Queries
- What are K, Kd [discharge coefficient?], Kw, Kb [back pressure coefficient?] and Kc and where should I use them?
- It seems that K Gas is 90% of Kd Gas and K Liquid is 90% of Kd Liquid. Why so?
- Why rated capacity is 90% of actual capacity?
- For inlet and outlet pressure drop calculations, which flowrate do I need to consider i.e. is it rated or actual capacity? I believe, I need to consider rated capacity [9980.002kg/hr] ?
- I am calculating maximum two phase flow capacity for the required relief valve using Leung’s method using formula G = CD * hfg / ( vfg * SQRT ( Cf To)). What discharge coefficient, CD [here it might be either Kd Gas or Kd Liquid?] should I use?
Before reading datasheet, you should read API-520 and API-521 carefully to understand the terminologies involved in relief system analysis and their significance. Here are the short answer to your question, however, I would suggest you to read API and very nice handbook uploaded by Breizh in his above post:
1. K, Kd are the discharge coefficients and those values are different for different phases (Gas/vapor, liquid and two phase). Preliminary values of Kd for sizing per API are 0.975 (for Gas/vapors), 0.62 (liquid non certified valves) and 0.65 (for liquid certified valves), 0.85 (for two phase mixture).
Kw and Kb are the back pressure coefficients, the significance of this factor is consider the effect of back pressure on the capacity relief device.
Kc is the combination capacity factor, which generally 1.0, however, if you have rupture disc at the inlet of relief valve it should be accounted based on resistance coefficient of the rupture disc (based on the type).
2. and 3. Manufacturer should be consulted for those.
4. General approach is to calculate the pressure drop on:
- Actual capacity for fluid with Gases/vapors/two phase at the inlet of relief device. It is actually due to pop-up action which tend valve to open fully irrespective of required capacity.
- Required capacity for fluid with liquid phase at the inlet of relief device. This is due to linear/proportional opening characteristic of with respect to required capacity.
5. Why Leung's method? Why you are not using universally accepted HEM method suggested by (DIERS).
Hope this helps you.