Relieving temperature vs Design temperature

[sheiko]

Dear members,

I have a relief valve installed on a line (outlet from an heat exchanger) sized for fire case (this is not the only scenario) with:
- relieving pressure = 15 barg
- relieving temperature = 206 C

But, the piping class (CL-150) of the line (on wich is placd the SRV) indicates that the maximum temperature that the line can sustain at 15 barg is 170 C (also corresponds to the design temperature of the heat exchager). Also, the maximum pressure that the line can sustain at 250 C is 12 barg.

What shall i do, change the piping class? or can i consider that external fire is so unusual that the piping class is OK?


"We don't believe things because they are true, things are true because we believe them."

 

[YBrooman]

Sheiko,
What you have to have in mind is that in a fire case the PSV should opens first than the temperature goes high and depending on the flowrate in the outlet of the PSV the temperature of the fluid don't increase anymore.
And, in a fire case you should be worried about some other things more important than the piping class of the PSV.

 

[CJKruger]

sheiko, my view is that the design temperature does not have to be equal to or greater than the fire case.

In general, the design temperature should be equal to or greater than the relieving temperature, but fire case is an exception.

 

[sheiko]

Thanks,

YBroman: To me, if the fluid is pure and the vessel partially filled, saturated vapor will relieve at the boiling point even once the SRV opens (in my case 206ºC at 15 barg), so i don't understand what you mean.

CJKruger: If i undestand well, the piping class is ok if fire case is the only issue (higher relieving temperature than piping class design pressure)?


"We don't believe things because they are true, things are true because we believe them."

 

[YBrooman]

The PSV is attached to the flare system or to environment? In which pressure operates your vessel? It's below 12barg? If so, why don't you set your PSV to open at 12barg? If the temperature go high the pressure will increase to 12 barg then your PSV opens before the saturated vapor and start to eliminate some fluid. Then the pressure goes up to 15barg and the most portion of your fluid will be vaporized and flow through the PSV easily.

If your vessel operates at a pressure above 12barg or near from the supported by the pipe at 206ºC I don't have any idea then to change the Piping Class.

 

[MortenA]

We have been over this many times:

The fire relief case is not the basis for chosing the design temperature.

The fire relief case is a lastprotection against escalation - not meant to protect the equipment as such but to prevent rupture and further spreading of the fire - damage to people on site etc. This is also why the premitted overpressure is 21%

The "first defense" is deluge. The delge is not meant to stop the fire - but to prevent the temperature to go up! Should the deluge fail then the equipment is cosidered damaged beyound repair.

The design should of course be checked that it wont burst at design P+21% at 206ºC but the design codes normally has so many margins that it will be the case if you dont care about re-using the equipment.

Best regards

Morten

 

[sheiko]

Thanks all,

Does anybody have any reference on this (code? standards?)



"We don't believe things because they are true, things are true because we believe them."

 

[MortenA]

API 521 section 5.15.1.1

Read 4. påaragraph carefully.

Then also read 5.15.2.2.1 & 5.15.4.2 and specifically the table. The table seems to go against my arguments since it assigns a environmental factor of 1 (e.i. no effect). But that is for the sizing of the valve - and the valve is there for the failure of the deluge system.

Best regards

Morten