stanier, When you say "vibrating", what is vibrating? Is it the pump and base, the piping (suction and discharge), both? Is the vibration noticeable along the entire pipe line? Does the vibration occur at various motor rpm's as the pump-motor combination comes up to speed or is this VSD controlled pump that vibrates at different motor speeds? This type of vibration is due to mechanical couples between the piping system and pump/motor set. and can be solved by proper pipe support analysis and pipe suppports.

There is another potential cause of line vibration/ flow surging  due to Hydrodynamic forces created while the material is being pumped. The first issue that comes to mind is this  material a non-newtonian fluid? This will affect viscosity as the pump comes up to speed and the shear rates change in the discharge and suction lines, changing the curve that the pump operates on which affects flow rate. If it happens to be a rheopectic fluid this stuff increases in viscosity as the shear rate increses over time, lowering flow, then as the flow decreases, shear rate drops, viscosity drops and the flow rate again increases. Weird stuff can happen when pumping non-newtonian fluids.Is there an issue involving stratification in the sludge well such that the pump is seeing different viscosity fluids flowing through it as it pumps through the stratifications?

These are just a few things to look at. Hope this helps.

Goodluck.
saxon    

Hi saxon,

Thanks for the thoughts.

The application is municipal sewage so non newtonian flow is not the problem. The fluid is basically water.

We are investing the coupling of speed and the vibration frequency and now believe that this may be a cause. We will proceed with changing the gear ratio to vary the speed. We do not have the luxury of VFDs.

The pipework is cast iron and is solidly supported. It is the piping that vibrates and makes the pump vibrate. We intend decoupling this by use of flexible ss hoses.

Also the force main has some negative sections. Our field investigation shows that the discharge pressure varies on start up and a leak is suspected in the main. We are investigating this further.

Stainer:

where is the pump on its curve?  And, any idea why they used a PC for this application?  If you have changing pressure on start up, then the air/vacuum valves are probably working right (it's probably a good thing to check to even see if there are any combo valves in the first place!!!)  I think you are heading in the right direction, but I think it's the pump vibrating the pipe and there isn't much you can do about it.  It seems like a poor pump selection for this duty.  Try getting a flow out of the pump by judging drawdown in the wet well over a period of time, maybe this will help get a grip on where its operating.  What is the design principal for the PC?  Stator or something else?

BobPE

The pump is a positive displacement pump and as such the curve is a straight line.

The application is municipal sewage so there are no air valves.

The pump has been selected becuase it is low flow and high head. There are many successful installations of this type of pump on similar duties.

The pump specified performance has been replicated on a test bed and by throttling a bypass back to the wet well.

The pump is a progreesive cavityu pump by Mono. Similar to those by Moyno, Robbins Myer, Bornemann, Seepex etc.

Seems like there should be air valves at a minimum at high points, espically with domestic sewage.  This is probably what is causing the surging since the area available to flow at the high points in reduced due to air accumulation and its cycling through compression and high head loss making the pump move on its curve (yes curve!!!)  These pumps have a curve and its that curve that makes them undesirable for this type of service.  Not that they cant be used, but they become troublesome for those that have to take care of them.  In my experience it seems like the sales vendors make a point to use them in these applications, and they are very pursuasive it seems.  I have run into more unsessful aplications that succesful ones.

Look to the high points and design a municipal sewage air release valve at them.  At least as a minimum, dig up the high points and tap a corp stop at them to release the air and test the system.

Good luck,

Bob

Hi Stanier,

The discharge piping on a PC pump vibrating can be a sign of the head pressure reaching its max for the volume. If you can put a pressure gauge on the outlet of the pump you can see if the pressure is bouncing because of air in the line or increasing due to viscosity changes. The best solution would be a pressure transmitter to a chart output so you can see the head pressure over time. A starved suction may cause pump vibration but probably just a lot of noise and damage to the rotor, stator and joints

Regards

Gord