compressor pulsation levels 1

[CadLady]

I have the following information. I am trying to find how the pulsation levels at the compressors flanges were derived.

suction flange info
suction pressure = 16.83 psia
discharge pressure = 53.23 psia
ACFM = 117.12
Cp/Cv = 1.26
Sonic velocity of the gas = 989 ft/sec
Cylinder bore = 7.5 in
Stroke = 4.5 in
Compressor speed = 890 RPM
Cylinder suction nozzle I.D. = 3.07 in
Vol. Efficiency = 57.1913
Clearance Ratio = 0.2680
X/S = 0.3965
Pk = 0.9783
Pulsation level at cylinder flange equals 22.24 per cent of line pressure.

discharge flange info
suction pressure = 16.83 psia
discharge pressure = 53.23 psia
ACFM = 47.59
Cp/Cv = 1.26
Sonic velocity of the gas = 1122 ft/sec
Cylinder bore = 7.5 in
Stroke = 4.5 in
Compressor speed = 890 RPM
Cylinder suction nozzle I.D. = 3.07 in
Vol. Efficiency = 23.2388
Clearance Ratio =1.4974
X/S = 0.1065
Pk = 0.6169
Pulsation level at cylinder flange equals 12.36 per cent of line pressure.

I do not understand how the 22.24% and 12.36% values were calculated.

Can anyone help?
Thanks

 

[sms]

I assume that you are reviewing a manual or a report from the compressor manufacturer? If so, then the levels were probably arrived at from a pulsation study conducted to API 618. Pulsation studies are done with either a analog computer, or with a digital technique such as FEA. Typically the compressor performance, cylinder passages, valves, and attached piping is included in the model, which makes it a fairly complex process. I'm sorry, but I can't give you a simple equation that gets you to that pulsation amplitude. By the way, I would consider a pulsation amplitude of 22.24% of line pressure to be too high, and design changes should be made to control the pulsation level. If you need to discuss it further, let me know.

 

[kahlilj]

sms,
i am interested in knowing more about this concept. i work for a gas tranportation company and we have several compressor stations. as such this problem (high pulsation levels) come up frequently. i don't have a copy of API 618 (have heard it referred to) and don't know the standards set forth in it.

i understand that u may not be able to give a "simple equation" to determine pulsation levels, but it seems u know quite a bit about the subject. can u please ellaborate?

thanks,
kgj

 

[CadLady]

sms,
Thank you for your response to my post.
I am very interested in learning more about compressor pulsations. Although I was hoping for one magic formula, I realize it is a bit more complicated than that.
Could you recommend learning resources? Books, web pages…
CadLady

 

[sms]

Here are some links that might be of interest

http://www.gmrc.org

http://www.engdyn.com/pulsanal.htm

http://www.swri.org/3pubs/brochure/d04/complnt/complnt.htm

http://www.energypubs.com/issues/html/we9103_004.html

API 618 is a long document put together by the American Petroleum Institute for the design and purchase of new reciprocating compressors. It includes detailed design requirements not only for pulsation control, but every other aspect of the machine design, materials, bolting, rods, packing, rings, cooling, lubrication, etc, etc, etc. The sections on pulsation provide some general guidelines for allowable levels, as well as what to specify when going to a consultant like EDI or SwRI for the study. As for tutorials on pulsation, I don’t know of a good resource on the web. But Jim Tison and Ken Atkins of EDI did a tutorial at this years Texas A&M Turbomachinery symposium that covers both the basics of pulsation, and how the latest edition of API 618 should be applied. If you can get that tutorial it would go a long way in giving you a basic understanding. I also have a paper by Larry Blodgett of SwRI on Fundamentals of Reciprocating Pulsation Analysis. Send me an e-mail with your fax number and I’ll fax it to you (as long as you are here in the USA). My e-mail is steven.schultheis@equistarchem.com.

The basic principal of pulsation in a recip compressor piping system is basically the same as a musical instrument such as a trumpet or a pipe organ. Flow variations are magnified by the geometry of the piping system. Imagine a piston pushing gas into a piece of ductwork, and the duct has a closed end. If the piston pushes in a bit of gas, that higher pressure pulse travels the length of the duct, reflects off the closed end, and comes back toward the piston. If we assume that the speed at which the pulse travels is constant (the speed of sound in the gas) then you could have a length of pipe such that as the reflected wave reaches the piston again such that it arrives just as the piston is pushing out the next pulse of gas. In that situation the new pulse builds on the reflected one, and now you have twice the pulse pressure going down the duct. This is where pulsation can become a problem. When the pipe length is resonant with the frequency of the pulses of gas coming from the compressor, the fluctuation in pressure becomes large, and the pipe begins to shake.

That is a very simple example, in a complex system the analysis also becomes complex.