ANSI-HI-9-6-1 Centrifugal and Vertical Pumps for NPSH Margin

9 Sylvan WayParsippany, New Jersey07054-3802 www.pumps.org

AN

SI/H

I9.

6.1-

1998

ANSI/HI 9.6.1-1998

American National Standard for

Centrifugal and Vertical Pumpsfor NPSH Margin

http://www.pumps.org

Copyright © 2000 By Hydraulic Institute, All Rights Reserved.

This page intentionally blank.


ANSI/HI 9.6.1-1998

American National Standard for

Centrifugal and Vertical Pumpsfor NPSH Margin

Secretariat

Hydraulic Institute

www.pumps.org

Approved March 3, 1998

American National Standards Institute, Inc.

Recycledpaper



Approval of an American National Standard requires verification by ANSI that therequirements for due process, consensus and other criteria for approval have been metby the standards developer.

Consensus is established when, in the judgement of the ANSI Board of StandardsReview, substantial agreement has been reached by directly and materially affectedinterests. Substantial agreement means much more than a simple majority, but not nec-essarily unanimity. Consensus requires that all views and objections be considered,and that a concerted effort be made toward their resolution.

The use of American National Standards is completely voluntary; their existence doesnot in any respect preclude anyone, whether he has approved the standards or not,from manufacturing, marketing, purchasing, or using products, processes, or proce-dures not conforming to the standards.

The American National Standards Institute does not develop standards and will in nocircumstances give an interpretation of any American National Standard. Moreover, noperson shall have the right or authority to issue an interpretation of an AmericanNational Standard in the name of the American National Standards Institute. Requestsfor interpretations should be addressed to the secretariat or sponsor whose nameappears on the title page of this standard.

CAUTION NOTICE: This American National Standard may be revised or withdrawn atany time. The procedures of the American National Standards Institute require thataction be taken periodically to reaffirm, revise, or withdraw this standard. Purchasers ofAmerican National Standards may receive current information on all standards by call-ing or writing the American National Standards Institute.

Published By

Hydraulic Institute9 Sylvan Way, Parsippany, NJ 07054-3802

www.pumps.org

Copyright © 1998 by Hydraulic InstituteAll rights reserved.

No part of this publication may be reproduced in any form,in an electronic retrieval system or otherwise, without priorwritten permission of the publisher.

Printed in the United States of America

ISBN 1-880952-25-4

AmericanNationalStandard



iii

ContentsPage

Foreword . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . v

9.6.1 Pump NPSH margin . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1

9.6.1.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1

9.6.1.2 Suction energy level . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1

9.6.1.2.1 Suction energy factors. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2

9.6.1.2.2 Suction energy determination . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3

9.6.1.3 Cavitation damage factors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4

9.6.1.4 NPSH margin ratio recommendations . . . . . . . . . . . . . . . . . . . . . . . . 4

9.6.1.5 Application considerations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

9.6.1.5.1 Petroleum process pumps. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

9.6.1.5.2 Chemical process pumps . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

9.6.1.5.3 Electric power pumps . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

9.6.1.5.4 Nuclear power/cooling tower . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

9.6.1.5.5 Water/wastewater . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8

9.6.1.5.6 General industrial . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9

9.6.1.5.7 Pulp and paper . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9

9.6.1.5.8 Building services . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9

9.6.1.5.9 Slurry . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9

9.6.1.5.10 Pipeline . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10

9.6.1.5.11 Waterflood (injection) pumps. . . . . . . . . . . . . . . . . . . . . . . . . . . . 10

9.6.1.6 Summary. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10

Appendix A Index . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13




v

Foreword (Not part of Standard)

Purpose and aims of the Hydraulic Institute

The purpose and aims of the Institute are to promote the continued growth andwell-being of pump manufacturers and further the interests of the public in suchmatters as are involved in manufacturing, engineering, distribution, safety, trans-portation and other problems of the industry, and to this end, among other things:

a) To develop and publish standards for pumps;

b) To collect and disseminate information of value to its members and to thepublic;

c) To appear for its members before governmental departments and agenciesand other bodies in regard to matters affecting the industry;

d) To increase the amount and to improve the quality of pump service to the public;

e) To support educational and research activities;

f) To promote the business interests of its members but not to engage in busi-ness of the kind ordinarily carried on for profit or to perform particular servicesfor its members or individual persons as distinguished from activities toimprove the business conditions and lawful interests of all of its members.

Purpose of Standards

1) Hydraulic Institute Standards are adopted in the public interest and aredesigned to help eliminate misunderstandings between the manufacturer,the purchaser and/or the user and to assist the purchaser in selecting andobtaining the proper product for a particular need.

2) Use of Hydraulic Institute Standards is completely voluntary. Existence ofHydraulic Institute Standards does not in any respect preclude a memberfrom manufacturing or selling products not conforming to the Standards.

Definition of a Standard of the Hydraulic Institute

Quoting from Article XV, Standards, of the By-Laws of the Institute, Section B:

“An Institute Standard defines the product, material, process or procedure withreference to one or more of the following: nomenclature, composition, construc-tion, dimensions, tolerances, safety, operating characteristics, performance, qual-ity, rating, testing and service for which designed.”

Comments from users

Comments from users of this Standard will be appreciated, to help the HydraulicInstitute prepare even more useful future editions. Questions arising from the con-tent of this Standard may be directed to the Hydraulic Institute. It will direct allsuch questions to the appropriate technical committee for provision of a suitableanswer.

If a dispute arises regarding the contents of an Institute publication or an answerprovided by the Institute to a question such as indicated above, the point in ques-tion shall be referred to the Executive Committee of the Hydraulic Institute, whichthen shall act as a Board of Appeals.


vi

Revisions

The Standards of the Hydraulic Institute are subject to constant review, and revi-sions are undertaken whenever it is found necessary because of new develop-ments and progress in the art. If no revisions are made for five years, thestandards are reaffirmed using the ANSI canvass procedure.

