Operating a Centrifugal Pump with a Variable Frequency Drive

It is increasingly common for pumps to be operated by motors that are controlled by variable frequency drives (VFDs). VFDs control motor speed by varying the frequency of the power being sent to the motor. So a 60 Hz motor operating at 1800 RPM will reduce to 1500 RPM if the frequency is varied down to 50 Hz. There are two primary reasons why someone might consider operating pumps on variable frequency drives:

1.To simplify the pumping system by minimizing the different sizes of pumps needed.

2.To ensure the pumps operate as efficiently as possible.

Lets explore each of these reasons independently.

Simplify the Pumping System

So let’s say a pumping system needs to produce a total flow of 10,000 GPM. However, it is also expected to operate regularly at flows of 2,500 GPM and 5,000 GPM. Easy enough right? Just size four pumps for 2,500 GPM each and run 1, 2, or 4 pumps depending on the system requirements! Not so fast.

Below is a list the customer provided that shows the heads needed to produce each flow.

1.Maximum condition: 10,000 GPM @ 200 Ft

2.Average condition: 5,000 GPM @ 100 Ft

3.Minimum condition: 2,500 GPM @ 50 Ft

By now the problem should be apparent. In order to hit the 10,000 GPM condition each pump will have to be sized for 2,500 GPM at 200 Ft which means they will overshoot the actual expected 2,500 GPM condition. There are a few options available to solve the problem:

The pumps could be sized with consideration only given to the maximum flow condition. The downside is that when one pump is in operation it will produce a flow far larger than 2,500 GPM. Also, because the pump will be operating beyond the end of the published performance curve there is a serious risk of significantly diminishing the life of the pump.

1.The pumps could be different sizes. Size one pump for 10,000 GPM at 200 Ft, a second pump for 5,000 GPM at 100 Ft, and a third pump for 2,500 GPM at 50 Ft. The problem here though is that the initial purchase cost just went up dramatically, and any redundancy between pumps of equal size has been lost.

2.The pumps could be sized for 2,500 GPM at 200 Ft but selected carefully so that they can also operate efficiently at 2,500 GPM at 50 Ft when the operating speed is reduced.

In this scenario option 3 is the best option. It allows us to retain the redundancy and simplicity of having four identical pumps and ensures we are operating the pumps at an acceptable point on the curve. So in this case VFD’s are an obvious choice.

Ensure Efficient Operation

Another reason someone might employ VFD’s is to maximize system efficiency. It’s no secret that pumps operate most efficiently within a small portion of their total potential operating range. This range has been dubbed the “Preferred Operating Range” and formally defined by the Hydraulic Institute Standards and the American Petroleum Institute. In most cases the POR extends from 70% to 120% of the flow at the best efficiency point on the pump performance curve. VFDs are often employed to adjust the number of pumps in operation and their operating speed to keep pumps operating as close to the POR as possible as system demands change.

Here’s an example of how this might work. Imagine that in the scenario discussed above we decided to maximize efficiency at the 5,000 GPM condition. In doing so we might decide that it makes the most sense to have two at reduced, three at a further reduced speed, or four pumps at a very slow speed in operation in order to achieve the best possible efficiency. VFD’s provide the flexibility to analyze a pumping system and tailor the operating scenario to maximize pump efficiency at all operating points thereby minimizing power costs, extending pump life, and increasing the mean time between failure of the pumping units.

Summary

VFD’s are an increasingly common technology that is applied to simplify pumping systems and maximize system efficiency. Understanding the most common reasons why VFD’s are used will enable you to provide intelligent assistance to pump system designers in selecting the right pumps for the job.