suction specific speed from pump-zone.com

Pump Ed 101

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Written by: Terry Henshaw, P.E.

The Eighth in a Series

Normalizing NPSHOur attempts to normalize or classify water turbine hydraulic performance culminatedin 1915 with the development of the specific speed concept, which was later appliedto centrifugal pumps (2).

Because NPSH is not included in specific speed, however, attempts continued tounderstand and normalize this elusive characteristic. In 1922 (1), the Thoma-Moodyparameter, Sigma, was introduced and defined as:

Sigma = σ = NPSH/H (13-1)

where H is the total head of the first stage impeller. Sigma was widely accepted andused for a number of years, but it had a significant shortcoming, since the NPSHR ofa pump is relatively independent of the head produced by the pump.

In 1937, three engineers at Worthington-Karassik, Wislicenus and Watson-wereassigned the task of developing a better concept than Sigma. Initially workingindependently, they joined forces. While discussing their problem over Saturday morning coffee in Karassik's kitchen,they developed the concept of Suction Specific Speed (3).

Definition of Suction Specific SpeedSuction specific speed , like specific speed, is not a speed at all. It is an index number, or "yardstick." It is based on theNPSHR of a centrifugal pump, normally the 3 percent head drop NPSHR and normally at its best efficiency point (BEP).The equation for suction specific speed is the same as specific speed, except that NPSHR is substituted for head, asfollows:

S = (13-2)

Where (in U.S. units):

S = Suction Specific Speed

N = RPM of Pump

Q = Pump Capacity*†, GPM

NPSHR = NPSH required by pump†, feet

*If the impeller is double suction, Q in the above equation is one-half the BEP capacity of the pump. This is a majordifference from calculating specific speed, in which we use total pump capacity, whether the impeller is single suction ordouble suction.

†Normally calculated at the BEP

The symbol Nss is often used in place of S for suction specific speed.

The value of S for most pumps is typically between 7,000 and 15,000. The higher values are more common in higherspeed, higher capacity units. (See next month's article for additional discussion of the effect of speed and capacity on S.)

Problem No. 1: Suction Specific SpeedCalculate the suction specific speed for the pump represented by the performance curve in Figure A (Figure 2 from theMay column).

N = 3,550 rpm

Q = 450 gpm

NPSHR = 14 ft

S = = =10,400 (RPM-GPM-FT)10,400 (RPM-GPM-FT)

(Our answer is different than the 9,000 stated on the curve.)

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Suction Specific Speed (Part One)

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Figure A. Typical published performance curve. Single-line NPSH curve.

Importance of Suction Specific SpeedEstablishing a Maximum Value for S

For a number of years, the push from users and competitors required pump manufacturers to continually strive for lowervalues of NPSHR. The philosophy was that "The lower the NPSHR, the better the pump." (NPSHR in centrifugal pumps isnormally reduced by increasing the diameter of the impeller eye, as shown in Figure 1.) That philosophy has nowchanged. Due to problems that have been attributed to oversized impeller eyes, pump users have established maximumvalues for S, which establishes minimum values for NPSHR.

Figure 1. To reduce NPSHR, the impeller eye diameter is increased.

Establishing a "Stable" Window of Operation

Every centrifugal pump would like to run at its BEP-always. All pump components would experience maximum life at thatcapacity.

Seldom does a pump run at its BEP, but component life will be significantly extended if it operates within its "stable"window of capacities.

To a large extent, suction specific speed indicates the size of that window. Pumps with lower values of S have largerwindows.

Suction Recirculation, or the "Big Eye Syndrome" (Monster or Myth?)For decades, industry recognized that centrifugal compressors would "surge" if operated below a certain capacity, butonly more recently have we recognized that centrifugal pumps have a comparable characteristic.

We now know that any centrifugal pump will experience recirculation in the impeller eye if the capacity is below a certainvalue. Larger impeller eyes and higher speeds (i.e., higher peripheral velocity of the eye-U ) produce higher energyrecirculation.

The large eye required to obtain low NPSHR leads to the problem of (higher energy) "eye recirculation" or "suctionrecirculation" (4). As shown in Figure 2, flow through the eye is proper at the BEP, but at some reduced capacity,recirculation starts in the large eye. As pump capacity is further reduced, the intensity of the circulation increases,sometimes resulting in a reversal of flow at the i.d. of the suction pipe, near the pump. If strong enough, this vortexcauses cavitation, noise and pulsations. The capacity, at which this recirculation starts, increases as the eye diameter isincreased.

