10 replies to this topic

[Ahsan67]

Dear All,

 

Ethylene is stored as a liquid at -100 C and transported to customers as a gas via a compressor.  The ethylene tank is equipped with a Pressure Safety Valve (pilot operated) and a vaccum safety valve. The compressor takes suction from the top of the ethylene tank and operates between 50-250 mbarg suction pressure ( below 50 mbarg compressor stops and above 50 mbagr it starts to transport).  A problem is accrued with a suction pressure drop that causes the compressor to trip.

 

To tackle this problem while running the tank's operation, the existing PSV nozzle is connected to tee with one end connected to the PSV and the other is used to lay new line tied-in to the existing compressor suction line.

 

My questions here are :

1. Is there any restriction for this type of modification in any Code or standard ( using tank's PSV nozzle for other operations, using a tee?
2. Will this modification affect the PSV operation?
3. What is the criteria for upstream line pressure losses for storage tanks PSVs (Like 3 % as in API 521)?

Regards

Ahsan

Edited by Art Montemayor, 28 June 2015 - 04:15 AM.

[fallah]

 

 

Ahsan67,

 

At first clarify how did you tackle the problem by using mentioned configuration for compressor suction line?
 

[Ahsan67]

We didn't exactly tackle but given interim solution, we are of opening that there must be some problem in compressor suction nozzle on tank, as tank can't be decommission at this time,to bypas this nozzle a line from psv nozzle where a new tee is intalled who's one end connected to PSV and other joined just after that compressor suction nozzle

[fallah]

Ahsan67,

 

Please upload a simple sketch of the system you are trying to describe...

[Zauberberg]

To me, these two look as completely unrelated topics - suction pressure drop and using another vessel nozzle for compressor suction line.

 

Apart from the sketch, you need to elaborate more on what measurements/conclusions have you made so far, and what has made you think that just swapping vessel nozzles will (or it already did) solve the problem.

[Ahsan67]

Dear

 

kindly find attached isometric for newly laid line with existing configuration as well as PFD of the same

 

Regards

 

Attached Files

•  Ethylene Tank top Isometric.pptx   37.27KB   15 downloads
•  Hydraulic.jpg   312.69KB   2 downloads
•  PFD ethylene.jpg   86.58KB   4 downloads

[Ahsan67]

Dear fallah and Zauberberg,

I am still looking for your kind response on the above query, your early suggestions , guidance will help us to properly design our system 

 

Regards

Ahsan 

[Zauberberg]

Ahsan,

 

I also didn't get an answer on my question why would you choose such piping arrangement. Where has the compressor feed gas nozzle been located prior to the decision to relocate it to the PSV nozzle? And why do you think that this will solve high suction pressure drop problem, if you are changing the nozzle only?

 

API 520 does not explicitly prohibit using process laterals connected to PSV inlet piping (see chapter 5.5 of the latest edition) but it also generally recommends not to use this arrangement. Exceptions should be analyzed carefully to ensure that the PSV will work as intended, and that there are no additional relief loads associated with this arrangement.

[Ahsan67]

Dear Zauberberg,

 

Thanks for prompt response, for your further reference i am attaching piping detail drawing, by doing this arrangement we have overcome our low suction pressure problem.  For now we are concern to the effects of this modification on PSV working, a hydraulic analysis has been carried out to find out pressure drop due to newly installed fittings at the upstream of PSV, as per recommended practices API-521 4.2.2 which limits the inlet piping losses to 3 % of set pressure. My question Do I need to consider the same for my system or some other criteria to be followed?

 

Regards

Ahsan

Attached Files

•  Hydraulic-1.jpg   148.47KB   1 downloads

[Zauberberg]

I do not see a problem with this setup, but I do not know all the details. I would also look for the real cause of high suction pressure drop, because blaming one nozzle for excessive head loss definitely does not seem correct.

[Zubair Exclaim]

Some commpanies ask for dedicated connections to PSVs .....  its the prefered choice ....

 

other than that adverse effects are not much