The Pump Handbook Series 87

Suction Side Problems - Gas Entrainment

BY: JAMES H. INGRAM

ultiple symptoms associatedwith noncondensable suc-tion side gas entrainment,such as loss of pump head,

noisy operation, and erratic perfor-mance, often mislead the pump opera-tor. As a result, entrained gas isgenerally diagnosed by eliminatingother possible sources of performanceproblems. To adequately control gasentrainment a user should first beaware of systems most likely to pro-duce gas, and then employ methodsor designs to eliminate entrainmentinto these pumping systems.

ENTRAINMENT VERSUS CAVITATION The audible pump noise from

noncondensable entrained gas willproduce a crackling similar to cavita-tion or impeller recirculation.However, cavitation is produced by avapor phase of the liquid which iscondensable, while noncondensableentrained gas must enter and exit thepump with the liquid stream.

To test for gas entrainment overmild cavitation, run the pump backupon the curve by slowly closing thedischarge valve. The noise will dimin-ish if it originated from cavitation andthe pump is not prone to suction recir-culation. In contrast, with entrainedgas, continued performance at thisportion of the curve will choke off orgas-bind the pump, causing unusuallyquiet operation or low flow.

M GAS BOUND IMPELLERSAs a process stream containing

entrained gas nears the impeller, theliquid pre-rotating from the impellertends to centrifuge the gas from theprocess stream. Gas not passing intothe impeller accumu-lates near the impellereye. As entrained gasflow continues to in-crease, the accumulat-ing groups of bubblesare pulled through theimpeller into the dis-charge vane area wherethey initiate a fall inflow performance. Thebubble choking effect atthe impeller eye pro-duces a further reduc-tion of Net PositiveSuction Head Available(NPSHA). At this stagelong term damage to thepump from handlingentrained gases is gener-ally negligible whencompared with thedamage due to cavita-tion. If the processstream gas volumeincreases, however, fur-ther bubble build-upwill occur, blocking offthe impeller eye andstopping flow (Ref. 1).

A pump in this gas bound state,will not re-prime itself, and the gas,with some portion of the liquid, mustbe vented for a restart against a dis-charge head. The effort to restart agas bound impeller depends on

Capacity in U.S. Gallons per Minute

Head

in F

eet

Size: no. 55; Type: SQ. Speed: 1750 Impeller Diameter: 11”

Air quantities given are in terms of free air at atmosphericpressure referred to % of total volume of fluid being handled.

FIGURE 1. ENCLOSED IMPELLER-ENTRAINEDGAS HANDLING PERFORMANCE

NO AIR HEAD2%

5%

8%

10%

12%15%

12001000800600400200050

60

80

100

120

140

160

The LaBour Company, Inc. Effect on head and capacity ofvarying quantities of air with water being pumped.

CENTRIFUGAL PUMPS

HANDBOOK

head loss at 5% gas volume, theGould’s open impeller experiences a12% head loss at this volume. Someopen impeller paper stock designscan actually handleup to 10% entrainedgas because clear-ance between thecase and impellervanes allows moreturbulence in theprocess fluid, whichtends to break upgas accumulationmore efficientlythan an enclosedimpeller with wearrings. In addition,other designs, suchas a recessed im-peller pump, mayhandle up to 18%entrained gas. Infact, most standardcentrifugal pumpshandle up to 3%entrained gas vol-ume at suction con-ditions withoutdifficulty. (Ref. 2

discusses open impeller pump modi-fications.)

SYSTEMS PRODUCING ENTRAINED GAS

The most common conditions ormechanisms for introducing gas intothe suction line are: 1. Vortexing

2. Previously flashed process liquid conveying flashed gasinto the suction piping.

3. Injection of gas, which does notgo into solution, into thepumpage.

4. Vacuum systems, valves, seals,flanges, or other equipment in asuction lift application allowingair to leak into the pumpagestream.

