19 replies to this topic

[dtmgo1]

Hello,

 

In general for a pressure vessel, how would you consider the relief load depressurization valve design peak load vs. relief valve design load to compare to each other?  Equal?  One bigger than the other?  There is no general rule?

 

This is case of fire for both (depressurization due to fire emergency, and PSV sized for fire case).  Suppose the vessel is equipped with both depressurization valve and PSV.

 

i would expect the loads to be not very different from each other.  Thoughts?

 

Thanks,
dtmgo1

[Bobby Strain]

Calculate each and then tell us.

 

Bobby

[fallah]

dtmgo1,

 

Supposing the incoming heat rate from fire case to be almost equal for both PSV relief and BDV relief in a common vessel and the vessel has already been isolated by SDVs leading to almost the same inventory for both situations, then in most cases the BDV relief is, and should be, higher than PSV relief, because in BDV relief contrary to PSV relief the pressure of the vessel should be decreased to 50% of initial pressure in a short time duration (let say 15 min) while in PSV relief the vessel's pressure would be maintained at design pressure and the PSV not to be sized to reduce the vessel pressure but to be sized just to prevent the pressure to be exceeded the relieving pressure (1.21*design pressure). 

 

Of course it is worth to mention that in real world with having both PSV and BDV on a vessel engulfed in a pool fire, it might rarely be happened the PSV can be activated before vessel failure and the above judgment just to be submitted for relief loads comparison as per OP request.

Edited by fallah, 03 June 2013 - 03:56 PM.

[dtmgo1]

Bobby Strain:

 

You missed the point.

[dtmgo1]

Thank you Fallah, very interesting..

 

The question is relevant in the real world.  We have to design depressurization systems together with PSVs for fire case even though fire may not be the controlling case for the PSV size.  The PSV is the last line of defense, and if the Blow Down Valve doesn’t work, it should handle loads due to fire.

 

Picture a case where the vessel is mostly full of liquid (e.g. a 3-phase separator with a small vapor space) and so it will not fail on the non-wetted surface given that the fire does not impact it (pool fire flames act from bottom up).

 

It seems to me that the pressure has low impact on the rate of vaporization due to fire heat flow. the volume of the vessel is fixed and relatively small compared to the volume of vapors generated, so any phase change will very quickly raise the pressure.  Furthermore, mass heat of vaporization is not a strong function of pressure.

 

In case of a fire, pressure is caused by phase change from liquid to gas, more than a decrease in density at a constant volume.  Liquids are incompressible.

 

dtmgo1

[fallah]

dtmgo1,

 

The first part of my reply is mostly related to your query and you didn't submit your viewpoint about it...

 

And yes, if BDV doesn't work and most inventory would be in liquid phase the PSV might be activated before vessel failure but because the vessel pressure would remain constant at design pressure (actually relieving pressure), if pool fire does continue, it takes no long time having all inventory in gas phase due to liquid vaporization and at that time the vessel would fail due to very high skin temperature will lead to passing allowable stress at corresponding vessel pressure.

[dtmgo1]

dtmgo1,

 

The first part of my reply is mostly related to your query and you didn't submit your viewpoint about it...

 

And yes, if BDV doesn't work and most inventory would be in liquid phase the PSV might be activated before vessel failure but because the vessel pressure would remain constant at design pressure (actually relieving pressure), if pool fire does continue, it takes no long time having all inventory in gas phase due to liquid vaporization and at that time the vessel would fail due to very high skin temperature will lead to passing allowable stress at corresponding vessel pressure.

 

THE ORIFICE OF EITHER THE PSV OR THE DEPRESSURIZATION ORIFICE (BLOWDOWN IS FOR LIQUIDS, DEPRESSURIZATION FOR GASES) DOES NOT DETERMINE THE LOAD.  THE LOAD IS DETERMINED BY THE RATE OF VAPORIZATION.

 

JUST TO CLARIFY, THE QUESTION IN THIS THREAD IS NOT WHETHER THE PSV OR BDV ORIFICE AREA IS LARGER.  

 

THE RATE OF VAPORIZATION IS A FUNCTION OF MASS LATENT HEAT OF VAPORIZATION, WETTED AREA AND FIRE HEAT TRANSFER.  PRESSURE DURING A FIRE IS CAUSED BY THE RELATION BETWEEN THE FIXED VOLUME VESSEL, AND THE VOLUMES OF GAS GENERATED BY THE FIRE, AND THE VOLUMES OF GAS ESCAPING THROUGH THE ORIFICE. 

