SERIES AND PARALLEL OPERATION OF CENTRIFUGAL PUMPS
A centrifugal centrifugal pump will pump fluid at the point where the system curve intersects the pump curve.
If you need more flexibility you can install another pump and operate it in either series or parallel with the first pump. SERIES OPERATION OPERATION
Centrifugal pumps are connected in series if the discharge of one pump is connected to the suction side of a second pump. Two Two similar pumps, in series, operate in the same manner as a two-stage centrifugal pump. Each of the pumps is putting energy into the pumping fluid, so the resultant head is the sum of the individual heads. ome things to consider when you connect pumps in series! •
"oth pumps must have the same width impeller or the difference in capacities #$%& or Cubic meters'hour.( could cause a cavitation problem if the first pump cannot supply enough li)uid to the second pump.
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"oth pumps must run at the same speed #same reason(.
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"e sure the casing of the second pump is strong enough to resist the higher pressure. *igher strength material, material, ribbing, or extra bolting bolting may be re)uired.
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The stuffing box of the second pump will see the discharge pressure of the first pump. +ou may need a high-pressure mechanical seal.
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"e sure both pumps are filled with li)uid during start-up and operation.
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tart the second pump after the first pump is running.
PARALLEL OPERATION
%umps are operated in parallel when two or more pumps are connected to a common discharge line, and share the same suction conditions. ome things to consider when pumps are operated in parallel! •
"oth pumps must produce the same head this usually means they must be running at the same speed, with the same diameter impeller.
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A%I , states that when pumps are run in parallel, /the head shall rise at least 0 of the head at rated capacity./#this is called a /stable curve because there is a continious rise to shutoff.(
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Two pumps in parallel will deliver less than twice the flow rate of a single pump in the system because of the increased friction in the piping. The shape of the system curve determines the actual increase in capacity. If there is additional friction in the system from throttling #see dotted line in the
following diagram(, two pumps in parallel may deliver only slightly more than a single pump operating by its self. •
If you run a single pump only, it will operate at a higher flow rate #A( than if it were wor1ing in parallel with another pump #"( because it will be operating further out on the curve re)uiring increased power. The rule is that if a pump is selected to run in parallel, be sure it has a driver rated for single operation.
Six truths about parallel pump operation 1. For any given discharge head 1-2, fows or parallel pumps are additive 2. The system fow rate will be determined by the intersection o the systemhead curve and the perormance curve o the parallel pumps 3. umps o di!erent hydraulic characteristics may be operated in parallel to the e"tent that they share common discharge head characteristics #. umps o di!erent hydraulic characteristics may encounter severe problems when operated in parallel $. %ll pumps have di!erent hydraulic characteristics &. To produce fow, a pump must generate a greater discharge pressure at startup than the pressure already present in the system
For any given discharge head, fows or parallel pumps are additive 'eerring to (gure 2) at any given discharge head fow * will e+ual the sum o the fow rom each pump %. Furthermore, the power draw o each pump will be the power draw at the contributing fow rate or each pump. t is generally desirable to use ust one pump where one pump can do the ob. ultiple small pumps will have a higher capital installation cost and will combine to draw more energy than a single properly designed larger pump. /owever, some other actors, such as limited 0et ositive uction /ead %vailable 0/%, may preclude the use o a single pump.
The system fow rate will be determined by the intersection o the system-head curve and the perormance curve o the parallel pumps %lthough the fow capability is additive or parallel pumps at any given discharge head, the actual output o the pumps will be determined by the intersection o the system-head curve with the parallel perormance curve. For a system where the system curve is dominated by rictional losses, parallel operation will generally mean a lower fow than twice the single pump fow Fig 3. 4hen the discharge head is variable, such as with a control valve, then fow will be controllable when within the range o the valve.
Pumps o dierent hydraulic characteristics may be operated in parallel to the extent that they share common discharge head characteristics o ar the discussion has been limited to supposedly identical pumps a scenario that we5ll (nd out shortly does not e"ist other than on paper. /owever, it is possible to operate very di!erent pumps in parallel provided that they share common discharge head characteristics in the region o parallel operation. Fig # depicts characteristics or two di!erent pumps % 6 *. Following rule 0o. 1, that or any given discharge head, fows or parallel pumps are additive7 the parallel operation curve consists o curve % to point 8, and curve 9. oint 8 corresponds to the shut-o! head o pump *. 9urve 9 represents the additive fows or % and * that share a common discharge head.