Figure 2. Although resulting in reduced NPHR at the BEP, the large eye creates eddies and recirculation atreduced flow rates.

When the vortices are strong enough to cause cavitation, the vapor bubbles collapse on the driving side, or pressure side,of the impeller vane, near the eye. If the vane twists as it enters the eye from the larger diameter, this part of the vanecannot be seen by looking directly into the eye, but must be viewed with the assistance of a small mirror.

Study by HallamNumerous technical papers and articles reported problems caused by suction recirculation, but none quantified thephenomenon until Hallam (5) reported in 1982 the results of a 5 year study of 480 centrifugal pumps in Amoco's Texas

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Tags: Centrifugal Pumps NPSH September 2009 Issue net positive suction head

City refinery. Most of these pumps were in hydrocarbon services. The remainder pumped water. The average powerrequirement was about 150 hp and the maximum was 1,000 hp.

Figure 3 shows the results of the study. The pumps were divided in groups according to their suction specific speed. Theaverage numbers of failures/year/pump were plotted for each group. A failure was defined as any problem with the pumpthat required service in the refinery repair shop. The graph shows an

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increase in failure frequency of almost 100 percent above an S of about 11,000.

Figure 3. Failure frequency vs. suction specific speed (J.L. Hallam, [5])

The logic contained in some of the papers on suction recirculation is not totally rigorous. Most of the symptoms attributedto recirculation can be attributed to other centrifugal pump phenomena. The high S attributed to some centrifugal pumpshas also been found to have been obtained by improper methods of NPSH testing. Ross (6) claims that the problem ofsuction recirculation has been distorted-that it simply boils down to inadequate NPSHA.

Regardless of the reason for the problem-whether it is suction recirculation, improper methods of testing or justinadequate NPSHA-Hallam's paper clearly shows that a pump with a suction specific speed greater than 11,000 shouldbe selected with caution.

This finding caused a number of companies to prohibit the purchase of a pump with a suction specific speed in excess of11,000.

Problem No. 2. Suction Specific Speed for a High Speed Multistage Waterflood Pump

A four-stage pump is operating on an offshore platform in waterflooding (secondary recovery) service. Figure 4 is aredrawn copy of the performance curve provided by the pump manufacturer. The pump is equipped with a double suction,first stage impeller.

Calculate the suction specific speed:

S = = = 12,000 (RPM-GPM-FT)

Would you suspect that this pump could be a problem? Yes, and it was. The first stage impeller would periodicallyexperience cavitation-erosion on the pressure side of the vanes, throwing the rotor out of balance and causing excessivevibration.

Figure 4. The vendor performance curve for the waterflood pump

Pumps & Systems , September 2009

Terry Henshaw is a retired consulting engineer who designs pumps and related high pressure equipment and conductspump seminars. For 30 years, he was employed by Ingersoll Rand and Union Pump. Henshaw served in variouspositions in the Hydraulic Institute, ANSI Subcommittee B73.2, API 674 manufacturers' subcommittee and ASMEPerformance Test Code Committee PTC 7.2. He authored a book on reciprocating pumps, several magazine articles and








Suction Specific Speed (Part One): Page 2 of 2

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Tags: Centrifugal Pumps NPSH September 2009 Issue net positive suction head

the two pump sections in Marks' Handbook (11th Edition). He has been awarded six patents. Henshaw is a registeredprofessional engineer in Texas and Michigan, is a life fellow of the ASME and holds engineering degrees from RiceUniversity and the University of Houston. He can be reached by e-mail at [email protected].

ReferencesGraber, P., "Solids Handling Pumps-Part 2", World Pumps Magazine, 1983

Stapanoff, A. J., Centrifugal and Axial Flow Pumps, John Wiley & Sons. Inc., 1948

Karassik, Igor J., "NPSH Characteristics of Centrifugal Pumps," paper presented at the Pump Workshop of the PacificEnergy Association, Long Beach, CA, Jan. 1981.

Shepherd, W.O. & Godin, R. L, "Face Up to Feedpump Cavitation", Power, May 1977.

Hallam, J. L., "Centrifugal Pumps: Which Suction Specific Speeds are Acceptable?", Hydrocarbon Processing, April 1982.

Ross, Robert R., "Theoretical Prediction of NPSHR for Cavitation Free Operation of Centrifugal Pumps", UnitedCentrifugal Pumps, about 1982.

