PD pump (Rotary lobe pump) -- loud noise

[Theerapat]

Hello,

I have a problem with the rotary lobe pump, it causes a very loud noise when running. The pump specs are as follows:

Liquid: Syrup (The liquid is in the 15,000 liters tank.)
Viscosity: 2200 cPs
Temp: Ambient
Flowrate: 15 m3/hr
Discharge pressure: less than 2 bar (open ended pipe)
Inlet pressure: 2.5-3 bar
Pump running speed: 350 rpm
Size: 65 mm

https://imgur.com/a/mpnmySw

I am quite sure that the problem is the insufficient liquid at suction side but I don't understand why. The pressure at suction side is very high, the suction pipe has the same size as the pump inlet. The discharge pressure is very low. Do you guys have any comments for this application?

Theerapat S.

 

[Asymptotic]

It is a good idea to oversize the suction line by one or two diameters and keep it as short as possible when pumping a high viscosity fluid. These considerations are probably mentioned in the pump user manual, and the manufacturer probably has other information resources to call upon.

A VFD is shown in the photo, and one way to proceed is to experiment with progressively lower pump speeds to find at what speed the noise goes away, make a plumbing change, and re-run the speed test to learn if and how it affected the onset of unacceptable noise.

 

[Theerapat]

It is a good idea to oversize the suction line by one or two diameters and keep it as short as possible when pumping a high viscosity fluid. These considerations are probably mentioned in the pump user manual, and the manufacturer probably has other information resources to call upon.

A VFD is shown in the photo, and one way to proceed is to experiment with progressively lower pump speeds to find at what speed the noise goes away, make a plumbing change, and re-run the speed test to learn if and how it affected the onset of unacceptable noise.

We have already tried lowering the pump speed, the noise went away at 25 Hz (around 175 rpm). What I wondered is that we have such a high inlet pressure and the liquid is still not entering the pump sufficiently.

If we can lift the liquid tank higher and increase the inlet pressure will it help?

 

[JBA]

I don't see any pressure transducers or gauges on the pump inlet piping beyond the hose; so, at what point are you measuring this "high inlet pressure"?
Also, have you done a flowing pressure drop analysis on the piping and hose between the tank connection and the pump inlet?

 

[Theerapat]

I don't see any pressure transducers or gauges on the pump inlet piping beyond the hose; so, at what point are you measuring this "high inlet pressure".

This is when we moved the pump nearer to the tank. Actually, the pump was connected to the stainless steel pipe about 10 m long with the pressure gauge on the suction line.

 

[Asymptotic]

We have already tried lowering the pump speed, the noise went away at 25 Hz (around 175 rpm). What I wondered is that we have such a high inlet pressure and the liquid is still not entering the pump sufficiently.

If we can lift the liquid tank higher and increase the inlet pressure will it help?

Yes, increasing suction head will improve the situation, but a larger part of the problem appears to be excessive resistance to flow due to suction plumbing. If you are considering raising the tank anyway, I'd recommend elevating it enough so the pump can be located directly under the tank discharge port, then plumb the pump suction as directly as possible to it (one or two pipe diameters oversized). Do whatever that can be done to keep suction line resistance to a minimum.

As to the inlet pressure you have measured, @JBA is pointing you in the right direction. Pump suction pressure must be measured directly at the pump suction port; pressure will indicate unrealistically high if measured farther downstream towards the tank. If I were nosing around, I'd mount one pressure gauge at the tank discharge, another at pump suction, and record both pressures as pump speed in raised in 10 RPM increments, then put the data into a spreadsheet and create speed vs. pressure curves.