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[thoriqmacto]

Dear all,

 

I'm newbie in PVSV (Pressure Vacuum Safety Valve). I want to ask about how to decide set pressure for PVSV on storage tank which already have a design from vendor. My PVSV is both pressure/vacuum combination vent type.

 

PVSV will locate on methanol injection storage tank which has 50 USGal capacity.

 

From PVSV vendor, they calculate with their software which given input (I don't understand from where they putted that value) for sizing data:

• Equivalent Volume Flow: 728 m3/h, which result to the relieving pressure: 66 mbarg
• Normal Volume flow for the vacuum side: 472 m3/h, which result to the relieving vacuum: 55 mbarg

I don't understand that value because the methanol injection pump has a design flow only: 10.7 USGPH (0.045 m3/h) which far from the value that PVSV vendor inputted to their software.

What is equivalent volume flow? how to calculate it?

 

As per API 2000, I justify that the cause of overpressure or vacuum primarily is "Liquid movement into or out of the tank". From that reason, I try to understand Section 4.3.2.1.1 which stated :

 

The requirement for venting capacity for maximum liquid movement out of a tank should be equivalent to
5.6 SCFH of air for each 42 US gallon barrel (0.94 Nm3/h of air for each cubic meter) per hour of maximum emptying rate for liquids of any flash point.

 

If we assume moving liquid is 10.7 USGPH it will require 1.42 SCFH (0.04021 m3/h) venting capacity minimum that require. It still far from vendor given value. Is my understanding wrong? Please advice if anyone have any idea regarding this. I'll appreciate any comments.

 

Thank you

[AlertO]

Dear thoriqmacto

 

From your doubt about the PV relief valve capacity, i have two presumption for giving you some idea.

 

1. Methanol is quite volatile compound. Vendor may includes some evaperated rate to the total breath out flow.

2. Typical PV valve (Direct acting type) normally has 100% overpressure, but for the tank design, there is no such big range as required. Therefore, vendor will use the big valve size to cope with the required flow at lower % overpressure. Then, as per the datasheet, you might see the large flow of the valve which is full capacity at the maximum overpressure.

 

However, i think if you have some question the vendor's datasheet. You can raise this question to them and let them answer you.

 

Hope this may help you.