2 replies to this topic

[eastorca]

Hello everyone

My system is designed to start-up fuel gas skid on platform (during about 8 hours only). It has two operating cases from two feed sources: one from Export Fiscal Metering (157 barg), the other from 1st Separator (65 barg).
Design pressure of Heater is 160 barg.
Please, see the attached file for more details

My question:
What is the real purpose of PSV?
Why the set pressure is so close to operating pressure (160 barg; 157 barg)?
Why the type of PSV is "THERMAL". But they (my seniors) told me that: "calculate it as FIRE CASE"?
Which criteria should I follow to sizing this PSV?

Thank you so much

Attached Files

•  System.bmp   1.15MB   35 downloads

Edited by eastorca, 29 December 2010 - 11:01 PM.

[daryon]

Here are my thoughts;
1. PSV governing case 'thermal' is probably a poor/inaccurate description. Thermal expansion PSVs are typically installed on liquid full systems, here we are dealing with a gas filled system. I think the PSV is probably for gaseous expansion which may occur if the heater is shut-in and left on; or possibly if the heater is shut-in and exposed to a pool fire. I would have described the PSV governing case as ‘fire’. Do you have access to the PSV calculation? This will tell exactly what relief cases have been considered and which the governing case is.

2. You are mixing two gas streams one a 65 barg and one at 157 barg. I would suggest that your operating pressure is actually closer or below the 65 barg gas pressure. So there is a large margin between normal operating and system design pressure.

3. As above I would call this fire case not thermal (this is for liquid filled systems). I think it will benefit you greatly to read through API RP 521 where there is a full discussion of potential sources of overpressure.

4. You should consider every conceivable scenario which may result in overpressure of the heater. If the upstream separator can be isolated from the PSV you may need a PSV on this pressure vessel also. Again API RP 521 is a excellent resource I and strongly recommend you read through it.

[fallah]

My question:
What is the real purpose of PSV?
Overpressure protection of heater's shell as per ASME Sec IV
Why the set pressure is so close to operating pressure (160 barg; 157 barg)?
Refer to relevant project specification for specifying the design pressure and margin with regard to operating pressure
Why the type of PSV is "THERMAL". But they (my seniors) told me that: "calculate it as FIRE CASE"?
May be fire case is only applicable scenario or governing scenario.Thermal may means overpressure due to being subjected to heating source like fire
Which criteria should I follow to sizing this PSV?
As per what we can see in your sketch you should size the PSV based on fire case

Edited by fallah, 02 January 2011 - 07:46 AM.