?relief valve can convert liquid release of vessel into vapour(flash)? 1

[Kkurt]

Dear process specialist,

I have a small horizontal vessel of about 60 liters.
It has a bottom exit pipe 1" that goes 12 meters upwards.
At this high point I want to connect a relief valve to
reduce pressure in case of external fire.

my question:
at this high point: can a relief valve discharge this liquid
from incoming (trough the upward pipe) into a existing header?
+++++++++++++++++++++++++
more details
vessel 60 liters
medium = methylene chloride
normal operating conditions: -25°C; 1 à 2 bara
MAWP of vessel 10 bara
relieving flow: approximately 200 kg/h needed in case of external fire @145°C (vapour pressure at 10bara of
methylene chloride
(no other case for releaving is dedected)
++++++++++++++++++++++++++++++++++++++++=
there is no space/room to connect a relief valve directly to
the top of the vessel wich would discharge a vapour (like an
ordinary safety valve)
+++++++++++++++++++++++++++++++++++++++
I dont think the max pressure drop of 3% by API has to be
followed because I will have a vertikal going flow of liquid methylene chloride connected to a PSV ;
after this PSV the methylene chloride will probably flash from 10bara towards atmospherically pressure

I would appreciate some advice of the feasability of this
particular design.

Kkurt

 

[Montemayor]

Kkurt:

Some quick and dirty advice: DON'T!

This is not only goes against ASME, it goes against common sense. You cannot expect to efficiently relieve an emergency over-pressure in a 2-phase vessel by discharging LIQUID. You must relieve the pressure source: the vapor.

If you are a Chemical Engineer you can understand and appreciate the fact that you are dealing with a 2-phase system. The liquid is not the source of your over-pressure. The over-pressure is created by conversion of the liquid into a vapor, and this is what must be relieved if you expect to counter-arrest the pressurization effect in due time.

If you are in the USA, you'd better follow the API Recommended Practices 520 and 521 - or else you'll have hell to pay. If you are in another country, I wish you luck in avoiding the local authorities. Your reasoning in avoiding the 3% ruling is erroneous and bad engineering practice. If you haven't read thoroughly and dominate API 520 and 521 as well as having at least several years in designing relief valve applications, I would strongly urge you to give it up and turn it over to an experienced process engineer instead of going ahead and doing what you are proposing. What you are proposing is dangerous and ill-advised and I advise you not to go through with it. I wish I could do more to help you, but with only this forum to act through, I can't give better advice.

Please have a safe day.

Art Montemayor
Spring, TX

 

[jalvarez]

Kkurt
Maybe you didn't mention to turn the drum up because you supposed that discharging the liquid would be the best to do.
As Art mentioned, the pressure is relieved relieving the gas phase.
Further, removing the liquid is not advisable because in most circumstances, having liquid is a manner to refrigerate the drum (heat absorption due to vaporization) delaying a possible BLEVE (Boiling Liquid Expanding Vapor Explosion).
So, can you rotate the drum? If yes, follow Art and design the PRV according API and the procedure of a qualified supplier. Crosby supply a good PRV's design soft at

http://sizing.tycovalves.com

Have a safe day
J.Alvarez

 

[25362]

In addition to the excellent advice given by Montemayor and Alvarez, from the VP mentioned: 1-2 bara at -25^oC, I have the feeling you are mixing up methyl chloride (monochloromethane) with methylene chloride (dichloromethane). Otherwise, either you have a presssurizing inert gas or the vessel is fully packed with liquid. Kindly comment.

 

[Kkurt]

Tnx all for the correct advice,

I will go for the conventional way = PSV on top of the vessel. I agree with yr arguments.

btw the medium is CH2CL2.

greets
KKurt

 

[meandmyostrich]

Have you considered insulating the tank to give you an extra layer of safety?
This would reduce the boil-up rate of your liquid in the fire engulfment case and allow you time to control the fire or transfer the tank contents before your emergency relief venting starts spewing hot toxic gas into the area.
I would recommend sizing your emergency relief vents without taking the insulation factor into account despite the calcs in API520 allowing a factor for it as there is a possibility of it becoming dislodged when you turn a fire hose on it

 

[Kkurt]

Yes,

You’re absolutely right
the tank will be insulated.
I have foreseen insulation for the tank and whole the piping to which this little tank is connected to.
The environmental facor of API = 1(= full effect of fire) has been taken into account.

Kkurt
chem. eng.