Scope

This standard applies to centrifugal and vertical pump types. It describes the ben-efit to pump life when the NPSH available is greater than the NPSH required by asuitable margin, and suggests margins for various applications.

Units of Measurement

Metric units of measurement are used; corresponding US units appear in brack-ets. Charts, graphs and sample calculations are also shown in both metric and USunits.

Since values given in metric units are not exact equivalents to values given in USunits, it is important that the selected units of measure to be applied be stated inreference to this standard. If no such statement is provided, metric units shall govern.

Consensus for this standard was achieved by use of the CanvassMethod

The following organizations, recognized as having an interest in the standardiza-tion of centrifugal pumps were contacted prior to the approval of this revision ofthe standard. Inclusion in this list does not necessarily imply that the organizationconcurred with the submittal of the proposed standard to ANSI.

A.W. Chesterton CompanyAgrico Chemical Corp.Ahlstrom Pumps, LLCAlden Research LabBechtel CorporationBlack & VeatchBrown & CaldwellCamp Dresser & McKeeCH2M HillChas S. Lewis & Co., Inc.Crane Pump & SystemsDeWanti & StowellDow ChemicalDuPont EngineeringElectric Power Research InstituteEngineering Devices Resource GroupENSR Consulting & EngineeringEssco Pump DivisionFairbanks Morse PumpFlorida Power CorporationFloway PumpsFlowserve Corp.Fluor Daniel, Inc.Grundfos Pumps Corp.Ingersoll-Dresser Pump

ITT Industrial Pump GroupITT Flygt Corp.Iwaki Walchem Corp.J.P. Messina Pump & Hydraulics

ConsultantJohn Crane, Inc.Johnston Pump Co.Lawrence Pumps, Inc.M. W. Kellogg Co.Malcom Pirnie, Inc.Marine Machinery AssociationNational Pump Co.Monsanto Co.Montana State UniversityMontgomery WatsonMWI, Moving Water IndustriesOxy ChemNational Pump Co.PACO PumpsPatterson Pump Co.PC Garvin & AssociatesPrice Pump Co.Raytheon Engineering & ConstructorsRobert Bein, William Frost &

Associates


vii

Sewage & Water Board of New OrleansSkidmoreSouth Florida Water ManagementSouthern Company Services, Inc.Sta-Rite IndustriesStone & Webster EngineeringSulzer Bingham Pumps, Inc.

Summers Engineering, Inc.Systecon, Inc.The Process Group, LLCUnion Pump Co.US Bureau of ReclamationUS Army Corp of Engineers

“Although this standard was processed and approved for submittal to ANSI by theCanvass Method, a working committee met many times to faciliate the develop-ment of this standard. At the time it was developed, the committee had the follow-ing members:”

CHAIRMAN - Allan Budris, ITT Industrial Pump Group

OTHER MEMBERS

Ronald Brundage, ITT Flygt

Fred Buse, Ingersoll-Dresser Pump Co.

Greg Case, Price Pump

R. Barry Erickson, ITT Industrial Pump Group

Herman Greutink, Johnston Pump

Al Iseppon, Sta-Rite Industries

Ray Perriman, Sundstrand Fluid Handling

Robert Stanbury, Flowserve Corporation




HI Pumps – Centrifugal and Vertical Pumps for NPSH Margin — 1998

1

9.6.1 Pump NPSH margin

HI Pumps – Centrifugal and Vertical Pumps for NPSH Margin9.6.1.1 Introduction

The noise, the vibration and possibly the reliability of acentrifugal or vertical pump and mechanical seal maybe significantly affected if an appropriate Net PositiveSuction Head (NPSH) margin is not provided by thesystem above the published Net Positive Suction HeadRequired (NPSHR) by the pump.

The NPSH Margin is defined as the NPSH Available(NPSHA) at the pump inlet, minus the NPSH Requiredby the pump. The NPSH Margin Ratio is the NPSHAdivided by the NPSHR. The Net Positive Suction HeadAvailable (NPSHA) is the total suction head available,over the vapor pressure of the liquid pumped correctedto the center line of the impeller (or impeller inlet vanetip datum if vertically mounted), and measured at theinlet to the pump.

NPSHA = hatm + hgs + hvs + Zs – hvp

Where:

hatm = atmospheric pressure head

hgs = suction gage head

hvs = suction velocity head

zs = suction elevation head

hvp = liquid vapor pressure head

See the ANSI/HI 1.6 Centrifugal Pump Tests for fur-ther details on the definitions of NPSHA and NPSHR.

By Hydraulic Institute definition, the NPSHR of a pumpis the NPSH that will cause the total head (first stagehead of multistage pumps) to be reduced by 3%, dueto flow blockage from cavitation vapor in the impellervanes. NPSHR is by no means the point at which cavi-tation starts; that level is referred to as incipient cavita-tion. The NPSH at incipient cavitation can be from 2 to20 times the 3% NPSHR value, depending on pumpdesign. The higher ratios are normally associated withhigh suction energy pumps or pumps with large impel-ler inlet areas.

The 3% head drop criteria was selected for theNPSHR value based on the ease of determining theexact head drop off point. Most standard low suctionenergy pumps can operate with little or no marginabove the NPSHR value, without seriously affecting

the service life of the pump. The full published pumphead will not, however, be achieved (by definition)when the NPSHA equals the NPSHR of the pump. Thehead will be 3% less than the fully developed headvalue (see Figure 9.6.1.1). It can take up to 2.5 timesthe NPSHR value just to achieve the 100 percent headvalue. Just because the definition uses the wordRequired, does not mean that providing that muchNPSHA will necessarily give satisfactory pump life. It isalso recognized that, as the suction energy of a centrif-ugal pump increases, so does the need for a largerNPSH margin above the 3% NPSHR of the pump, toavoid excessive noise, vibration, and possible cavita-tion erosion and seal damage.

