PVSV design

[ChemenggEIT]

I have read many threats about low pressure tank; I just want to confirm “rule of thumb” for designing vents. I have worked on vent six month back, but never decided set point, it was just revalidation, but just for information I am posting this………plz let me know if I m right…….
1. Set pressure on PVSV= ½ of design pressure
2. Set vacuum on PVSV= ¼ of design vacuum
3. API-620 tanks= one can go 10% over the design pressure (I mean you can overpressure vessel based on scenario)
4. API-650= can’t go above design pressure
5. Best way to design a new PVSV is combination of calculation, based on overpressure and size of vent.

 

[pleckner]

Your points #1 & #2 have no basis of fact. It was probably the company guidelines you were following. Point #3 is correct and you can add 20% due to a fire. Point #4 is correct.

Point #5: First you need to consult with API 2000. Then you need to take into consideration the vessel MAWP and vacuum rating. Then you need to take into consideration the mechanical device you will use as each manufacturer has their own criteria for vent rate-vs-size-vs-pressure. In general, the pressure side of low pressure relief deivces won't achieve full device capacity unless you reach 100% overpressure. The good news is that often you don't need to reach 100% full device capacity, only enough to meet your maximum required relieving rate. This will happen at some overpressure specific to that device. For vacuum, there will be some pressure drop requirement between the external source and inside the vessel at the time of maximum required inlet flow.