Valve Recommendations to regulate Pressure Surges on Pneumatic Rail Car Offloading System

[admirza215]

Hey,

This might not exist but does anyone know about a possible valve that only allows liquid through and closes when gas enters? At our plant, we're using a pneumatic rail car offloading system where fluid is moved out of a rail car to storage with the use of an air pad. The issue with this is that at the end of the offloading process, air builds up and surges through the storage tank and into the scrubber. The scrubber can't handle the gas surge and releases fumes through the vent into the plant. Does anyone have any experience with a similar issue or have any suggestions on how to avoid this surge without replacing it with a pump system?

Thanks

 

[LittleInch]

We've talked about options before for a gravity feed but it's similar. Have a good read through these.

https://www.eng-tips.com/viewthread.cfm?qid=435582

https://www.eng-tips.com/viewthread.cfm?qid=444637

But getting gas blowby should be a design issue caught before operation.

This sounds like a poorly designed system to me or the pneumatic system was a retrofit?

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

 

[admirza215]

Thanks Inch, that's exactly what I'm looking for!
Documentation shows it was designed like this. The issue might've been overlooked.

 

[LittleInch]

Gas blow by is a well understood issue so fairly surprising if it wasn't considered.

Getting the last dregs out of tankers by pushing it is not easy as you have some form of two phase flow towards the end.

Or get a bigger scrubber.

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

 

[georgeverghese]

There are liquid drainers for this sort of service, which are typically used in air compressor wet air receivers and compressor intercoolers to dran out liquid and which autoclose when air flows through. But I dont think these vendors make them for low dp and high flow which is what you need here for this operation.

 

[Compositepro]

In the links that LittleInch provided there was no air pad involved and there were pumps used for unloading the cars.

In this case there are two approaches that would work.

One would be to use an excess-flow valve located near the discharge end of the pipe. This is similar to a spring loaded check-valve except that the spring keeps the valve piston open rather than closed. When the pipe fills with air the flow velocity of the liquid rises rapidly. When the velocity exceeds the set point the the liquid drag on the piston causes the piston to move and shut the valve. It will stay closed until pressure in the pipe is reduced significantly, to allow the spring to push the piston back. Often there is a small hole in the piston to allow the pressure on the piston to equalize and automatically reset after an upstream valve is closed. There may also be a manual reset lever so that it does not reset automatically. You would have to design for the water hammer that occurs when the valve closes. This type of valve is often required for safety on hoses that may break.

The second option is to install a float valve that works just like a steam trap. The float close the outlet when there is no liquid in it. You will need a vent line to allow air to go somewhere when you hook-up to a full tank car