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THE FLARE SYSTEM

FLARE [GAS] "SEALS"
LABYRINTH TYPE

Labyrinth Type Gas Seals (which are also known under the trade name "Molecular" seals and "Flarex" Seals)  cause a double flow path reversal in the flare, forming a complex 3-dimensional labyrinth or double "U" for the flow of relief gases. Gases flowing up the stack riser impinge upon the various baffles and closure plates and are directed through two consecutive 180 degrees turns before continuing to flow up through the exit of the seal.  [typical sketch here]

Buoyant decanting of light gases from the stack, with its associated atmospheric replacement, is arrested at the bottom closure plate of the seal thereby removing the ability for buoyant movement of air into the stack and "trapping" a leg of light gas in the inner leg of the seal.

In normal operation the only theoretical forward flow required by such a seal is that required to overcome diffusion at the air/gas "interface". Although a flow rate as low as 0.01 fps can overcome diffusion alone in the available space and for common gases, using this rate will only maintain a stable interface condition. However, the turbulent effect of wind and the possible normal variations in atmospheric pressure and temperature will cause the position of the interface to change such that it may be driven back into the inner sealing leg of the seal. Thus, to ensure a safe condition for the flare, any oxygen interface in the inner leg of the seal must be driven toward the exit using a positive purge flow.

It is, therefore, necessary to exceed the diffusion based purge rate in this inner sealing leg in order to maintain the interface at the commencement of the final upward leg and, thus, prevent wind and atmospheric effects from pushing the interface progressively backward and overcoming the forward flow rate. The excess purge velocity in the seal effectively becomes an inverse measure of the time taken to restore the position of the interface to the bottom of the upward leg following a displacement due to changing ambient conditions.

It should also be noted that the oxygen concentrations in the final outgoing leg may be outside "safe" concentrations even whilst the presence of the inversion leg acts as a barrier, permitting zero oxygen in the incoming leg of the seal.

This type of flare seal has a slight additional safety factor in that loss of the sealing purge gas flow is not an immediate hazard and the stack may remain "safe" for a short period of time in "no wind" conditions and for a stable system temperature.

The labyrinth seal's overall usefulness is most apparent in the larger range of tip sizes (greater than 24") where reductions in long term operating costs override the capital investment in a complicated fabrication.

For general safety concerns, any piping which can effectively "short circuit" the seal, such as an external drain pipe, must be designed to resist both outflow of internal positive pressure and possible inflow of atmospheric air.