Pumps in parallel P006_1
The 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
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:
- And any fittings installed in the system
If 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.
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:
We sometimes hear complaints that when three pumps 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/D4 shaft 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 application
Assume 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 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.
- 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.
- Most plants read only total flow and cannot see the differences in individual pump performance
- Parallel pumps are notorious for operating at different flows. Often a weaker pump is operating close to its shut off point while a stronger pump is operating to the far right of its curve and running out of NPSHA.