3 replies to this topic

[Fr3dd]

Hello Everybody,

 

I have a question related to Restriction orifices and choked flow. I've been searching in the forum and a lot of discussion related to this matter has been going on; however, I was not able to find an answer for my question. I would love to have your insights about it.

 

I have gas line discharging to LP Flare, in case that something goes wrong upstream, there will be a stream of really high pressuregoing through this line. That's the reason why a Restriction Orifice is required. The pressure upstream is expected to be around 320 bara and downstream (LP flare) around 1.5 bara.

 

Obviously, the device needs to be sized for choked flow conditions; after reading API 520, I concluded that the orifice needed to be sized for critical flow. Looking at the equations it can be seen that the estimation of the orifice area is only dependant on the upstream pressure (and fluid properties), temperature and required flowrate. However, what is the role of the downstream pressure?

 

In this thread: http://www.cheresour...iction-orifice/ this issue is discussed and, in post #2 it is mentioned:

 

"In the situations such as you mentioned above,chocked flow downstream of RO is a common occurance.
Size the RO for dP=9.6 bar and maximum flow of 4.4 MMSCFD."

 

This is exactly my concern, can the design of the RO affect the downstream pressure in choked flow conditions or are these conditions given by the downstream system (LP flare system backpressure)?. Is there any way to influence the downstream pressure with the design of the RO?.

 

Thanks in advance for any insight you can give me on this.

 

Regards,

[PingPong]

The purpose of a Restriction Orifice (RO) is to limit the FLOWRATE between a given upstream pressure and a given downstream pressure, not to reduce pressure, or whatever. It is just a stupid plate with a stupid hole in it.

 

If you do not know what FLOWRATE you want to achieve there is no point in further discussion.

[latexman]

However, what is the role of the downstream pressure?

 

This is exactly my concern, can the design of the RO affect the downstream pressure in choked flow conditions or are these conditions given by the downstream system (LP flare system backpressure)?. Is there any way to influence the downstream pressure with the design of the RO?.

 

With a small hole in the orifice, there will be less flow and less backpressure on the downstream side of the orifice.  With a large hole in the orifice, there will be more flow and more backpressure on the downstream side of the orifice.  So, yes, there is a way to influence the downstream pressure.

Edited by latexman, 15 January 2015 - 07:33 AM.

[breizh]

Hi ,

Consider reading this paper .

 

Hope this helps

 

Breizh