Most pump manufacturers use the industry standard3% head drop for NPSHR values and provide theNPSH Margin recommendations separately. A fewmanufacturers do include the NPSH Margin in theirpump NPSHR curves which then supersede theguidelines spelled out in this standard. Unless advisedotherwise, however, the user must assume that thereis no margin in the published NPSHR, and that it isbased solely on the 3% head drop criteria.

9.6.1.2 Suction energy level

The suction energy level of a pump increases with thecasing suction nozzle size, the pump speed, the suc-tion specific speed and the specific gravity of thepumped liquid. Anything that increases the velocity inthe pump impeller eye, the rate of flow of the pump, orthe specific gravity, increases the suction energy of thepump. The suction nozzle size is used for simplicitybecause it approximates the impeller eye diameter andties to the rate of flow of the pump. The rpm ties directlyto the inlet tip speed of the impeller and relative inlet

Figure 9.6.1.1



2

velocities, and the suction specific speed is alsodependant on rpm and rate of flow. The NPSHR in thesuction specific speed is appropriate as a measure ofsuction energy in that larger impeller eye diametersare normally required for lower NPSHR values, whichincreases the impeller tip speed.

9.6.1.2.1 Suction energy factors

Many factors are known to contribute to the suctionenergy level, and resulting NPSH margin requirementsof a pump, more than used in the above definition.Those used in the above definition are factors whichare typically available from standard pump manufac-turer’s technical literature. Manufacturers of customengineered pumps may use alternate evaluation meth-ods to establish NPSH margin requirements and thesewould supersede the guidelines spelled out in thisstandard. For general information a list of suctionenergy factors is provided below:

• The peripheral velocity at the O.D. of the impel-ler eye. Values below approximately 15 m/sec. (50ft/sec) are generally considered low suctionenergy, while values above approximately 35 m/sec. (120 ft/sec) are considered high suctionenergy.

• The suction specific speed of the pump (S = n× Q½/(NPSHR)¾). Suction specific speed valuesbelow approximately 8,000 metric (7,000 U.S.units) generally represent low suction energy,while above approximately 23,000 metric (20,000U.S. units) are considered high suction energy.See Figure 9.6.1.3 for suction specific speedsbetween these values. (Note: Q is the BEP rate offlow entering the impeller eye. In double suctionpumps, use one half total rate of flow. NPSHR isbased on 3% head drop at BEP.)

• The specific gravity of the liquid pumped. Thehigher the value the higher the suction energy.

• Thermodynamic properties of the liquid. Coldwater has one of the highest energy levels forimploding cavitation bubbles. See section on Elec-tric Power pumps for more details.

• The geometry of the pump inlet. The greater thevariation in velocity across the impeller inlet andthe higher the magnitude of velocities, the higherthe energy level. For this reason, radial inlets, asfound in split case pumps have higher suctionenergy levels due to the right angle turn in front ofthe impeller.

• The overlap of the impeller vanes. Overlap val-ues less than approximately 15 degrees, such asfound on two or three vane impellers (seeFigure 2), can allow the high discharge pressure(energy) to recirculate into the impeller suction atlow rates of flow. Overlap is defined as the angularamount that the trailing edge of one vane (lowpressure side) overlaps the inlet leading edge ofthe following adjacent vane (at the outer diameter).

• The incidence angle between the inlet impellervanes and the approaching liquid. Typically animpeller is designed to have a “zero” incidenceangle at design rate of flow. Higher or lower ratesof flow cause a mismatch between the angle of theapproaching liquid and the impeller vane inlet tips.The greater the incidence the greater the turbu-lence and suction energy.

• The geometry of the inlet piping to the pump.The turbulence (added suction energy) that is gen-erated at the pump inlet from piping turns andlarge changes in pipe diameter adds to the suctionenergy at the pump inlet.

• Operation away from the best efficiency point(BEP) of the pump. At reduced rates of flow thepump may operate in its suction recirculationregion. Operation off BEP rate of flow alsoincreases the incidence angle to the impellervanes, and suction recirculation adds to the suc-tion energy level. See ANSI/HI 9.6.3-1997, Centrif-ugal and Vertical Pumps for Allowable OperatingRegion, for more information.

Figure 9.6.1.2

ROTATION

VANEOVERLAP

15°

IMPELLERVANE



3

9.6.1.2.2 Suction energy determination

This is a complex situation and a single equation orrelationship has not been developed, which will accu-rately tie all of these factors together to predict pumpnoise, vibration, erosion, and reduced mechanical seallife from cavitation, and the NPSH margin levelrequired to avoid these undesirable effects. Recom-mended margin ratios can typically range from one tofive times the NPSHR value of the pump, with thehigher values applying to high and very high suctionenergy pumps, and continuous operation outside thepreferred operating region of the pump. The attachedgraph (Figure 9.6.1.3) is a simplified method for identi-fying high suction energy pumps. Pumps above theappropriate suction specific speed curve as shown inFigure 9.6.1.3, are considered high suction energypumps. Very high suction energy pumps can bedefined as pumps whose actual impeller operatingspeeds are in the range of 1.5 to 2.0 times the valuesshown in Figure 9.6.1.3, or higher. As an example, anend suction pump with a 10" suction nozzle size and9,500 suction specific speed is shown to start highsuction energy at 1,800 RPM. If this pump were to beoperated at 3,600 RPM (2 times 1,800) the pumpwould be considered to have very high suction energy.

