VIBRATING SEWAGE PUMP

[Berko]

We are currently faced with a vibrating sewage pump issue & I was wondering if anyone had any ideas as to the problem.

The situation is a an 8M deep wet well sump with a dry well sump interconnected to it via 375mm suction pipework.

Due to the semi-tropical climate, the 2 x 6" (55 kw) wet well submersibles operate (via VSD) upto Approx 260 L/s @ 16M (during dry weather).

Once a storm hits, 1 x 10" vertical, dry well submersible (also on VSD) kicks in upto (530 L/s @ 22M) & the pump station can handle a max duty of 1500 L/s @ 26M (3 pumps in parallel)

Static Head of the system is between 14 & 18M.

The wet well pumps work well on commissioning, however the dry well pumps have a vibration issue. When tested singularly, each pump operated fine upto 40 Hz (Approx. 330 L/s) & then began to vibrate. The vibration got progressively worse as the pumps ramped upto their max. frequency of 50 Hz (vibration reading at 50 Hz = 16mm/sec).

At BWL, the water level is 1.2M above the dry well pumps suction. NPSH A at this point equates to 7M, with the pumps NPSH R value at max duty flow of 550 L/s equating to 5.8M.

All suction pipes are free from blockages & all valves are fully open. Judging by the flow rates being encountered, the pumps are not blocked.

I have ruled out cavitation ??? & think that we may be looking at a Vortexing problem. Although with over 2M of water above the suction pipe inlet this should not occur.

This has got me baffled, if anyone has any suggestions, similar experiences, or advise on solving this issue your input would be appreciated.

 

[micjk]

Have you performed any vibration analysis on this unit. If you run the pump out to 60 hz does the vibe taper off. If so I would guess that you are exiting the first natural frequency of the equipment. If you have the capability, a 2 channel vibe data collector and force hammer, do a bump test and look for spikes at or around 50 Hz.

Roy Gariepy
Maintenance and Reliability Dept.
Dorlastan Fibers LLC.
Goose Creek, South Carolina USA

 

[Berko]

Hi Roy,

Yes we have done some vibration analysis:

Results are as follows,

35 Hz - Flow = 260 L/s, Vibration = 5.6 mm/s
40 Hz - Flow = 310 L/s, Vibration = 9.0 mm/s
45 Hz - Flow = 455 L/s, Vibration = 13.5mm/s
50 Hz - Flow = 530 L/s, Vibration = 16.3mm/s

Pump has a 50 Hz, 1500 RPM motor, therefore we cannot ramp upto 60 Hz.

Do you have any suggestions based upon the above ?

Berko

 

[smckennz]

Hi Berko

Is it possible to run the pumps up dry? (you will need to figure out how to protect the seals and wear rings; a dribble of water from a hose should do it).
If there is vibration then you have balance issue.
If no vibration issue then the hydraulics need to be looked at.

Cheers

Steve

 

[Artisi]

How is the inflow introduced into the wet-well. Is it piped in at the top and then allowed to free fall into the wet-well. If this is the case then it is possible you are entraining a lot air into the sump, this could / will result in vibration of the pump units. Also check for vortexing in conjunction with air entrainment.

International College
Naresuan University
Phitsanulok
Thailand

 

[piffer]

Hello Berko,
I wanted to point you back to what "micjk" was talking about. Do you have spectrum analysis capability? Have you performed "ring" tests on pump (at the top of motor) and on the suction and discharge piping? This will tell you if pump speed (or vane pass) coincides with any structural natural frequencies. Also, Artisi's response (entrained air) is another thing to consider.
Piffer

 

[stanier]

Problem could be as a simple as the VSD . Had the same problem not so long ago. Had to get the VSD manuafcturer in to fix it.

 

[Berko]

Hi Guys,

Other thing to mention is that the pump volute is making a rumbling "hydraulic" noise. Also nothing is loose inside the volute.

The highest vibration occurs at the volute, with lesser figures seen at the top of the motor.

The sewage in the wet well is pretty turbulent when viewed from above.

Berko

 

[Artisi]

something is wrong with your flow figures or my analysis of what I reads is incorrect, are you saying that each 6" submersible is operating 530 l/s @ 16.3 m head - highly unlikely - even 530 l/s between 2 x 6" pumps sounds unlikely.
Can you clarify

Naresuan University
Phitsanulok
Thailand

 

[Berko]

Artisi,

To clarify, at the 530 L/s flow, the 6" submersible pumps will not be working. The 10" pump operates at this flow.

Regards,

Berko

 

[tstead]

Do you have a single vane sewage pump impeller? Who is the pump mfgr and what model is it?

Tim