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Graph of Minimum Continuous FlowsIn 1982, Richard Dubner of Chevron developed a graph for use in establishingminimum continuous flow rates for centrifugal pumps (see Figure 1 ). It allows alower flow rate for pumps handling hydrocarbons than for those pumping water, and itrequires larger pumps to operate closer to the BEP than smaller pumps.

Figure 1. Dubner's (Chevron) Chart for Minimum Capacity

Note that Figure 1 cautions against S values greater than 11,000, and prohibits operation of any pump larger than 100gpm (BEP) from operating continuously at less than 20 percent of BEP.

This graph may be used to establish operating guidelines for existing pumps, and in the selection process for new pumpsto eliminate offerings that have a minimum flow above that anticipated for the intended service.

Problem No. 1. Minimum Continuous Capacity

Using Dubner's graph ( Figure 1 ), determine therecommended minimum continuous capacity for thepump in Problem No. 2 ).

Q = 1,250 gpm

S = 12,000

Pumpage is water. From Dubner's graph, we read afactor of 0.76.

Q = (0.76)(2450) = 1,860gpm








Suction Specific Speed (Part Two)

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"Stable" Window of OperationIn 1985, Lobanoff and Ross (1) provided the graph shown in Figure 2 . It provides not only a minimum capacity, but alsoa maximum capacity for "stable" pump operation. It represents test results of a 4 in (discharge) pump with eight differentimpellers. The onset of instability was determined by measuring pump vibration. The minimum flow values obtained fromthis graph compare favorably with Dubner's water values ( Figure 1 ).

Figure 2. "Stable" Flow Range vs. Suction Specific Speed for a 4 in Pump

Figure 3 is an adaptation of the Lobanoff-Ross graph. Dick Allen developed the graph in 1990 to assist in evaluatingpump bids. It provides a method for evaluating the size of the stable window by penalizing pumps that have smaller stablewindows (higher suction specific speeds).

Figure 3. Centrifugal Pump Stable Window of Operation Based on Suction Specific Speed (Courtesy Dick Allen)

The penalty values are arbitrary, based on the judgment of the graph's author, and may be revised to suit the experienceand philosophy of a given company.

Key points made by this graph:

1. Do not buy a pump to operate continuously at a capacity within the "unacceptable" area (roughly any capacity lessthan 50 percent of Q ).

2. To operate in the "excellent" hydraulic range, capacity must be higher than the "penalty" range (roughly 75 percentof Q ).

3. Do not buy a pump to operate continuously at a capacity higher than Q .

4. Do not buy a pump rated at a capacity higher than about 115 percent of Q .

Increased ReliabilityReliability of existing pumps can be increased by maintaining flow rates within the guidelines established by Figures 1and 2 .

Reliability of future pumps can be increased by using these graphs to establish a maximum suction specific speedacceptable for the intended service.

Effect of Speed on Suction Specific SpeedAlthough specific speed does not change as a pump's speed is changed, suction specific speed does change. Becauseof the 1.5 exponent relationship between NPSHR values (Understanding NPSH, August 2009), a pump will have a higherS when it runs at a higher speed (because the NPSHR is based on the 3 percent head drop).

To establish the effect of speed on S, substitution of NPSH = NPSH (N /N ) into a ratio of S equations,

S =

yields the following relationship:

S = S (13-3)

Where:




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Tags: Centrifugal Pumps NPSH October 2009 Issue net positive suction head

S = Suction specific speed at second speed

S = Suction specific speed at first (base) speed

N = RPM of pump at second speed

N = RPM of pump at first (base) speed

Suction specific speed, based on the 3 percent head drop criterion for the same pump, varies as speed to the 3/8power. (Doubling the speed increases S by 30 percent.)

Although a correction for a speed difference would seem to have value, the author is unaware of any speed adjustmentnow made in suction specific speed evaluations. The following is therefore offered for consideration.

Most process pumps, at least in the United States, are tested and rated at 3,550 rpm. Because NPSHR is measured at3,550 rpm, S is calculated for that speed. To normalize all suction specific speeds to 3,550 rpm, the following equation,from Equation 13-3, can be used:

S = S (13-4)

Where:

S = Suction specific speed, normalized to 3,550 rpm

S = Suction specific speed established by testing (NPSHR for a 3 percent head drop)

N = Test speed, rpm

Example:

Test speed = 1,770 rpm; S = 8,500

S = 8500 = 11,000

Effect of Capacity on Suction Specific SpeedYedidiah (2) plotted (on log-log paper) values of S versus Q for several hundred pumps made by 10 companies, allrunning at 1,750 rpm. The scatter of points showed about a 2:1 variation in S for all capacities from 30 gpm to 4,000 gpm.He drew a single line though the approximate center of these points and measured the slope at about 0.18, indicating

S=KQ , but stated that this exponent would be 0.125 "under ideal conditions."

