Blocked Outlet on a Vessel: Liquid Relief

[Ojit]

Hi everyone.

For a blocked outlet on a production slug catcher I need to determine what my releif load through the PSV when discharge lines are blocked. Do i size the PSV based on vapour rate or based on liquid rate?

For a Blocked Outlet API states that (a) For Liquid, capacity is maximum pump-in rate. (b) For liquid-vapour system, capacity is total entering vapour plus any generated in vessel.

Since my system is a liquid-vapour system do I only consider vapour rate through the PSV?

Thanks for your time in advance

 

[Montemayor]

You only mention a blocked outlet.

What about the inlet? Does it continue to enter the Slug catcher as the latter relieves? I believe it does under the scenario you briefly describe.

If that is the case, then you will surely go into a 2-phase relief and a possible subsequent liquid relief as the vapor is evacuated through the relief device (assuming you place it on the vapor space). The times the vessel is subjected to either one of the 3 types of flow depends on the GOR (Gas to Oil Ratio). The worse case for designing the relief device will probably be the 2-phase mixture.

 

[Ojit]

Thanks for the response.

There is a control valve on the outlet vapour line and an ESD valve both on the outlet liquid line and on the feed line.

I was guessing the feed ESD valve will trip after the ESD valve on the outlet liquid line trips.

What do you think?

 

[Montemayor]

Ojit:

Any relief case is a serious situation. Please be explicit and detailed in describing your scenario. You've originally offered a simplistic, general case of a blocked outlet. Now, the story continues on with another slant and more important basic data. Is there moe to come?

Why do you insist on drawing out what is a presumably serious case into a long, debated and speculated discussion when we could be of better help were we to have ALL the basic data? If you are indeed a professional engineer, you can surely appreciate the concern here for being detailed, accurate, and precise in your communications.

Await the rest of the story.

 

[Ojit]

Apologies for your time.

The dimensions and conditions of one of the Production Slug catcher (3x50%) are as follows:

Vessel: Horinzontal
Size: 4900mm ID x 19600mm T/T
Design Press: 3500 kPag
PSV Set Press: 3500 kPag
HHLL: 4000mm
Discharge Location: Hp Flare Header

As explained earlier the vessel has a failed closed control valve on the vapour discharge line (this control valve is linked to a failed open control valve on an upstream line that bypasses the PSV joining the Hp flare line) and a failed closed ESD valve on the liquid discharge line. The feed line from an inlet manifold also has a failed closed ESD valve.

We have been asked to size the PSV for blocked outlet with the relieving line (upstream of the PSV) mounted on top of the vessel specified as a produced gas service line.

I need to know what will be the worst sizing case for the relieve load through the PSV.

I hope the above clarify things as English is not my first language.

Thanks again for your help

 

[Skoogen]

O.k.

The things I am not sure is there is two outlets here.. Gas and liquid. I hope this is correct.

Firstly, if both outlets are blocked then you will have to calculate the liquid releiving capacity through the PSV.

This is done by looking at API 521 Appendix twelve. It is quite straightforward. Please ask if you have any questions.

Consider a mixed flow through your PSV. Not just vapour.

Kind regards

Alex Chatwin