Increase System Capacity Using Pumps in Parallel

Q: Can I double my system capacity by installing a second supply pump in parallel?


A: The short answer to this question is “No, not exactly”; however, this answer does not sufficiently explain the reason for the negative response. While it is true that adding one or more pumps in a parallel configuration will increase the system’s capacity, it is a common misconception that two identical pumps in parallel will deliver twice the amount of flow as a single pump.

The extent to which a system’s capacity may be increased by adding pumps in parallel is entirely dependent upon the system’s resistance to flow.  Consider the simple system below in Figure 1.

 


Figure 1:  Liquid transfer with a single pump


The boundaries of this system consist of two tanks with fluid surfaces separated by 80 feet of elevation. Water needs to be pumped from the Source Tank to the Outlet Tank. With the system configured as it is here, with 2.5 inch suction piping and 2 inch discharge piping, this transfer pump will supply 72.09 gpm of flow.  Figure 2 below shows the system resistance curve plotted against the transfer pump’s performance curve. The point where the two curves intersect determines the amount of flow through the system.
 


Figure 2:  System resistance curve plotted against pump curve.

Notice the flow velocities in the pipelines. The suction lines have a velocity of 4.831 ft/s and the discharge lines have a velocity of 6.893 ft/s. These are reasonable velocities, and the system has been appropriately designed for a flow of around 72 gpm.
If the same pump were duplicated and installed in parallel with the first pump, the results would be as shown below in Figure 3.
 


Figure 3:  Liquid transfer with two parallel pumps

Notice that the total flow rate has only increased from 72.09 gpm to 83.4 gpm.  This is because the system’s resistance to flow is fairly significant in this case.  The increased flow rate significantly increases the resistance in this system.  Figure 4 below shows the system resistance curve plotted against both a single pump, and two pumps in parallel.
 


Figure 4:  System resistance curve plotted against single and two-pump curves


Therefore, it is quite apparent that this system was not designed for much more flow than the 72 gpm achieved with one pump. The dynamic losses increase rapidly as flow is increased. The only way to get a significant increase in flow rate by adding an additional pump is if the system resistance curve is much flatter or gradual. And the only real way to achieve a more gradual resistance curve in this system is to increase the pipe sizes.

If the suction pipe nominal sizes were bumped up to 4 inches and the discharge pipes to 3 inches, then this would greatly reduce the amount of resistance in the system. Figure 5 below shows a system resistance curve with the larger pipe sizes. Notice how much more gradual this curve is than before. The curves now intersect at about 98.5 gpm.
 


Figure 5: System resistance curve with larger pipe sizes plotted against a single pump


If a second pump is installed in parallel with the first pump in this system with the larger pipe sizes, a much more significant increase in flow rate will be achieved. This is because of the larger pipe sizes. Notice where the system resistance curve intersects with the pump curves in Figure 6 below.



Figure 6: System resistance curve with larger pipe sizes plotted against single and two-pump curves

With the larger pipes, a second parallel pump installation increases the flow rate from 98.5 gpm to 145.5 gpm, a much more significant increase than with the smaller pipelines.

So adding pumps in parallel can increase the flow rate in a piping system, but the amount of that increase depends on the system resistance.  Larger pipe diameters can allow for a much greater benefit with multiple parallel pumps, however systems with smaller diameter pipes might not see a significant increase in flow rate at all.

So, unless you are looking to replace all the pipes in the system downstream of the pumps, a simple parallel pump installation might not be the most economical way to increase the systems overall capacity. Each system will have different results but if you are a user of the PIPE-FLO or FLOW of FLUIDS software, you can model multiple scenarios, and test which changes to a system’s design will yield the best results.

Attachments:

AAE.pipe

AAE.xlsx