VIBRATING SEWAGE PUMP
VIBRATING SEWAGE PUMP
(OP)
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.
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.





RE: VIBRATING SEWAGE PUMP
Roy Gariepy
Maintenance and Reliability Dept.
Dorlastan Fibers LLC.
Goose Creek, South Carolina USA
RE: VIBRATING SEWAGE PUMP
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
RE: VIBRATING SEWAGE PUMP
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
RE: VIBRATING SEWAGE PUMP
International College
Naresuan University
Phitsanulok
Thailand
RE: VIBRATING SEWAGE PUMP
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
RE: VIBRATING SEWAGE PUMP
RE: VIBRATING SEWAGE PUMP
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
RE: VIBRATING SEWAGE PUMP
Can you clarify
Naresuan University
Phitsanulok
Thailand
RE: VIBRATING SEWAGE PUMP
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
RE: VIBRATING SEWAGE PUMP
Tim