Water Seal in Dry Gas Flare

[EmmanuelTop]

What is the purpose of water seal in dry gas flare - considering that the upstream KO vessel is equipped with hydrocarbon vaporization system?

I consider this to be quite hazardous solution - rather than it serves some purpose. Why there is a water seal in dry flare system (marked with red color)?

Thanks,

 

[MortenA]

I would assume that the seal is there to prevent the escape of gas when no liquids enters the stack while allowing liquids to be drained from the stack? A liquid height of 4.6 metres is almost half a bar. Not knowing the tip design i would still assume that the risk of overpressure so much that the liquids will be pushed out of the seal is very low? Then theres the problem if draining the liquids is OK, and if the liquids from the vessel evaporates more easily than water risking the drying out of the seal. I assume that one of note 8,9 or 10 mentions something about filling the seal periodically?

Best regards

Morten

 

[25362]

http://www.gba.com/ancillaries1.htm

 

[EmmanuelTop]

What liquids one can expect in Ethylene system flare?
Ethylene boiling point at 1 bar is -104C, and the ambient temperature is +30C.

I would say this system poses much higher risk of gas blowout in case of flare overpressure, than it offers the protection against some imaginary "liquids" that may appear.

 

[25362]

Sorry, it should read:

http://www.gba.com/ancilliaries1.htm

 

[JLSeagull]

What do you mean by an ethylene system? What process unit?

My recollection is that some olefins units includes two flares; one may be called the cold flare. The P&ID segment lacks the name of the drum and lacks the notes. Those notes could be interesting on the line from the bottom of the flare. Also the unit moves lots of vapor to the flare during startup.

An olefins unit includes lots of hot steam joined with the hot feed stock for the reaction. Downstream of the reactor or TLE is a vessel that catches most of the solid stuff, lots of water etc. This is a low pressure vessel. However the cracked gas is still saturated until dehydration. A flare header may also exist in the hot section. These are process licensor details that vary from process to process.

 

[Flareman]

EmmanuelTop
Regardless of the gases in the flare system, the loop seal shown in your PID is essentially a drain which you need because the bottom plate of the flare is below the inlet nozzle and can collect atmospheric moisture as well as liquids from the line (as suggested by MortenA). The 4.6 m depth is needed to prevent the external seal from blowing when there is a high back pressure in the stack. The water supply is needed because you cannot rely on having any liquids to seal the loop provided by the process or from rainwater.

If you put the stack sealing plate immediately at the inlet nozzle and slope it back, making a Bend rather than a Tee, you could drain the stack directly back into the incoming line from the KO drum and avoid the use of the drain shown in the PID. (line needs to slope back as well as suggested by note 3)

Regards
David

 

[EmmanuelTop]

Thanks

For that purpose, I would expect to see an ordinary drain valve at the bottom of flare (including the level transmitter), as I have seen in other installations. Messing up with water in cryogenic flares doesn't look like a good idea, and in addition it is hazardous as well.