Pumps o dierent hydraulic characteristics may encounter severe problems when operated in parallel 'eerring again to Fig. #) point : corresponds to the minimum fow rate o pump *. %s long as the system curve intersects the parallel operation curve between points : and the ma"imum allowable fow, everything is (ne. or any reason the system head curve should shit to the let o point :, pump * will be compromised either mechanically or thermally. the system head curve should shit to the let o point 8, pump % will start to run singly and pump * will operate at ;ero fow. %s will be discussed urther in rule & below, it is important to note that pump * can never come on line against pump % unless pump % is operating at a fow greater than that at point 8. The paramount +uestion is whether the operator has the capability to right up until ailure. To operate dissimilar pumps in parallel, one would need to ma
ll pumps have dierent hydraulic characteristics ost pump hydraulic components are built rom castings. :ven when new, di!erences in casting suraces, clearances, and tolerances usually result in slightly di!erent perormance curves. For well-made industrial pumps these di!erences are generally small, and given continuously rising curves and similar shut-o! heads, parallel operation is not a problem. n contrast, or cheaply made products the perormance di!erences can be substantial. =ne should also be cogni;ant o the act that any stage variances within multistage pumps are cumulative across the number o stages.
9urve shape is very much a actor in evaluating supposedly similar pumps or parallel operation. For e"ample, the pumps shown in (gure $ have very similar discharge head characteristics, but in parallel operation they will have the same issues outlined above or the dissimilar pumps in (gure #.
pump must generate a greater discharge pressure at start-up than the pressure already present in the system robably the most commonly discussed but not necessarily the most prevalent problem concerns supposedly identical pumps with a non-continuously rising head curve to shut-o! Fig &. the system curve intersects the pump curve anywhere to the let o point %, a second pump with similar characteristics cannot be brought on line. t can be seen rom Fig & that any o the fow-rates to the let o point %, e"cept or ;ero fow, have a head that is greater than the shut-o! head o the second pump. =bviously, the same issue e"ists or dissimilar pumps such as in Figure #. ump % o Figure # can always start against a running pump *, but pump * can only start against pump % i pump % is operating to the right o point 8. n summary, parallel pump systems are more e"pensive, less eAcient, and create problems with load sharing that single pumps do not. There are some valid reasons or parallel pump operation such as when switching over pumps, 0/% considerations, and handling intermittent pea< load situations that cannot readily be satis(ed with a single pump. 4hen aced with spending constraints, one might also be driven towards parallel pump operation to increase system throughput as an alternative to purchasing new pumps. =perating pumps in parallel is viable providing that it is done with a ull understanding o the individual characteristics o the pumps involved and the ability to monitor or ensure minimum fow thresholds are met or each pump
Pumps in parallel !"-#! The headB capacity curve or a centriugal pump will be supplied to you by the pump manuacturer. The curve he supplies describes the relationship between the head and capacity o that particular model. %s you loo< at his drawing you should note
that the *: best eAciency point is located somewhere between CDE and C$E o the shut o! or ma"imum head. To ma"imi;e the lie o the pump you should operate the pump as close to the *: as you can. lease note that in each o the ollowing diagrams use the same terminology) •
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/ /ead or height, measured in eet or meters G 9apacity measured in gpm, m3Bhr or any other units you are comortable with. % description o the system curve supplied by the consumer •
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Hnless the internal pump clearances go out o speci(cation you will always pump on the pump curve. %s the centriugal pumpIs capacity increases the head will decrease or as the capacity decrease, the head will increase. you change one you always change the other. The pump curve does not e"tend out to intersect the capacity a"is at some point. *eyond the noted limit the pump will go into cavitation because o e"cess fow.
n other papers we learned that a system curve is a description o the various heads the pump will encounter at the customerIs desired capacities. The system curve is generated by the pump user and supplied to the pump manuacture to assist him in selecting the correct pump or the application. The head shown on the system curve is always a combination o) •
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The static head. The vertical distance between the discharge o the pump and the ma"imum height o the piping, minus the siphon a!ect The pressure head. The amount o pressure in the tan< to be (lled, converted to head units. The head loss caused by riction in the)
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iping
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Jalves
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%nd any (ttings installed in the system
you are not comortable with these head terms please reer to paper 1#-1D H.. customary units or paper DK-D1 metric units or a detailed e"planation. /ere is a diagram o a typical system head curve.
lease note that the static and pressure heads remain constant in most systems. t is the riction head that varies with the pumpIs capacity. The higher the fow, the more riction or head loss in these components.
