Rated Capacity of Fire Case Relief Valves in Aspen Flare System Analyser 2

[Sirius P.Eng.]

Here goes...

Having sized a fire case (Max. Accumulation = 1.21 x MAWP) relief valve using API 520 equations.

I selected a preliminary API 526 orifice size; in this case a "T".

I obtained the manufacturer's actual orifice area (which was larger than the API 526 orifice area) and the certified derated coefficient of discharge.

I calculated the certified/rated capacity of the relief valve using the relevant equation for any gas/vapour as specified in Mandatory Appendix 11 of ASME BPVC Section VIII Div. 1, at 10% overpressure [Pset x 1.10 + Patm].

I have developed a model of the flare system in Aspen Flare System Analyser to calculate backpressure, velocities, etc in the tailpipes and headers of the relief system.

Now my questions

1. Why is the ASME capacity calculation not carried out at 21% overpressure since it is a fire case?

2. Why does Aspen Flare System Analyser use the required reliving load in place of the actual/rated capacity of the relief valve for FIRE CASE RELIEF VALVES?

Thanks.

 

[MortenA]

1) Because you dont size for fire case when calculating the rated capacity?

2) Because that would lead to a gross oversizing of the flare system? I think you can force it to do this. Also, i think its only the lead pipe you need to size for rated capacity (to avoid chatter) - so it shouldnt impact your flare system

 

[The Obturator]

ASME VIII Certified Flow is based on 10 % overpressure - that is the default. 10% is used under laboratory conditions when manufacturers perform their capacity testing to establish a flow coefficient. Consequently, if a PRV is to be "NB/UV" Code Stamped, the rated capacity (air/steam/water) is based on 10%. This would also apply even if the PRV is be used for a fire case where 21% has been taken as the overpressure. The PRV has been certified only for 10%.

Appendiz 11 in ASME VIII, is generally showing how to use the formulae for different fluid compositions, using a standard minimum of 10% overpressure. There is no reason why you cannot use 1.21 in place of 1.10 for your calculation.

I cannot comment on the ASPEN program as I am not a user.

Per ISO-4126, only the term Safety Valve is used regardless of application or design.

 

[Sirius P.Eng.]

@MortenA

I understand your point of not grossly oversizing the flare network.

In a flare system rating study, Will you recommend forcing Aspen Flare System Analyser to use the rated flow or leave it to run on the required relief load.