5. Gas evolution from an incom-plete or gas producing chemicalreaction.

If a particular application pro-duces entrained gas or has the poten-tial to do so, the best solution is toeliminate as much entrainment aspossible by applying corrective pumpsystem design and/or a gas handlingpump. If liquid gas mixing is desired,employ a static mixer on the dis-

impeller position, type and valvingarrangement, among other variables.Degassing is easier to accomplishwith a variable speed driver, such asa steam turbine, than with a constantspeed electric motor drive. In addi-tion, a recycle line to the suction ves-sel vapor space is often an effectivemethod for degassing an impeller,since with this arrangement thepump is not required to work againsta discharge head. (Ref. 1 describesmethods for venting gas on modifiedpumps that are gas bound.)

As a rule, if the probability ofentrained gas exists from a chemicalreaction, the inlet piping designshould incorporate a means to ventthe vapor back to the suction vessel’svapor space or to some other source.

EFFECTS OF ENTRAINED GAS ONPUMP PERFORMANCE

Figures 1 and 2 illustrate theeffect of entrained gas on a LaBourenclosed impeller and a Gould’spaper stock open impeller. As illus-trated by the figures, 2% entrainedgas does not produce a significanthead curve drop. Note that while theLaBour impeller experiences a 22%

Gallons per Minute0 500 1000 1500 2000 2500 3000

FIGURE 2. OPEN IMPELLER-ENTRAINED GAS HANDLING PERFORMANCE

Gould’s Pumps, Inc. Approximate Characteristic Curves of Centrifugal Pump

4%

0%2%

6%0%2%4%6%

6%0%Bhp@ sp gr=1.0

Size: 6x8-18 Speed: 1780 rpm Impeller Diameter: 17 1/4”

50

60

70

80

Effic

ienc

y %

250

200

150Brak

e Ho

rse

Pow

er (B

hp)

150

200

250

300

350

Head

in F

eet

FIGURE 3. DEVELOPMENT OF A VORTEX

A. Incoherent surface swirl

B. Surface dimple with coherentsurface swirl

C. Vortex pulling air bubbles tointake

D. Fully developed vortex with air core to nozzle outlet

AIR

(c)

(a)

(b)

(d)

88 The Pump Handbook Series

The Pump Handbook Series 89

charge of the pump. In addition, ananticipated drop in pump head due toan entrained gas situation may be off-set by oversizing the impeller.

Of the five aforementionedmechanisms, vortexing is the mostcommon source of entrained gas.Therefore, a user should be especiallycautious employing mechanicalequipment, such as tangential flashgas separators and column bottomsre-boilers, likely to produce a strongvortex.

VORTEX BREAKER DESIGNThe extent of gas entrainment in

the pumped fluid as the result ofvortex formation depends on thestrength of the vortex, the submer-gence to pump suction outlet, andthe liquid velocity in the pump suc-tion nozzle outlet. Vortices form notonly through gravity draining vesselapplications, but also in steady statedraining vessels, and in vesselsunder pressure or with submergedpump suction inlets. Vortex forma-tion follows conservation of angularmomentum. As fluid moves towardthe vessel outlet, the tangentialvelocity component in the fluidincreases as the radius from the out-let decreases. Figure 3 shows variousstages of vortex development. Thefirst phase is a surface dimple. Thisdimple must sense a high enoughexit velocity to extend from the sur-face and form a vortex. (For experi-mental observations regardingvortex formation see Refs. 3, 4.)

The most effective method to

eliminate entrained gas in pump suction piping is to prevent vortexformation either by avoiding vortexintroducing mechanisms or by em-ploying an appropriate vortex break-er at the vessel outlet. A ”hat” typevortex breaker, illustrated in Figure4, covers the vessel outlet nozzle toreduce the effective outlet velocity.This design doesn’t allow a vortex tostabilize because the fluid surfacesenses only the annular velocity atthe hat outside diameter (OD). Inaddition, the vanes supporting thehat introduce a shear in the vicinityof the outlet to further inhibit vortexformation. An annular velocity of1/2 ft/sec at the hat OD produces aviable solution. Variations in hatdiameters from 4d to 5d and hatannular openings of d/2 to d/3 areacceptable when annular velocity cri-teria are met. Annular design veloci-ties of more than 1 ft/sec are notrecommended.

