Back-flow Prevention Systems – Purge Gas

Typically, flare headers are continuously purged with a flow of non-oxidizing gas compatible with the fluids expected to be in the header and with the materials of construction of the header and all component equipment. The purge gas should normally be introduced at the upstream end of each branch of the header system.

The purge gas system is normally designed to enable additional flow for the following occasions.

– Start-up of the flare system after being opened to atmosphere (e.g., for maintenance).
– Contraction of the vapors in the flare system as they cool following a hightemperature venting contingency. The maximum required purge flow rate will normally be dictated by this contingency, if it can occur.
– Release of low molecular weight fluids (e.g., H2) to the flare system.

For flare stacks without purge reduction seals, the minimum purge gas flow through the stack is usually calculated using the methods described in H. W. Husa, “How to Compute Safe Purge Rates,” (Hydrocarbon Processing and Petroleum Refiner, vol. 43, no. 5, pp. 179-182 (1964)). For flare stacks equipped with purge reduction seals, the appropriate normal purge flow should be determined in consultation with the manufacturers of the seal and of the flare tip. The volume of gas is typically enough to maintain a velocity of about 0.1 foot per second in the relief header with no other gas flow into the relief header. This injected purge gas is adequate to maintain the gas seal.

In older relief systems, the amount of purge gas is flow controlled. Today, to conserve energy, the rate of purge gas injected is often controlled by using the relief header operating pressure as the measured variable. When there is enough PSV leakage or process venting to maintain the desired back pressure, no purge gas is injected.

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