PSV Relief Temperature for a Blocked Flow Case

[sjackson]

I am trying to determine what the relieving temperature is for a Blocked Flow PSV on a Fuel Gas Drum. The Operating Pressure is 20 Barg and the PSV set pressure is 25 Barg. I don't expect there to be a large temperature increase but I need an accurate value for the relieving temperature. Is there a way to determine this temperature using HYSYS? Any advice would be much appreciated.

 

[TD2K]

It's hard to say without understanding how your system works and therefore what happens in a blocked outlet case.

In most fuel drums I've seen a variety of streams flow to it from various sources under usually pressure or flow control. From the fuel drum, the mixed gases flow to the various users. Natural gas is usually the final variable to control fuel gas pressure with a vent valve to flare if the pressure gets too high.

Given this, the temperature doesn't usually vary much if I was to envision a blocked outlet. The incoming streams would (typically) be assumed to continue at the same flow rate even if the controllers would take some action because typically, you do not take credit for instrumentation (however, if an instrumentation should take action that would make the relief case worse, I assume it does respond and include that in the required relief flow).

Now, how this compares to your system and what is the overall control system is something you need to consider. Temperature isn't a huge factor in sizing a relief valve.

I'd size the relief valve and then based on the difference in areas between the required area you calculate and the next available PSV orifice size, I'd see just how critical the temperature and other assumed properties are (MW for example, flow, etc) for being satisfied you have sufficient relief capacity.

 

[Homayun]

Sjakson,

Normally when sizing a PSV for a blocked outlet case, one would use the values from the Heat and Material balance. So that's basically how you would set your relieving temperature.

Having said this, the majority of the PSV's on Fuel Gas drums I have seen are sized for only fire case! If there is a source of pressure upstream the drum that can over-pressure your drum, you should set the design pressure of your drum equal to maximum expected pressure caused by the upstream source, which might be in this case a compressor or a grid where gas is being imported. It is therefore worthwile to:
1) check whether there is a PSV upstream your Drum and check the settings. If the same setting, then blocked outlet case is not applicable.

2) if no PSV exists upstream your drum, consider increasing the drum design pressure to 25 Barg and get rid of blocked out let case.

hope this was of any help.