SERIES AND PARALLEL OPERATION OF CENTRIFUGAL PUMPS A 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 OPERATIONCentrifugal pumps are connected in series if the discharge of one pump is connected to the suction side of a second pump. 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.Some things to consider when you connect pumps in series: Both pumps must have the same width impeller or the difference in capacities (GPM or Cubic meters/hour.) could cause a cavitation problem if the first pump cannot supply enough liquid to the second pump. Both pumps must run at the same speed (same reason). Be sure the casing of the second pump is strong enough to resist the higher pressure. Higher strength material, ribbing, or extra bolting may be required. The stuffing box of the second pump will see the discharge pressure of the first pump. You may need a high-pressure mechanical seal. Be sure both pumps are filled with liquid during start-up and operation. Start the second pump after the first pump is running.

PARALLEL OPERATIONPumps are operated in parallel when two or more pumps are connected to a common discharge line, and share the same suction conditions.Some things to consider when pumps are operated in parallel: Both pumps must produce the same head this usually means they must be running at the same speed, with the same diameter impeller. API 610, states that when pumps are run in parallel, "the head shall rise at least 10% of the head at rated capacity."(this is called a "stable curve because there is a continious rise to shutoff.) 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 working in parallel with another pump (B) because it will be operating further out on the curve requiring 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 operation1. For any given discharge head1-2, flows for parallel pumps are additive2. The system flow rate will be determined by the intersection of the system-head curve and the performance curve of the parallel pumps3. Pumps of different hydraulic characteristics may be operated in parallel to the extent that they share common discharge head characteristics4. Pumps of different hydraulic characteristics may encounter severe problems when operated in parallel5. All pumps have different hydraulic characteristics6. To produce flow, a pump must generate a greater discharge pressure at start-up than the pressure already present in the system

For any given discharge head, flows for parallel pumps are additiveReferring to figure 2: at any given discharge head flow B will equal the sum of the flow from each pump A. Furthermore, the power draw of each pump will be the power draw at the contributing flow rate for each pump. It is generally desirable to use just one pump where one pump can do the job. Multiple small pumps will have a higher capital installation cost and will combine to draw more energy than a single properly designed larger pump. However, some other factors, such as limited Net Positive Suction Head Available (NPSHA), may preclude the use of a single pump.

The system flow rate will be determined by the intersection of the system-head curve and the performance curve of the parallel pumpsAlthough the flow capability is additive for parallel pumps at any given discharge head, the actual output of the pumps will be determined by the intersection of the system-head curve with the parallel performance curve. For a system where the system curve is dominated by frictional losses, parallel operation will generally mean a lower flow than twice the single pump flow (Fig 3). When the discharge head is variable, such as with a control valve, then flow will be controllable when within the range of the valve.

Pumps of different hydraulic characteristics may be operated in parallel to the extent that they share common discharge head characteristicsSo far the discussion has been limited to supposedly identical pumps (a scenario that well find out shortly does not exist other than on paper). However, it is possible to operate very different pumps in parallel provided that they share common discharge head characteristics in the region of parallel operation. Fig 4 depicts characteristics for two different pumps A & B. Following rule No. 1, that for any given discharge head, flows for parallel pumps are additive; the parallel operation curve consists of curve A to point D, and curve C. Point D corresponds to the shut-off head of pump B. Curve C represents the additive flows for A and B that share a common discharge head.

