Horns on the PRVs? 2

[SamChem]

In a petrochemical plant I'm working at, there are a number of vessels topped with PRVs. Besides the main relief line, the top of the PRV also has a small elbow fitting, usually pointed downward, usually bigger at the exit than the connection to the body of the valve.

What is this piece for or why is it there? I do LDAR work, and we've occasional detected leaks from this fitting where the valve has either lifted or has not re-seated properly, but is that the intended purpose?

 

[Ashereng]

Are you looking at the bonnet vent?

"Do not worry about your problems with mathematics, I assure you mine are far greater."
Albert Einstein
Have you read FAQ731-376 to make the best use of Eng-Tips Forums?

 

[SamChem]

I believe so - I just wasn't sure it was still called a bonnet for a PRV, like a more common valve.

 

[Montemayor]

Sam:

If you are working in a Petrochemical plant as a chemical engineer, you should easy access to API RP 520, Part I. Look at and study page 5 where you find Figures 2, 3, 4, & 5. In fact, you should be reading all of API 520 - Parts I & II - as well as API 521. Return and describe what you have in detail and we'll be in better shape to answer any question(s) you have. You state these type of leaks are "occasional". Does this mean you have fugitive emissions? If so, you must be fixing them and in doing so, you must have first hand knowledge of exactly what type of PRV you have and every internal part that makes it up. Am I correct?

 

[SamChem]

I should have been more specific about my role, I suppose, Montemayor; I didn't realize there was a great deal of variation in the possible answers to my question. I am not a petrochemical engineer employed at a plant; I am a third-party firm hired to do LDAR only - except that just now my company is branching out to doing some PRV leak detection to try and prevent huge losses up the flare.

I do not, therefore, have access to the API 520 and 521, nor do I have a detailed knowledge of the internal parts. I have some very generalized knowledge of two or three different internal mechanics of operation for this general type of valve, but I am not experienced. The extremely basic nature of my original question reflects a desire to learn more, not some obscure and rude way of quizzing if I were already expert in the matter of PRV construction.

I just wondered what the opening/vent's possible purpose could be, given that these valves already have an outlet to the flare - and the chemicals within are not the kind one would usually want to vent to atmosphere. I thought perhaps it was a double-fallback, that if so much pressure were to arise as to blow the PRV not only to the release pipe, but also further, going to release to atmosphere as well.

 

[Montemayor]

Sam:

When we get a question on a leaky Pressure Relief Valve (PRV), my immediate instinct is to try to help stop this type of hazardous incident – ASAP. With your handle, I don’t believe I’ll receive any criticism for assuming that you are a Chemical Engineer by training. As it is, it turns out you are not, and since I am one (with 46 yrs experience) I can easily start out talking well over your head and we wouldn’t understand each other from that point on. I appreciate your candor and ask you to bear with me if my statements or questions seem kind of “stupid”. Our intent is to help, not to confuse. But because the nature of the beast is one of a technical engineering product, it inevitably turns out to be very technical and as such, tough to understand without the necessary background.

Regardless whether you’re an engineer or not, your concern is of importance since you clearly indicate that there are emissions involved and they are not being directed to the intended flare system. In fact, these emissions may be potentially hazardous. You are wise in seeking help and a solution to these abnormal occurrences – even though they may be “occasional”.

First, let me clearly define the basic operation of a PRV:

1) These devices are intended as pure safety devices to relieve any sudden or gradual buildup of internal process pressure due to process upsets, accidents, human error, equipment failure, etc. Their operation is to relieve excess pressure and once pressure relief is effected, the PRV re-seats back to the closed position – without any subsequent leakage through the seat (or the body) of the PRV.
2) If leakage occurs, then the PRV is clearly defective or operating in a defective manner;
3) Once a PRV starts to leak, it must be removed and repaired before being used again;

The best way to get a clear understanding and gather basic knowledge about the methodology of how a PRV works and how it is designed (although you may not be interested in the engineering design portion) is to carefully read and study the API Recommended Practices I noted above. From what you describe as your work function at the plant you are working at, I would caution you to not tolerate being in the vicinity of a leaky PRV. The chemicals being relieved may be toxic or flammable – or both. Since you are an outside contractor doing work within the plant, you would be wise to educate yourself as much as possible regarding the basic equipment involved in your work – especially PRVs.

I really wish you could furnish us with concise and accurate technical details so that we could be of definite help to you. As it appears to me, you may be able to seek local help from plant engineers who should be aware of the leaky PRVs and be able to explain the nature of the fluids involved and the safety situation. If we can be of further or additional help, please follow up this thread with additional, detailed information that you may gather in the field.

Good luck and stay safe.

 

[SamChem]

Montemayor:

I am a chemical engineer by training, but I would not criticize you for rendering me assistance and advice. I just got ahead of myself a bit. I've only put in about 18 years thus far, and most of that in semiconductor or other industries, thus my questions. I agree, it is a hazardous situation. These leaks are being dealt with immediately upon discovery, by the proper plant personnel. I have monitoring equipment and safety procedures to follow to make the work as safe as possible. I don't want to worry you unnecessarily. I just wanted to have a better idea of what I'm working around, both for safety's sake and curiosity alone.

I'll be going to the library to see if the reference desk can provide those API codes. Since I'm in the industrial/port corridor of Houston, I think that might be possible.

Thank you for the tip. I'll try to come back with more specifics once I've self-educated a bit more!

Sam

 

[jaxonjin]

Sam,
I guess that this little elbow fitting pointing downward could be the pilot vent for a pilot-operated PSV.

Per API520 Part I, "the pilot vent can be either directly exhausted to atmosphere or to the main outlet depending upon the pilot's design and user's requirement." However, the vent should start to function only when set pressure is reached, in other words, no leak under normal condition.

Hope this helps.

 

[proinwv2]

With the small amount of detailed information, it is really conjecture as to what that fitting is.

It might be a downward facing vent above the diaphragm and the liquid indicates a failed diaphragm.

It may well be a serious situation.

PAUL

Paul Ostand

http://www.ostand.com

 

[sulphuric98]

I am not sure but it is for mannual operation if PRV fails then you can open if it is possible to do that to release pressure from the vessels and to avoid serious situation

 

[proinwv2]

If that is so, the fitting must be in direct communication with the tank, ie. the valve inlet.

The fitting must also have a shutoff valve.

In general would one want to get on top of a tank in a failure mode to manually open a vent to atmosphere?

This guy wouldn't...my hero days are past

Paul Ostand

http://www.ostand.com