7 replies to this topic

[abir]

Hi

 

In our project we are dealing with 3phase PSV. PSV outlet is routed to atmosphere without any flare. PSV is conventional type. PSV set pressure is 100 barg. 

 

We have sized the PSV based on rated flow and 18" outlet size is calculated with 0.9 mach and back pressure of 2.5 barg.

 

Client is asking to reduce the outlet size by putting a reducer or restriction orifice at the end of the pipe. By that they are looking to choke flow at the outlet of piping and creating more back pressure at the PSV outlet. Client's argument is for flare tip you will get 1.0 mach. Comparing with flare they want to design our system with 1.0 mach at the outlet.

 

My question,

1. Is this acceptable to put a reducer or orifice at the end of open pipe. I could not find any direction in API which says it is not acceptable.

2. Will this work for 3 phase flow? I have never this sort of arrangement in tail pipe.

 

regards

ak

[Bobby Strain]

Discharging liquid to atmosphere? Something seems wrong with this.

 

Bobby

[paulhorth]

Abir,

 

Bobby is right - but I will add something rather important.

The purpose of a flare is to dispose of flammable hydrocarbons SAFELY, without exposing everyone in the vicinity to fire and/or explosion. If you don't have a flare, what do you think will happen when that huge PSV opens, discharging through 18 inches at Mach 1?  A flare tip will not burn at Mach 1 unless it is very carefully designed.

 

YOU are responsible for designing a safe system which does not expose the workforce and the public to risk (above a very low threshold).

 

Never mind about the reducer and the API code - if your client wants to do this, find another client.

 

Paul

[flarenuf]

i totally agree with Paul here. This relief should be routed to a safe disposal through a flare stack/ground flare or horizontal burner, properly designed.

 

The days of just venting HC reliefs into the air are hopefully behind us

 

flarenuf

[shan]

Sonic flare tip is to obtain high jet velocity (Mach = 1) to reduce grid radiation level.  What is the benefit of unfired reducer at the top flare stack?  Liquid droplets at sonic velocity like bullets to penetrate your pipe cap or pipe elbow and form a large area HC/water rain around when the liquid droplets fall down back to the ground eventually.   

[fallah]

IMO, before any judgment about OP query and submit a response some vague points should be clarified by him as follows:

 

-If the relieving fluid is flammable HC? It's not specified in OP query...It might the fluid to be other than HC especially because it is routed to the atmosphere...

 

-OP mean when refers to three phase isn't clear...if the solid phase is accompanying suspended solid?...or two dissimilar liquids accompanied by a gas?...

 

Bottom line is that, it might OP refers to sonic flare for using its similarity with a short tail pipe only in sonic velocity standpoint...

Edited by fallah, 27 October 2014 - 02:32 PM.

[abir]

Thanks everybody for chipping in. We have now sorted the issue with client. To clarify the forum following is project outline.

 

This is an onshore field development project which comprises of 300 wells in the desert. As you can understand there is no individual flare for each well head. Well head outlet pipe till choke valve is ±10000 rating and after that ±600 rating which includes gathering headers. PSV is provided for each well head to protect ±600 system.

 

1. This is HC system. Also CGR &WGR is low. So no rainout is occurring at the PSV outlet. 

2. No flare only open pipe to atmosphere as individual flare (300 nos) is not cost effective.

 

It has been widely discussed about the open to atmosphere but client is ok with that. So no more further discussion on that.

 

Now we have sorted this mach no issue with client. They have agreed that mach 1.0 will not be applied to the tailpipe.

 

regards

ak

[paulhorth]

Abir,

 

 

It has been widely discussed about the open to atmosphere but client is ok with that. So no more further discussion on that

Here is some further discussion for you to consider.

The relief to atmosphere could be acceptable if this is a genuinely remote area, no roads or buildings in the vicinity .

 

The maintenance crew should be able to shut the well wing valve remotely before approaching within  a pre-defined sterile radius, this procedure needs to be established and enforced.

Have you considered what do do if the relief valve opens? How do you shut off the well? I expect you have a hydraulic package to actuate the upper master valve, actuated by some kind of cable signal, but both the hydraulic unit and the cable need to be protected from the fire that could easily result from a relief event, so that they can continue to function. This could be achieved by putting the relief vent at sufficient distance from the wellhead.

 

So, two reasons to establish a safe distance between the wellhead and the vent.

 It is for you to determine what distance that should be.

 

Paul