Very basic PRV calcs

[UBbaja]

After going through many threads and from my previous knowledge I have a good idea of what is going on, but I would like some concrete calculations to back me up. I am designing a very simple pressure relief valve. It consists of a spring, a disc, and a nozzle. The spring and disc experience atmospheric backpressure here, which is 14psia.

I found that the valve opens up 23psig . If I operate the set up at a high altitude, thus lower atmospheric pressure, 3psia for example, will the valve now open at 14psig? Perhaps I am over simplifying? What would the calcs be?

 

[insult2injury]

The forces on the disc need to be balanced. The atmospheric pressure plus the spring force (@ the correct opening height) must equal force acting from the system pressure/relieving flow.

I2I

 

[UBbaja]

Thanks for the feedback insult. Last night while brushing my teeth I suddenly realized what was going on, I am always amazed at the simple little things that are easily overlooked. Sometimes its best to go back to the basics and go over that stuff.

 

[israelkk]

UBbaja

As you wrote in your post "..the atmospheric backpressure here, which is 14 psia.. (needs to be 14.7 psia instead) where psia is "absolute" presssure. For the valve you used "...I found that the valve opens up 23 psig.." psig is "gaged pressure", therefore, the absolute pressure inside the valve was 23+14.7 = 37.7 psia.

In high altitude the gage pressure will still be 23 psig which is determined by the spring load therefore, the absolute pressure inside the valve to open it in high amplitude will be 23+3=26 psia

 

[insult2injury]

Atmospheric pressure is only 14.7 psia at sea level...at a higher elevation, 14 psia may be the correct value.

I2I

 

[israelkk]

Sorry for the typo error it has to be "high altitude" instead of "high amplitude"

 

[JoeWong88]

UBbaja,

I think the following posts may help you :


i) How Back Pressure Affect Conventional PSV Set Pressure Subject to It Vent

ii) Back Pressure Affect Conventional PSV Set Pressure : Case Study #1 - Bonnect Vent to ATM

iii) Back Pressure Affect Conventional PSV Set Pressure : Case Study #2 - Non-Bonnect Vent



JoeWong
Chemical & Process Technology

 

[UBbaja]

Thanks a lot for those links Joe. Insult you are correct I used 14psi originally more for making calcs easier, but also as you said, I am not at sea level. For here its ~14.3

Also, is pressure safety valve the correct term? I have always called them pressure relief valves. Is this to differentiate from something easily confused?

 

[april11]

I am working on a gas station for a power station. I want the testing procedure for a Pressure relief Valve at a set pressure of 2000psi

 

[skearse]

UBb-If using ISA standards (ISA S5.1), the "correct" designation is PSV-Pressure Safety Valve. Pressure relief valve is commonly used, as is simply safety valve (API & ASME terminology). The confusion with using pressure relief valve is that when abbreviated as PRV, that is also commonly used to mean pressure reducing valve (i.e., a regulator). Which, if using strict ISA standards, would be PCV-pressure control valve.

 

[UBbaja]

Thanks for the info skearse! That makes a lot of sense. From know on in my vocabulary I will refer to the the respective items correctly. I am all for global standards.

 

[na5rc]

A relief valve is, at its basis, a on-off orifice. If you are looking for a simple calc, I have worked up an orifice calc spreadsheet for gas that allows you to substitute a relief valve orfice designation. It should not be used for final relief valve sizing of course but it does have its uses.