The derated PSV capacity using the certified combination factor is greater than my required capacity so I'm okay there. My question is with respect to the inlet 3% line loss guideline per API 520. The conservative approach is to calculate the inlet line losses using the PSV's capacity flow rate (without the rupture disc rerate factor) and include the effect of the rupture disc in the piping losses (for this disc, the vendor has certified resistance coefficient) as just another component. If I'm below 3%, I'm okay. Alternatively, I was thinking whether the rupture disc/PSV should be viewed as a combined device given the derating factor and the inlet line losses are calculated using the PSV's derated capacity and without the rupture disc being considered as a piping component. Sometime back I downloaded two good papers from Fike but unable to locate those from their website now. One of the papers deals with combinations of rupture disc and safety valve. Let me know if you are interested to have a look at them.

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