 

HAVE YOU NOTICED HOW LONG IT TAKES TO BOIL OFF LIQUIDS IN A PRESSURE COOKER WITH A BURNER UNDER IT?  IT CAN EVEN TAKE HOURS.   IT IS NOT A SHORT PERIOD OF TIME.   ONE OF THE FUNCTIONS OF THE PSV FOR FIRE IS TO GIVE TIME TO THE FACILITY IN CASE OF THE EMERGENCY BEFORE THE VESSEL FAILS, BECAUSE A VESSEL FAILING ENTAILS MANY MORE SERIOUS HAZARD POTENTIAL THAN THE POOL FIRE THAT MAY CAUSE IT TO FAIL.   SOME TIME TO EVACUATE, SHUT DOWN, ISOLATE, ALARM, ETC. IS PRECIOUS.

 

 JUST FOR THE RECORD AND TO STATE A TRUISM, BY LOAD I MEAN MASS FLOW, E.G. KG/HR OF VAPOUR.

 

DTMGO1

[flarenuf]

DTMGO

 

a couple of pointers for you .

It is not necessary to post messages in capital letters and coloured , in many instances people on this forum see it as shouting and rude..

secondly a lot of your posting here seems to imply that fallah does not understand anything about Flare and Relief systems and you talk to him like he is a child. I assure you he is not, he has posted many replies here to help  Junior Engineers, of which you are one, and he , like me , has many years experience in this topic. If he has misunderstood your question then that is simply because you didn't post it correctly in the first place.

We all try to assist younger engineers in their quest for knowledge on this important subject.

 

you shout "THE ORIFICE OF EITHER THE PSV OR THE DEPRESSURIZATION ORIFICE (BLOWDOWN IS FOR LIQUIDS, DEPRESSURIZATION FOR GASES) DOES NOT DETERMINE THE LOAD.  THE LOAD IS DETERMINED BY THE RATE OF VAPORIZATION"

 

i beg to differ. If by load you mean the flowrate into the flare system then  the orifice size of the DP valve or PSV will set this.
The PSV orifice size will be sized so as to prevent max accumulation pressure reached to be = to or < 121% of set. The peak flow conditions, T, MW, mass flow from the fire generated load at the max P will then size the orifice, This could be several minutes into the fire.

The DP valve, depending on its activation mode, could be opened on general fire alarm at the perceived start of the fire and hence will depressure the vessel quickly, removing a lot of the liquid inventory through flashing as the Pressure drops, resulting in a colder fluid relief. This will be well  before the fire has had chance to raise the content temperature appreciably.

Determination of the correct flare loading in both these cases can only be done by dynamic analysis and the answer will not be the same for each case studied.

So in answer to your original paraphrased question was

  " is the relief load, kg/h , the same or different for a DP valve and a PSV valve under a vessel fire scenario? "

and the answer is   Generally Different  depending on thr fluids involved.

i hope this clarifies things for you now

 

flarenuf







 

[dtmgo1]

Flarenuf,

 

I am very aware of the posts of Fallah, and he is a very good poster here.   But that does not make everything he writes right.   I like that he sticks to the topic, something you have not done in replying to my posts.   I disagree with your reply, and I stated why in my previous post (what the loads are a function of).  The load cannot be a function of the orifice.  An orifice fitting simply cannot generate heat/mass transfer.  Furthermore, if the liquid was water, it will not flash because the pressure is reduced. 

 

I am glad to know that most people on this Forum will not share your point of view, which assumes many wrong things about me and my post (that I am junior, that I am shouting).  My scientific education tells me not to assume things that I have no evidence of.  And yes I like to write in plain English, I think if all did that, we would all be better off.  I do not mean to condescend to anyone. 

 

I am junior member to this forum, but so is any type of engineer that joined recently.  There is now way this forum can check credentials, licence to practice, etc. 

 

You may (or not) be "senior" or "veteran", but no one knows everything about everything, and you are a begginer/novice/junior in many areas, even within your scope of experience or education.   That is the difference between a specialist and a generalist career, some keep moving to learn new things of new areas, some don't.

 

Albert Einstein - "If you can't explain it to a six year old, you don't understand it yourself."

[fallah]

dtmgo1,

 

You mentioned in your last post: "...i like that he sticks to the topic..." but appears you yourself don't like to stick to your own topic. I am wodering if you think you know the response of your query better than everyone in this forum why do you ask it here. Unfortunately forum admin (or if somone else did i don't know) would select your answer to your own query as best answer on this topic!. 

However i responded to your statements wouldn't any relation to my viewpoints submitted as reply to your query, in red color as follows:

 

 

dtmgo1,

 

The first part of my reply is mostly related to your query and you didn't submit your viewpoint about it...

 

And yes, if BDV doesn't work and most inventory would be in liquid phase the PSV might be activated before vessel failure but because the vessel pressure would remain constant at design pressure (actually relieving pressure), if pool fire does continue, it takes no long time having all inventory in gas phase due to liquid vaporization and at that time the vessel would fail due to very high skin temperature will lead to passing allowable stress at corresponding vessel pressure.

