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# Vapor recirculation line

## Vapor recirculation line

(OP)
We are pumping MeOH from a railcar to a storage tank, and venting the vapors from the tank back to the rail car. I have run flow calculations from the tank to the rail car to determine the maximum vapor rate through the OH line - to not overpressure the tank. Then I back calculated the liquid flow rate that matches the maximum allowed vapor rate (then limiting the pump in rate). I am second guessing myself.. Since the vapor back to the RC matches the displacement of the MeOH being pumped out of the RC, should I not be limiting the pump out rate?

### RE: Vapor recirculation line

I think you want to limit the backpressure in the tank, so as to not lift/crack the vent unit with the lowest set pressure. Say, limit the backpressure to at least half the lowest set pressure vent unit. Tell us what those pressures are, because I’ve made some assumptions, but I doubt that will limit the pumps liquid flow appreciably.

Good Luck,
Latexman

### RE: Vapor recirculation line

Vapor recirc. line must be free draining, else local liquid pools within the line may invalidate pressure drop calcs. Use a generous roughness value for dp calc since carbon steel corrodes with methanol vapor. API650 tank PSV usually is set at 80% of tank upper design pressure - ask PSV manufacturer what the simmer / crack open pressure is. Say 80-90% of PSV simmer pressure would be the permissible upper limit of operating backpressure on the tank during RC pump out operations.

### RE: Vapor recirculation line

(OP)
Thanks for the input!

### RE: Vapor recirculation line

I don't fully understand your question or what yu are doing?

You say you're pumping methanol into the tank and getting a vapour return line back into the railcar. So far so good, though normally the pressure available for the vapour is very low as atmospheric tank design pressure is usually inches of water.

But then you mention pump out rate? From the storage tank?? Is this always going to happen or how does it fit into the process.

Also you need to be very carefully about the negative pressure in the rail car. They really don't like it and can collapse very easily.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

### RE: Vapor recirculation line

(OP)
Inch,
Thanks for the comments. We are pumping from the RC to the tank, and running the vapors back to the RC.

Regarding the vacuum problem you are correct - RCs don't like vacuum. These are vacuum protected.

Thanks.

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