Restriction Orifice in Condensate Line

[sshep]

Hello,

Last year a condensate line failed just downstream of a restriction orifice in a 12" line going to a 350kPa flash drum. A field check revealed the orifice tab stamped as 40.75mm, but pictures from the failure showed it to be a three hole design with considerable downstream erosion. Maintenance just raised a concern that the line could fail again after less than 2 years service. We may have a chance to remove the orifice which is what I would like to do.

In the history of this line a 4" bypass around the orifice was installed to alleviate high back pressure- currently closed. The line serves the returns from reboilers and traps for a plant designed originally for 55,000kg/hr of mixed phase condensate, but now operating at higher rates. This is one of those "why is it there" questions where there is no documentation. I have never seen a restriction orifice put in a condensate line like this- it seems risky because of the need to size for poorly estimated values of 2phase flow. I would like to remove it, but hesitate to do so without understanding why it is there.

Does anyone have any ideas or experience of why someone would put a restriction orifice in such a major condensate line?

best wishes,
sshep

 

[katmar]

Good Morning Sshep, I agree that the place to start is "why is it there" and until this has been resolved it is a bit of a shot in the dark to propose any remedy. I believe the most likely reason for its original installation would be to limit flashing and two phase flow in the line, and therefore to reduce the line diameter.

I have seen a similar installation where the orifice was installed right at the flash drum to avoid flashing in the line, but the high velocities generated by the flash caused severe noise and vibration where the condensate impinged on the opposite side of the drum (which was a bit small for the duty).

If you have only 5% flashing through the orifice, you could get to velocities of about 30 m/s in a 12" pipe. It will probably be necessary to increase your line size significantly at the orifice to keep the two phase flow velocity below the erosion limit. Or install the orifice right at the drum if the drum is large enough to prevent the impingement problems I mentioned above.

Katmar Software
Engineering & Risk Analysis Software

http://katmarsoftware.com

 

[25362]

This article might provide some clues:

http://www.cuservices.net/NPCS120.pdf

 

[SeanB]

Without more information I too would have to guess that the RO was there to hold back pressure in the line and prevent flashing. However, the condensate will definitely flash across the RO. If possible, why don't you take a pressure survey upsream and downstream of the RO and then determine how much flashing you are actually getting and then take a look at the velocity in the line.