7 replies to this topic

Dear Friends,

I am working on design of Pressure relief valve for column. but I face problem for calculating vapor load for PRV. this is simple system where steam is provided to column and vap of column is condensed. operating pressure of column is 2.2 kg/cm2 and vapor load is 17000 kg/hr.

Failure may possible due to cooling water supply failed or steam control failed in this case what flow may I consider? how to calculate relief of vapor how much vap relief and what time is to be taken to relief the vapor.

Waiting for reply

Mahendra

[JoeWong]

Mahendra,
Your post is too general and lacking of technical information to offer further assistance.
I guess what you are facing (like many other young engineers) is derivation of relieving scenario. For a column with condenser and reboiler, two common scenario are :

i) Loss of cooling
Loss of cooling lead to overhead vapor passing cooler without condensation. Vapor will accumulate in the reflux drum, downstream of condenser and pressure will increased. Pressure continue to increase until PSV set pressure and finally open. Thus, all vapor generated in the column at relieving condition will be relieved through this PSV.

ii) Excess heating by reboiler (blocked-in-heat-on)
Uncontrolled excess heating into reboiling fluid in the reboiler i.e. hot oil supply control valve failed to full open position lead to excess hot oil into the reboiler. The excess heat will generate extra vapor which potentially lead to overpressure in the column, and subsequently PSV open. The worst case is vapor generated by all inflow of heat and feed vapor will be relieved via the PSV.

iii) Blocked discharge
Either the reflux pump discharge or the vapor outlet control valve failed to closed position lead to blocked discharge. All inflow shall be relieved via PSV...

iv) Feed Control valve failed to open position
Feed control valve failed may lead to gas blowby via the feed control valve. Excessive flow shall be relieved via the PSV

v) Fire contigency
In the vent of fire, heat input from fire into the vessel will vaporize liquid and vapor expansion. Excess vapor generated shall be relieved via the PSV.

and others...

Thanx for reply,

I would like to known which condition should be consider for relief and how to calculate maximum vap relief rate.

waiting for reply

mk

[JoeWong]

Mk,

I think in order to assist you in effective way, you should tell us
- how much you have done on the subject
- what methods you have used
- what is your opinion on the subject
- how confident you are on the subject
- which problem you are facing
- how much efforts you have put in
- did you search for the similar query in this forum
- did you search in Google, Yahoo, etc for similar subject..
- what are technical parameters for your system...
- did you ask the same question to your senior or superior

With these info, may be CheJedi in this forum can assist effective.

By answering to above questions, you at least have taken the effort to assist yourself to understand your problem, assist yourself to appreciate the problem...

Sincerely...
I guess no people will take the spoon to feed you if you are normal. Those "guru" who take the spoon to feed you, please refuse to accept the offer as he/she has hijacked your chance to learn...
Good luck.

[djack77494]

I guess no people will take the spoon to feed you if you are normal. Those "guru" who take the spoon to feed you, please refuse to accept the offer as he/she has hijacked your chance to learn...
Good luck.

Excellent comment JoeWong, especially the last sentence. Mahendra, now that I've been warned not to "spoon feed you" (not that I really needed the warning), let me say that the simple answer to your last question is "all". Your question, "which condition should be consider for relief and how to calculate maximum vap relief rate" is answered by saying that you calculate each and every relief rate from hazards that may arise and that you then select the largest relief flow as the governing case (except that the fire case gets special consideration). Joe has also given you a start as to how to calculate the individual rates. Please provide additional particulars is you wish further guidance.

- how much you have done on the subject
I work on critical operating parameter, I study different failure of column for my system.

- what methods you have used
For calculation I refer book "crossby pressure relief valve engineering handbook"

- what is your opinion on the subject
I think I have to consider max flow in failure of different condition for relief flow

- how confident you are on the subject
I am working first time on relief system, hence I need confirmation for confidance what I do is on right direction, hence I am on forum.

- which problem you are facing
As I mention above I work on differnt failures & for these failure I calulation max vap relief flow but I don't know that is right path to work or not.

- how much efforts you have put in
i mentioned above what I have done

- did you search for the similar query in this forum
I search atricles written on rapture disc which is availble on this forum, if any other articles available then please provide me path.

- did you search in Google, Yahoo, etc for similar subject..
Ya have done

- what are technical parameters for your system...
details I will provide.

Caculation for orifice design and other design is easily get on net but process parameter how to freeze or guidline for process parameter finalization is not found me, if you have any guidline then please provide me link or document.

MK

[JoeWong]

MK,
Well...I like your positive response... Keep up.

I have given you the scenarios and outline in reponse post #2. You have to take some effort to conduct the calculation. Once you have done that you may post you SPECIFIC problem in this forum to obtain OPINIONs from all CheJedies.

I would like to known which condition should be consider for relief and how to calculate maximum vap relief rate.

You may not expect others to advise you which condition to be considered and how to do the calculation as ANY scenario can be the controlling case of the PSV.

You have to calculate ALL cases.

Since you have not done any similar calcs before, you have to do ALL and familiar with them.

Infact, after you have done several times similar calculations, you will have some "clue" which scenario will be "mostly" the controlling case. This subject to experiences and justification.

I don't want to tell you which scenario will be "mostly" the controlling case as i don't want to pre-set your mind and i don't have any parameters. Do the calculation and find out yourself. After that can discuss you findings...

There are many references out there for you to read.
Useful Documents Related to Pressure Relief Valve (PRV) - Part 1
Useful Documents Related to Pressure Relief Valve (PRV) - Part 2
Useful Documents Related to Pressure Relief Valve (PRV) - Part 3
GOOD LUCK.

[JoeWong]

Excellent comment JoeWong, especially the last sentence. Mahendra, now that I've been warned not to "spoon feed you" (not that I really needed the warning), let me say that the simple answer to your last question is "all". Your question, "which condition should be consider for relief and how to calculate maximum vap relief rate" is answered by saying that you calculate each and every relief rate from hazards that may arise and that you then select the largest relief flow as the governing case (except that the fire case gets special consideration). Joe has also given you a start as to how to calculate the individual rates. Please provide additional particulars is you wish further guidance.

Doug,
Thanks for your compliment...