Evaluating the same set of points, this author measures a slope of about 0.15, acknowledging that a slope of 0.125 wouldalso fit the points satisfactorily.

Effect of Capacity and Speed on Suction Specific SpeedYedidiah (2) also stated that as a function of speed, S "should" vary as S=AN .

If we let the capacity exponent be 0.125, and the speed exponent 0.25, and if we assume that these exponents applywhile holding the







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Author Bio:Terry Henshaw is a retired consulting engineer who designs pumps and related high pressure equipment and conductspump seminars. For 30 years, he was employed by Ingersoll Rand and Union Pump. Henshaw served in variouspositions in the Hydraulic Institute, ANSI Subcommittee B73.2, API 674 manufacturers' subcommittee and ASMEPerformance Test Code Committee PTC 7.2. He authored a book on reciprocating pumps, several magazine articles


other variable constant, we can write the following equation:

S = C Q N

Solving for C for the best fit with the second Yedidiah graph (and it fits reasonablywell) results in

S = 550 Q N (13-5)

Where (in U.S. units):

S = Suction specific speed of a "typical" pump

Q = Pump capacity* at BEP, GPM

N = RPM of pump

*If the impeller is double-suction, Q in the above equation is one-half the BEPcapacity of the pump.

This equation can be used to calculate S for a "typical" pump, realizing that published performance data may show avalue 40 percent above or below the "typical" value.

A more useful purpose may be to further "normalize" suction specific speed. Equations 13-3 and 13-4 provide fornormalizing S for a particular pump to a common speed, such as 3,550 rpm. Equation 13-5 can be massaged to providenormalization of speed and capacity for different pumps, resulting in Equation 13-6:

S =S (Q /Q ) (N /N ) (13-6)

Where:

S = Suction specific speed of second pump

S = Suction specific speed of reference pump

Q = BEP capacity (per eye) of second pump

Q = BEP capacity (per eye) of reference pump

N = RPM of second pump

N = RPM of reference pump

Note that if both pumps are the same pump, we can substitute from the Affinity Laws:

Q /Q = N /N

Equation 13-6 reduces to Equation 13-3, confirming the 0.375 exponent.

If we choose to "normalize" S to 3,550 rpm and to, say, 1,000 gpm (per eye), Equation 13-6 becomes:

S = S (13-7)

Where:

S = Suction specific speed, normalized to 1,000 gpm and 3,550 rpm

S = Suction specific speed established by testing (NPSHR for a 3 percent head drop)

Q = BEP capacity of pump (per eye), GPM

N = Test speed, RPM

References1. Lobanoff, Val S. & Ross, Robert R., Centrifugal Pumps: Design & Application, Gulf Publishing, Houston, Texas, 1985.

2. Yedidiah, S., "Factor Pump Size into NPSH Comparisons," Power, June 1973.








Suction Specific Speed (Part Two): Page 2 of2

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and the two pump sections in Marks' Handbook (11th Edition). He has been awarded six patents. Henshaw is aregistered professional engineer in Texas and Michigan, is a life fellow of the ASME and holds engineering degrees fromRice University and the University of Houston. He can be reached by e-mail at [email protected].

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Pump Ed 101

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Tenth in a SeriesA number of technical papers (1, 4, 7, 8) have shown that the maximum damage rateto centrifugal pumps, at least in water services, typically occurs when the NPSHA isin the range of two to three times the NPSHR . Therefore, although providingadditional NPSHA will increase pump head and efficiency, it will also push theNPSHA toward, or into, the range where maximum erosion rate occurs to theimpeller.Vlaming (3) concluded that it was not reasonable to eliminate all cavitation in animpeller, and settled on 40,000 hours (five years) as a reasonable life of an impeller.His NPSH recommendations for obtaining 40,000 hours with a properly designed,stainless steel impeller pumping cool water can be condensed to the followingequation:

NPSHR = 1.2(13-8)

This equation applies only to the capacity (flow rate), which results in non-prerotating, shockless entry of the pumpageinto the impeller vanes.* That capacity is normally about 20 percent higher than the best efficiency capacity, but can beeven higher (1).

