5 replies to this topic

[J_Leo]

Dear Fellow Engineers,

 

I used to use the rated flow rate of the controlling relief scenario for PSV inlet and outlet line sizing. Now I was told to size the line based on the rated rates of all cases. 

 

For example, I have a blocked outlet case which need an Orifice size M and a liquid overfilling case which needs an orifice size of D. However, I need to size the lines based on the rated capacity of Orifice Size M for liquid relief as well. Based on my calculations, the inlet line size is governed by the liquid relief.

 

Would you please share your practice for the PSV line sizing? 

 

Thank you.

Best regards,

Leo

[Saml]

Leo, can you provide a little more detail?

It is difficult to imagine what equipment are you working with. It is a separator drum?

I guess you have gases going out, and if you block the outlet you have the first case.

Now, if you overfill the vessel so that liquid reachs the PSV,  what is the scenario?  two phase flow to the next equipment?, an outlet valve closing and a two phase relief scenario? Gas feed stopped and only liquid relief?

[J_Leo]

Saml,

 

Thank you for your response. My question is just the general practice and it is not really related to details. If you have two relief cases, of course, one of them is the controlling relief scenario for orifice sizing. Do you need to consider the non-controlling scenario for inlet and outlet line sizing? Especially when the other case relief rate is much smaller than the controlling one.

Edited by J_Leo, 07 May 2017 - 10:41 PM.

[latexman]

Do your inlet and outlet calculations for the worst case, credible scenario for each phase (gas/vapor, liquid, two phase) that you have.

[Saml]

Now, you said that your liquid relief case requires a D oriffice. That means that the excess liquid that has to be relieved is limited. If a M valve opens what you will have is a burst of liquid for a short time thru that M oriffice and then a reclosing when the pressure falls below blowdown. If the liquid keeps going into the vessel you will have a valve cycling. 

 

How you valve will respond to liquid, how you piping and downstrem collection will behave during this event, etc,  are things that have to be considered. Because a case is non controlling for orifice size, it does not mean that is non controlling for some other part of the system.

[J_Leo]

Thank you, Laxeman and Saml for your inputs.