It must be stressed that the impeller eye diameter isactually a better factor for identifying the suctionenergy level of a pump than the suction nozzle diame-ter. The nozzle size was chosen for Figure 9.6.1.3because it is more often available to the pump user,and normally has a close relationship to the impellereye. Therefore, reducing the suction nozzle size, with-out a corresponding reduction in the impeller eyediameter, will not reduce the true suction energy of apump. It could even increase cavitation.

Generally speaking, high suction energy pumps aresusceptible to noise and increased vibration, but willnot suffer significant erosion damage (especially withmore erosion resistant impeller materials) when suffi-cient NPSH Margin is not provided. Very high suctionenergy pumps will more likely experience erosiondamage from cavitation under inadequate NPSH mar-gin conditions.

Notes for Figure 9.6.1.3:

• For two vane impellers and impeller trims with lessthan 15 degrees vane overlap, (see Figure9.6.1.2) increase suction nozzle size by one or twosizes before using Figure 9.6.1.3.

• Inducers, which are generally beyond the scope ofthis document, should have the suction nozzledecreased by at least one size before usingFigure 9.6.1.3.

• For axial split case (side Suction) pumps,decrease nozzle size by one size, before usingFigure 9.6.1.3.

Figure 9.6.1.3A (metric)

Figure 9.6.1.3B (US units)



4

• For pump speeds higher than 3600 rpm, the suc-tion nozzle sizes should be increased, propor-tional to the increase in speed, and enter thegraph at 3600 rpm. For example, increase thenozzle size by 2 times if the speed is doubled.

• For vertical turbine (line shaft diffuser) typepumps, the Impeller Inlet eye diameter should beobtained from the supplier and used as the suctionnozzle size, when using Figure 9.6.1.3.

• Multistage pumps, such as used for boiler feedand pipeline services, are excluded from this fig-ure due to the typically large shaft diameters in theimpeller eye, which distorts the relationshipbetween the impeller eye diameter and the suctionnozzle size.

9.6.1.3 Cavitation damage factors

There are other factors which, although not affectingthe suction energy of the pump, will affect the degreeof cavitation erosion damage (and sometimes noise)within a pump when sufficient NPSH margin is not pro-vided above the NPSHR of the pump. These non-suction energy factors are:

• The impeller material. Rigid plastics and com-posites are normally the least cavitation resistantmaterials. Cast iron and brass will experience themost damage of commonly used metals, whilestainless steel, titanium and nickel aluminumbronze will have much less damage, under thesame cavitation conditions.

• Pump size. Large pumps (impeller inlets over450 mm (18 in) in diameter can be more prone tocavitation damage than smaller pumps.

• The gas content of the liquid. Small amounts ofentrained gas (1 to 2%) cushion the forces fromthe collapsing cavitation bubbles, and can reducethe resulting noise, vibration and erosion damage.The lack of any entrained gas can have the oppo-site effect. Warmer liquids tend to release less dis-solved gas, which increases the noise level of apump. On the other hand gas can collect in theinlet of a pump which will block portions of the flowarea, thus increasing the inlet velocity of the liquidand creating even more cavitation. This increasesthe apparent NPSHR of the pump. The net resultof these two counter effects of gas content onpump noise and vibration will vary based on thesuction energy level of the pump. In the case oflow to high suction energy levels, the net effect of

gas may be to quiet the pump, since the cushion-ing may more than offset the added cavitation.However, with very high suction energy pumps,the force of the collapsing cavitation bubbles maybe too great for any real cushioning, so the noiseand damage will increase with increasing gascontent.

• Additives in the liquid. Additives in the liquidwhich increase vapor pressure can increase cavi-tation damage. For example, cooling tower watertreatment agents.

• The corrosive properties of the liquid. This canaccelerate the damage.

• Solids/abrasives in the liquid. Adding abrasivesto the high implosive velocities from the collapsingvapor bubbles increases the wear rate.

• The duty cycle of the pump. Cavitation damageis time related. The longer a pump runs under cav-itation conditions, the greater the extent of dam-age. Fire pumps, which run intermittently, rarelyhave a problem with cavitation damage for thisreason.

9.6.1.4 NPSH margin ratio recommendations

Field experience is the most accurate predictor offuture performance. Table 9.6.1.1 offers suggestedminimum NPSH margin ratio guidelines (NPSHA/NPSHR), within the allowable operating region of thepump (with standard materials of construction). Thetable is based on the experience of the many pumpmanufacturers with many different pump applications.

Vertical turbine pumps often operate without NPSHmargin without damage, but with slightly reduced dis-charge head.

High and very high suction energy pumps that operatewith only the minimum NPSH margin values recom-mended in Table 9.6.1.1 will normally have what isconsidered “acceptable” seal and bearing life. Theymay still be susceptible to elevated noise levels anderosion damage to the impeller. This can require morefrequent impeller replacement than otherwise wouldbe experienced had the cavitation been totally elimi-nated. It will typically take an NPSHA of 4 to 5 timesthe 3% NPSHR of the pump to totally eliminate cavita-tion. This ratio can reach 20 for very high suctionenergy pumps, and a low of 2 for some pumps with lowsuction energy levels. There are studies that show thatthe maximum cavitation damage can actually occur at



5

NPSHA values twice the NPSHR or more for very highsuction energy pumps.

