API 541 Vibration Limit

[Andy49]

I am reading our spec which states
"Vibration tests and acceptance criteria shall be conducted per API 541, except that the overall vibration limits in all directions on the bearing housing shall not exceed 0.03 - 0.05 inches/second peak-to-peak at no load, with the frame bolted to a massive foundation”

Looking at API 541 Figure 1, it shows almost a flat line graph with a magnitude of 1.5 mils peak to peak. I think our spec may be too stringent. I called a motor salesman and he told me to google it. I looked at previous posts and I was not able to fully understand it. Any explanation would be appreciated. Just as an example, an application may involve 3500 hp motor, 3600 rpm. Thanks.

 

[electricpete]

You have a bit of apples and oranges going on.

API 541 Figure 1 describes shaft displacement limits.

Your spec describes housing vibration limits.

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(2B)+(2B)' ?

 

[Strong]

Andy,

If you convert 1.5 mils p-p at 3600 cpm (assumes dominant vibration frequency at motor speed), then the velocity is 0.28 in/sec peak or 0.2 in/sec RMS. Your accepatance spec (for whatever reason) is well above API 541. There is no such thing as a "massive foundation" for a large frame 3500 hp motor; speaking dynamically of course!

Walt

 

[electricpete]

Figure 2 of the same standard gives housing limits 0.1 ips above 1200rpm for machines rigidly fastened to massive foundation.

NEMA MG1 requires 0.12 ips bearing housing vibration for similar condition. That is just general run of the mill motors, not necessarily large or critical. Many users request lower. IEEE841 requires 0.08ips if I remember right.

Some would say that you are running blind if you only measure housing vib and not shaft vib on sleeve bearing machine (which I assume such a large machine would be). Lowering your housing limit may be a small counter-measure to add small degree comfort if the shop is not measuring shaft vibration (still not as good as measurign the shaft). But 0.03 ips could be tough to achieve for some machines even well balanced. It's not unusual for me to see 0.05 ips at 2*LF.

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(2B)+(2B)' ?

 

[electricpete]

Some points of discussion Walt
1 - Wouldn't you agree about apples/oranges to compare bearing housing to shaft movement? (I assume you posted to soon to see my earlier message)
2 - If you ignore the apples/oranges, his limit is lower, not higher, correct?
3 - Massive foundation is defined in the standards. In API541-1995, it means the vibration near the machine feet are <=30% of that on the housings. Massive foundation is different aspect than rigid mounting which of course means mounted resonant frequency far above operating speed as you know.


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(2B)+(2B)' ?

 

[Andy49]

Thanks to all for replying. Is there a basic book or a technical paper that will make me somewhat conversant in this area? I would like to to learn (without calculus) a little more. I read a paper by Seimen's engineer but it kind of went over my head.

 

[Strong]

El'Pete,

You are correct about the apples-oranges or shaft-housing vibrations. My conversion was of course assuming that both displacement and velocity were measured on the bearing housing. Andy has the problem to resolve why the imposed velocity limit range is lower than the API 0.1 value. The API filtered values at common shaft speeds appear to be about (0.03 in/sec true peak velocity from the API graph), so perhaps their is some consistancy after all! I am certainly not going to argue the descritions of foundation rigidity, especially for a test stand. API definition or not, the motor foundation in the plant or test stand is either acceptable or not.
Walt