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t should also be noted that riction loss varies by appro"imately the s+uare o the resistance. Twice as much fow produces almost our times the riction losses
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=nce he has the customerIs system curve in his possession, the pump manuacturer will place his pump curve on top o this system curve and the pump will then operate where the two curves intersect . /opeully this is close to the pump *:
The ne"t diagram shows two centriugal pumps connected in parallel.
These pumps could be either centriugal or positive displacement types. The terminology remains the same. 4e connect pumps in parallel because we are trying to increase the capacity gpm or m 3Bhr o the system
The ollowing s
The pumps will pump where they each intersect the system curve. lease ta
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4ith two pumps running they intersect at a higher head * and a greater capacity than i one pump was running. To determine the fow o an individual pump while both are running, trace bac< at that combined head to the single pump curve and read the fow or each pump at LML. 4ith two pumps running,
the system head is higher causing each pump to reduce its capacity a little bit.
4e sometimes hear complaints that when three pu mps are run in parallel the third pump oten does not seem to be ma
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The ne"t diagram is an e"ample o three di!erent si;e centriugal pumps running in parallel. 'unning di!erent si;e pumps in parallel is seldom a good idea because the larger pump can throttle the smaller pump causing it to run too ar o! o its *: best eAciency point. This can cause shat defection and possible premature bearing and seal ailure. Oour best protection against e"cessive radial movement o the shat caused by operating o! the *: best eAciency point is to e+uip the pump with a low N3B8# shat number.
either % or * is running alone, it will intersect the system curve at the point shown on the diagram. % and * pumps are running at the same time, the capacities are additive at the same head. The resultant curve gives a new intersection point on the system curve or the combined capacity. To determine the fow contribution o each pump in this arrangement, trace bac< to the intersection with curves % and *, Oou must be sure that the pumps will run individually in the system as well as in parallel. lease ta
%ssume that when the pumps are running together, the combined pump curve intersects the system curve within the operating range o the pumps. %6*. the pumps are run individually neither o them can develop enough fow to intersect the users system curve. *ecause the pump is running at the right hand side o itIs curve the pump will cavitate and e"perience all o the problems associated with severe shat defection. Nets tal< or a minute about what happens when you run 8 positive displacement pumps in parallel. 'emember that the word LheadL is not used with 8 pumps. 4e will be using the term LpressureL instead. ositive displacement pumps connected in parallel should have the same ma"imum pressure capabilities. they incorporate internal relie valves the valves should be set to the correct anticipated pressures.
The rules are the same as running centriugal pumps in parallel. Oou add the capacities o the two pumps at the same pressure.
0ow go bac< and loo< at the ourth diagram. n constructing these e"amples used the same diameter piping or the suction and discharge sides o both pumps, so the discharge head or pressure would be identical coming rom each o them. n practice the two pumps could be using di!erent si;e piping and the discharge head or pressure coming rom the pumps would be di!erent. •
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the piping or pumps P1 and P2 are identical, the head at the discharge o each pump would be the same. the piping or pump P1 were smaller than the diameter or pump P2, the only common diameter would be where they discharge into pipe P3. /ow would the fow be a!ected in this second caseQ The higher riction loss in piping P1 would meet the head at the intersection o 1-3, The head rom pump P1 would drop when the fow encountered this larger diameter and the fow would increase. *oth pumps P1 and P2 are running independently, with the system curve controlling, so pump P2 would continue to provide fow at a rate limited by the riction in the system
There are several reasons why you might want to use pumps running in parallel) •
Two smaller pumps could be less costly than running one large pump.
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n critical applications you need a bac<-up pump.
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Hse parallel pumps to satisy the demands o a changing fow system.
There are some considerations you must address when using parallel pumps) •
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The pumps should run at the same speed with the same diameter impellers. Hse installed hour meters to assist you in determining the service hours on each pump i you alternate them in operation.
