Letter from a Reader

Dr. Pump, I have been posed with a question. One of our paper coating progressing cavity pumps causes pipe vibration. I have been told that running two pumps in parallel at slightly different speeds could reduce or eliminate this vibration. Is that possible? How is that done? Your help is very much needed. Brian Huza, Sales Engineer Van Bergen & Markson

Nelik’s Response

The first thing you need to do is understand the cause of the problem. Pipe vibrations can happen for many reasons. A few examples are:

• acoustic resonance
• structural resonance
• poor support
• poor foundation

A pump acts as a force, but there must be something in the system to be forced. In other words, you must look at both the pump itself and the system it is interacting with. Progressing cavity (PC) pumps are known to run rough. This is a feature of their rotor design and is often caused by connection of the rotor to the drive shaft. Such “nutating” rotors can create problems, which is why PC pumps usually run at slow speeds. There are also other reasons for low speeds, such as rotor/stator interference fit. If the problem is more with the piping and not as much with the pump, then two pumps simply exaggerate the problem and force the piping twice as violently. Reducing the speed of one pump could, in theory, result in combining frequencies, with resultant harmonics being at a new frequency. If the pipe resonance happened to hit at a pump’s running frequency, then “shifting away” into new harmonics can “tune the system out.” There is good mathematics behind it, such as Fourier FFT series, but we will stick with simpler and more immediate possibilities for now. In practice, such an approach is a bit like scratching a left ear with a right foot. It may work, but looks kind of strange. To execute this approach, you can change gearbox ratios or variable frequency drive (VFD) speed, which is a simple thing to try. And if it works, you solved it. Unfortunately (or perhaps fortunately?) VFDs are not often used with PC pumps, so getting one is an added expense at this point. Changing gearbox ratio is easier, but it still takes some doing. Look at the root cause. It is usually findable, although not always straightforward. You may have someone knowledgeable join you to help troubleshooting, as this may save you time. I have recently published a series of articles in Pumps & Systems magazine regarding pumps reliability and energy/efficiency evaluation using the PREMS-2A systems technology (see Pumps & Systems December 2016-June 2107). You can read more on more advanced vibration applications methods in these articles, now online at pumpsandsystems.com. Let me know if these simple suggestions I noted here help. If not, give me a call or send me an email and we can take a closer look at the problem.