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[pye_cint]

Hye there,

I have one case of flare purging using production gas at an unmanned drilling production platform; tapping off the gas production line (which is multiphase flow line - 900# pressure rating) and supplying the gas to purge the flare system (to avoid air ingress to the flare system). Hence, we're using stand pipe to settled out possible liquid dropout from the gas phase before purging. We're proposing RO to control the purge rate and also a throttle valve (upstream of the RO) to let down the pressure therefore can cater the lower pressure rating (150#) for the RO.

I've seen this design in several projects but I couldn't manage to find what is the basic of sizing the standpipe to ensure that there is no liquid entrainment to the purge gas line.

Pls advise.

[daryon]

Hi there,
You can't really expect to separate out liquid droplets carried in the gas phase using a standpipe. To disengage liquid droplets you need to slow the gas down, the slower the gas the smaller the droplet diameter that will separate out in given distance. You need a two phase gravity separator to achieve this. I think the purpose of the stand pipe installed at the low point in the piping is to collect any liquid forming in the header to prevent slugs of liquid being push up the flare stack.

To size the stand pipe you'll need to estimate how much liquid will condense or be carried through into the flare piping and determine the residence time you need. I presume you will be installing a level switch or transmitter that will be monitored at a manned facility close by. If the stand pipe fills up you need sufficient time to allow someone to come and drain it, and you also don't want it to fill up to quickly