My questions relate to multiple relief scenarios on relief valves. I have attached Table 1 and Table 2 from API 520, Pt II below. In sizing relief devices we all most always have more than one scenario. Let us assume one scenario is liquid and one scenario is vapor, both need to be evaluated for capacity.
- Do I choose the trim according to which scenario will require the largest orifice or since multiple phases can go through the valve choose liquid? For the question, assume the vendor does not make dual certified trim.
2a. Case 1: Assuming we have an existing valve and the trim is vapor, according to Table 1, if the device is vapor certified and I need to evaluate the liquid capacity I should use non-certified equations – no issue, I believe I understand what to do. I would use the gas ASME Kd and ASME/API gas equations for gas sizing and the liquid ASME Kd and non-certified API liquid equation for liquid sizing.
2b. Case 2: Assuming we have an existing valve and the trim is liquid, according to Table 1, if the device is liquid certified and I need to evaluate the vapor capacity, API specifies to talk to the manufacturer for estimated capacity. Do I still use the gas ASME Kd and ASME/API gas equations for gas sizing and the liquid ASME Kd and ASME/API liquid sizing equation for liquid sizing?