Pumps of different hydraulic characteristics may encounter severe problems when operated in parallelReferring again to Fig. 4: point E corresponds to the minimum flow rate of pump B. As long as the system curve intersects the parallel operation curve between points E and the maximum allowable flow, everything is fine. If for any reason the system head curve should shift to the left of point E, pump B will be compromised either mechanically or thermally. If the system head curve should shift to the left of point D, pump A will start to run singly and pump B will operate at zero flow. As will be discussed further in rule 6 below, it is important to note that pump B can never come on line against pump A unless pump A is operating at a flow greater than that at point D.The paramount question is whether the operator has the capability to know what the actual flow for each pump is. Operating pumps blindly in a configuration like that shown in figure 4 is very risky. Pumps wear; heat exchangers foul; and control valves open and close. A dead-headed3pump, especially a vertical dead-headed pump, can run amazingly smooth right up until failure. To operate dissimilar pumps in parallel, one would need to make sure that pump B was equipped with a minimum flow recirculation line and preferably, flow monitoring instrumentation. It should also be noted that the pumps might be of the same size and model and yet be dissimilar simply because one is badly worn and the other is not.All pumps have different hydraulic characteristicsMost pump hydraulic components are built from castings. Even when new, differences in casting surfaces, clearances, and tolerances usually result in slightly different performance curves. For well-made industrial pumps these differences are generally small, and given continuously rising curves and similar shut-off heads, parallel operation is not a problem. In contrast, for cheaply made products the performance differences can be substantial. One should also be cognizant of the fact that any stage variances within multistage pumps are cumulative across the number of stages.Curve shape is very much a factor in evaluating supposedly similar pumps for parallel operation. For example, the pumps shown in figure 5 have very similar discharge head characteristics, but in parallel operation they will have the same issues outlined above for the dissimilar pumps in figure 4.

A pump must generate a greater discharge pressure at start-up than the pressure already present in the systemProbably the most commonly discussed (but not necessarily the most prevalent) problem concerns supposedly identical pumps with a non-continuously rising head curve to shut-off (Fig 6). If the system curve intersects the pump curve anywhere to the left of point A, a second pump with similar characteristics cannot be brought on line. It can be seen from Fig 6 that any of the flow-rates to the left of point A, except for zero flow, have a head that is greater than the shut-off head of the second pump. Obviously, the same issue exists for dissimilar pumps such as in Figure 4. (Pump A of Figure 4 can always start against a running pump B, but pump B can only start against pump A if pump A is operating to the right of point D.)In summary, parallel pump systems are more expensive, less efficient, and create problems with load sharing that single pumps do not. There are some valid reasons for parallel pump operation such as when switching over pumps, NPSHA considerations, and handling intermittent peak load situations that cannot readily be satisfied with a single pump. When faced with spending constraints, one might also be driven towards parallel pump operation to increase system throughput as an alternative to purchasing new pumps. Operating pumps in parallel is viable providing that it is done with a full understanding of the individual characteristics of the pumps involved and the ability to monitor or ensure minimum flow thresholds are met for each pump

Pumps in parallel15-01The head/ capacity curve for a centrifugal pump will be supplied to you by the pump manufacturer. The curve he supplies describes the relationship between the head and capacity of that particular model. As you look at his drawing you should note that the BEP (best efficiency point) is located somewhere between 80% and 85% of the shut off or maximum head. To maximize the life of the pump you should operate the pump as close to the BEP as you can.Please note that in each of the following diagrams I use the same terminology: H = Head or height, measured in feet or meters Q = Capacity measured in gpm, m3/hr or any other units you are comfortable with. S = A description of the system curve supplied by the consumer Unless the internal pump clearances go out of specification you will always pump on the pump curve. As the centrifugal pump's capacity increases the head will decrease or as the capacity decrease, the head will increase. If you change one you always change the other. The pump curve does not extend out to intersect the capacity axis at some point. Beyond the noted limit the pump will go into cavitation because of excess flow.

In other papers we learned that a system curve is a description of the various heads the pump will encounter at the customer's desired capacities. The system curve is generated by the pump user and supplied to the pump manufacture to assist him in selecting the correct pump for the application. The head shown on the system curve is always a combination of: The static head. The vertical distance between the discharge of the pump and the maximum height of the piping, minus the siphon affect The pressure head. The amount of pressure in the tank to be filled, converted to head units. The head loss caused by friction in the: Piping Valves And any fittings installed in the systemIf you are not comfortable with these head terms please refer to paper 14-10 (U.S. customary units) or paper 07-01 (metric units) for a detailed explanation.Here is a diagram of a typical system head curve. Please note that the static and pressure heads remain constant in most systems. It is the friction head that varies with the pump's capacity. The higher the flow, the more friction or head loss in these components. It should also be noted that friction loss varies by approximately the square of the resistance. Twice as much flow produces almost four times the friction losses

