8 replies to this topic

[jprocess]

Dear all,
When we want to set design pressure for compressor suction drum we should consider settle out pressure.I want to know about the concept of settle out pressure,its calculation and is this concept limited to compressor application?Is there any other application?
Thanks in advance.
Warm Regards.

[Nirav]

It is the equilibrium pressure when your compressor system is tripped. Theoritically, the entire system will be at same pressure, if there is no venting takes place anywhere from the system.

In brief, it can be calculated quickly as...

Settle out pressure = SIGMA ( Pi x Vi) / SIGMA (Vi)
where,
Pi = Operating pressure of equipment
Vi = Volume of equipment

You can take into account the margin for total volume of piping.

If you want detailed & accurate calculations, refer to a very good article on this subject in recent issue of Hydrocarbon Processing, November 2006.
"Calculating settle-out pressure in compressor loops"

I don't have idea about any other application other than compressor loop wherein such concept is required to decide the minimum design pressure.

Thank you,

[Sandeep01]

A very apt description provided by Nirav.

Don't forget to work-out the various settle-out scenarios, particuallry for multistage compressors with more than two stages. Different scenarios arise with check valves between stages holding and not holding.

Also account for the temperature of each section.

Regards
Sandeep

[Radionise]

Can't add any better to Nirav's and Sandeep01's replies.

But just to comment on the article in Hydrocarbon Processing, November 2006 - "Calculating settle-out pressure in compressor loops",

Firstly, the method outlined in that article is to roughly approximate the settle-out pressure, not a 'detailed and accurate calculation' . Clearly, the method suggested by Gorji and Jari presents a much more simplified approach when compared to the API method (I can't remember the exact API method name) which is more rigorous.

Secondly, there are some assumptions made in the article about the percent volume of vessel occupied by the vapour/gas phase. Do consider if these assumptions are applicable to the context of your work. For example, is assuming only 50% of the volume in the interstage cooler consists of gas across a two-stage compression skid for an offshore gas platform realistic?

As a whole, the article published is brilliant! But use it with care; "applicability to the context of one's work" is the key phrase here.

[SuechinLim]

Hi, I am new to this forum. I found this forum interesting and helpful.

I am currently about to perform a settle out calculation and I saw the discussions on this topic. Can someone share with me the article of Settle Out Pressure Calculation from the Hydrocarbon Processing Magazine, Nov 2007?

Your kindness is highly appreciated.

[JoeWong]

Hi Lim,
Welcome...

The article is copyrighted (as of now) and it is against Forum policy (as far as i understood) to park copyrighted document here...

Since you are performing settle out calculation and i guess you are the potential "client" or "driver" to those advertiser in Hydrocarbon Processing, i would suggest you to ask for a complementary copy from HP.

Good luck.

[SuechinLim]

Hi JoeWong,

I appreciate your advice. Thank you very much. Will do so.

Cheers

[jprocess]

About the effect of different scenarios with check valves:

The designer can locate the valve in positions to lower the settle out pressure (as close as possible to the compressor package or as close as possible to anti surge line take off point) and so, having better design in terms of cost. But, can we rely on proper operation of check valves?

[cea]

Nice article by Nirav-

However on concept part, I would like to add some more clarification.

For centrifugal compressors, while following the compressor curve, there shall be designated flow for the required discharge head. At the time before compressor stops / compressor runs on no load condition, the flow is less than the desired. In that case, desired head cannot be maintained. To achieve this, antisurge valve (discharge to suction) will be opened to have minimum required flow at specified head, without blowing any gas to atmosphere.

If this is required to operate for longer time, the compressor suction & discharge loop including spill back line, comes to a pressure which is neither compressor discharge pressure nor a suction pressure. It is generally more than suction & less than discharge pressure. This is called settle out pressure.

In short, settle out pressure is the pressure at which compressor will have stable operation, at the time when the compressor is not operated under normal conditions.

Regards