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

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.

Thanks for the input!

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.

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.