In addition to the minimum NPSH Margins recom-mended in Table 9.6.1.1, extra margin may be requiredto account for changes in the pump geometry whichcan increase NPSHR, such as wear that can openimpeller wearing ring clearances and increase theinternal flow through the impeller eye. The NPSHRmay also be affected by the gas content of the liquidpumped. Added NPSH Margin may be needed tocover uncertainties in the NPSH available or the actualoperating rate of flow. If a pump runs further out on thecurve than expected, the NPSHA of the system maybe lower than expected and the NPSHR for the pumpwill be higher, thus giving a smaller (or possibly nega-tive) NPSH Margin. (See ANSI/HI 9.6.3-1997, Centrif-ugal and Vertical Pumps for Allowable OperatingRegion). All pumping systems must be designed tohave a positive margin throughout the full range ofoperation. Optimum pump performance also requiresthat proper suction/inlet piping practices are followed,

according to the Hydraulic Institute Standards (seeANSI/HI 9.8-1998, Pump Intake Design), to ensure asteady uniform flow to the pump suction at therequired suction head. Poor suction piping can resultin separation and turbulence at the pump inlet, whichdecreases the NPSHA to the pump and causes addedcavitation. NPSHA Margins of two to five feet are nor-mally required (above those shown in Table 9.6.1.1) toaccount for these uncertainties in the actual NPSHRand NPSHA values, and this added margin require-ment could be even greater depending upon the sever-ity of the conditions. If the application is critical, afactory NPSHR test should be requested.

NPSH Margins are not normally a consideration formost standard vertical turbine pumps, since they gen-erally have Low Suction Energy, and cavitation noise isnormally not an issue. NPSHA must, however, beequal to or larger than the NPSHR over the allowableoperating region of the pump, including at low waterlevel. The determination of the minimum submergencerequired to avoid the formation of sump vortices

Table 9.6.1.1

Minimum NPSH margin ratio guidelines (NPSHA/NPSHR)

Suction energy level

Application Low High Very high

Petroleum 1.1a

a) Or 0.6m (2 feet), whichever is greater.

1.3c

c) Or 1.5m (5 feet), whichever is greater.

Chemical 1.1a 1.3c

Electric power 1.1a 1.5c 2.0c

Nuclear power 1.5b

b) Or 0.9m (3 feet), whichever is greater.

2.0c 2.5c

Cooling towers 1.3b 1.5c 2.0c

Water/waste water 1.1a 1.3c 2.0c

General industry 1.1a 1.2b

Pulp and paper 1.1a 1.3c

Building services 1.1a 1.3c

Slurry 1.1a —

Pipeline 1.3b 1.7c 2.0c

Water flood 1.2b 1.5c 2.0c



6

around the pump inlet must be considered indepen-dently from NPSHA, since they are a separate phenom-ena. (See ANSI/HI 9.8-1998, Pump Intake Design).

9.6.1.5 Application considerations

9.6.1.5.1 Petroleum process pumps

Pumps used for petroleum (hydrocarbon) services canusually survive with relatively small NPSHA marginsfor several reasons:

1) Processes are typically steady, with few sys-tem upsets (transients) or quick flow changedemands.

2) Process requirements are typically well knownand demands can be planned and predicted.

3) Most hydrocarbon liquids have relatively lowvapor volume to liquid volume ratios. Thismeans that, if the liquid should vaporize at ornear the pump suction (impeller inlet), the vol-ume of the resulting vapor does not choke theimpeller inlet passages as severely as doeswater vapor during cavitation. This results in asmaller drop in developed head for the sameNPSH margin.

4) Less energy is released when hydrocarbonvapor bubbles collapse (velocity from implo-sion is less), and this means less damageoccurs as a result of cavitation. It is, therefore,not as critical that cavitation be avoided, asmight be the case with other liquids.

Hydrocarbon liquids, especially mixtures of hydrocar-bon liquids, because of their relatively low vapor vol-ume, are sometimes associated with a “hydrocarboncorrection factor.” This “correction factor” is applied tothe water NPSHR values to “correct” for the fact thatthe vapor volume of “flashed” hydrocarbon liquid issubstantially less than that of “flashed” water and,thus, has the effect of reducing the amount of NPSHrequired by the pump at a given rate of flow beforecavitation results in a 3% drop in the developed head(first stage head) of the pump.

This favorable vapor bubble size situation with hydro-carbons should be taken into account when determin-ing the NPSHA Margin requirements for petroleumpumps. The margins can be lower than for other appli-cations. Typical NPSH Margins for pumps on hydrocar-bon services are as follows:

• Low Suction Energy Single Stage Overhung, Verti-cal and Multistage Pumps: For all hydrocarbon liq-uids use an NPSH Margin Ratio of 1.1.

• High and Very High Suction Energy Single StageOverhung, Single Stage Double Suction Multi-stage Pumps: For all hydrocarbon liquids use aNPSH Margin Ratio of 1.3.

The majority of vertical turbine pumps in the petro-chemical industry are normally installed in a barrel orcan as shown in Figure 2.6 of the Hydraulic Institutestandard ANSI/HI 2.1-2.2, Vertical Pumps for Nomen-clature and Definitions. The NPSHA must exceed theNPSHR over the expected range of operation. Nor-mally, the customers will give a margin value which willvary from 0 to approximately 1.5m (5 feet). TheNPSHA is normally given at ground level or pump inletlevel. The manufacturer then determines the length ofthe pump required to achieve sufficient NPSHA at thefirst stage impeller inlet to account for the NPSHR,pump inlet losses (inlet to eye of first impeller) andmargin.

9.6.1.5.2 Chemical process pumps

Pumps for these applications frequently share the fol-lowing characteristics:

1) Operation frequently occurs at a wide varietyof rates of flow.

2) Materials of construction are often stainlesssteel impellers.

3) They may operate with relatively low NPSHA.

4) Operators are frequently located remotelyfrom the pumps.

These factors emphasize the need to apply largeNPSH margins when selecting pumps.

Taking these issues into consideration, the followingNPSH Margin guidelines are proposed for ChemicalProcess pumps to account for the many uncertainties:

• For low suction energy pumps, the margin shouldbe 10% of the NPSHR or 0.6m (2 ft), whichever isgreater.

• For high suction energy pumps the margin shouldbe 30% of the NPSHR or 1.5m (5 ft), whichever isgreater. NPSH tests are recommended if the



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pump specific speed is above 2,300 metric (2,000U.S. units).

