Vibration analysis of piping connected to reciprocating compressor

Hi, can any one let me know that CAESAR II can perform vibration analysis of piping connected to reciprocating compresssor operating at diferent loading conditions (25 %, 50 %, 75 % and 100 % compressor loads)?

Is there any method in CAESAR II to perform analysis as per API-618 and checking the adequacy of pulsation dampners ?

Your vibration analysis can be accomplished by defining harmonic loads - you will have to define the loads to CAESAR II.

At the moment, CAESAR II does not address API-618.

Also you may require a Pulsation study by a Dynamics expert(most of the compressor vendors do) due to the Acoustic pulsations . One has to have a estimate of the natural frequecy of piping for the spacing of supports,before these studies are taken by the Specialist. Practice is to stay above the Fourth harmonic..

Hi, Richard Ay

How i will know harmonic loads of existing compressor and its piping.

I know operational data of compressors like pressures, temps, flow rates etc. and geometry of piping but how i will calculate harmonic loads ?

Zahid,

You don't know. This is a guessing game, where you simulate (or guess) the load, run the analysis and compare the CAESAR II response to the response of the real system. If the responses don't match, in shape, move the simulated force. Once the shape of the response matches, scale the force up or down to match the displacements.

Once you have "simulated" the load, you can evaluate the system to determine:
a) if you have a problem
b) if you do, how to alter the system so it doesn't respond to the load.

Oops....

Richard Ay,

Let me brief you our problem before I go in the detail analysis of piping and ask you to advise me some guidelines.

We have two fertilizer plants, plant-1 and plant-2. At our plant-1 we have two feed gas booster compressors (reciprocating), which were standby since 1981 and at plant-2 such compressor is not available. Plant-1 is designed in such a way that one compressor feeds that plant and other remains on standby.

We had decided to operate the second (standby) compressor to feed plant-2 after laying new suction and discharge lines. Currently both compressors are operating and independently feeding plant-1 and plant-2. We have to change the loads on compressors to 50%, 75% or 100% keeping in view the plant operations.

The problem is this; when the compressor of plant-2 is operated at 75% or 100% loads, it’s newly installed suction and discharge lines vibrate excessively. Vibration is excessive at the elbows, tees and dead pockets of PSVs & off takes. Most sever area is located near compressor and ~ 250 meters away from compressor.

We think piping vibration is mainly due to flow because at 50% compressor load the vibration is not so sever.

Please tell me, what kind of analysis I should perform to arrest the problem. Either harmonic analysis or acoustical analysis or any other analysis you recommend. I am using CAESARII.

Hope to have some guidelines to resolve the problem.

Mr Zahid

You are mentioning about some NEWLY installed lines to the existing compressor circuit. Were these lines along with the whole circuit analysed for vibrations(mechanical, acoustic etc).?

You may not be able to perform an acoustic anlaysis with C2. I would suggest you contact the compressor vendor and take advise (in case they have the analysis competence). You may have to submit the new piping circuit rouitng to the Vendor and coordinate . For more information, you may try the follwing link of Engineering Dynamics : http://www.engdyn.com/index.html

CAESAR II does not address "acoustic analysis", which may be required here.

If the problem is "flow induced", then the only way you can approach the issue is with the "guessing game" I described above. This is not a trivial task, and if the response is not dominated by a single mode of vibration (natural frequency) you may never match the response of your actual system. In this case, an analysis by someone like Engineering Dynamics is a must.

Your piping has changed its characteristics both acoustically and structurally... start with an acoustic analysis which CAESAR II does not do....

We have not performed Acoustical and mechanical analysis on new and old piping.

Thanks to all for your advice.

Before perform a vibration analysis may be interesting to check some items related with piping support design in your plant:
- Concrete slabs in both reciprocating compressors must be independents.
- Suction and discharge lines must be supported out of compressor concrete slab to avoid amplified vibrations.Normally are supported at the ground out of slab.
- Antivibration guides (clamps+rubber+spring) in suction and discharge lines are recomended.

Hi, JDM

we have supported the piping independently from compressor and the compressors have independent foundations.

Can any one tell me about the software which can perform acoustical analysis.

Zahid,
Perhaps you may find something in "Controlling the Effects of Pulsations and Fluids Transients in Piping Systems" presented by Southwest Research Institute in November 4-5-6, 1981
Attempt it in the next direction:
http://www.swri.edu/default.htm

.............. you can read this olde classic paper for starters:

http://www.engdyn.com/download/57.pdf

the new link:

http://www.engdyn.com/images/uploads/57-piping_vibration_analysis_-_jcw&kea.pdf

In fact there are more:

http://www.engdyn.com/resources/category/technical_publications/

I guess the answer is hidden in how you make the load changes. Do you play with RPM of the compressor to achieve the load reduction/increase in the system, I guess you do. So you change the forcing frequency without consulting the originator of the system, and now some of the forcing frequecies might be coinciding with the natural frequency of the piping.

If you need to work in all those loads you really needed to discuss it with the piping design engineer and the pulsation specialts.

I believe it is still not late to do it if you want to see the plant working under those conditions. They might be making some or substantial modification in the supporting system of the piping to get away from the cooincidental frequencies. Or you may be giving us some of the operations in case the cost become enourmous.

Plaesa note that the reciprocation compressor piping is fined tuned piping, changing the operating conditions without consulting to the specialist might be very costly.

Good luck.

Ibrahim Demir

danb,

Thanks a lor for the link. Very useful to a starter!