Back-flow Prevention Systems – Seals

Purge Reduction Seals

A flare tip is normally specified with a purge reduction seal located immediately below the tip. This seal may be of either the molecular or the velocity design. A gas seal (molecular seal) can replace the liquid seal drum on an elevated flare, but not on a ground flare. Figure 1200-37 and Figure 1200-38 show diagrams of typical molecular and velocity seal designs. Refer to API Recommended Practice 521 for further discussion of purge reduction seals. Note that the pressure drop induced by any purge reduction seal should be included when performing a flare system hydraulic analysis.

The gas seal is located just below the flare tip and serves to prevent air entry into the stack. It is supplied by the flare supplier. The gas seal is often called a molecular seal because it depends on the density difference between air and hydrocarbon gas. A continuous stream of purge gas is required for proper functioning of the gas seal, but the amount of purge gas is much less than would be required without the seal.

Gas seals have major advantages over liquid seals: they do not slosh and they produce much less oily water. However, the gas seal must be drained and the drain loop has to be sealed. Figure 1200-37 shows a gas seal.

Perhaps several examples will serve to underscore the importance of keeping the gas seal drains clear and functioning.

In an incident in a cold climate, a boiler plant failure interrupted the steam supply to several key turbine drivers. As the hydrocracker makeup compressor slowed down, excess hydrogen was vented to the flare. But no increased flame was observed and the crude unit pressure “… went off the chart!” Quick shutdown of the entire complex prevented potentially catastrophic failures. The gas seal was plugged with ice. Witnesses saw debris flying out of the flare. Ice boulders weighing 50-60 pounds were found at the base of the flare. Also found were the 3-inch steam center pipe and nozzle.

In another incident in a milder climate, when the drain-plugged gas seal began to fill with water (presumably steam condensate and rain) it became a good stripper of the H2S in the relief gas bubbling through. The concentration built up and, eventually, a relatively large gas release expelled sour water onto personnel below. Such incidents serve to emphasize the importance of keeping the seal drain line free and clear.

In another incident in a milder climate, the drain line plugged and the seal filled
with water.

The water came mostly from the steam system which was used to assist in smokeless flaring. The liquid filled seal was calculated to have about 10 tons of water in it and as this water was off-center and sloshed back-and-forth during small gas releases, it caused the 120 foot flare stack to shake violently and. The shaking actually resulted it the guy-wire turnbuckles to loosen; one of the turnbuckles had loosened up over 1 foot when it was noticed and only had another few inches of working length before it would have let go.

The plant handled sour fluids and flare system was the only relief system, thus in order to clear the flare it was necessary to simply blow out the water with a high release rate. This meant that the plant to take the chance that the fluid in the molecular seal was indeed water and not LPGs which commonly accumulated in the flare knock-out drum. Luckily they were right as liquid water rained down on the plant and not flaming liquid hydrocarbon.

Since a gas seal is required with an elevated flare to keep air out of the flare stack, the liquid seal is usually omitted from an elevated-only flare system. If a vapor recovery compressor is used, a liquid seal is used to provide a minimum header back pressure.

The fluidic seal is an alternative to the gas seal. It uses an open wall-less venturi that permits flow out of the flare in one direction with very little resistance but strongly resists counterflow of air back into the stack. This venturi consists of a series of baffles, like open-ended cones in appearance, mounted with the flare tip. The fluidic seal is smaller and less expensive than a molecular seal and, since it weighs less, there is less structural load on the flare stack. However, fluidic seals require more purge gas. The Company has used fluidic seals in offshore applications, but not in refineries. Figure 1200-38 shows a fluidic seal.

fig 1 - Back-flow Prevention Systems – Seals