8 replies to this topic

[sunil_chemical]

Hi Everybody,

My question is related to Operating condition if changed then what should we take care or what importatnt point we should hilight for Relief load calcution.
Perticularly for revamp or modiifcation Job.

Can anybody help me for this?

Thank you

[fallah]

Type of existing relief valve?
Specifying condition(s) (Temperature,Pressure,.....) to be changed? First value? change percent?

[sunil_chemical]

Thanks for the reply..

The existing relief valve is conventional relief valve ?

Scenario : Power failure, control valve failure, exernal fire, block outlet.

Previous operating condition :
Pressure : 540 psig
Temp : 120 F

Present condition :
Pressure : 545 psig
Temp : 87 psig

And I want general things i.e if operating condition is changed for fire case but if the composition is not changed then there is not much more effect on relief load.

Can you tell me like wise for other scenario?

If yes then It would be highly aAppreciate, bcs it is very useful for me,

Thank you.

[fallah]

Sorry,
Set pressure?
Fluid to be relived?
First relief temperature?

[sunil_chemical]

fallah,

Set pressure is 600 psig
Design Pressure 600psig
Design temp 200 F
Relief temp 87 F
Fluid : Khuff Gas:

Thanks

[fallah]

sunil_chemical

Going back to your first post, if we assume 545 psig (540 psig in previous operating condition) is maximum operating pressure, seems there is no considerable change in relief load calculation (relief load may be increased, a little, due to temperature reduction, 120F to 87F, leading to increase gas density).


What you should take care about,is checking for blowdown value of PSV in new condition.Maximum operating pressure is changed from 540/600 = 0.9 of set pressure to 545/600 = 0.908, means the blowdown of PSV should not be more than 0.091 of set pressure.

If the blowdown already has been 0.1 of set pressure,it would be decreased at least to around 0.09 by adjusting through blowdown ring.

Hope this help

[sunil_chemical]

fallah,

Thank you very much.

This is very good and valid point during the any modification or shutdown Job.

Once again, Thanks.

I have one more question regarding Total Power failure case.

Do you have any calculation for relief load for Total power failure case?

[fallah]

There is no specified criteria for value of blowdown in different PSVs.
Almost all ASME Sec. VIII spring loaded PSV have 10% blowdown,and therefore will reseat at 90% of the set pressure.Pilot PSVs can have lower blowdown usually around 3%.

The main function of blowdown is protection of PSV against chattering.Maximum operating pressure should be equal to 90% of PSV set pressure,or in some case lower.Usually,maximum operating pressure plus blowdown should not exceeded PSV set pressure.

[sunil_chemical]

Thanks fallah,

Spending time to give ans in very detail.

There is no specified criteria for value of blowdown in different PSVs.
Almost all ASME Sec. VIII spring loaded PSV have 10% blowdown,and therefore will reseat at 90% of the set pressure.Pilot PSVs can have lower blowdown usually around 3%.

The main function of blowdown is protection of PSV against chattering.Maximum operating pressure should be equal to 90% of PSV set pressure,or in some case lower.Usually,maximum operating pressure plus blowdown should not exceeded PSV set pressure.