”Cross” type breakers, installedabove or inserted in vessel nozzle out-lets as shown in Figure 5, work forsome applications by providing addi-tional shear to inhibit a mild vortexfrom feeding gas into a nozzle outlet(providing enough submergence isavailable). However, this design willnot stop a strong vortex and willdecrease NPSHA. A user should beaware of these limitations.

COLUMN VORTEXINGIf a column draw-off pump is

erratic and/or nearly uncontrollable, avortex may be feeding gas into the

draw-off nozzle of the pump as illus-trated by Figure 6a.

It may be difficult to understandhow a pump with 60 ft of verticalsuction could be affected by en-trained gas, but in this real caseexample Murphy’s law applied twice.First, since the pump system in ques-tion has a NPSHA greater than 50 ft,the piping designer employed a small-er suction pipe with a liquid velocityof 10 ft/sec. Second, the columndraw-off nozzle was sized accordingto normal fluid velocity practice. As aresult, the tray liquid had an exitvelocity of 5 ft/sec with a liquid level6-in. above the top of the draw-offnozzle and a vortex formed, feedinggas into the draw-off nozzle.

As in the above example, due to alack of proper submergence, gas is car-ried into the pump suction piping as ahigh liquid downward velocity exceedsthe upward velocity of a gas bubble.

Many draw-off vortexing prob-lems may be eliminated by properpump system design or by one of twovortex breaker designs illustrated byFigures 6b and c. The selection of thebreaker design may depend on thedowncomer arrangement and spacelimitations. The most effective vortexbreaker is the slotted pipe designshown in Figure 6c.

Application of these correctivepump systems designs or installa-tion of an appropriate gas handlingpump can solve suction side gasentrainment problems, resulting in asmoother process operation. ■

FIGURE 4. “HAT” TYPE VORTEX BREAKER FIGURE 5. “CROSS” TYPE VORTEX BREAKER

90 The Pump Handbook Series

REFERENCES1. Doolin, John H., ”Centrifugal

Pumps and Entrained-Air Problem,”Chemical Engineering, pp.103-106(1963)

2. Cappellino, C.A., Roll, R. andWilson, George, ”CentrifugalPump Design Considerations andApplication Guidelines forPumping Liquids with EntrainedGas,” 9th Texas A&M PumpSymposium 1992

3. Patterson, F.M., ”Vortexing canbe Prevented in Process Vesselsand Tanks,” Oil and Gas Journal,pp. 118-120 (1969)

4. Patterson, F.M., and Springer, E.K.,”Experimental Investigation ofCritical Submergence for Vortexingin a Vertical Cylinder Tank,”ASME Paper 69-FE-49 (1969)

5. Kern, Robert, ”How to DesignPiping for Pump Suction Con-ditions,” Chemical Engineering,pp.119-126 (1975)

James H. Ingram is an EngineeringTechnologist with Sterling Chemicals inTexas City.

Figure 6a. Tray take off nozzle with vortex from lack of correctsubmergence and too high exit velocity.

10”6”

BUBBLE CAP

10’/sec.

DOWNSPOUT ORDOWNCOMER FROMTRAY ABOVE

Figure 6b. Plate extension over outlet nozzle lowers high outletvelocity.

1/2’/sec.

EXTEND PLATE FROMVESSEL WALL. CHECK VELOCITY AT PLATEEDGE ≤ 1/2’/sec.

Figure 6c. Slotted pipe vortex breaker.

AREA OF SLOTS—3XPIPE CROSS SECTIONAREA. CHECK VELOCITY INTO SLOTAREA ≤ 1’/sec.

FIGURE 6. DESIGN MODIFICATIONS FOR A SYSTEM EXHIBITING A LACK OFADEQUATE SUBMERGENCE AND PROHIBITIVELY HIGH EXIT VELOCITY