If a pump is applied to the right of BEP, careful consid-eration should be given to ensuring that, at the maxi-mum flow rate permitted by the system, and itscontrols, the NPSHA is in excess of the NPSHR of thepump.

If the above criteria cannot be met and there is no priorexperience with the specific pump in the application,NPSH tests should be conducted on the pump. Onetest should be conducted at the rated conditions andmust demonstrate that the NPSHR (3%) is equal to orless than the rated NPSHR. Tests should also be con-ducted at four additional rates of flow at approximatelyeven intervals from the minimum to maximum antici-pated rates of flow to fully define the NPSHR (3%)characteristic curve.

9.6.1.5.3 Electric power pumps

Power plant pumps are water pumps. Cold water isone of the most difficult liquids to pump in that cavita-tion can cause severe damage. Unlike hydrocarbonliquids handled by petroleum pumps, water, when itvaporizes (flashes), expands tremendously. Thisresults in higher impact velocities when the vapor bub-bles implode, thus higher suction energy. One poundof water at room temperature which occupies 4.5×10–4

cubic meters (0.016 cubic feet), will flash to over34 cubic meters (1200 cubic feet) of vapor. This is avolume ratio of 75,000 to 1. For typical hydrocarbonliquids, this volume ratio is one-half to one-tenth that ofwater.

Hot water, on the other hand, can act similar to hydro-carbon liquids. When water is heated to 250-300° F,the vapor volume characteristics become similar tothat of a typical hydrocarbon. This means that theeffects of flashing are diminished; however, the oppor-tunities for system transients increase significantlywith temperature.

In addition to possible severe vaporization effects, typ-ical power plant operating cycles are not stable. Mostpumps in these services do not remain at constantflow rates for extended periods of time. The pump flowdemands vary widely with power demands. Becauseof varying power demands, system upsets may occurwhich result in rapid changes in pump flow demandsand, many times, severe changes in pump suctionpressure. This is especially true for pumps in the boilerwater systems such as boiler feed pumps and boilerfeed booster pumps. It is not unusual, during such

system upsets, or transients, for flashing to occur inthe suction line to the pump, causing loss of suctionflow and allowing the pump to “run dry”. A commonside effect of a pump running dry is rapid mechanicalseal face wear, general seal deterioration and prema-ture, sometimes catastrophic failure.

Other pumps in the power plant are not usuallyexposed to such severe transients as those in theboiler water system. Condensate pumps and heaterdrain pumps are usually isolated from severe systemupsets. They too, however, have special demands oroperating requirements which impact on NPSH andNPSH Margin requirements. Since they are typicallyrequired to operate with very low NPSHA, they aredesigned to function, and survive, with a certainamount of cavitation present. Some systems operateon what is termed “cavitation control,” i.e. the pumpsoperate with cavitation at all times.

In such a system, the pump is constantly under somedegree of cavitation which results in a reduced pumpdeveloped head. The quantity of flow through thepump, and system, is “controlled” by the intersection ofthe pump reduced head–rate-of-flow curve and thesystem curve. For such an application, there is noNPSH margin; and the pump must be designed towithstand constant cavitation. This means it must be ofrugged construction to offset the detrimental effects ofcavitation related vibration, and the materials of con-struction must be capable of withstanding the erosionassociated with cavitation.

Vertical turbine type pumps used as condensatepumps are normally installed in a barrel or can asshown in Figure 2.6 of ANSI/HI 2.1-2.2, VerticalPumps for Nomenclature and Definitions.

9.6.1.5.4 Nuclear power/cooling tower

Pumps in nuclear power plants share the followingcharacteristics and requirements:

a) Nuclear Reactor Duty:

1) Users are more frequently requesting NPSHRcurves based on a 1% head drop.

2) The NPSHA Margin, over NPSHR (3%), isoften incorporated in the NPSH Requiredcurve by the manufacturer.

3) High horsepower reactor cooling pumps, alsocalled primary heat transport pumps, are oflow to high suction energy levels.



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4) Reactor cooling pumps normally operate athigh temperatures and suction pressures, theywill operate at ambient temperature and lowsuction pressures during transients andcommissioning.

b) Boiler Feed Duty:

1) NPSHR based on a 3% head drop is specified.

2) Suction energy levels are between low to veryhigh.

c) Cooling Tower Duty:

Cooling tower water typically has modified chemis-try due to water treating agents. These additivescan increase the vapor pressure, which results ina lower NPSHA than calculated for pure water.This reduction can be as high as 1.8 meters (6.0feet), although the exact number must be experi-mentally determined.

9.6.1.5.5 Water/wastewater

The following considerations apply to pumps for thisapplication:

1) During variable speed operation, all possiblewet well levels, pump speeds, and rates offlow exist. It is important that the pump canfunction properly over the full operating rangeof the system curve. A flow duration diagramcan be used to determine where the pump willoperate most frequently. In the on/off mode ofoperation, the speed and rate of flow will berelatively constant, but the sump level will varybetween a maximum water level and a mini-mum water level. The change of the waterlevel in the sump will also change the pump’stotal head, also slightly changing the speedand rate of flow of the pump, but the duty pointwill be nearly constant compared to variablespeed operation.

2) Actual system head curves often differ fromthe calculated values. This will cause theNPSH Margin calculation to be incorrect. It is,therefore, very important to ensure that thecalculated system head curves be as close aspossible to the actual. For existing systems, itis possible to measure the head at a numberof points to develop the system head curve.Two system curves should be calculated fornew installations: one for the system as it will

be installed; and a second to represent thecondition of the system after some increase inpipe roughness has occurred.

3) Many pumps are installed in wastewater appli-cations with elbows mounted in front of theimpeller eye. When suction elbows are neces-sary, it is best to use reducing or long radiuselbows.