Equation 13-8 can be rewritten, in terms of U and β , as follows:

NPSHR = (13-9)

Converting to suction specific speed results in:

S = 8150 (13-10)

Equation 13-10 is plotted in Figure 1 , for Dh = 0 and β = 17˚(which is the angle for which S is maximum). Vlamingstated that his experience did not exceed a U of 220 ft/sec, so the S values above 220 ft/sec must be recognized asextrapolated.

Conditions that apply to the 40,000 hour curve are:

1. Cool water

2. Stainless steel impeller

3. Impeller vanes properly twisted and tapered. (For plain vanes, S is lower.)

4. β = 17˚

5. No-prerotation, shockless-entry capacity








Suction Specific Speed (Part Three): UsingSuction Specific Speed to Establish AdequateNPSHA, net positive suction head, NPSH

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6. No hub, shaft or fastener blocking part of the eye. If the eye is partially blocked by a hub, shaft or fastener, multiply S

from the figure by (1-(D /D ) )

This author's work (9), supported by Yedidiah (5), demonstrated that, based on the 3 percent head drop, the approximatesuction specific speed, calculated at Q , varies with U as follows:

S = C U (13-11)

Gongwer's work (10) allowed this author to determine that S reaches its peak value when β = 10˚. With β = 10˚,Equation 13-11 becomes:

S = 2520 U (13-12)

Equation 13-12 is also plotted in Figure 1 .

Note that the 3 percent head drop curve is based on Q and β = 10˚, and the 40,000 hour curve is based on Q andβ = 17˚, so the two curves are not directly comparable. However, the figure does provide a general comparison, and itemphasizes the diversion of S values as U increases. Any S value selected from the top curve could result in aproblematic pump.

Vlaming provided a set of curves for estimating NPSHR for capacities above and below the non-prerotatingshockless capacity, which showed that the NPSHR is higher at all capacities other than shockless.

Figure 1

NomenclatureSYMBOL DEFINITION UNITS EQUATION

β

The angle of the inlet edge of the impellervane, at the point where the vane joins thefront shroud, measured in a plane tangent tothe shroud surface.

Degrees

AEye area. The net flow area just upstream ofthe impeller vanes.

SquareInches

(π/4)( D12- Dh2)

BEPBest efficiency point. The capacity (flow rate)at which pump efficiency is a maximum.

CThe meridional component of the velocity ofthe liquid just upstream of the impellervanes.

ft/sec 0.321 Q/A

D

Diameter of the circle prescribed by theinlet edge of the impeller vane, at the pointwhere the vane joins the front shroud(Typically equal to the diameter of theimpeller eye)

Inches

DDiameter of impeller hub, shaft or fastener,in the plane defined by the outer points ofthe leading edges of the impeller vanes.

Inches

g Acceleration of gravity32.2

ft.sec

N Rotative speed of impellerrev/min(rpm)

NPSHThe NPSH available to the pump coincidentwith a 3 percent pump head loss Feet

NPSH Feet




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Tags: Centrifugal Pumps November 2009 Issue NPSH net positive suction head

As defined by Vlaming (3), the NPSHrequired by a pump, with a stainless steelimpeller, pumping cool water, to obtain animpeller life of 40,000 hours

P

The meridional velocity through theimpeller eye required to obtain a righttriangle with U and W

ft/sec U tan β

QCapacity (flow rate) through each impellereye GPM

QCapacity (flow rate) of the pump when pumpefficiency is at its peak GPM

Q

Capacity (flow rate) through eachimpeller eye, when C = P (noprerotation, shockless entry)

GPM 3.12 P A

SSuction specific speed withcavitation sufficient to cause a 3 percent lossof pump head

RPM-GPM-FEET

N Q

NPSH

S

Approximate suction specific speed withcavitation sufficient to cause a 3 percent loss

of pump head, for β = 10

RPM-GPM-FEET

SSuction specific speed required to obtain40,000 hour life, as defined by Vlaming

RPM-GPM-FEET

N Q

/NPSH

UPeripheral velocity of the inlet edge of theimpeller vane, at the point where the vanejoins the front shroud

ft/sec D N/229

WVelocity of liquid, relative to the impeller, justupstream of the impeller vanes ft/sec

References1. Stepanoff, A. J.,








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