PSV on a heat exchanger

[AtleK]

I'm working on project where LNG shall be gasified. To achieve this, we use a shell&tube heat exchanger to heat the pressurized LNG from -155 degC to 10 degC, where the natural gas is in gaseous state. The LNG is on the tube side of the exchanger.

On the shell side we use a water/glycol mix as heating medium. This liquid is run in a closed loop with pumps providing the circulation. It is heated with HP steam (22 barg) and cooled in the LNG heater.

The problem is: Need there be a PSV on the shell side of the LNG heater? I can imagine the steam system running at the same time the LNG is stopped, may cause a situation where the water/glycol may overheat and start boiling... Is this far fetched? Do we actually need a PSV?

To cater for tube rupture, I believe a rupture disk would suffice.

Any thoughts?

If the problem is not enough defined, please tell, and I will expand.

 

[gerhardl]

(Warning! I have no experience with LNG and your exact or similar application - but although):

a) 22 bar G steam to indirectly heat water-glycol???? More sensible to use low pressure????

b) If failures can occur, failures will soner or later occur. Safety devices necessary!

(The engineers practical/ethical test: Would I like my 'underengineering', or a failure because of this, to hit the media?

 

[AtleK]

Thanks for your reply, gerhardl.

a) the steam is what is available on the plant
b) question is: do we need both PSV and burst disk? Is the water/glycol likely (i.e can it happen) to be heated by the steam so it'll exceed design pressure of the heat exchanger?

 

[gene2007]

In answer to your questions:
Yes, you need to protect both sides of the heat exchanger.

If you can show that the rupture disk is adequately sized to protect the shell side of the heat exchanger from all overpressure scenarios, then you do not need to place both a PSV and a rupture disk on the shell side of the heat exchanger.

 

[dcasto]

You need to look at a tube failure. The typical scenerio is to have high pressure LNG entering the shell through a single burst tube (gas coming from both sides of the break).

 

[AtleK]

There is necessary to protect both sides, yes. But from what?

Tube side is protected by a PSV, in the event of a blocked outlet and running pumps.

Shell side shall be protected against tube failure. Bursting of two tubes actually.

Question is, do I need to examine two events -
i)failure of two tubes and
ii)heating and boiling of the water/glycol as the steam system gives full throttle simultaneously with a stop in LNG flow?

Is this (ii) a probable event?

 

[dcasto]

It's you companys call, on how many failures you will cover, do you add a fire at the site too? How about 6 tubes?

 

[Tamer123]

dear Atlek
you need to know if the amount of energy in the steam which heating water/glycol mixture will raise mixture Temp to its evapouration temp or not, because you need high energy (latent heat+sensible haet) to evapourat the mixture, if it is yes then you need to calculate the amount of vapour which will create due to that temp and finaly calculate you PSV size on shell

 

[AtleK]

Thanks all.

I've checked a fire case and two tubes (flow from both ends of each tube)and last, the energy input from the steam, as Tamer123 pointed out.