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N2 Purge Velocity

N2 Purge Velocity

N2 Purge Velocity


This is my first post to the forum so please excuse if this thread is in the wrong spot.  
I am currently in a summer internship as a process engineer.  One of my projects is to install N2 purge rotameters on Cl2 PSV piping.

Here is some additonal info on the process.  For our process Cl2 needs to be in a vapor form, so we have two chlorine vaporizers with the same relief design.  Cl2 is very aggressive and we continuously have to send PSV's & PSE's out for PM.  Each vaporizor has two PSV's, one that relieves to an emergency vent header line and the other relievs to ATM.  The reason we have two PSV is when one is out for PM we still have one and service.  Only one is in service at a time, normally the one that relieves to the EVS. We continously have issues when it rains and when the one to ATM is in service; water will back in and sit on top of the PSE. We have a small run of bypass piping with a FO to prevent pressure from building between the PSV and PSE causing an insufficient dP across the PSE (vessel could exceed MAWP) and also the PSE could fail inward causing material to enter the vaporizer(NOT GOOD!).  If there is any small leak of Cl2, the mix with it and water is very aggresive and corrode the PSE.  Another problem is when the one to the vent header is in service; if the vent header gets backed up then whatever is in there will back in and sit on our PSE, again causing corrosion.  

Ok, so finally my quesiton.  I am putting N2 purge rotameters on the vent stacks to prevent moisture from sitting on the PSE supplying it with a slight N2 sweep.  What should be my N2 purge velocity?

Also I am putting a similar N2 purge on the vent header to sweep the material away, preventing it from sitting on the PSE as mentioned before.  Same purge velocity??

I took a similar application in the plant where there is a N2 purge on a vent header.  With the flow set at 6CFM, I calculated a velocity of only .2 ft/sec or ~12 ft/min.  That seemed pretty low to me.  

Any thoughts on this matter is much appreciated.

Thanks in advance,

RE: N2 Purge Velocity

I worked in a vinyl chloride plant many years ago. As I remember this problem was addressed by placing a rupture disc downstream of the relief valve. I can't recall if the disc was rated lower than the relief pressure or not. There was a pressure gauge before the disc to check for valve leakage.  

RE: N2 Purge Velocity

most people use rupture disks under PSV's in that service.

RE: N2 Purge Velocity


We do have rupture disks under the PSV's, but we have bypass piping that stays car sealed open.  That bypass piping allows moisture/corrosive material to back in and sit right on top the rupture disk.

RE: N2 Purge Velocity

If you are going to continuously purge, just be sure the flow guarantees you have turbulent flow in your largest pipe, say NRe > 10,000.  See how that compares to your 6 CFM.

Good luck,

RE: N2 Purge Velocity

i am surprised that you are having trouble with the disc on the rainwater side.  any metallurgy able to withstand chlorine should hold up to water.

but maybe do not let the water there in the first place with a raincap such as these http://www.weathercap.com/

if you are trying to purge just the space between the PSD and the PSV, that should only be ~1/2" piping and the nitrogen should be in the CFH range and not CFM.

do you have a venting excess flow device between the PSD and PSV?  http://forums.fike.com/dslookup.php?datasheet=r.1.16.01   

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