5 replies to this topic

[ahyong]

Hi, I have a question regarding the image which I attached below. I am not sure but I think there is something wrong with the drawing. For your information the stream comes from LP Steam header to a slug surge vessel. The spec of the vessel is as follow:

Size: 20’IDx135’T/T
Capacity:46600 CU Ft
DP: 1100psig /7.5psig@300F
DT: 130F
MDMT:-7F
TRIM: 6NS0P1/6CA4P1

During the incoming from the LP Steam header there isn’t any Sour Service and the OP is only around 60psig. But at the piping end it connected to another class of pipe which is 6NS0P1. The fluid here suddenly having OP of above 900 psig. So my question is since there isn’t any equipment in between such as expander, is it possible to have such big gap of pressure difference? Any solutions or clarification provided will be much appreciated.

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[Pilesar]

The P&ID is intended to represent accurately the piping that is installed. I would not assume the drawing is inaccurate. Pipe classes identify more than just the maximum pressure rating -- they describe the maximum temperature rating, the material, flange type, gasket type, connection type, etc. I do not know your process or what your pipe specs signify. There could be a very good engineering reason that the pipe spec break is located where it is shown. I suspect the downstream line can possibly be fed by higher pressure steam or steam with a much higher temperature. Sometimes the reasons for piping spec breaks cannot be determined from the drawing itself. There may be an abnormal operation case that requires the different pipe specification. Sometimes a section of pipe is already installed and used in another service than originally intended due to plant modifications. There would be no reason to remove a higher pressure class of pipe to replace it with a lower pressure class. Another option is that a spec break was put in the wrong place during the design phase. Depending on when the mistake was caught, it may be more cost effective to leave a section of pipe with a higher than necessary specification than to change the drawings. Your pipe designer may have documentation of the line that might help explain the cause of the spec break.

[fallah]

ahyong,

 

It could be logical...

 

Appears the LP steam injection facilities to be a utility service (purging, cleaning,... agent to be used in maintenance period) and don't relate to the main process, because two 3" valves around two check valves are to be normally closed. Obviously, if main process pipe class to be higher than the LP steam pressure rating then the class break before second check valve is to be needed due to possibility of inadvertent opening of downstream closed gate valve while the main process is working and LP steam facility isn't in service...

Edited by fallah, 29 March 2013 - 01:11 AM.

[ahyong]

Thanks for the explanation,

Fallah, could you please elaborate further on your sentence of "due to possibility of inadvertent opening of downstream closed gate valve while the main process is working and LP steam facility isn't in service..." I don't really understand about this phrase

Thanks!~

[fallah]

 
ahyong,
 
It means: If downstream gate valve, which is closed in normal operation, left wrongly open let say by the operator, the piping between second check valve and the gate valve would be subject to higher pressure rating existed in downstream and can transfer backward through opened valve. Then the class break should be considered at least upstream of the second check valve instead upstream of the normally closed gate valve...

[Dacs]

As explained by Pilesar and fallah, there are numerous reasons why you'd have tapoffs from lower rating to higher rating system.

 

My guess is that the destination is designed to handle a higher pressure stream than the LP Steam line (which looks like not being used normally, maybe for startup/shutdown or maintenance), hence the need for:

1. A higher rated class

2. having check valves on the LP blowdown line (to prevent backflow)

 

I don't think you'd be using this line when you have a higher pressure on the destination.