4 replies to this topic

[mortino]

Dear all,

A vessel is a former reactor effluent separator from a dismantled hydro cracker and has a mechanical design pressure of 180 barg. The vessel will be reused as a storage drum for water in a revamp of another plant. It has to be protected against overpressure during external fire, while blocked in. The pressure relief valve load is calculated per API Recommended Practice 521 and the orifice is sized per API Recommended Practice 520, using a pressure accumulation of 20%. It appears, that a 1E2 relief valve is just adequate. Unfortunately, in the construction stage of the revamp a 1D2 relief valve was installed. This error remained unnoticed, because both the inlet and outlet nozzle of the valves are of the same size. The Questionis; Will the vessel still be protected against overpressure during external fire and why?

[pleckner]

This raises all sorts of red flags.

One thing you didn't mention and should is that the vessel needs to be re-certified since you are using it in a change of service. If it hasn't been re-certified then it needs to be (unless you are in Texas, which I believe may not have this requirement, but after some very serious recent accidents I hope that will change soon).

The allowable over pressure for a fire scenario is 21% of set pressure, not 20% but OK, if you didn't take full credit you're just cheating yourself.

No, you must never use a relief valve with an orifice size less than what is necessary as you won't get the required flow. This is as good as not having any relief protection at all. Change the valve (or orifice if you can)...period!

And one more thing, this sounds like it could be an academic question so I hope I didn't just answer your homework assignment.

[mortino]

Dear Phil ,

First of all I would like to appreciate for your answer but please take it easy and be not worried from where the question is, we are here to learn from each other and reduce risks of the our world.

You are the only person who can decide to answer or not,so if you like ,do please and never talk to people how you like and just answer the question technicaly.

[pleckner]

I was almost going to ignore your last comment but I think a couple of things need to be said.

The question was written as if it were by a native English speaking person but your very rude response is obviously from someone who is not. And this makes me question again whether this is just a homework problem that you stated verbatim. You are correct that it is my choice to answer or not and I chose to answer so we can all indeed learn from each other. But if you are a student, I only ask that you post in the Student Forum and refrain from asking us to do your homework for you. This is why the Student Forum on this website was created. If you are a practicing engineer in need of help, a simple, "No, I can see where this looks like it could be an academic question but this is a real world problem and thanks for taking the time to address it.". is all you need to respond with. Believe it or not many questions we get on these message boards are real world questions but sound like homework problems. You should not be offended if we question that.

[Nirav]

I have one query about Mortino's question.

You said that the drum was designed at 180 barg in it's original application. But you have not mentioned as what is the system design pressure of the new system wherein you are "re-using" it as storage of water.

Why i ask it because, 180 barg design pressure is of very large magnitude. What is normal operating pressure & temp of your "water storage" drum? And what is design pressure & design temperature of water storage system?

In any case, your smaller orifice needs to be replaced.
Becasue,
In your case, the rate of vapor generation due to fire will be higher than discharge. So, pressure will keep on rising above set pressure rather than going down !!