABSOLUTESYSTEMINLET SIDEPRESSURE

PUMP INLET PRESSURE

MAXIMUM LIQUIDVAPOR PRESSURE

0a b d e f

LOCATIONRELIEF

ATMOSPHERICPRESSURE ATJOBSITEALTITUDE ELEVATION

DIFFERENCE

L MINIMUMLIQUID LEVEL

MAXIMUMLIQUIDVISCOSITYANDVAPORPRESSURE

Figure 2. Factors impacting net inlet pressure available

imum inlet pressure. The lower caseletters in the diagram correspond tothe horizontal axis locations in thegraph. Pump manufacturers haveconducted extensive tests and deter-mined the empirical equations usedto calculate the required minimuminlet pressures for their products.

So from where does the requiredminimum inlet pressure come? Itcomes from either an upstreampump or atmospheric pressure push-ing on the free surface of the fluid

upstream of a pump in question.Atmospheric pressure can be the nat-ural pressure exerted by the columnheight of air above the pump, or itcan be the artificially maintainedpressure above the fluid surface,such as a deliberately maintainedvacuum or pressure in a process ves-sel. If natural atmospheric pressure

VAPOR PRESSURE- BARA0.5 1.0 1.5 2.0 2.5

I130

20

5

-30

6-50

1205

15

FEETABOVESEALEVEL10X1000

TEMP.of 110

- TEMP.DC

4 METERSABOVESEA

3 LEVELX1000

2

-40

100

90

40 50 60 70 80 90 100ATMOSPHERIC PRESSURE,%

Figure 3. Effects of altitude on atmos-pheric pressure

10 15 20 25 30VAPOR PRESSURE- PSIA

Figure 4. Effect of temperature on vapor

pressure

is used, then the job site altitudeabove sea level is an important fac-tor. Figure 3 illustrates the reductionin atmospheric pressure with alti-tude. Higher elevations have lesspressure available for use in pushingfluids into a pump, and this oftenoverlooked factor can make or break

an application. The idea that a pumpcan "suck, IIwhile seemingly obvious,is in fact incorrect. The pressurereduction at the inlet of the pump issimply the result of frictional pres-sure loss due to the flow of fluid from

its source to the pump and into thepumping element(s).

If fluids always remained intheir liquid state, establishing theminimum required inlet pressurewould be somewhat simpler. Howev-er, many liquids exhibit a vapor pres-sure of sufficient magnitude atpumping temperature - a factor thatmust be taken into consideration for

proper pump operation (Figure 4).Vapor pressure is the inverse of boil-ing temperature. As we all learnedlong ago, water boils at 100De(212DF). This boiling temperature isonly correct when the water is at apressure of one atmosphere (oneinternational atmosphere is equal to101,325 Pascals, 1.01325 bar, 1.03323kg/cm2, 14.696 psi). At an elevationof 3000 meters (9842 feet), the atmos-pheric pressure is only 69% of whatit is at sea level. At this reduced pres-sure, water will boil at about 90De(195DF). The inverse way of lookingat this is to say that the vapor pres-sure of water is 1 atmosphere at100De (212DF). If you wish to pumpwater in its liquid state and the waterhappens to be at a temperature of100De (212DF), the inlet side of thepump must not be exposed to a pres-sure below 1 atmosphere or the liq-uid will begin to convert to a gas(steam) and the pump will enter acavitating region of operation, a veryundesirable condition. If liquid wateris to be pumped at 160De (320DF),then the inlet side of the pump mustbe maintained at or above the 6.1

atmospheres that represent the vaporpressure of water at this tempera-ture.

Many liquids handled at theirnormal pumping temperature exhibitsuch a low vapor pressure that thisfactor can be ignored for all practicalpurposes. Refined lubricating oils,for example, at normal operating

I

Rotary Pump Inlet Pressure Requirements

Photo 1. High pressure gear pump destroyedby severe cavitation

temperatures up to 82°C (180°F)have vapor pressures in the range of0.01 atmospheres. On the otherhand, volatile liquids such as gaso-line and alcohol will readily evapo-rate (boil) at ambient temperatures.Propane is kept liquid at ambienttemperature only because it is storedin a pressure vessel. The vessel mustcontain the propane's vapor pressureat the vessel's temperature. Vaporpressure invariably increases withtemperature. It is this very fact that isput to use in refining petroleum andin many other petrochemical andchemical processes. While it isalmost always important to know afluid's minimum vapor pressure atthe maximum pump suction temper-ature, the pumping of high tempera-ture fluids should involve a careful

analysis of the possible impacts ofvapor pressure.

