Eng-Tips is the largest engineering community on the Internet
Intelligent Work Forums for Engineering Professionals
design pressure of KO drum of flare siystem 2
- Thread starter JWengtips
- Start date
-
1
- #2
-
1
- #3
JWengtips
MortonA correctly identifies the operating pressure as the accumulation of the flowing pressure losses back along the system to the drum but that doesn't always cover a suitable design case unless the drum is actually part of the flare and always open to the atmosphere.
Although a flare KO drum sounds like a drum at the flare, sometimes it might be a local plot blow-down vessel quite a distance away. If your vessel can be isolated downstream or blocked in, you need to look at additional conditions such as the potential upstream pressure against a blocked outlet or the pressure which can be developed due to liquid in a blocked in drum and a fire case. You can get more information from API RP-520 and RP-521.
If your question is related to an anticipation of an explosive flash-back, there is NO CLEAR AGREEMENT on an appropriate set of design conditions. First of all, you have to apply operational procedures (purging) to do your best to avoid that condition, but even then, it is not an unknown possibility.
A couple of major Oil companies take the theoretical view that the pressure uplift in a flame front is about 7 times base presure and use 125 psig or 150 psig as a design pressure.
I can personally go along with that on the independent basis that even though the theoretical detonation pressure can go quite high (>40 bars ?) the wave is travelling very quickly so the circumferential stress effects are mitigated by time and strain energy accumulation and the problems historically occur in circumferential joints under longitudinal stress, where the velocity pressure of the wave accumulates, (e.g. bends fall off). Using a slightly convoluted argument and assuming a detonation pressure which goes as high as 1000 psig, take the conventional 4:1 safety factor on UTS applied by ASME along with the 2:1 difference between circumferential and longituginal stresses to provide 8:1 on UTS for longituginal stresses, which would be a design condition of 125 psig to just prevent total failure. Experience again shows that vessels designed this way WILL bulge and distort if hit by such a condition.
Just so that we all understand, this is NOT an endorsed condition by any authority and I have NOT rigorously tested the theory, so you're on your own when it comes to responsibility.
I anticipate a number of people might want to jump in on this and look forward to a spirited debate.
![[ponder]](/data/assets/smilies/ponder.gif "[ponder] [ponder]")
David
MortonA correctly identifies the operating pressure as the accumulation of the flowing pressure losses back along the system to the drum but that doesn't always cover a suitable design case unless the drum is actually part of the flare and always open to the atmosphere.
Although a flare KO drum sounds like a drum at the flare, sometimes it might be a local plot blow-down vessel quite a distance away. If your vessel can be isolated downstream or blocked in, you need to look at additional conditions such as the potential upstream pressure against a blocked outlet or the pressure which can be developed due to liquid in a blocked in drum and a fire case. You can get more information from API RP-520 and RP-521.
If your question is related to an anticipation of an explosive flash-back, there is NO CLEAR AGREEMENT on an appropriate set of design conditions. First of all, you have to apply operational procedures (purging) to do your best to avoid that condition, but even then, it is not an unknown possibility.
A couple of major Oil companies take the theoretical view that the pressure uplift in a flame front is about 7 times base presure and use 125 psig or 150 psig as a design pressure.
I can personally go along with that on the independent basis that even though the theoretical detonation pressure can go quite high (>40 bars ?) the wave is travelling very quickly so the circumferential stress effects are mitigated by time and strain energy accumulation and the problems historically occur in circumferential joints under longitudinal stress, where the velocity pressure of the wave accumulates, (e.g. bends fall off). Using a slightly convoluted argument and assuming a detonation pressure which goes as high as 1000 psig, take the conventional 4:1 safety factor on UTS applied by ASME along with the 2:1 difference between circumferential and longituginal stresses to provide 8:1 on UTS for longituginal stresses, which would be a design condition of 125 psig to just prevent total failure. Experience again shows that vessels designed this way WILL bulge and distort if hit by such a condition.
Just so that we all understand, this is NOT an endorsed condition by any authority and I have NOT rigorously tested the theory, so you're on your own when it comes to responsibility.
I anticipate a number of people might want to jump in on this and look forward to a spirited debate.
David- Thread starter
- #5
Thanks all.
Our client has an existing flare stack with a KO drum. The KO drum is designed based on the back pressure of the stack, which is around 7 psig.
However, when they operated the flare system, the KO drum was failed (a crack occurred on a nozzle).
I've used Hysys to analyse this flare system. With the main pipeline pressure 1170 psig, the KO drum pressure can possibly hit as high as 700 psig based on the Hysys model. I truly agree what David said in his reply.
An orifice has been proposed on upstream of KO drum to constrain the flow rate and bring the pressure down. Even though it will result the blowtown time longer, it may be a cost effective way to use this 7 psig design pressure KO drum.
David, do you have any comment/suggest on this matter?
Thank you all again!
Regards,
Joan
JWengtips
I am somewhat surprised by your report of a failure based on 7 psig, ... on the nozzle ??
Even a large vessel (12 ft diameter) would have to be paper thin to fail under pressure. I'm guessing that the crack is in the weld which then would sounds like poor fabrication, an unrecognized nozzle load or stress corrosion (or something like that !!!)
Before you rush off and make process changes make sure YOU KNOW how the problem was created. Don't allow someone else's potentially inaccurate interpretation to guide you (if that might be the case).
