Correcting a Common Pump Myth About Flow Rate

One common misconception I hear is that a pump is rated for a certain flow rate and the pump is bad if it does not deliver. While this may be true when you are discharging to a spillway and there is little resistance, most pumping systems are much more complicated. I was recently dealing with a situation where multiple pumps in parallel and the operators and engineers were confused as to why they were not getting the flow or pressure they thought they should. In fact, they were only operating with one of the two cooling towers available due to the lack of flow. The system had been expanded many times, and they were considering expensive repairs to the pump manifold that would total more than $100,000.

Since the system had been in operation for many years, there were not many places to collect data. There was a pressure gauge on the pump manifold. It was known that the cooling tower would overflow at 2,200 gallons per minute (gpm), so a flow rate of around 2,000 gpm was assumed. While we looked at modeling the entire system, the crux of the problem can be seen in just the model of the pump manifold. There were two larger pumps on the ends (Pump 4070) with three slightly smaller pumps (Pump 830) in the middle (Image 1). The pumps were operating at just over 82 pounds per square inch (psi).

Image 1. Pump manifold under normal operation. (Images courtesy of the author)

Normally, all of these pumps were in operation. The end users thought something was wrong because they could not see a difference in discharge pressure when one of the smaller pumps was off.

When the system was modeled with shutting down the middle pump, it resulted in a pressure drop at the gauge of less than 2 psi (Image 2). This drop could easily be missed. It also shows that it may not be the rest of the system at fault, or even the manifold itself, but how the pumps were interacting with each other.

Image 2. Pump manifold with one pump shutdown

Pump professionals agree it is generally not a good idea to have different sized pumps operating in parallel. At one point during an expansion, the plant decided they needed more flow and installed the third small pump expecting to get the rated flow. It appears the efforts were largely wasted. So why is it that the extra pump did not deliver the desired results? We need to look at the pump curves.

The larger pumps may have overpowered the smaller pumps, but there is another aspect to observe. Normally, the smaller pumps put out just over 177 feet of head (Image 3). The impeller has been trimmed to 7.125 and the pump is operating far to the left of its best efficiency point (BEP). While the desired output was around 600 gpm, the pump was only putting out just over half this amount. We can also see that the pump curve is flat, so we should not expect a large change in pressure with a change in flow rate.

Image 3. Pump 830 under normal operation.

When we see how the pump is operating while one of the pumps is shut down, it confirms that there is a small change in pressure due to the flow change. While the total system flow is the same, the pressure barely drops. The small pump’s flow rate changes from around 350 gpm to almost 400 gpm. Because it is operating at an almost flat section of its curve, barely any change at the pressure gauge is observed. If we repeat this process with the larger 4070 pumps, we would see the same thing.

Image 4. Pump 830 with one pump shut down

This scenario demonstrates why more attention needs to be paid to pump curves. To be clear, there is no issue with the specific pumps, just how they were applied. Here, I ran the simulation with all five pumps the same.

Where the pump was operating shifted, but the manifold exit pressure did not change much. If all the pumps were the larger model, the exit pressure would only increase by about 1 psi total to 83.5 psi. This would lead to a power increase, and the pump would be almost outside of the manufacturer’s recommended operating range. Even with all five pumps at the smaller Pump 830 model, the pressure would only drop about 2 psi to 80.4 psi.

While it may seem a new pump is needed, there may be an easier, less expensive solution. Remember that the impellers were trimmed, so there is the potential of ordering larger impellers. When modeled, this could significantly increase the pressure and flow rate.