why the load noise for the double screw pumps

[760106]

I have met a big problem with the double screw pumps(total no. is 6) in ship(150,000DWT). Each pump rated capacity is 400m3/h, rated discharge pressure is 1200kpa. Rated voltage for the motor is 6300V. The pumps are used to pumping produced water which temperature is about 60centi-degree. The pumps were designed with the opposed thread arrangement and double suction.
During the commissioning in shipyard with the seawater, the double screw pumps were all good. But after putting into production, when the pumps used to pumping the produced water, big noise was happened inside the pumps for most of the pumps. Only for one pump, some times is ok, but sometimes same thing happen. The noise like some thing hammering, somebody thought it maybe two screws abraded with each other, other body thought it maybe the screws abraded with the pump case.
when we dismantled one pump, we found it is like the axial faces of thread of the two screw abraded with each other.
After grinding the axial face of the screws, assembly and alignment, same thing happened. but when we use the pumps to pump seawater, all the pump is ok. So we suspect the temperature or the impurity of the fluid has some influence to the pumps.
After using the heated sea water with temperature about 60centi-degree, all the pump is ok.
Has any body met the same problem? Can some body tell me why it is?

 

[smckennz]

Sounds like the bearings at each end of each shaft are locked. Have you checked that the design permits thermal expansion?

Cheers

Steve

 

[760106]

we have ever checked the design temperature of the pumps, which it is the 75centi-degree. So we don't know what happened for this. we just asked many mechanical engineers to diagnose it.

 

[colinp]

Could cavitation in the inflow be collapsing on the impeller screws? Cavity collapse would be more violent in distilled deairated water than in seawater (more nuclei available in seawater etc.) Was most of the damage on the intake portion of the screws?

-colin pitts

 

[pablo02]

Please confirm one small detail -- I have read your post several times and I have arrived at a couple of interpretations:

a) the pumps still have problems with produced water, but work OK with seawater (even heated seawater)...

or b) the pumps gave you problems before, but after using heated seawater now works OK with produced water?

I have assumed that a) is the correct interpretation, but wanted to confirm...

Another aspect, but one I'm having some trouble rationalizing, is that the two waters will boil at different temperatures -- the sea water should boil at a slightly higher temperature -- is there a possibility that sufficient heat was being generated by the screw lobes or the bearings to cause some localized boiling of the (produced) water -- causing either a differential thrust in the screws or cavitation?

also, check your bearings...especially the thrust bearings on the screws...

 

[imok2]

If sea water has a higher viscosity then producible water it may have a bearing on the slippige factor of the pumps ..in other words the sea water may be acting more as a lubricant on the screws thereby cushining the slippige(noise)

 

[760106]

pablo02,
I am sorry to make a big mistake in the first message. I should confirmed that
using the heatd sea water, the pumps were not ok, the noise was happend again.
Either using the seawater or using the process water, the suction pressure of the pumps is all about 1.1bar, and the discharge pressure is normal.
Now, we are sure it is the mistake of manufacturer.
The manufacture engineers also not sure about it. they just suspect the material of the screw. They only admited to manufacture new screws to us.and this is the only method we can do.
Can anybody tell me why? Is it the not enough hard for the material of the screws? or the expansion of the material?

 

[smckennz]

The two screw pump is normally fitted with external timing gears and, in theory there should be no contact between the screws. If you have a timed pump and are getting face contact, then either something is assembled wrong, something is made wrong, or the pump is running outside its design condition range. Suggest you check;

Turn the pump by hand. Rotation should be smooth with no tight spots or metal to metal contact felt.

Timing gear to screw phasing.As a rough check, there should be measurable backlash at the timing gears. The backlash should normally be measurable on both sizes of the engaged tooth - no preload. The manual may give tolerances.

Axial location/clamping of thrust bearings, normally at timing gear end.

End float on screw shafts. Movement should agree with manufacturer spec.

Provision for axial expansion of screw shafts. Make sure nothing is clamped that should be floating.

Cheers

Steve