Once he has the customer's system curve in his possession, the pump manufacturer will place his pump curve (P) on top of this system curve (S) and the pump will then operate where the two curves intersect (I). Hopefully this is close to the pump BEP

The next diagram shows two centrifugal pumps connected in parallel.These pumps could be either centrifugal or positive displacement types. The terminology remains the same.We connect pumps in parallel because we are trying to increase the capacity (gpm or m3/hr) of the system

The following sketch shows what happens when two identical centrifugal pumps, connected in parallel, intersect a system curve.The pumps will pump where they each intersect the system curve. Please take note of the following: With two pumps running they intersect at a higher head (B) and a greater capacity than if one pump was running. To determine the flow of an individual pump while both are running, trace back at that combined head to the single pump curve and read the flow for each pump at "G". With two pumps running, the system head is higher causing each pump to reduce its capacity a little bit.

We sometimes hear complaints that when three pu mps are run in parallel the third pump often does not seem to be making any difference. Look at the following diagram for an explanation:Take a good look at the diagram and you will see that the third pump (C) is intersecting the system curve at just about the same point as the second pump (B).All of this means that the capacity of three pumps running will not be greater than that of two pumps running.

The next diagram is an example of three different size centrifugal pumps running in parallel. Running different size pumps in parallel is seldom a good idea because the larger pump can throttle the smaller pump causing it to run too far off of its BEP (best efficiency point.) This can cause shaft deflection and possible premature bearing and seal failure.Your best protection against excessive radial movement of the shaft caused by operating off the BEP (best efficiency point is to equip the pump with a low L3/D4shaft number.If either (A) or (B) is running alone, it will intersect the system curve at the point shown on the diagram.If (A) and (B) 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 for the combined capacity.To determine the flow contribution of each pump in this arrangement, trace back to the intersection with curves (A) and (B),

You must be sure that the pumps will run individually in the system as well as in parallel. Please take a look at the next diagram to see a problem applicationAssume that when the pumps are running together, the combined pump curve intersects the system curve within the operating range of the pumps. (A&B).If the pumps are run individually neither of them can develop enough flow to intersect the users system curve. Because the pump is running at the right hand side of it's curve the pump will cavitate and experience all of the problems associated with severe shaft deflection.

Lets talk for a minute about what happens when you run PD (positive displacement) pumps in parallel. Remember that the word "head" is not used with PD pumps. We will be using the term "pressure" instead. Positive displacement pumps connected in parallel should have the same maximum pressure capabilities. If they incorporate internal relief valves the valves should be set to the correct anticipated pressures.The rules are the same as running centrifugal pumps in parallel. You add the capacities of the two pumps at the same pressure.

Now go back and look at the fourth diagram. In constructing these examples I used the same diameter piping for the suction and discharge sides of both pumps, so the discharge head or pressure would be identical coming from each of them. In practice the two pumps could be using different size piping and the discharge head or pressure coming from the pumps would be different. If the piping for pumps #1 and #2 are identical, the head at the discharge of each pump would be the same. If the piping for pump #1 were smaller than the diameter for pump #2, the only common diameter would be where they discharge into pipe #3. How would the flow be affected in this second case? The higher friction loss in piping #1 would meet the head at the intersection of 1-3, The head from pump #1 would drop when the flow encountered this larger diameter and the flow would increase. Both pumps #1 and #2 are running independently, with the system curve controlling, so pump #2 would continue to provide flow at a rate limited by the friction in the systemThere 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. In critical applications you need a back-up pump. Use parallel pumps to satisfy the demands of a changing flow system.There are some considerations you must address when using parallel pumps: The pumps should run at the same speed with the same diameter impellers. Use installed hour meters to assist you in determining the service hours on each pump if you alternate them in operation.