gjmorin
Mechanical
- Aug 18, 2011
- 5
I have a number of pressure and level connections on a vessel that has a lot of particles and gaseous products that could condense at low temperatures into a gummy mess (technical term)
We therefore purge the taps to keep a little reverse flow on the taps to keep them from being plugged. No problem there for the standard process,, whihc is tolerant of the little bit of O2 that comes in with the purges.
We take a tap off the air header, run it through a minimum diameter orifice (2mm) and past the pressure connection into the vessel. My process is 8 psig, the air header pressure upstream of the orifice is 90 psig or so, guaranteeing choked flow and a more or less constant flow.
The problem is that I'm trying to minimize the flow of O2 into the system for a new process.
The first fix is obvious. I'm putting a PRV on the header such that the pressure upstream of the orifice is say 40 psig (8 psig = 23 psia. You'd need ~46 psia plus a little margin to ensure choked flow & therefore flow independant of process pressure,, so i arrive at 40 psig)
The problem is that I'm still leaking 20 lb/hr of air into my process through each of these taps.
I can't really go down in orifice size, i'd like to keep choked flow, and ganging the taps up downstream of the orifice would result in not knowing that each tap has positive flow. I could switch the taps to inert gas, but there's a cost to that.
Has anyone run across this kind of issue?
Thanks
Greg
We therefore purge the taps to keep a little reverse flow on the taps to keep them from being plugged. No problem there for the standard process,, whihc is tolerant of the little bit of O2 that comes in with the purges.
We take a tap off the air header, run it through a minimum diameter orifice (2mm) and past the pressure connection into the vessel. My process is 8 psig, the air header pressure upstream of the orifice is 90 psig or so, guaranteeing choked flow and a more or less constant flow.
The problem is that I'm trying to minimize the flow of O2 into the system for a new process.
The first fix is obvious. I'm putting a PRV on the header such that the pressure upstream of the orifice is say 40 psig (8 psig = 23 psia. You'd need ~46 psia plus a little margin to ensure choked flow & therefore flow independant of process pressure,, so i arrive at 40 psig)
The problem is that I'm still leaking 20 lb/hr of air into my process through each of these taps.
I can't really go down in orifice size, i'd like to keep choked flow, and ganging the taps up downstream of the orifice would result in not knowing that each tap has positive flow. I could switch the taps to inert gas, but there's a cost to that.
Has anyone run across this kind of issue?
Thanks
Greg
