Flashback protection

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. 

Flare systems are subject to potential flashback and internal explosion since flammable vapor/air mixtures may be formed in the stack or inlet piping by the entry of air, and the pilot constitutes a continuous ignition source. Flares are therefore always provided with flashback protection, which prevents a flame front from travelling back to the upstream piping and equipment. Design details are described later. 

Inlet Piping and Flashback Protection - In non-freezing climates, for services... 

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. 

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. 

Requirement 20 % excess air or 35 % excess air (67 mol % unsats.) Flashback protection of flare systems. 

Alternatively, one can introduce either NH3 or H2 with air and either decrease or eliminate the need for preheat. These reactions will proceed at much lower temperatures and provide the heat necessary to warm the catalyst before switching to the desired fuel. When the catalyst temperature is high enough, NH3 or H2 can be incrementally replaced with the desired fuel. It should be stressed that the feed composition should always be outside the flammable range in order to avoid upstream flames. In fact, in practical design, flashback protection and temperature alarms are common in the upstream portion of CPO reactors. 

Fig. 2.7-2 a) Flare with integrated flashback protection (acceleration path and switchable flashback protector. b) Three types of flashback protection [Mendoza 1998]. 

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 tests incorporate a flame arrester on top of a tank with a large plastic bag surrounding the flame arrester. A specific gas mixture (e.g., propane, ethylene, or hydrogen at the most sensitive composition in air) flows through and fills the tank and the bag. Deflagration flames initiated in the bag (three at different bag locations) must not pass through the flame arrester into the protected vessel. On the unprotected side, piping and attachments such as valves are included as intended for installation a series of tests, perhaps 10, is conducted. 

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. 

Some vaporization may occur in the spray chamber, and the largest droplets will condense onto the walls of the chamber and go to waste. The waste tube leads to a liquid trap which prevents the gases from escaping and ensures a small steady excess of pressure in the spray chamber. Because a relatively large volume of flammable gas is in the chamber, it is a potential source of danger. Modern AA instruments, however, are equipped with gas control systems which give protection from flashback of the flame. Spoilers are often employed inside the spray chamber to improve the change between the sample mist and the tube walls. 

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. 

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