gents,

I have a scenario here. Could a natural frequency affected the harmonic(s) of the running speed?

For simple example, let say a motor running at 1500 Rpm, and the natural freq (1st or 2ns critical) believe to be near 50hz(3000rpm). So could the alignment will be so much affected??

Yes it can and it happens, if there is a force with the correct frequency that can excite a resonance, it will do that. Olov

Forces must not only have the frequency match, but also they must have the mode shape match.

Folks,

Yes i agree with you. Seems like my equipment having prob with resoance and hence i am 90% correct that its come from the beam/structure resonance.

Coupling shims is the culprit

Thanks

CBM Analyst,

Remember, in the world of diagnostics, the 10% of uncertainty, may get you, proving you wrong.
If you suspect it is indeed the coupling shims then I would use a strobelight to freeze the coupling to take a look at the shims...make sure you got the permits cleared first.
In my exposure in the Offshore platforms (Miri/Brunei), I found that alot of the resonance fault is not the structure itself but, loose/failed bolts that cause the resonance especially, where Recips are installed. This can contribute significantly to your misalignment, as you'd be aware
Food for some thought, my friend!
Cheers

Rajan Muthukrishnan

Rajan,

Thanks again for your input.

Yes i agree that old platforms in Miri,Brunei and Sabah having lots of resonance problem. But the operator could not believe by changing vibration isolator/dampener/holding bolts could improve the vibration. Furthermore the engineers stressing that it is misalignment problem.

Rajan,
Do you monitor those COTPs? What is your vibration limit for the engine, gearbox and pumps? Currently searhing from the net and manuals.

Btw, where's your location now?

CBM Analyst,

If the Engineers stress on misalignment, then ask them to justify their call and like wise, you'd have to be able to justify yours.
I generally use 8 mm/s Pk as Alarm and 12 mm/s Pk as Danger on the COTPs, or other equipment. Key thing is, refer to baseline data on specific machine, if available; or past levels and trend.
A word of advise, don't completely rely on vibration data alone, always be alert by using your senses; which may lead to using other tools to close-in on problems. I currently reside in Mississauga, Canada; close to Toronto.

Cheers...Rajan Muthukrishnan

Rajan,

Monitoring a cotp for me is very unpredictable. Furthermore, the engine speed is variable and depends on the tank level. Usually i request for manual speed around 1200 Rpm.

Most of the time, the pump give abnormality first before we trigger the genuine problem. (since prediction is somewhat diff)

Enjoy your weekend
thanks

CBM Analyst,

Crude Oil Transfer Pumps are no different from any other pumps. Remember they are pumping crude oil and hence, the rotor is operating in hostile environment; vulnerable to accelerated wear and tear. Early indication would be seal leakage.
Being installed on skids/I-Beams, coupling misalignment between Engine-Gearbox-Pump are common issues; again anchor bolts are culprits here...frequent torquing to spec is req'd.
Engine journal bearing wear is also common due to above reasons.
Just got to be mindful of these when you are collecting routine vibration data and be observant.

Cheers...Rajan Muthukrishnan

Hello all,

Again my crude oil pump having knocking sound on the gearbox, previously at high speed of 1500rpm. Currently shift to lower speed of 1000 rpm.

Knocking on the gearbox? Would it be improper backlash? Alignment of coupling lock??

You posted on the wrong thread
Try the one you started on the knocking before.
Dave