8 replies to this topic

[Vishal78]

I have 1 stage reciprocating compressor package which compresses 480 psig suction gases to 630 psig discharge pressure. During maintenance case blow down, my suction auto valve and discharge auto valve will be closed and immediately auto blow down valve opens. The discharge temperature is around 131 deg. F. We ran Hysis considering adiabatic flow. By blowing down gas in 2”line (through 2” full port blow down valve) to atmospheric pressure, temperature goes down to -170 deg. F in less than a minute. Calculation was done in two steps: depressurization from 630 psig to suction pressure 480 psig (only discharge side of the volume considered), then depressurization from 480 psig to atmospheric pressure (only suction volume considered, as compressor valve act as a check valve) Material of construction of pipe and flanges are A-106 and A-105 which have MDMT of -20 deg. F (even I take credit from ASME section VIII Div.1 for stress ratio, material MDMT for these carbon steel will not go upto -170 deg. F at any lower pressure). Vessel material is SA-516-70 which has similar MDMT, though during depressurization temperature of vessle goes down to less than -100 deg. F. My question is: Does ASME or API have any guide line indicating using normal carbon steel (A-105. A-106 or SA-516-70) on such type of very short time (less than a minute) of depressurization with lower temperature than material MDMT should be acceptable?

[Art Montemayor]

Before discussing the safe and proper manner of unloading a gas compressor - especially a  reciprocating one - it is mandatory to do it with a detailed and accurate P&ID.  Since you haven't furnished one, I am submitting a simplified PFD that can at least focus on the principal equipment and instrumentation involved.  A picture is worth a thousand words - and your written description is not enough to fully understand what all is involved in what you have or propose.

 

Please review the sketch and make any comments or notation and resubmit it as accurate or modified with your additions as well, labeling it with a Rev #1.  I note that you fail to mention the capacity unloading of the compressor prior to actuating the suction and discharge block valves and that you also don't make mention of check valves in the discharge line downstream of the discharge block valve.  Please clarify the detailed steps you employ in your compressor shutdown.

 

You can't just make a decision based on the quality (or temperature) of the blowdown.  You have to quantify the amount of refrigeration as well as the rate that you are applying to your steel mass in the blowdown line.  What is the quantity and rate of flow of the blowdown?

 

 Reciprocating Compressor Unloading.xlsx   444.89KB   170 downloads

[Vishal78]

Art,

Thank you very much for the response. I have made modification and put all the necessary information on your excel spread sheet. However, could not find any attachment icon. I am trying to find on this website how to upload, once I will find I will upload the spread sheet.

Again thanks

Edited by Vishal78, 24 June 2014 - 05:53 PM.

[Vishal78]

Art,

Here I have attached the excel spread sheet rev. 1. I did modification and put more information for your reference. I have suction scrubber, suction bottles and discharge bottles. No coolers.

Thanks

Attached Files

•  Reciprocating Compressor Unloading Rev. 1.xlsx   47.17KB   86 downloads

[Art Montemayor]

Vishal:

 

From what you describe, I see no problem using the specified pipe for blowdown to atmosphere.  You don't tell us what gas you are compressing that has such a high Joule-Thomson effect on cooling, but since your downstream piping is going directly to atmosphere, the pipe has no stress imposed on it.  Additionally, the blowdown is not continuous and all refrigeration will be distributed to the pipe's steel mass.

 

I do have some issues with the manner in which you depict your installation and that is why I requested you send us a sketch.  Please read my notes and comments on the attached Rev2 and you should understand.  My point is that I have more concern with the way you are configured for an eventual compressor shutdown and subsequent blowdown than I have for the carbon steel blowdown piping.

 

If you want to discuss the issues, I'll be happy to explain in detail - provided that you furnish all the basic data regarding your designed shutdown operations.

 

 Reciprocating Compressor Unloading Rev2.xlsx   76.88KB   138 downloads

[Vishal78]

Art,

Thank you very much for the response. Unfortuneatly, I am not able to give detail gas composition, however I can tell that gas has most of the content of Methan, few Ethan and less than 1% of heavy C3, C4 and so on. No H2S. I am appologizing not indicating the cooler in my recycle line. Yes I do have small cooler to cool the hot gas from discharge going back to suction scrubber.

You mentioned that I can use carbon steel piping since I am not continuoulsy blowing down, and since I am blowing down to atmoshpere, I am not stressing out the pipe material, however do you happen to know or can quote the ASME para. or API para. that indicate the acceptability of material or anything related to this subject? I have HAZOP action item open and I am in between purchasing stainless steel material or carbon steel material.

Thanks Again

[Art Montemayor]

Vishal:

 

In my opinion, if you are only conducting the blowdown gas downstream to the atmosphere (for an estimated 3-7 meters, well braced and supported) then you are not subjecting the conducting pipe to any internal pressure or stress.  The pipe is merely acting as a conduit.  The ASME code (or any other safety code) is involved when you depend on the pipe for containment of a pressurized fluid - and this is not the case in this application.  The concern around this application is that there is a substantial temperature drop due to the free, adiabatic expansion taking place when you open the blowdown valve and the resulting low temperature will cause embrittlement of the carbon steel subjected to this temperature.  While this is exactly true as to the consequences on the carbon steel, as long as you do not put any stress on the pipe during the blowdown (and don't subsequently use the pipe for a pressure application), then you are OK with regards to a safe procedure.

 

I don't think the use of carbon steel in the downstream blowdown line (after the customary restriction orifice) will present any HAZOP problem if you explain the case.  I presume you will employ an extra large diameter for the exhaust pipe to lower velocity and noise level.  This will add more mass to the pipe and give you a larger heat sink.

[Vishal78]

Thank you very  much for the explanation Art.

[Bobby Strain]

On the other hand, installing a 2" stainless blowdown valve and a few feet of pipe would pass anyone's test for a safe system.

 

Bobby