4) Materials of construction are typically cast iron(wastewater) or cast iron / bronze fitted(water). These materials are preferred forwater/wastewater, but they do not stand upwell under heavy cavitation. The protectivelayer that is built up under normal operation isdestroyed by cavitation, causing abnormalmaterial removal rates. It is advisable tochange to tougher materials such as stainlesssteel or aluminum bronze alloys if the pumpmust withstand destructive cavitation levels,however, this will not help the seals or bearings.

5) Pump stations often operate unattended, andthe malfunction of a pump must be avoided. Afailed pump station processing water or waste-water will cause considerable inconvenienceto the public, and should be designed to be astrouble-free as possible.

6) Single, two and three vane impeller designsare common in wastewater applications, withno or minimal vane overlap. Increase the suc-tion nozzle sizes by one or two sizes forpumps with one to three vane impellers beforeusing Figure 9.6.1.3.

7) Vertical Turbine barrel or can type pumps onwater booster services are generally appliedwith little or no NPSH Margin, since they aremostly low suction energy applications.

The above items are listed to illustrate the uncertainiesrelated to the NPSHA calculations, and at the sametime demonstrate the importance of accuracy whendetermining the required NPSHA. It seems as thoughthe simple answer would be to over-compensate byadding margin on top of margin, guaranteeing that thepump would run far from the point of cavitation. Eventhough an excessive amount of NPSHA is often notdetrimental to the pump, putting margin on top of mar-gin would add to the cost of the pump stations. It isalso important to note that there are a number of peo-ple involved in the supply chain from the specifier tothe end user, and each one may add a margin of their



9

own. Some pump manufacturers include a margin intheir published NPSHR curves. If everyone was to adda margin, the result of this excess margin wouldincrease the cost of the pump stations dramatically.

9.6.1.5.6 General industrial

Pumps for this application are used to pump a greatvariety of liquids, ranging from water to concentratedchemicals. These pumps are often sold as standardcatalog, pumps. They are generally low suction energydesigns.

Due to the variety of liquids pumped through anextreme range of temperatures, the specifier mustcarefully calculate the NPSHA in the system, takinginto account the vapor pressure of the liquid at theextreme operating temperature. The use of hose con-nections and the associated piping bends must beaccounted for. The use of hose or tubing connectionswith internal diameters smaller than the pump suctioninlet should not be used on the suction side of thepump.

NPSHA on tank draining applications should be calcu-lated for the lowest possible level of the liquid in thetank during the pumping process.

Another consideration in the NPSH Margin of catalogtype pumps is the common changes in flow ratesexperienced during process changes, as well as thephysical expansion of process systems to meet higherproduction rates. In general an NPSHR versus rate offlow curve has a parabolic shape. This may causelarge changes in NPSHR especially if the pump isbeing run to the right of the best efficiency point.

Due to the low suction energy of most general indus-trial pumps, operation of the pump without any NPSHmargin does not normally cause substantial damageto the internal components of the pump. Typical prob-lems are frequent replacement of the mechanical sealas well as the front motor bearings (on close coupledpumps) due to the intense vibration caused by the col-lapsing bubbles, when in fully developed cavitation.

9.6.1.5.7 Pulp and paper

For horizontal end suction stock process pumps situ-ated close to the suction chest, and operating in thecontinuous allowable operating region, it is normal toadd sufficient NPSH Margin to account for the uncer-tainties in the actual NPSHR and NPSHA from poorsuction piping and entrained air. The following minimum

NPSH Margins are suggested for stock consistenciesup to 6%:

• For Low Suction Energy pumps use an NPSHAMargin Ratio (NPSHA/NPSHR) of 1.1 or a marginof 0.6m (2 ft), whichever is greater.

• For High Suction Energy pumps, or pumps havingSpecific Speeds greater than 2300 metric (2000US units), use an NPSHA Margin Ratio of 1.3 or amargin of 1.5m (5 ft), whichever is greater.

9.6.1.5.8 Building services

Fluid systems for the building trades or HVAC Industryare comprised of both closed and open pumping sys-tems. NPSH is generally not a concern when design-ing closed pumping systems. The typical closedsystem is filled and then pressurized to a “fill” pressureof 4 to 10 psig. If an inadequate NPSH available(NPSHA) condition should occur, it can usually beremedied by increasing the fill pressure.

For open systems, NPSH margin is a very importantconsideration. As a guideline, the NPSHA for opensystems should exceed the pump manufacturer’sstated NPSH-required (NPSHR) by a minimum of0.6m, (2 ft) or 1.1 times the NPSHR for Low SuctionEnergy Pumps. For High Suction Energy pumps themargin ratio should be increased to at least 1.3, or aminimum of 1.5m (5 ft). Pumps operating at theseestablished minimum NPSH margins may experiencesome degree of impeller erosion and/or noise butthese effects should be minimal. System constructionmay contribute to the problem of noise, and cavitation.Increasing the NPSH margin will improve pump opera-tion and reliability.

9.6.1.5.9 Slurry

Pumps used in slurry service are frequently con-structed of either hard metals or elastic materials. It isalso common for the slurry concentration and flowrates to change rapidly, imposing significant loads onthe impeller, shaft and bearings. Because of this, andthe erosive nature of many slurries, slurry pumps areof an extremely rugged design, making them relativelyinsensitive to the mechanical effects of cavitation.

Also, to minimize erosive effects, slurry pumps oftenoperate at low speeds (less than 1200 RPM). As aresult of this, they normally fall into the Low SuctionEnergy category, and have NPSHR values below 6m(20 ft).



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Slurries are typically water based and at ambient tem-peratures. Suction flow is usually gravity fed. Conse-quently the NPSHA is normally in excess of 9m (30 ft),giving NPSHA/NPSHR ratios in excess of 1.5.

