Purge Gas Rates

The purpose of using purge gas (fuel gas or inert gas) is to ensure a positive flow of gas in the stack and thus to prevent flame flash-back. Without purge gas, air may enter the stack as a result of diffusion. This may bring about flame flash-back or even an explosion in the stack. 

The following assumption was made the purge gas rate, cfh (cubic feet per hour), depends on the molecular weight, M, and the diameter, d (inches) by the relation, cfh =, where b = 0.214 

Flare stacks and connecting piping emd drums can be kept nonflammable by a small flow of oxygen-free gas adequate to prevent air from backing in. Hydrocarbon gas (fuel gas) and steam are the purge gases most often used because of their low cost and ready availability. Inert gases may be used but are more expensive and normally require special facilities to make them available in reasonable quantities. 

The operating cost of a 2-ft. diameter flare stack, when purged at the minimum rate, is nearly 1,500 a year (about 4 a day) if fuel gas is valued at 30 per million Btu. 

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Normal operation purge gas flows are ealeulated based on the American Petroleum Institute (API) Standard 521(1) to provide less than 6% Oxygen (O2), 25 feet fix)m the top of the flare tip. However, purge gas rates at the mega-trains maintained above API 521 guidelines to result in 1% O2, one meter below the flare tip in order to reduce bum back and increase tip life. The Mega-Train Purge Gas Reduction Project was therefore initiated in 2012 in order to achieve ... 

With a fuel purge gas rate of 5 cfm, approximately 350 kw of beta heating from the decay of the fission-product gases and their daughters is deposited in the fuel and on metal surfaces of the fuel expansion tank. This heat is partly removed by the bypass fuel circuits and the balance is transferred through the expansion tank w alls to the blanket salt. 

The vacuum plate drwer is provided as pari of a closed system. The vacuum dryer has a cylindrical housing and is rated for fiill-vacuum operation (typical pressure range 3-27 kPa absolute). The exhaust vapor is evacuated try a vacuum pump and is passed through a condenser for solvent recovery. There is no purge-gas system required for operation under vacuum. Of special note in the vacuum-drying system... 

A gas seal shall be furnished if required to reduee purge gas requirements. The gas seal shall be self draining. The gas seal must be qualified by test data showing oxygen eoneentration below the seal at proposed purge rate and at least 50 mph simulated wind eondition. 

Minimum purge gas flow rate required to prevent tip degradation when operating at mm down condition, even under maximum wind velocity,... 

The sweep-through purging process adds purge gas into a vessel at one opening and withdraws the mixed gas from the vessel to the atmosphere (or scrubber) from another opening. This purging process is commonly used when the vessel or equipment is not rated for pressure or vacuum the purge gas is added and withdrawn at atmospheric pressure. 

The siphon purging process starts by filling the vessel with liquid — water or any liquid compatible with the product. The purge gas is subsequently added to the vapor space of the vessel as the liquid is drained from the vessel. The volume of purge gas is equal to the volume of the vessel, and the rate of purging is equivalent to the volumetric rate of liquid discharge. 

Velocity seals are more recent developments in air seal design. They use conical baffles to redirect and focus the purge gas flow field just below the flare tip to sweep air from the flare stack. Some velocity seal designs can reduce the purge gas flow rate requirement to about 1/10 of the rate needed without the seal. Also, some velocity seal designs reportedly require only about 25 to 33 percent of the purge gas used in diffusion seals (AICliE-CCPS, 1998). More details about air (purge reduction) seals may be found in API RP 521 (2007). 

Figure 3. Recoveries of selected compounds as a function of purge gas flow rate...

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