I have a query related to Demethanizer Column overhead protection system. We have control valve, blowdown valve (BDV) and relief valve.
Kindly refer the sketch for better understanding.
My question is :
1) Licensor has specified the line sizes for control valve, relief valves and blowdown valve. While doing Preliminary FEED verification, I need to check the basis for all three valves. I can do the blowdown valve calculations but for control valve, I am unsure as what flowrate shall I consider (or how to calculate). There is no project specific guidelines. Random searches provide result as 30% of overhead but I need to understand the basis.
2) For Relief valve sizing (for blocked outlet case), can I take the credit of control valve opening ? I mean initially control valve will open then blowdown valve and if still pressure increases the relief valve will open.
3) If answer of point 2) is NO then can the flowrate from PSV should be equal to BDV + Control Valve flowrate ?
4) Staggered pressure set points are specified on Relief Valves. Also in the relief valve datasheet, I can see the flowrates are different (PSV-A is having less flowrate then PSV-BCD). As per my understanding, we can divide equally the overhead flowrate by 4 (if 4 valves are required, based on sizing). Please clarify my understanding.
Kindly provide the correct path forward. If more information is required, then I can share.