The recommended NPSH Margin Ratio for slurrypumps is 1.1 or a margin of 0.6m (2 ft) whichever isgreater. For applications where the margin is less,characteristics of the slurry, and the NPSHR perfor-mance of the pump, should be reviewed to assure sat-isfactory performance.

9.6.1.5.10 Pipeline

For this paper, pipelines are defined as hundreds ofmiles in length for the transport of hydrocarbons orwater. Pumps used for pipeline service normally sharethe following application criteria:

1) Customers more often request the NPSH“Required” values to be based on a 1% headdrop.

2) The NPSHA Margin, over NPSHR (3%), isoften incorporated in the NPSH “Required”curve by the manufacturer.

3) Some pipeline designers and operatorsrequest two NPSH Required curves. Onebeing the conventional NPSHR curve basedon a 3% head drop, and a second based onthe NPSH required to guarantee a 40,000hour impeller life.

4) Specifications frequently require that theNPSHA exceed the NPSH “Required” (40,000hrs) over the full Allowable Operating Regionfor the pump (Minimum to Maximum Flow).

5) There is no standard method for determiningthe NPSH “Required” for 40,000 hours impel-ler life, however it is a function of:

i) Suction Energy Level.

ii) Material of impeller.

iii) Acidity of pumpage (pH).

iv) Temperature of pumpage.

v) Suction Specific Speed.

vi) Operating rate of flow vs pump best effi-ciency point.

vii) The NPSH “Required” (0%) vs NPSHR(3%) ratio throughout the Allowable Oper-ating Region flow range.

9.6.1.5.11 Waterflood (injection) pumps

Water injection pumps for flooding of oil wells typicallyoperate against relatively constant systems. The sys-tem requirements vary with time, but normally thesevariations are gradual and do not impact on operatingNPSH Margins. For sizing of the pumps initially,NPSHR considerations are based on a) expected flowrate requirements (changes) over the planned life ofthe injection project and b) the nature of the suctionsource for the pumps. Assuming that any changes inthe nature of the suction source would also be gradual,the NPSH Margins required by the pumps are rela-tively small in order to ensure satisfactory, consistentpump performance.

Typical NPSH Margins for injection pumps are setbased on the following criteria, considering variationswhich could occur during the life of the injectionproject:

1) Pump NPSHR at maximum expected flowrate.

2) Minimum NPSHA expected at this maximumflow rate.

The NPSH Requirement based on 40,000 hours mini-mum impeller life is being requested more frequentlyin this market.

9.6.1.6 Summary

In summary, the following key points should be under-stood about cavitation in a centrifugal pump, NPSHMargin requirements, and how they are affected by theSuction Energy level of the pump:

• Cavitation exists when NPSHA is at and substan-tially above the NPSHR of a pump.

• The Suction Energy level of a pump (as installed ina system) determines if the cavitation that fre-quently exists in a pump will cause noise, vibrationand/or damage to the pump.

• Low Suction Energy pumps can normally operateat or near their NPSHR with little or no problemsfrom cavitation, except for the 3% head drop.



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• High Suction Energy pumps are likely to be noisywith higher vibration and will possibly experienceless than optimum pump life, if sufficient NPSHMargin is not provided.

• High Suction Energy pumps are more susceptibleto problems from poor suction inlet piping.

• Entrained air, or dissolved air which comes out ofsolution in the impeller eye, can quiet the noiseand vibration of High Suction Energy pumps at lowNPSH Margins.

• Very High Suction Energy pumps will be noisy, willhave high vibration and are likely to experiencereduced pump life if sufficient NPSH Margin is notprovided. Very High Suction Energy pumps arevery susceptible to problems from poor suctioninlet piping.




HI Pumps – Centrifugal and Vertical Pumps for NPSH Margin Index — 1998

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Appendix A

Index

This appendix is not part of this standard, but is presented to help the user in considering factors beyond thisstandard.

Note: an f. indicates a figure, and a t. indicates a table.

Additives in liquid, 4

BEP See Best efficiency pointBest efficiency point, 2Building services pumping systems, 9

Cavitation, 3, 6, 10damage factors, 4

Chemical process pumps, 6Cooling towers, 7Corrosive properties of liquid, 4

Electric power pumps, 7

Gas content, 4

Impeller eye diameter, 3, 4Impeller material, 4Impeller vanes

incidence angle, 2overlap, 2f., 2

Industrial pumps, 9Inlet geometry, 2Inlet piping geometry, 2

Multistage pumps, 4

Net positive suction head available, 1, 1f.Net positive suction head margin

See NPSH marginNet positive suction head required, 1, 1f.NPSH margin, 1, 10

building services pumping systems, 9chemical process pumps, 6cooling towers, 7definedelectric power pumps, 7general industrial pumps, 9

guidelines, 4, 5t.nuclear power pumps, 7petroleum process pumps, 6pipeline pumps, 10pulp and paper pumps, 9ratio, 1slurry service pumps, 9and vertical turbine pumps, 6water/wastewater pumps, 8waterflood (injection) pumps, 10

NPSHA See also Net positive suction head availableNPSHR See Net positive suction head requiredNuclear power pumps, 7

Peripheral velocity, 2Petroleum process pumps, 6Pipeline pumps, 10Pulp and paper applications, 9Pump duty cycle, 4Pump size, 4

Slurry service pumps, 9Solids/abrasives in liquid, 4Specific gravity, 2Suction energy, 10

determination, 3, 3f.factors, 2

Suction energy level, 1Suction specific speed, 1

Thermodynamic properties, 2

Vertical turbine pumps, 6and inlet eye diameter, 4and NPSH margin, 6

Water/wastewater pumps, 8Waterflood (injection) pumps, 10


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ANSI-HI-9-6-1 Centrifugal and Vertical Pumps for NPSH Margin

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