4 replies to this topic

[vikramltv]

Dear all
I want to know that while performing flare load calculation only rated flows of different PSV's for the respective governing case will be added up or relief scenarios that are not governing for PSV design but will effect the KOD sizing should also needs to be considered ?

For example:
selection criteria for PSV-1 was fire case :

Case-1 - Fire case, required reliving area=2 sqr inch, rated flow = 5000 lb/hr of gas
Case-2 - Blocked discharge (liquid relief), required reliving area = 1.5 in2, rated flow = 200 gpm of liquid

&
Selection criteria for PSV-2 was blocked discharge liquid relief. as:
Case-1 fire case, required relief area = .5 in2, required flow = 2000 lb/hr
Case-2 - Blocked discharge (liquid relief), RRA = 1in2, RF = 2000 gpm of liquid

Now my Q is whether flare load is 5000 lb/hr of gas and 2000 gpm of liquid (considering only govrning cases)
or it should be 7000 lb/hr (i.e. 5000 +2000) of gas and 2200 gpm (i.e. 200 + 2000) of liquid.

or i have to look for the simultaneous relief scenarios ?
Kindly suggest.

[fallah]

Dear all
I want to know that while performing flare load calculation only rated flows of different PSV's for the respective governing case will be added up or relief scenarios that are not governing for PSV design but will effect the KOD sizing should also needs to be considered ?

For example:
selection criteria for PSV-1 was fire case :

Case-1 - Fire case, required reliving area=2 sqr inch, rated flow = 5000 lb/hr of gas
Case-2 - Blocked discharge (liquid relief), required reliving area = 1.5 in2, rated flow = 200 gpm of liquid

&
Selection criteria for PSV-2 was blocked discharge liquid relief. as:
Case-1 fire case, required relief area = .5 in2, required flow = 2000 lb/hr
Case-2 - Blocked discharge (liquid relief), RRA = 1in2, RF = 2000 gpm of liquid

Now my Q is whether flare load is 5000 lb/hr of gas and 2000 gpm of liquid (considering only govrning cases)
or it should be 7000 lb/hr (i.e. 5000 +2000) of gas and 2200 gpm (i.e. 200 + 2000) of liquid.

or i have to look for the simultaneous relief scenarios ?
Kindly suggest.

vikramltv,

In general, the largest flare loads from multiple relief valves result from general failures such as fire, power failure or cooling water loss.

Obviously, in each scenario relevant PSVs would relieve based on that scenario regardless of whether it is governing case for each of them or not.

Hence, to obtain flare load for each failure you should sum up relief loads of those PSVs would simultaneously be opened due to such failure.

Fallah

Edited by fallah, 26 November 2011 - 07:46 AM.

[vikramltv]

Thank u sir..
one more thing I want to know if we can route an liquid relief PSV discharge directly to close drain system instead of sending it to flare header..and if it is done then what will happen in fire case when the same PSV will relief gases/vapor? If it is a common industry practice or violation of some standard?

[jrtailor09]

Dear vikram,

You are considering relief valve for two cases. One is liquid discharge & other is gaseous discharge.
Common KOD is used to prevent liquid carry over in flare system. The liquid collected in KOD is finally discharge to closed drain system or slope header. You should connect the PSV discharge to flare header only not Closed drain system. It is not safe to divert the gas to closed drain system.

Regards,

[vikramltv]

Noted...