Dear all,

I have a vertical pump for sea water inhale to cooling system. The pump driven by motor with 400 kW, 6 poles, 50 Hz and 6.3 kV. The pump shaft is 6.5 meters long (inside sea water). The vanes number of impeller is 5 vanes and the bowl has 8 vanes.The highest overall vibration at 1V and 1H around 5.45 mm/s rms at 105.63 Hz. 1xrpm is lower (2.1 mm/s rms.
The overall vibration increased day by day around 0.2 mm/s (I already took date since 14 November 2005)
All direction (1V,1H,2V, 2H, 2A, 3V,3H,3A,4V,4H and 4A)this spectrum (105.63 Hz) is appear.
The 2nd harmonic also appear (211.25 Hz) and it have sidebands (space 16.679 Hz).

Do you have any suggestion about this problem?

Your prompt reply will be highly appreciated.

Regards,
Jono

Hi jono,

U gave lot of information about the Motor, but forget to gave the RPM.

Jenish,

He said it was a 2 pole motor, so speed is 1200rpm.

Dear Jenish,

I already mention 50 Hz, 6 pole so speed is 120f/p = 120*50/6 = 1000 rpm.

To Dnicejr,

The motor is not 2 pole , but 6 pole

Regards,

Jono

Dear Pak Jono,

Motor with 50Hz,6 pole, would u please ensure that the motor speed is 1000rpm or it's speed is 750 rpm ? Can u check on nameplate of motor.

B'cause in Indonesia Generraly as far a I know the AC motor speed is 3000 rpm,1500rpm,750rpm,300 rpm,etc.

If the motor speed is 750 rpm. the frequency 105,63Hz can be coming from 8xvanes of bowl(I don't know what is the bowl?)
But if i see the sidebands 16,67Hz,i've got confuse here,this sidebands is equal to 1000rpm.

So,would u please give me the correct speed for ur machine?

Th'k

Umbro.

Jono,
As per your information your synchronus speed is 1000rpm and motor running speed will be around 925-975 rpm due to slip. We shall discuss regadring the problem.

I think it is best to dismantle the pump and ckeck each impeller as well as bearings. There can be out of balance in one of the impllers and there can be bearing wear. When this type of problem occurs, in causes to wear the barings faster and causes to increse the vibration.

Best regards,

Jagath.

Jono,
I dont know whether you already solved the problem.If not please check the base level before going for any major action.
Regatrds
Anis

Dear Jono,

The best guess would be a nicked impeller, what kind of strainer o you have arround the well?
As for procedure I would check alignment first.
Then decouple the motor and check the vibration there, and if none, proceed to pull the pump. I suspect a single impeller blade with a chip.
Look to better protect the inlet to the pump.

Hi Jono,
For a 6 pole motor with some slip, 105.63 Hz is around 6.4x shaft speed and therefore not something to do with shaft speed such as imbalance, misalignment or looseness, or anything on it like blades or vanes.

If this motor and pump have rolling element bearings then this is most likely a "ball pass frequency" typically 3.x to 8.x times shaft speed depending on whether it's an inner or outer race, or ball spin fault.

Because it's a vertical shaft machine the vibration gets more severe as you go up so it could appear worse at brg 1 (assuming top motor bearing) but could be from the lower motor brg or even the pump top bearing.

Either way it's high and needs replacement. Try de coupling the pump, and if it's still there, it must be in the motor.

Cheers

John