Piping Vibration Software

[denniskb]

I have a client with sometimes severe piping vibration problems at a number of gas pipeline compressor stations and need some advice re suitable software to use.
I want to be able to model various beam configurations with combinations of lumped mass and spring restraints at various locations and determine natural frequency and response to various source frequencies.
The simple text book solutions don't touch the surface and FEA seems a bit over the top. I have tried some add ins to Mathcad and Matlab but they typically solve only one configuration.
Can anyone suggest suitable software that will help with an initial analysis of the problems and suggest possible remedial actions?


Dennis Kirk Engineering

http://www.ozemail.com.au/~denniskb

 

[GregLocock]

Actually I'd use linear FEA, even though it is over the top. It can't handle discrete dampers, so the transfer functions will be wrong in detail, but the frequency estimates are good. The advantage is that lumped masses, discrete springs, and continuous beams are all well within its capabilities.

The sort of package you describe used to be known as system analysis by building blocks, I think these days most people would use Simulink, or grind through the maths in Matlab or Scilab.

The other approach is to model the mechanical system 'properly' in ADAMS or Working Model, but again they may be over the top, and are non trivial to learn.

Cheers

Greg Locock

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

 

[Geerhead]

A little off topic but.......

My first position out of school was the analysis of acoustical (inside pipe) and vibration of recip gas compressors.

We used Ansys for the determination of MNFs and a proprietary program for the acoustical analysis.

I guess the first question to be asked is, what exactly is causing the "sometimes" severe piping vibration? Don't overlook the unbalanced forces caused by pressure pulsations. Is it possible to determine the exact conditions in which the vibration occurs? ie: pressures, flow rates, gas composition, run speed, etc.

Is your client running recips or others? Provided the piping is adequately supported (and does not have a MNF coincidental to an order of run speed), vibration caused by recips (due to unbalanced forces caused by excessive pressure pulsations) can usually be solved by the use orifice plates and/or pulsation dampers.

I guess my point is look for the cause before you try to discover a cure.

 

[LSThill]

Geerhead (Mechanical)but did you do the API 618 CHECK?

Reference API 618, and BOS Fluid
BOS Fluids Reciprocating Compressor Pulsation and Mechanical Analysis
Reciprocating compressors produce pulsations in the uction and discharge piping that can bedamaging both to the piping and to the machine. API 618 gives allowable pulsation levels anddirectives for the type of analysis required based on compressor power and discharge pressure.
(See API 618 Figure 4.) Even though API requirements are satisfied, piping systems may still vibrate when reciprocating compressors are running. Pipe vibration is a function of the strength and frequency of the pulsation, and the natural frequency of the pipe and supporting structure. BOS Fluids makes it easy to check the mechanical shaking forces and the API 618 allowables to be sure that the compressor installation runs smoothly.

 

[Tmoose]

It was helpful for me to identify vibration frequency, and understand if the forcing funtion was external (compressore vibration) or internal (pressure induced).

If there are flex joints or pipe loops close to the compressors to serve as isolators (for external vibration) the piping must be anchored securely at the discharge end of the isolator. Many flex joints are about as stiff as 10 feet of pipe.

Each pressure pulsation makes an elbow straighten out a little, whixh can really take advantage of piping resonance near pulsation frequency. If the process can be stopped and started, the operating vibration can be measured, and "bump tests" of troublesome sections are easier with compressors off. Might not be necesssary to model the whole pipe system.

 

[flaco1967]

Hi Dennis

I´ve solved a similar problem for a number of gas compressor stations. My experience suggest the next sequence of events:

1st. measure piping vibration and pulsation levels
2nd. measure dynamic strains

this would help to sort and troubleshoot most vibration issues.

If it turns out that solution is mechanical redesign. I´m afraid that you´ll need ansys - abaqus and a lot of expertisse in transference factors to couple fluids and pipes.

Let me know if this was helpful or if you need anything else

best regards

Andres Rivas
GIE

 

[PulseMaster]

CAESAR from Coade for a quick beam element representation of the mechanical system. It is primarily for statics, but does a fine job for frequencies of simple pipe spans. The interface is made for building pipe. Ansys builds pipe beam models reasonable well also. I don't think you should waste your time modeling what you can measure. Look for the excitation mechanism.

However, these guys are right in that you probably have a pulsation problem. What you are looking for is pulsation that corresponds to the vibration frequency. Your looking at trying to troubleshoot it yourself with orifice plates, or going to a specialty company like Southwest Research, EDI, Beta, etc.