Vent / Flare Stack Question

[meanderson3d]

I am designing a NGL processing facility and need some help with the vent system design. Since the facility is essentially liquid packed, purge gas is not available to keep a positive pressure on the vent system to prevent air ingress. The only time the vent system will see a flow is in the case of PSV release. I found this thread - thread124-219538 - which does have one idea that has possibility - detonation arrester. Does anyone have any other ideas to prevent air ingress into the system? Thanks.

 

[cheute79]

This is a bit out of my experience, but is the pressure at release enough to overcome a short water column pressure, even 2 to 6 inches... if so, design a p-trap or gooseneck into the vent, and keep water in the trap, with a sensor to indicate when it empties or dries out?

Another possiblity if the vent is small diameter, bring in a small compressor or gas cylinders to supply the positive pressure on the stack. Design the system as you would a blanket on a tank, and they overall use of gas is small. I did this last month for a sulfuric acid storage tank, and the blanket gas consumption (with a 6 inch vent) was small enough that the plant "only" uses a standard nitorgen cylinder every 5 to 7 days.

 

[ash9144]

You would still be able to use a nitrogen sweep on relief header. It is I assume connected to the outlet of your PSV. This header would not stay liquid filled. And with NGL in a relief scenario the liquid would vaporize across your RV.

 

[VikingUK]

So long as you have a flame arrester at the vent and it is positioned away from plant and any ignition source you will be OK as you are.

 

[JimCasey]

If you are venting liquid, the liquid or flashing liquid/vapor in the vent line will not ignite.

When the valve closes, the liquid remaining in the vent stack will evaporate/flash, displacing all the air and it will not ignite.

With the valve closed, there may be a brief time that a flammable mixture exists in the vent stack. If you keep the vent stack as short and straight as possible the highly improbable ignition will just produce a momentary flame and then be gone. It cannot propogate back through the valve.

I don't think you want a det arrestor or a flame arrestor on the line since the primary function of the line is to relieve liquid flows. The FA or DA are not designed for liq flows.

It is important to keep the stack short and straight. I was taught that a flame front in a pipe doubles in speed every 10 diameters or every elbow. What starts out as a harmless deflagration can turn into a destructive detonation when the flame front goes sonic, and that is caused by long or contorted piping.

 

[dcasto]

Please give us an idea of what kind od NGL plant is all liquid and not cryogenic when depressued or have two phase flow. in a vent header.