8/17/2019 Screw Pump Characteruistic Curve

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THE OHIO STATE ENGINEER

C H A R A C T E R I S T I C S OF SCREW PUMPSBy R. K. ANNIS B.M .E. 21

The screw pump has a number of very impor- entrance losses of the suction pipe and the lossestant advantages compared with centrifugal due to recovery of velocity head at the d ischargepumps . In order however to app recia te fully pipe are not as greatw h a t the screw pum p will do as compared with the centrifugal pump par ti-cularly for low head operation it isnecessary to have a thorou gh knowledgeof the character is t ic curves of bothtypes of pumps.

The three curves which are repro-duced here show an actual comparisonbetween a screw pump and a 36-in. cent r i fugal pump. A great deal of carehas been taken to make th is comparisonas fair as possible; but owing to the dis-similarity of the character is t ics of thetwo pumps a perfect comparison ispractically impossible. For this reason

wherever it is impossible to make theconditions coincide exactly for the twodifferent pumps the centrifugal pumphas been given every advantageyeteven under rather severe handicaps thescrew still maintains its supremacyunder low head conditions.

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SCREW PUMP

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4OOO 8OOO I6OOO 16000 20OOO 24000 28OOO 32OOO 360OO 4 0000US GAL LONS P R MINUTE

Fig. 1. Complete performance curve showing 42-in. screw pump at 360 r.p.m.

The combined curve shown in Fig. 3i l lustrates that at all heads lower tha n 12.6 ft.the screw pump is the more efficient of the twopumps . It will be noticed t ha t the 42-in. centrifu-gal has not been compared with the 42-in. screwpump because the screw pumps are designed forsuch very low heads that the suction and dischargesizes are made larger than the connections forcentrifugal pumps which handle the samecapacity. In other words the 42-in. centrifugalpump would have a capacity so much greatert h a n t h a t of the 42-in. screw t h at comp arisonwould be impossible. All of the total head s w hichare shown in the three curves are total dynam icheads and this includes the velocity head. There-fore the wa ter delivered from the screw pump ismoving at a lower velocity because of the size ofpipe and hence it is in a more usable form. The

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4OOO 8OOO I2OOO I6OOO 20000 240OO 32OOO 320OO 36OOO 40000

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Fig. 2. Complete performance curve of a 36-in. centrifugal pump withFrancis runner at 224 r.p.m.

I t has often been stated t ha t the speed of ascrew pump can be much higher than tha t of acentrifugal operating under the same conditions.These curves demonstrate this beyond a doubt.It will be noted tha t the centrifugal pump oper-ates at 224 r.p.m. while the screw pum p oper-ates at a speed more th an 50 per cent in excess ofthis namely 360 r.p.m. The advantage in thecost and the efficiency of a motor for operat ingthese two pumps is distinctly in favor of the screwpump.

Fur thermore the screw pum p is a much l ighterpump requiring less expensive foundations andit is easier to install . The 42-in. screw pum pweigh s 9 000 pound s while the 36-in. cen trifugalpump weighs 21 000 pounds. This shows th atthe body of the screw pump is much smaller t ha n

t h a t of the centrifugal p ump in spiteof the fact tha t the pipe sizes arelarger than the la t ter.

In addition to this the a r range-ment for pumping over levees or between canals at different levels is

much more simple for the screwpump than for the centrifugal pum p.The property through which thecanal runs is always long and narrowan d the screw pump together withits prime mover makes a long narrow installation which lends itself tothe shape of the property in whichit is to be installed.

The centrifugal pump is usually amore costly pum p to produce tha n thescrew pump and this is especiallytrue of the pump with the character-istics shown in Fig. 2 because of thefact th at this 36-in. pum p has aFrancis runner.

N O V E M B E R 1928

8/17/2019 Screw Pump Characteruistic Curve

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T H E O H I O S T A T E E N G I N E E R

The Francis runner is known for its efficiencyat low heads and in this case the curve reachesthe unusually high maximum point of 90 per cent.This is partly due to the special de-sign and partly due to the very care-ful workmanship and careful testingof the unit in question. The screwpump on the other hand had a cast-

steel runner whose surfaces wereonly partially smooth. No grea t ef-fort was made to bring up its pointof maxim um efficiency and thereforeit does not exceed 76 per cent at anypoint. Yet even in the face of thes ehandicaps of workmanship and finishon the pa rticular units which were se-lected for this comparison the screwpump is shown to be inherently amore efficient pump at low heads. Itdoes not take a great deal of imagina-tion to see how the screw pump effi-ciency curve of Fig. 3 would comparewith the centrifugal pump efficiencycurve had its blades been m ade ofbronze and highly polished so as toreach a maximum point somewhereabove 80 per cent.

The curves shown in Fig. 3 are out of theordinary in that they are plotted against the totalhead rather than against the conventional gallonsper minute. This is done because th e compari-sons are at low heads rath er than at a givencapacity. By using this method of plotting itcan readily be observed that at low heads the effi-ciencies of the two pumps can be read directlyfrom the cha rt. For example at a head of 6 ft.the efficiency of the screw pump is 57 per centwhile that of the centrifugal pump is only 42 percent. This same information could be taken fromthe other curves but it would be inconvenient todo so. To get the efficiency from Fig. 1 it isnecessary first to read the capacity. At 6 ft. thecapacity is approxim ately 37 000 g.p.m. The effi-ciency is quite indefinite on account of the steep-ness of the curve; but it is apparent that it checks

approximately with the reading given in Fig. 3;namely 57 per cen t. Also in Fig . 2 it is necessaryfirst to read the capacity and then the efficiency

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TOTAL HEAD IN FSBT

Fig. 3. Combined curve of screw pump and centrifugal pump drawn with inverted co-ordinance to show the variat ion of the pump efficiencies under low head conditions.

which checks with Fig. 3 and is 42 per cent. Inaddition to this Fig. 1 and F ig. 2 would not makea graphical comparison even if plotted on thesame sheet.

The important point which should be kept inmind is tha t these curves may not show up th escrew pump to advantage when read in their en-tirety but that at the extreme right where thepoints of low working heads exist the advantagesof the screw pump begin to assert themselves. Itis under these conditions tha t a screw pumpshould be used.

In general these curves have demonstrateddirectly or indirectly most of the advantages ofthe screw pump over the centrifugal as follows:

Higher efficiency at low heads higher speed lighterweight sma ller dimensions lower first cost lower cost ofinsta llatio n cheape r motor more efficient motor low headinstallation.

Section through a reversible type screw pump. Pumps of this type have been built 12 ft. in diameter with a capacity of

360 000 gallons per minute each. They are also built in sizes about down to 24 in. Pumps of this type are used particul arlyin connection with large irri gation and drainage projects or in any work where large volumes are to be handled at low heads.All of the rainfall on the area covered by the City of New Orleans is pumped over the levees with equipment cf this t ype.

NOVEMBER 1928