 

THE ORIFICE OF EITHER THE PSV OR THE DEPRESSURIZATION ORIFICE (BLOWDOWN IS FOR LIQUIDS, DEPRESSURIZATION FOR GASES) DOES NOT DETERMINE THE LOAD.  THE LOAD IS DETERMINED BY THE RATE OF VAPORIZATION.

Where did i mention the PSV/BDV orifice area determines the relief load? Of course what would determine the relief load for a vessel engulfed in a pool fire and having liquid inventory is liquid vaporization rate, but on the other hand for example if we would face with an existing PSV we can determine its maximum capability in relieving viewpoint.

 

JUST TO CLARIFY, THE QUESTION IN THIS THREAD IS NOT WHETHER THE PSV OR BDV ORIFICE AREA IS LARGER.  

Where did i mention a statement to force you to submit above clarification?

 

THE RATE OF VAPORIZATION IS A FUNCTION OF MASS LATENT HEAT OF VAPORIZATION, WETTED AREA AND FIRE HEAT TRANSFER.  PRESSURE DURING A FIRE IS CAUSED BY THE RELATION BETWEEN THE FIXED VOLUME VESSEL, AND THE VOLUMES OF GAS GENERATED BY THE FIRE, AND THE VOLUMES OF GAS ESCAPING THROUGH THE ORIFICE. 

Obvious statements just show you know about the mechanism of liquid vaporization inside a vessel in fire case. It is very well.

 

HAVE YOU NOTICED HOW LONG IT TAKES TO BOIL OFF LIQUIDS IN A PRESSURE COOKER WITH A BURNER UNDER IT?  IT CAN EVEN TAKE HOURS.   IT IS NOT A SHORT PERIOD OF TIME. Where did i mention in this regard other than "no long time"? Obviously it doesn't mean short period. Of course latent heat value, amount of the liquid,...are among the parameters would determine the time of no liquid situation. ONE OF THE FUNCTIONS OF THE PSV FOR FIRE IS TO GIVE TIME TO THE FACILITY IN CASE OF THE EMERGENCY BEFORE THE VESSEL FAILS, BECAUSE A VESSEL FAILING ENTAILS MANY MORE SERIOUS HAZARD POTENTIAL THAN THE POOL FIRE THAT MAY CAUSE IT TO FAIL.   SOME TIME TO EVACUATE, SHUT DOWN, ISOLATE, ALARM, ETC. IS PRECIOUS.

Again obvious statements...........................[same as above]

 

 JUST FOR THE RECORD AND TO STATE A TRUISM, BY LOAD I MEAN MASS FLOW, E.G. KG/HR OF VAPOUR.

 

DTMGO1

Edited by fallah, 04 June 2013 - 02:55 PM.

[paulhorth]

Dtmgo1,

Sigh.

Fallah and Flarenuf are right, and you are wrong.This is only to be expected due to their greater experience.

A couple of examples for you:

You state

The load cannot be a function of the orifice.

Actually, the flow through an orifice is of course a function of the orifice size, and also of the upstream pressure. If this flow exceeds the vapour generated by a fire, then the pressure will fall. This is exactly what happens when a BDV opens, if its orifice has been sized correctly. In the case of a PSV, if the upstream pressure falls it will close until the set pressure has been restored, then it will open again.

 

Secondly, you state

Furthermore, if the liquid was water, it will not flash because the pressure is reduced.

I can assure you this is not correct. I run a model steam engine, and when I open the drain valve on the level gauge under pressure, the water flashes to create a great cloud of steam. In any case if you don't believe me just find a Mollier chart and run your finger along a constant enthalpy line from the water phase boundary to a lower pressure.

 

Paul

 

 

[dtmgo1]

Paul:  how can both be right when each is saying a different thing to answer the Original Post question?   One is saying "in most cases the BDV relief is, and should be, higher than PSV relief".  another one is saying "Generally Different  depending on thr fluids involved."  

 

Paul: I think you are confusing two concepts "determination of relieving rates" (API 521) with sizing a relief valve area (API 520).  The load relieved is variable, of course, but the design load for a given orifice  of a relief valve, and the peak load for depressurization is one and one only. 

 

Paul:  are you saying that a vessel with water and vapour space at say 20 degrees C will flash water vapour if you depressurize it?   If it was hot, yes, but at 20 degrees C, no.  sometimes liquid flash with a drop in pressure, but sometimes not, that is the point.

Edited by dtmgo1, 05 June 2013 - 09:05 AM.

