6 replies to this topic

[dgoyal]

Fire case relieving temperature for gas filled vessel
I have some cases where vessel are normally gas filled vessel and this are normally operating at very lower pressure( 35 Barg) than set pressure (96 barg) .

As per project specification we need to provide PSV on each vessel . Except fire case there are no other reliving scenario.
Now problem is that reliving temperature in fire case is too high ( 633 Deg C) , How can i report this temperature because maximum allowable temperature for carbon steel is 593 deg c .

Also i have come to know that in this cases set pressure can be reduced so that maximum temperature does not exceed.
Is this allowed please give a clear view on this.
thanks

[gvdlans]

Look at the big picture.
You have a scenario such as:

A nearby vessel or pipe leaks flammable liquid, resulting in a liquid pool. When this liquid ignites you have a pool fire that starts to heat up your vessel. Because of this heating, temperature of the steel increases and both pressure and temperature of the gas inside the vessel increase. At some point of time, the vessel wall will not be strong enough to withstand the internal pressure. As a result the vessel ruptures (Loss Of Containment). This may lead to consequences such as injuries, fatalities, environmental damage, asset loss, damage to reputation.

Now it is your challenge to find a strategy that makes sure that the risk (=combination of likelihood and consequences) of this scenario is acceptable and As Low As Reasonably Practicable (ALARP). For this, you can provide risk reduction measures such as:

- relocating the nearby vessels or pipes that contain flammable liquids
- reduce inventories of flammable materials
- ensure that flammable liquids are drained away from your vessel
- reduce likelihood of ignition by removing possible ignition sources
- fire and gas detection
- provide active fire protection system (e.g. based on foam)
- provide fireproofing
- provide water cooling on your vessel
- emergency depressurisation of vessel
- site emergency plan
- community emergency response
- etc.

Your relief valve will hardly do anything in this case (since it will probably only open after your loss of containment, and it will only limit the internal pressure to the set pressure plus accumulation).

So my advise is: focus on those measures that are effective and efficient in reducing the risk. If the authorities or your client or your company requires that every vessel must have a safety valve, just provide a safety valve with a small orifice area.

Note that adding a safety valve increases CAPEX and OPEX (since valve must be tested and maintained), and adds potential leakage sources therefore increasing risks. This money could be better spent on more effective measures.

Good luck!

[JoeWong]

Just add extra points to excellence response by gvdlans

Some additional point in "Protective Measures against FIRE other than Pressure Relief Device (PRD)"

[Art Montemayor]

Guido:

Please receive my warmest and heartiest applause and KUDOS as earned recognition for an excellent and expert recital of what true safety engineering is! Every professional engineer reading this thread should recognize and take time to appreciate your expressing exactly what the true and logical intention of every safety code represents: the application of common, logical engineering sense in a systematic and designed manner.

To all reading this thread:

What Guido has written should be copied and posted on every engineering company’s bulletin board. He has expressed what I have known and tried to expound for many years regarding the safety around a gas-filled pressure vessel in a pool fire. Everyone should take careful note of all the details and steps that he outlines. His analysis and procedure is the kind of stuff that all process engineers should apply on a daily basis.

Note how he starts by defining the BASIS of what constitutes the need for a pressure relief device. In doing so, and subsequently applying engineering logic and training, he points out the obvious and practical obstacle to obtain quick and sure pressure relief safely: The well-known inferior thermal conductance value of a gas film coefficient.

As Chemical Engineers, we should all be well aware of the inferior value of the gas film coefficient in transferring heat. That characteristic is what forces us all to install fins on air-cooled exchangers and what increases the size of a gas heat exchanger’s surface area. Just remembering that the Overall Heat Transfer Coefficient (“U”) for gas exchangers is in the order of less than 10 Btu/hr-ft-oF should start to raise the hairs on the back of our neck when we contemplate the pool fire heating the gas in a vessel, raising its pressure, and causing a PSV to relieve. Don’t get me wrong. The internal gas pressure WILL increase – but more often than not, by the time this happens the vessel steel will have already reached its ultimate tensile stress allowed at THE FLAME TEMPERATURE! It will be too late to save the vessel. As we say in Texas, the horses will already have left the barn. From a practical safety point of view, an engineer cannot allow such a fatal scenario to unfold – as Guido well points out.

The main point I have gathered from Guido’s comments is that we all have to do our engineering mental “homework” up front, before we start to generate PSV orifice areas. If we fail to recognize what the true, credible fire scenario can be – and cause – all our engineering efforts are lost and we will have created a disaster waiting to happen. All of us – especially the younger engineering generation – should be well aware and mindful of the fact that WE ARE RESPONSIBLE for the results. That’s why we are paid the big bucks, we get all the pretty girls, and we are very popular at cocktail parties. In the end, we are totally responsible for taking ALL credible scenarios into consideration and designing appropriately. There will be no excuses accepted afterwards, should a disaster (God Forbid) happen.

I hope all who read this appreciate the depth and importance of Guido’s post.

Thank you, Guido.

[maldini]

[Regarding the above mentioned if i have an instrument air vessel and it is not near to any source of hazard (flame or pool fire) so i will not install a safety valve for fire case
but i can't trust any instrument (i.e control valve) which it may fail and the pressure increase so i have to install safety valve

[fallah]

[Regarding the above mentioned if i have an instrument air vessel and it is not near to any source of hazard (flame or pool fire) so i will not install a safety valve for fire case
but i can't trust any instrument (i.e control valve) which it may fail and the pressure increase so i have to install safety valve

If you see the first post you will consider that "Except fire case there are no other reliving scenario."
Therefore your case isn't relevant to current discussion.
If in your case the governing scenario is other than fire,of course you shall consider the PSV based on that scenario (control valve failure).

[ARAZA]

Hi There,

Rather late to respond but I thought it will be valuable to add my two cents.

On one of my recent projects, I was dealing with heavy hydrocarbons with some of the pressure vessels operating at 1500 psig and higher. As it happens, when you deal with heavy HC, the relieving temps most of the time exceed MAWT. To deal with this scenario, we decided to with a fusible plug design at the bottom of the vessel which will open before the vessel reaches MAWT and drains the liquid to a outside storage vessel.

Sure you can do this if its practicle as such when your vessels sizes are relatively smaller and will require a small hold-up tank. Another way of dealing with this high relv. temps are automatic depressurization to the flare header systems.

Regards,

ARAZA