Hi Pete

Are positions 1 and 2 mtr ? 3 and 4 pump?
has strainer been checked ok? curve plotted and compared to orig spec? efficiency maybe down, check recirculation possibilities, amps slightly higher than normal for same flow? non return valves on standby machines passing? (block them in one at a time to eliminate), worn wear rings? impellor maybe operating well away from bep creating higher than normal bpf (usually evident in impellors with acute blade angles and recirculation), with bpf vibration higher at the thrust end and acting axially may indicate some recirculation back into the eye, how else would the axial increase?
The machine maybe delivering adequate flow/pressure but could still be operating off spec, is it far enough away from that point to worry about it at this stage??
Was there an explanation for the high 1A pos between ’96 and ’02 was this a bad coupling and has it been repaired?

captnb

Just a thought, but a broken diffuser vane could cause a directional vane pass vibration without significantly affecting other vibration parameters or pump efficiency. Other problems that cause high vane pass such as wear at the A or B gaps and the accompanying recirculation or bad flow angles will typically affect pump performance.

Thanks again for all the good suggestions.

The mysterious high blade pass frequency vibration has been resolved.

There is a shaft driven lube-oil pump connected to the outboard end of the pump as shown in attached slides.

It was noticed that the oil pump and associated oil piping were vibrating heavily around 1.0 ips on the oil pump and up to 1.5 ips on the piping. The initial reaction was that this was a consequence (not a cause) of the high 0.8 ips vibration on the pump bearing housing.

Then a breakthrough observation by our vibration engineer - putting your hand on the oil pressure gage reduced the vibration on the main pump bearing housing to 1/2 (!).

We couldn't find a resonance near blade pass frequency, but decided to replace the aux oil pump. During the process of oil pump changeout, some minor oil piping repairs were required.

Upon completion of the repairs, the highest housing vibration is much lower - 0.2 ips (still at blade pass frequency and harmonics) where it had previously consistently been 0.5-0.8 ips.

The inescapable conclusion is that the oil pump or piping was contributing to the high vibration on the main pump (probably some kind of resonance). This is also consistent with the observation that the vib was highest on the outboard end of the main pump. It That seems quite strange that the tiny oil pump/piping could cause large vibration on a much much bigger and more massive main pump, but that's what appears to have happened. What caused the initial sudden jump... not sure.

This message has been edited. Last edited by: electricpete,

SUFP_OilPump.ppt (3,580 Kb, 32 downloads)

could it be due to excessive internal recycling inside shaft driven pump? what was the lube oil discharge pressure during abnormality in comparison with previously normal operating pressure?

Our theory is that it was a mechanical resonance aided by that shaft driven pump.

There were no anomalies in the lube oil system other than pressure gage needle swinging wildly, which we assume was a result of the vibration.

The thing that argues against it being a function of the oil pump is the frequency. It is blade pass frequency for the main pump (5x). Could oil pump by pure coincidence have same number of blades or gears or something? Doesn't seem likely, but could be. I'll ask if anyone knows the construction features of that shaft driven pump.

pete, we experienced a case on excessive vibration at shaft driven lube oil pump discharge line - it is about similar in size with yours here. but ours is attached to medium size steam turbine.

what my senior did was increase the oil pressure by closing the internal relief valve of the lube oil pump (gear pump) after more or less confirmed it is flow induced vibration. the vibration diminished straight away.

my understanding in resonance is not that good. assume that the stray frequency (regional) generated from the instable lube oil flow exiting the pump is having highest peak at the main pump BPF. can this dominant frequency resonate along the shaft and it amplifies the main pump BPF?