Location of Safety Valve on a Pressure vessel

[rollingstein]

Is there a specific location (say top, bottom etc.) that a safety relieving valve is put on a pressure vessel? Say there's a tall refining separation column (operating above atmospheric pressure), where would logic (or codes) indicate that a pressure relief valve ought to be ideally placed.

Or does the exact location not matter?

 

[AlephZero]

Well, common sense says you shouldn't put it in a place where somebody might be standing in front of it when it blows!

More seriously, you would have to consider the transient effects (e.g. thermal, mechanical, internal gas or fluid flows, chemical reactions, etc) when it blows. All those things could depend very much on its position.

This isn't my specialist subject, so I don't know about design codes - but I would be very surprised if there weren't any.

 

[rollingstein]

Well, common sense says you shouldn't put it in a place where somebody might be standing in front of it when it blows!

Of course! No, I sort of assumed that. (In fact a lot of them don't vent to the atmosphere at all but to a relief header)

Here's what motivated my question: In most situations I recall they were installed either on the top head of a vessel or somewhere close to the top of the vessel on a nozzle.

So I somehow assumed that as the standard location without thinking much about it.

And then just today I was reading the accident report about BP's 2004 Texas refinery explosion and there seem three relief valves all installed on a vapor line coming off the top of this distillation column and they seem 150 feet lower down from the top of the vessel.

That intrigued me: (a) The fact that relief was provided through an attached pipe and not on the body of the vessel itself. & (b) The vertical location was so much lower.

Yes, this if of course a specialized science, but I'm wondering what the reasoning might be...

 

[Bystander]

Yes, this if of course a specialized science, but I'm wondering what the reasoning might be...

Tough to even speculate without design, engineering, fabrication, and use histories of the vessel. You could be looking at recycle, feed to another part of the plant (no longer in use) modified as pressure relief, just about anything.

 

[Baluncore]

Presumably the safety valve will be operated by gas pressure. The gas will be at the top of the pressure vessel. Any liquid will be at the bottom. The valve should be above the level of any possible liquid in the pressure vessel. The liquid will not be compressible but the gas will be. Venting the gas may retain all the liquid while relieving the pressure. The total mass released to lower the pressure will be less if it is the gas component that is released.

Gas has lower viscosity than liquid. Gas will cool as it is wasted, so it may freeze up the safety valve. A liquid on the other hand may boil as it is released which can provide a hazard and a back pressure that chokes the valve due to increasing volume.