VIBRATION LIMITS IN DRY WELL SUBMERSIBLE SEWAGE PUMPS
VIBRATION LIMITS IN DRY WELL SUBMERSIBLE SEWAGE PUMPS
(OP)
Hi All,
We have a single vane, dry well submersible sewage pump (4 pole speed, 220 kW) which has an operating duty of 500 L/s @ 40M.
The pump is vertically mounted on the OEM's purpose built suction stand.
When the pump runs, the following vibration levels have been recorded.
Vibration at suction stand/volute = 5.5 mm/s (RMS)
Vibration at top of motor = 20.2 mm/s (RMS)
Top of motor is approx. 3.2M from the floor level where the Suction Stand is bolted.
Is this figure 20.2 mm/s considered excessive ?
Can anyone advise on any documents/reports/thesis, etc where I can find out what vibration limits would be considered acceptable for the motor ?
Awaiting your reply,
Cheers,
Berko
We have a single vane, dry well submersible sewage pump (4 pole speed, 220 kW) which has an operating duty of 500 L/s @ 40M.
The pump is vertically mounted on the OEM's purpose built suction stand.
When the pump runs, the following vibration levels have been recorded.
Vibration at suction stand/volute = 5.5 mm/s (RMS)
Vibration at top of motor = 20.2 mm/s (RMS)
Top of motor is approx. 3.2M from the floor level where the Suction Stand is bolted.
Is this figure 20.2 mm/s considered excessive ?
Can anyone advise on any documents/reports/thesis, etc where I can find out what vibration limits would be considered acceptable for the motor ?
Awaiting your reply,
Cheers,
Berko
RE: VIBRATION LIMITS IN DRY WELL SUBMERSIBLE SEWAGE PUMPS
RE: VIBRATION LIMITS IN DRY WELL SUBMERSIBLE SEWAGE PUMPS
RE: VIBRATION LIMITS IN DRY WELL SUBMERSIBLE SEWAGE PUMPS
The pump in question is new & has just been commissioned.
Clearances were checked before installation. Pump came with a balancing certificate which checked out OK. Wear is not an issue.
VANSTOJA
Due you have any rules of thumb for acceptable vibration ratios you mentioned (for a pump of this height).
Thanks Guys,
Berko
RE: VIBRATION LIMITS IN DRY WELL SUBMERSIBLE SEWAGE PUMPS
Vertical pumps mounted like cantilever beams often have a 1st beam bending mode in the range of synchronous speeds of 30Hz and 60 Hz for 4 and 2 pole motors. You might do well to estimate the 1st cantilever mode frequency using the total weight of the combined pump and motor with weight both concentrated at the motor's center of gravity and distributed along the total beam length and to compare this with actual running speeds including motor slip. If the pump is mounted on a shell-like structure, the shell ID and OD control the beam inertia used to calculate beam frequency. If the pump has feet mounting, try using casing/volute diameters in the inertia equation. For more reliable verification of the cantilever beam bending frequencies, whack the top of the motor in several radial azimuths and record accelerometer responses on a portable noise analyzer using peak-hold mode to capture spectral "bursts". I had a 17,000 lb., 36in, diameter, 120 in. high vertical pump-motor unit mounted on a shell of 1-2in. wall thickness that calculated to be 30-55 Hz for distributed to point loading in the cantilever mode. It ran at both 30 and 60Hz synchronous speeds so the exact location of the beam frequency became very important.
RE: VIBRATION LIMITS IN DRY WELL SUBMERSIBLE SEWAGE PUMPS
Tim
RE: VIBRATION LIMITS IN DRY WELL SUBMERSIBLE SEWAGE PUMPS
There is an IRD vibration severity chart that has been around for a long time. I think if you do a web search you may be able to get it.
Anyway I have several questions.
Questions:
1) Is the vibration directional? Is it vibrating more in the direction of the discharge piping?
2) What is the primary vibration spike? It at 1X running speed? Is it sub-synchronous? Is it super-synchronous?
3) What balanace grade was it balanced to? Was it balanced to ISO grade 2.5 or 1.0 or API 4W/N?
We have pumps here at our power plant that would occasional get to 27 mm/s . This is not normal but the vibes we noted were related to the cleaning of the travelling screens. As the screens became plugged the vibes went up. The majority of the spike was subsynchronous and not related to the balance of the pump.
A thorough vibration analysis is required and possibily determination of any resonace as per Vanstoga's recommendation.
Regards,
James Chin Yut
James Chin Yut
RE: VIBRATION LIMITS IN DRY WELL SUBMERSIBLE SEWAGE PUMPS
RE: VIBRATION LIMITS IN DRY WELL SUBMERSIBLE SEWAGE PUMPS
I would also invrestigate the suction conditions as there may well be imbalance on the impeller caused by submersible eddies, swirling etc.
Your problem may not be the pump but the manner in which it is installed or the duty performed.
You need to work with the pump manufacturer re the design of the installation.
I would refer to Pumping Station Design by Sanks for information on station design. The book contains some godd and bad examples. You can download the book by subscribing to www.knovel.com