The use of an orifice needs careful analysis (I'm sure you know) because of the potential back pressure generated on the relief valves as well as the extended relief time. It depends on what you are relieving and whether you can afford to slow down the output. Also, the relief rate is usually controlled by the relief valve and you are dealing with a low presssure system in the first place. If the relief presently generates 10 psig at the point where you might put the orifice, and you have plenty of upstream pressure, you will probably just get the same rate with a greater overall pressure drop, which doesn't do you any good.
I would go back to the nozzle problem itself and debug that first.
Good luck
David
I am somewhat surprised by your report of a failure based on 7 psig, ... on the nozzle ??
Even a large vessel (12 ft diameter) would have to be paper thin to fail under pressure. I'm guessing that the crack is in the weld which then would sounds like poor fabrication, an unrecognized nozzle load or stress corrosion (or something like that !!!)
Before you rush off and make process changes make sure YOU KNOW how the problem was created. Don't allow someone else's potentially inaccurate interpretation to guide you (if that might be the case).
The use of an orifice needs careful analysis (I'm sure you know) because of the potential back pressure generated on the relief valves as well as the extended relief time. It depends on what you are relieving and whether you can afford to slow down the output. Also, the relief rate is usually controlled by the relief valve and you are dealing with a low presssure system in the first place. If the relief presently generates 10 psig at the point where you might put the orifice, and you have plenty of upstream pressure, you will probably just get the same rate with a greater overall pressure drop, which doesn't do you any good.
I would go back to the nozzle problem itself and debug that first.
Good luck
David
- Thread starter
- #7
A catastropic failure in a flare drum due to overpressure where the falre was not "shut-in" sounds very unlikely also to me! First ting to check is if there are ANY valves downstream the drum - both P&ID - but in this case i would also do a line walk! It would be highly irregular but i guess stranger things has happened. If there IS a valve then i would suspect that not matter what you are told then this valve has been shut at one time or another.
Bestregards
Morten
Bestregards
Morten
GL431
Chemical
- Aug 22, 2003
- 73
- 0
- 0
The pipeline pressure is 1170 psi. If that pressure is reduced to flare pressure there would be about 30 to 40°C temperature drop by the Joule-Thomson effect. Could this temperature cause the flare drum nozzle material to go below its brittle limit and therefore cause a failure?
This seems to be a real problem also on other natural gas projects I have seen. The pipeline material is mostly specified correctly (because of ASME B31.8), but the material specification of the blow down systems, especially of low pressure vessels connected to the vent system, often does not consider the low temperature upon blow down. You say it is an existing flare stack (I presume existing KO drum as well). What is the minimum allowable operating temperature on that system?
I presume the flare is for depressuring of the pipeline or for pigging. If it is for depressuring, would a globe valve not be a better solution than providing an orifice? That way one could control the rate to flare. Note, if it is indeed a low temperature problem you are dealing with, the low temperature would go all the way back to the orifice or blow down valve.
As for the design pressure of the flare drum, I can second what has been said before. Minimum 3.45 barg if flash back is effectively prevented from the flare or 10 barg if flash back is a contingency you have to deal with.
On the other hand, I have often seen the blow down systems of natural gas pipelines designed for 600 pounds or a pressure class commensurate with the pipeline pressure. This included all blow down piping all the way up to the vent stack tip. This was to cater for the high flow expected during a pipeline depressuring case. (Your HISYS calculation of a pressure of 700 psi in the flare/vent system upon blowdown, is an illustration in point.) In those cases, a dedicated vent stack was chosen. The lower pressure relief cases and vents, went to a normal flare of the type you seem to be describing.
For understanding your system better. What do you need the KO drum for? If it is pipeline depressuring, there would be only gas, no? Or are there some other cases involved? In the application you use it for, is the flare actually a flare (i.e. with flame at the tip) or is it just a vent (i.e. without a flame)? I can imagine, with 700psi in the flare drum, the flame would be blown out.
This seems to be a real problem also on other natural gas projects I have seen. The pipeline material is mostly specified correctly (because of ASME B31.8), but the material specification of the blow down systems, especially of low pressure vessels connected to the vent system, often does not consider the low temperature upon blow down. You say it is an existing flare stack (I presume existing KO drum as well). What is the minimum allowable operating temperature on that system?
I presume the flare is for depressuring of the pipeline or for pigging. If it is for depressuring, would a globe valve not be a better solution than providing an orifice? That way one could control the rate to flare. Note, if it is indeed a low temperature problem you are dealing with, the low temperature would go all the way back to the orifice or blow down valve.
As for the design pressure of the flare drum, I can second what has been said before. Minimum 3.45 barg if flash back is effectively prevented from the flare or 10 barg if flash back is a contingency you have to deal with.
On the other hand, I have often seen the blow down systems of natural gas pipelines designed for 600 pounds or a pressure class commensurate with the pipeline pressure. This included all blow down piping all the way up to the vent stack tip. This was to cater for the high flow expected during a pipeline depressuring case. (Your HISYS calculation of a pressure of 700 psi in the flare/vent system upon blowdown, is an illustration in point.) In those cases, a dedicated vent stack was chosen. The lower pressure relief cases and vents, went to a normal flare of the type you seem to be describing.
For understanding your system better. What do you need the KO drum for? If it is pipeline depressuring, there would be only gas, no? Or are there some other cases involved? In the application you use it for, is the flare actually a flare (i.e. with flame at the tip) or is it just a vent (i.e. without a flame)? I can imagine, with 700psi in the flare drum, the flame would be blown out.
- Status
Similar threads
