Safety Factor for Rotation Equipment

[sloth4z]

How close can a resonance frequency safely be to the operating speed of rotating equipment (centrifugal fans)? I have seen two methods and I am unsure of which one to believe.

The first states 15% above and below the operating speed should be avoided.

The second uses a chart dependant on temperature. It states 5% below the operating speed. The "above" factor varies depending on the temperature (5% for <200C, and adds 2% for every 100 degrees above 200).

Can someone point me in the right direction, or tell me a book of standards to look in (AMCA, etc.). Any help is appreciated.

 

[CESSNA1]

SLOTH4Z: I suggest not exceeding 30% of the resonant frequency. If, say, the resonant frequency is 1000 Hz then stay at 300 Hz or below. If you want to operate above resonance then go through the resonant frequency very, very, rapidly both increasing and decreasing. Damping can affect the resonant frequency and allow slightly higher frequencues, but I would suggest you ignore damping.

Regards
Dave

 

[GregLocock]

With driveshafts we try not to operate within 15% of the resonance. However, you can say that has a fair old safety factor built in.

Bear in mind that damping will broaden the peak as well as reducing its height, so increasing the damping can actually increase the response at a particular speed.



Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[electricpete]

API 684 has some criteria similar to Greg's.

a. If the amplification factor is less than 2.5, the response is considered critically damped and no separation margin is required.
b. If the amplification factor is 2.55-3.55, a separation
margin of 15 percent above the maximum continuous
speed and 5 percent below the minimum operating
is required.
c. If the amplification factor is greater than 3.55
critical response peak is below the minimum operating
speed, the required separation margin (a percentage
minimum speed) is equal to the following:
100-[84+6/(AF-3)] %
d. If the amplification factor is greater than 3.55
critical response peak is above the trip speed, required
separation margin (a percentage of maximum
continuous speed) is equal to the following:
[126-6/(AF-3)]-100 %

They note that standards for specific machines supersede these general guideliens. For example API Standard 613 requires that gear units operate below their first critical speeds with a 20 percent margin (unloaded case). API Standard 541 requires that special purpose induction motors (motors driving unspared critical equipment) operate below the first critical speed with only a 15 percent margin. I'm sure there is some special guidance for pumps as well. I don't know about fans.

Dave-your thumbrule doesn't sound right. Operating speed 50% of critical speed would be unsatisfactory?

=====================================
Eng-tips forums: The best place on the web for engineering discussions.

 

[rob768]

For propulsion installations, classifications societies such as Lloyds, ABS, DNV etc require a margin 20%, unless sufficient damping is proven (which they often require to be proven through trials).
According to out measuring experience, a maring of ± 15% is usually sufficient, providing there are no variables that might alter the frequency (for instance, some rubber couplings have a load dependant torsional stiffness, the inertia of an impeller might differ dependant on medium to be pumped, etcetera).
With large damping, the area of flank excitation is much broader than with little damping, where a margin of 5% may be sufficient. However, with large damping there is generally not a problem.