5 replies to this topic

[David A.]

Help please,

 

I need a better understanding of why an orifice plate is required on a manual blowdown line where you have a globe valve lined up with a ball valve. I appreciate your urgent response.

 

Dave

[Bobby Strain]

Someone wanted to limit the flow with both valves open.

 

Bobby

Edited by Bobby Strain, 04 May 2013 - 04:40 PM.

[Art Montemayor]

Dave:

 

The reason behind orifice plates being used on GAS blow down lines is that the gas stream is forced into a state of Sonic flow - and under Sonic flow, the gas can only flow at constant mass flow.  Therefore, by using the diameter of the orifice, you can fix the gas mass flow rate while it is in sonic flow conditions.  NOTE: this only applies to gas lines, not to liquid lines.

[David A.]

Thanks Art and Bobby for your responses.

 

But is there a possibility for potential overpressurization upstream piping connection to flare header when you manually blowdown a gas line operating at say 140 barg without the RO in place. Your responses have been great so far.

[Lai.CY]

Hello Dave,

 

When you say 'manually' blowdown, I assume that this is not a fire case. In the event of emergency, the shutdown valves closes. Now there are isolatable sections through out the system. You can depressurize the sections manually and slowly (not too fast because it'll have a high peak flow to the flare and might result in a very low temperature), The pressure will only go down regardless if the condensate starts to flash.

 

The RO is there to depressurize the pressure vessel in 15 minutes (depending on thickness) and some might want to consider 30 seconds delay. Anyway, say in 15 minutes you want to bring your pressure down to 6.9barg (as per API and I believe that 6.9barg is the maximum achievable backpressure) in the event of fire...

 

There's actually a procedure, to check if your pressure rupture due to fire, and you need to reduce the blowdown time and so on... But RO is preferred, because you do not need to throttle them and you can make sure that in the event of emergency depressurization, you vessels are not at high pressure.

 

Also, with those calculated flowrates, you can also know your flare, header, vessel sizes and so on!

[gatrongkieuky]

This issue in also mentioned in one of Company Standard in my current project: if any possibilites of exceeding the capapcity of vent/flare during manual depressuring exists, then RO may be installed or replaced the globe valve by a smaller size