Pulsation from a progressing cavity pump 1

sales literature aside, all progressive cavity pumps have some degree of pulsation at high delivery pressure.

Thought so - just not been exposed to the problem before.

Thanks

Hello,

Single screw moyno type pumps will experience a high thrust load on the rotor end at high operating pressures. The vibration you are experiencing could be caused by a small amount of mechanical looseness in the pump front end. This would not necessarily show up as pump vibration since the movement is axial, but it could evidence itself as a flow pulsation similar to what you are experiencing.

From left field:

As you slowly increase speed does the vibration seem to somewhat increase in a linear fashion or does it just all of a sudden start? Point being did you find a critical speed with your pump / system?

Good luck!

d23 brings up a good point, but a very rare occurance in a PC pump. More info would be helpful as my experiance agrees with the manufacturers claims. What is the suction and discharge pressurers, speed range of the pump, how many stages (rotor/stator), what is the noise like at top speed, viscosity of the material pumped, size and configuration of the first 15' to 20' of pipe. The application issues need to be checked.

CB2:

Just so you know it has been over 15 years since I've looked at Holtzer (not sure of spelling) calcs. I did good to remember the term Critical Speed. In my personal opinion a lot of mechanical problems get blamed on this to deal with what could be warranty considerations for a manufacture, but it is always possible. My post is from way out in left field and your commit is politically correct.

Thanks!

Hi - thanks for comments so far.

Some extra info as follows

Number of stagres = 1
Discharge pressure - 2 to 2.5 BarG - This isn't high pressure for a PCP pump is it ?
Suction line 6"NB and 30 m long drawing from an atmospheric tank.
Fluid - Sludge - (water with 5% dry solids)
Pump is driven by variable speed drive - problems occur when the inverter drive is between 40 and 50Hz, no problems below these frequencies - can't tell you rotor speeds as waiting for this info from the manufacturer.

I am now looking at the suction piping to calculate the NPSHA and compare this against what the manufacturer says is the NPSHR.

Can you / do you consider acceleration head in some way like you would for reciprocating pump system or am I thinking down the wrong lines.

We have 5 installations with these pumps and only two show problems - so I guess I also need to do examine the pipe support arrangement for critical frequencies / distances between supports / types of supports.

Can anyone help on methods / standards / info ?

maxh:

I do know your system so I'm not sure of the practically of this responce:

You have 5 systems. Can you trade places with two of the pumps? If you put a pump that you know operates good in the place of the vibrating pump it will identify (separate) system and/or pump problems.

Have you tried to change (natural) vibration characteristics of the discharge piping? In other words try changing its stiffness. Or consider installation (if possible) of some "dumpers" ("hydro-accumulators&quot to suppress those hydraulic pressure oscillations (more expensive solution).

I suppose there are specialized companies dealing with such matters so you may consult those experts too. Provide some work for fellow engineers.

Thank you for all your suggestions

I am going to look at the pipe supports, do some calculations and see if its a natural frequency, or multiple thereof, that is being excited - if so simple fix, change supports and/or hydraulic accumulator/snubber.

If no answer from this then I will try moving pumps as suggested by d23, might even try and do this even if I think its a pipework resonance problem, before making any mods, just to convince myself.

maxh:

If you would please post what you found or email it. Just curious.

Thanks
davidlsus@yahoo.com

Max:

I have seen this problem solved by increasing suction side diameter and, in difficult circumstances, by addition of suction side pulsation dampeners. The vibration in such cases can be instantanious acceleration of the suction fluid column upstream of the pd pump. In your case, it is probably exacerbated by higher effective viscosity (think you noted solids). Short, straight, and big suction lines may be a good bet.

srfman

Some more info - a bit subjective in nature unfortunately.

The pulsation definitely ramps up after passing 40Hz, fingertip tingle, on the invertor and reaches its worst at full speed (just shy of 50Hz) where it leaps about.

I am going to do some vibration measurements next week.

Will come back with some more comments then.

We recently experienced problems with a PCV pump on municipal sewge service. the force main had a few high points. Problem was solved by fitting VSDs to the drive and starting the pump slowly. Sharing knowledge is a way to immortality

maxh:

If you have a vibration all the way from 40 hertz to 50 hertz critical speed should not be the problem.

Have you been able to switch pumps or is that not possible? It would be nice if you could eliminate the pumps, but that may be a big problem for you.

Reading your post I was wondering is the inverter (VSD) vibrating or the pump/motor or both?

Thanks for your update!!! Please continue.