5 replies to this topic

[Afshin445]

I try to design a water seal drum for HP flare system. The header pressure need to maintain at 70 kpag.
With this pressure water level in seal drum at least need to have 7m hight and seal drum dimension will
be large. In other hand we need to design a loop for controling water level in seal drum as per API-521.
With this dimension, I don't know is it economical to use this type of loop because the loop hight need to be at least in same elevation of water level in drum and we need some structure for such a elevated pipe.

Is anybody has any experince about HP flare Seal drum sizing, with such a pressure?


Best Regards
Afshin

[ankur2061]

Afshin,

Water Seal drums are cumbersome to install and operate. Shell Standards allow a flare system to be installed without a water seal drum. In fact they specifically recommend not to install water seal vessels in flare systems of LNG, NGL and other gas plants due to the potential of cold release and problems associated with freezing of the water in the seal drum.

As an alternative to a water seal drum you could install a bursting disk or a buckling pin device in parallel to a full size emergency control valve (PZV). The parallel operation ensures 100 % availability of the flare and relief system and, by giving the pressure switch of the PZV a lower setting than the bursting pressure, prevents premature rupture of the bursting disk.

Hope this helps.

Regards,
Ankur.

[fallah]

I try to design a water seal drum for HP flare system. The header pressure need to maintain at 70 kpag.
With this pressure water level in seal drum at least need to have 7m hight and seal drum dimension will
be large. In other hand we need to design a loop for controling water level in seal drum as per API-521.
With this dimension, I don't know is it economical to use this type of loop because the loop hight need to be at least in same elevation of water level in drum and we need some structure for such a elevated pipe.

Is anybody has any experince about HP flare Seal drum sizing, with such a pressure?


Best Regards
Afshin

Afshin,

A procedure for sizing vertical seal drum has been included in "Flare Design Manual" by:Fluor Daniel Sec. 8.3.2.1,but nowadays vendors of Flare Systems use simple facilities such as velocity seal along with continuous purging for establishing positive pressure,instead seal drum.

Fallah

[Afshin445]

Dear Sirs,

The existing system is using bursting disk parallel with PZV but because of PZV passing client want to change this system to seal drum to keep positive presure in HP flare header and aslo prevent Air ingress from
flare system to flar recovery facilities. But as i mentioned before the drum size will be huge because of
high flare pressure. In other hand possibilty of cold flaring in this plant is negative.

Also as project purpose is to reduce HP flaring to zero,I think using velocity seal is not good idea, because we need positive pressure in flare header and there is no continuous purging in flare system.


Regards
Afshin

[ankur2061]

Afshin,

A passing PZV does not mean a bad design, it only means a bad or faulty selection of the PZV. In the North Sea operations by the major oil & gas companies this system is widely adopted successfully.

As earlier mentioned a water seal is cumbersome to install, maintain and operate. You need to ensure that the there is always adequate water level in the water seal drum (proper design of filling line and adequate pressure and flow of water for make-up). You also need to ensure adequate removal of the hydrocarbon-water mixture using pumps and automatic level control. Too many controls and instrumentation required for the water seal drum.

Instead you should simply replace the PZV with a more reliable valve during a shutdown or maintenance turnover.

Hope this helps.

Regards,
Ankur.

[fallah]

I think using velocity seal is not good idea, because we need positive pressure in flare header and there is no continuous purging in flare system.

Afshin,

Yes,air lock seal (velocity type) needs continuous purge gas to be established,but reduces gas consumption.If your client wants to change existing system to seal drum,why don't you go ahead regardless of how much would be the size of the drum!

Fallah