Which rated flow for DP calcs 3

[Justice100]

Hi,

I have just got some vendor quotes back and looking at reviewing the inlet/outlet line sizing based on the chosen vendor. The project is in Europe with vessels to EN 13445.

One vendor states an actual Kd and orifice area and then apply a de-rating factor of 0.9 to calculate the rated capacity (presumably for ASME code requirements). A different vendor states two capacities: 1."certified" flow and 2."max" flow, where the max flow does not apply the 0.9 factor.

My question is which to use for pressure drop purposes? This is probably a minor point as will only have an impact if the pressure drops are tight on the limits but just wondering what others thought.

 

[Latexman]

Use the 0.9 derated/ASME/certified flow. That's Code, with ASME.

Good Luck,
Latexman

 

[RVAmeche]

Not sure for EU codes, but ASME systems needs to evaluated at the installed device's capacity.

If you have a required flow of 50 lb/hr from your calculations and a relief valve rated for 200 lb/hr at that set pressure, the piping would need to be designed for 200 lb/hr.

This is to ensure the piping isn't inadvertently assumed to be "good" for the installed device's rating. You could have a situation where that overpressure scenario changes to requiring 75 lb/hr, and people may assume the system is still good since the existing device was 200 lb/hr.

 

[The Obturator]

You'll find that some vendors will advise on 2 capacities:
i. Rated Capacity - This is the rated capacity of the fluid at actual conditions (MW, relief temperature etc). It should be a calculation based on ASME actual orifice area and coefficient of discharge. The calculation will show a 0.90 derating factor.
ii. Certified Capacity - This is the capacity that the PRV has been certified to pass at the stated set pressure. It will be either an air, steam or water capacity as used when certifying the PRV - It is also a capacity that will be stamped on the PRV nameplate should it be required to be UV stamped.
For your purposes, use the rated capacity of the actual fluid.

*** Per ISO-4126, the generic term 'Safety Valve' is used regardless of application or design ***

*** 'Pressure-relief Valve' is the equivalent ASME/API term ***

 

[Justice100]

Ok thanks all.

 

[don1980]

Since this is for a European facility, I would think that you're using an EU harmonized standard (e.g. ISO 4126) for the relief design. Compliance with PED is easier when using harmonized standards. However, one can also use a non-harmonized standard such as ASME Sec VIII - it just requires a little more paperwork for the Notified Body.

For inlet/outlet pressure loss calculations, it's advantageous to use ASME Sec VIII, rather than ISO 4126, because ASME uses of the rated capacity (flow capacity result, after applying the 0.9 multiplier) for calculating pressure drop. ISO 4126 requires the use of the actual flow capacity (flow value before the 0.9 factor is applied) for calculating inlet/outlet pressure loss. And that makes a pretty significant different in the calculated pressure drop, because friction loss increases square to the flowrate (i.e. if flow increases 2X, the pressure drop increases 4X).

 

[Justice100]

Thanks don1980 - your PSV knowledge is extensive!

For anyone interested in 4126 part 3 there is the following applicable to safety valves in combination with bursting discs.

"Calculate the total inlet pressure drop (difference of stagnation pressures, i.e. non-recoverable losses) using the actual flowing capacity, which is the capacity of the safety device calculated using the certified coefficient of discharge, divided by the derating factor 0,9 and multiplied by the certified combination discharge capacity factor Fd or a value of Fd = 1,0 if a certified value is not available"