One of my customers has a centrifugal water pump, center-hung, horizontal split case, reportedly with a six vane impellar (I think it's a five). No other information is available except that it is driven by a 75hp motor with a gear tooth coupling. They mothballed it because it "always gets airlocked". The conclusion on the previous survey was that the pump was not moving water (increased vane pass and increase in speed) - assuming air lock. Now it's got acc up to 54Gs and possibly a harmonic pattern of 3.3x orders but that is not borne out by the peakvue spectrum. The peakvue shows mostly 1x components in the spectrum and wf. There are some 1x sidebands at non-synchronous harmonics but they fall at different orders depending on which point you are looking at. I know the elevated broadband from 15 - 30x is telling me something, but what?
Also, I always assumed airlocked meant that the volute was full of air and could not prime. What would be the effect of a discharge pipe full of air with a primed volute pumping against a closed head? Is that also considered air locked?

This message has been edited. Last edited by: Kurt Danford,

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Kurt:

With limited information, this sounds like more of a system problem. Possibly suction issues (not enough supply) and/or a fair amount of air/gas in the water. This will have the impact of damaging the impeller, which will compound the problem. If pumping against air and a closed head, then recirculation can occur which will result in flash (water to steam), loss of prime, and additional damage to the impeller.

With the vibration frequency, this type of pump normally has packing, lantern ring and wear (shaft) sleeve. The packing is inserted in the packing gland, with the lantern ring positioned (about half way) so that water can lubricate the surfaces. Damaged surfaces, worn packing, etc. can cause some high frequency noise, which will most likely occur at something other than an exact multiple of running speed.

When running, some water should be coming from the packing gland (or a supply and return water system), which is a sign of good seal lubrication. If excessive (packing not installed properly) it can also allow air into the pump system, and generate the 'air lock' problem. If too tight, it can cause the motor to have to work much harder, damaging the seal surfaces.

If you have additional information, I would be happy to review it. howard@motordiagnostics.com

Howard