Pressure tapping vibration

[Cootey]

We have recently had a fatigue-related failure of a 3/4" ANSI 1500# pressure gauge tapping on the discharge pipework of a centrifugal gas compressor. The tapping point was a simple stub with a flanged ball valve attached and was alongside 2 other pressure tappings and three 1 1/2" thermowell connections.

A repair is to be made and the stub will be clamped, however there remains concerns over similar tappings on the other compressor trains and elsewhere on the site.

Is anyone aware of any simple guidelines that can be applied to vibration of pressure/temperature tappings that we use during a site survey to assess acceptability.

Many thanks

 

[CESSNA1]

COOTEY: From your description this sounds like a classic cantilever beam problem. The spring (stub out) and the mass (pressure gauge) are being excited by a frequeency in the system.

Try restraining the gauge and connect to it with flex line.
Change the length of stub or position/weight of gauge to move the frequency out of the exciting frequency.

Regards
Dave

 

[btrueblood]

I agree with Cessna, as from your post it sounds like the failure was in the 3/4" pipe or fitting, not in the flange?

 

[Cootey]

Thanks for the response,

In answer to the question, yes the crack formed in the 3/4" stub pipe, not in the flange.

To avoid recurrence in the future we are intenting to fabricate a clamp to support the stub from the pipe.

My concern was more related to other stub connections - we intend to survey with a portable vibration data collector to assess magnitude and spectral information, however we currently do not have any guidelines as to what is an acceptable level of vibration.

Cheers
MC