Some fluids will exhibit multiplevapor pressures. Raw crude oil is anexample. This fluid is composed ofmany different complex molecules. Itis a mixture and, as such, its compo-nent fluids will each have its ownvapor pressure. The lowest dis-cernible component vapor pressureis the one to use for net inlet pressurecalculations if pumping this fluid.Alcohol mixed with water will exhib-it two vapor pressures: that of alco-hol and a different one for water. The

way to separate these two liquids isto apply heat. The alcohol will boilfirst at a lower temperature thanwater's boiling point. The alcoholcan be collected as a gas, then cooledto its liquid state. This process, calleddistillation, is a good example ofvapor pressure at work.

Cavitation causes its damage bythe abrupt, violent compression ofthe vapor (gas) back into liquid at thepump discharge. This compression

occurs very rapidly as animplosion. There is enoughenergy to erode minute metalparticles from the rotatingand stationary pumping ele-ments. Such erosion is fre-quently visible on outboardmarine engine propellers inwhich the propeller velocityexceeds the water velocity,thus cavitating the blades.Given enough time, a bladefailure is inevitable. Photo 1shows cavitation damage to apump. Most rotary positivedisplacement pumps use

incremental pressure staging withinthe pumping elements to withstanddifferential pressure. Examples ofthis staging include multiple wrapsof screw pump thread, multiple teethon gear pumps and multiple vanes invane pumps. This staging is onlyeffective if the fluid pumped is near-ly incompressible, i.e., a liquid. Intro-duction of gas, air or vapor causesthe fluid's compressibility toincrease, and this compressibilitydefeats the staging benefits. Most ofthe pressure rise across a pump han-

wa:::Jenenwa:c.I-w...J~Cwa::50wa:

VISCOSITY

Figure 5. Effect of inlet velocity andviscosity on required inlet pressure

VISCOSITY. CST200 300 400

130

12050

TEMP. 110

of

TEMP.

°c40

100

9030

1000 1250 1500 1750

VISCOSITY -ssu

Figure 6. Effect of temperature onviscosity

dling compressible fluids occurs atthe last stage, overloading the unit.

The minimum required inletpressure of a pump also dependsupon its size and speed. The productof size and rotational speed is veloci-ty. Fluids moving at high velocitiesentering the pumping elements willconsume more energy (pressure)than slower moving fluids. Conse-quently, large and/or high speedpumps will require a higher mini-mum inlet pressure than smallerand/or low speed pumps. Fluid vis-cosity (fluid resistance to shear) willalso adversely affect minimumrequired inlet pressure. Friction loss-es within the pump suction side cas-ing (minimal) and friction lossesentering the first pumping chamberincrease with increasing viscosity.Thus, pumps will require higherminimum inlet pressures when han-dling higher viscosity fluids. Figure 5shows the effect. Velocities are

labeled V 1 increasing in magnitudeto V4. One solution to the highrequired minimum inlet pressure isto use a larger pump operating at alower speed to reduce the internalvelocity.The price paid is, of course, a larger,more expensive pump and a moreexpensive driver.

Getting the inlet side of thepump correctly specified, and pro-viding as much net inlet pressure aspossible, will result in an optimallysized, minimum cost pump selectionthat can be expected to operate wellfor a long time. Excessively conserva-tive inlet pressure specifications willresult in larger, slower and moreexpensive - and perhaps even lessefficient - pumps..

James R. Brennan is Market Ser-vices Manager for Imo Pump {Monroe,NC}.

His responsibilities include world-wide marketing and technical supportfor pumping applications. He is a 1973graduate of Drexel University inPhiladelphia and a member of the Soci-ety of Petroleum Engineers.

Compliments of

IMO Industries Inc.P.o. Box 5020

Monroe, NC 28111-5020, USATel: (704) 289-6511Fax: (704) 289-9273