[dtmgo1]

Paul: furthemore, i hope you don't believe that having more experience makes you always to be more correct than less experienced people.  Say this openly in a professional environment and you will be frowned upon. 

[dtmgo1]

Fallah: no offense, but I have trouble following your English, especially in your first paragraph.

 

Fallah:  I digress in response to those that digress, because I will chose when not to remain silent to posts that need to be addressed, even though they may be off topic.

 

Fallah:  What is obvious to you may not be obvious to someone else.  I can state the obvious to avoid misunderstandings and to make a point.  This is an open forum to be read by others, and others may be interested in the OP question put forward.  And that includes not only engineers, this is open to the general public.   If all this was obvious, why is there no consensus amongst "experienced" members?

[fallah]

dtmgo1,

 

My viewpoints are as following red color statements:

Fallah: no offense, but I have trouble following your English, especially in your first paragraph.

I think it might relate to understanding the concept not English...However if you would specify the matter i will clarify...

Fallah:  I digress in response to those that digress, because I will chose when not to remain silent to posts that need to be addressed, even though they may be off topic. I did recommend what i should do...the rest up to you...

 

Fallah:  What is obvious to you may not be obvious to someone else.  I can state the obvious to avoid misunderstandings and to make a point.  This is an open forum to be read by others, and others may be interested in the OP question put forward.  And that includes not only engineers, this is open to the general public.   If all this was obvious, why is there no consensus amongst "experienced" members?

There are some common sections in this site such as Blogs, Articles, Library,.., in which you can upload your general/obvious inputs instead in such professional discussion...

[dtmgo1]

Fallah:  

 

However if you would specify the matter i will clarify...

I already did say that your first paragraph's English is hard to follow.  

 

"I did recommend what i should do...the rest up to you..."

It will always be up to me as it always has been.

 

There are some common sections in this site such as Blogs, Articles, Library,.., in which you can upload your general/obvious inputs instead in such professional discussion...

I don't think its in your role to decide what kind of things can be said and where.   Even less in an internet forum.   You can keep trying though, I am not going to tell you what to do, or even make unsolicited suggestions, unlike you.

[paulhorth]

dtmgo1,

Paul: furthemore, i hope you don't believe that having more experience makes you always to be more correct than less experienced people.

Would you say the same about a surgeon, or an airline pilot, who you were entrusting your life to?

I would say it's a pretty reliable rule of thumb.

 

Your other points:

You said in post no. 7:

THE ORIFICE OF EITHER THE PSV OR THE DEPRESSURIZATION ORIFICE (BLOWDOWN IS FOR LIQUIDS, DEPRESSURIZATION FOR GASES) DOES NOT DETERMINE THE LOAD THE LOAD IS DETERMINED BY THE RATE OF VAPORIZATION.

You now say

 The load relieved is variable, of course

Compare and contrast. That is precisely the difference which Flarenuf was pointing out. You got there in the end.

 

You said in post no 9:

Furthermore, if the liquid was water, it will not flash because the pressure is reduced.

You didn't mention temperature, just that water will not flash.

You now say

If it was hot, yes,

Good, quite so. Well done.

Just don't be so quick to assert you are right.

 

Paul

[dtmgo1]

Paul:  Let see if I understand you.  So in your view, more experienced people are right over less experienced people.  hm hmmm  I guess you think no genius old experienced guy can be wrong.  The history of science proves the contrary.  Experience is only one part of the equation.  There is also intelligence, education, training, vision, imagination, etc.  Its not just about experience Paul. 

 

Paul: again, the design load for the relief valve, and the peak load in the BDV are one and one only.  The OP is the question of comparing them.  kg/hr vs kg/hr.  In a relief event due to a fire, the load to the relief system is obviously variable due to many factors.

 

Paul: Flarenuf digressed in many ways, and in one of them he said above that the liquid would be depleted due to flashing when pressure drops during a fireless blowdown.  This is true sometimes, sometimes it is not the case, for example, with some heavy oils or water at room temperatures.  He digressed because the OP is about fire case, not a case of a non fire case, which he talked about.  But I have now shown that his statement is wrong.  

[fallah]

dtmgo1,

 

No need to be in offensive position...

 

However my first paragraph says that: relief load in a fire case for a vessel with liquid inventory is liquid vaporization rate at relieving conditions. But for an existing PSV with specified orifice area and set pressure we can determine its maximum capability for relieving.

 

About the other points: You are right and you are free to say/upload anything in everywhere and someone like me just can recommend in this regard as i did...

Edited by fallah, 04 June 2013 - 05:05 PM.

[flarenuf]

Fallah , Paul

Suggest we stop posting on this topic

dmt2go obviously knows more than we all do about Relief systems, but less about Professional Courtesy

 

lets spend time helping other less fortunate engineers

 

end