Flashback seals

All flares must be provided with flashback protection to prevent a flame front from travelling back to the upstream piping and equipment. A number of different flashback seal designs are available, of which the seal drum is used in nearly all applications. Key design details are summarized below ... 

Conventional Flare System - The majority of pressure relief valve discharges which must be routed to a closed system are manifolded into a conventional blowdown drum and flare system. The blowdown drum serves to separate liquid and vapor so that the vapor portion can be safely flared, and the separated liquid is pumped to appropriate disposal facilities. The blowdown drum may be of the condensible or noncondensible type, according to the characteristics of the streams entering the system. Selection criteria, as well as the design basis for each type of blowdown drum, are detailed later in this volume. The design of flares, including seal drums and other means of flashback protection, is described later. 

For services where ambient or inlet temperamres may fall below 0°C, flashback protection is provided by a special seal drum or loop seal in the inlet line. This equipment is designed specifically for the particular liquid and vapor materials being flared. In these cases a 150 mm minimum water layer is included in the bottom of the pit to prevent oil seepage into the ground, and the hydrocarbon inlet distributor is mounted 150 mm above the water surface. Details of the inlet distributor are shown in Figure 5. 

An important design detail is that the inlet line should be sloped at least 30" from the horizontal and the diameters of the vertical and sloped legs of the Y-seal are sized such that a 3 m slug of water is pressured back up the sloping inlet line, without spilling over into the header, in the event of flashback. Note that this requires an enlarged diameter section in the vertical leg of the Y-seal. The seal is maintained by a continuous flow of water at 1.26 dm /s and the water makeup is provided with steam injection, if required for winterizing or cold releases. 

In the case of heavier-than-air purge gas, there is no buoyancy mechanism causing air entry into the stack, and there is thus no incentive to include a dry seal. Unlike a water seal, a dry seal cannot prevent a flashback from traveling upstream if a combustible mixture has been formed by the entry of air into the safety valve or flare headers. It only protects against internal burning flashback... 

Flashback protection is required for H2S flaring systems, either by water seal or continuous gas purge. If a water seal is used, special requirements apply to the disposal of the effluent seal water. In the case of an HjS flaring system handling a flow of HjS which in uninterrupted throughout the period that a plant is in operation, and which stops only when the producing plant is shutdown, then flashback protection is not required. However, steam or inert gas connections are required to permit purging the flare line startup and shutdown. 

A peak velocity through the flare end (tip) of as much as 0.5 mach is generally considered a peak, short term. A more normal steady state velocity of 0.2 mach is for normal conditions and prevents flare/lift off [58]. Smokeless (with steam injection) flare should be sized for conditions of operating smokelessly, which means vapor flow plus steam flow [33c]. Pressure drops across the tip of the flare have been used satisfactorily up to 2 psi. It is important not to be too low and get flashback (without a molecular seal) or blowoff where the flame blow s off the tip (see Ref. 57), Figure 7-71. 

Air seals (also called purge reduction or gas seals) are often recommended to prevent air from entering the stack, which could possibly cause flashbacks and explosions. More information about air seals can be found in API RP 521 (2007) and the AIChE-CCPS book Guidelines for Pressure Relief and Effluent Handling Systems. 

Liquid seal drums Emergency vent streams are usually passed through a liquid seal, commonly water, before going to the flare stack. The liquid seal drum is usually located downstream of the knockout drum, and some vendors designs include them in the base of the flare stack. A liquid seal drum is used to maintain a positive pressure in the vent header system and upstream system. It also reduces the possibility of flame flashbacks, caused when air is inadvertently introduced into the flare system and the flame front pulls down into the stack it also acts as a mechanical damper on any explosive shock wave in the flare stack. Figure 23-58 is a schematic of a typical flare stack liquid seal drum, designed per API RP 521 criteria. 

Acetone from leaking bottle contacts paper material in feeder No—feeder is enclosed and above ambient temperature NR Acetone not reactive with paper material by common experience feeder is hot but below autoignition temperatures seal should prevent flashback... 

A liquid seal uses a liquid (usually water) trap to isolate vapors or gases to one part of the drainage system and to block them from being conveyed to other areas served by the drainage system and the flashback of any subsequent ignition. 

The seal drums provide protection against flashback into the blowdown lines. They also make it possible to set up a simple staging system for the burners. 

Maximize primary air Flame lifts off burner Open secondary dampers to control yellow flame Cool box with excess air Seal convective section doors Burner flashback... 

Flares have the added consideration of being designed to always have a flame present, even when there is a very low flow rate.They are typically equipped with molecular or fluidic seals and a small amount of purge gas to protect against flashback. 

---