Flare header system

Potential vibration problems of this type should be considered early in the design stage of the flare header system. The following screening criteria have been developed to assist the designer in recognizing services with potential vibration problems requiring further detail evaluation ... 

To handle the methane gas generated from the 42.5-acre site, an active interior gas collection system was installed as shown in Figure 11. The installation consisted of 42 recovery wells, a gas collection header system, condensate traps, blower station and a flare station. In addition, a methane monitoring system consisting of thirty-two 2-inch wells was installed around the site (U.S. EPA, 1987). 

Closed Disposal System - This is the discharge piping for a PR valve which releases to a collection system, such as a blowdown drum and flare header. However, a closed system can also be a process vessel or other equipment at a lower pressure. 

Pumps and Furnaces - It is not always necessary for a PR valve which must discharge to a closed system to be tied into a flare header. For example, PR valves on furnaces frequently discharge to the vessel downstream of the furnace, and PR valves on pumps normally discharge to the pump suction or pump suction vessel. 

When applying low temperature requirements, one should consider safety valve and flare headers to be subject to "shock chilling" if they can be exposed to cold liquids released into the system. This includes flare headers from blowdown drums into which cold liquids are discharged. Where laterals of different piping material are combined, the material of the lower-temperature header is continued for the rest of the combined line, and is also extended back into the other lines for 6 m. 

The first vessel in the blowdown system is therefore an acid-hydrocarbon separator. This drum is provided with a pump to transfer disengaged acid to the spent acid tank. Disengaged liquid hydrocarbon is preferably pumped back to the process, or to slop storage or a regular non-condensible lowdown drum. The vented vapor stream from the acid-hydrocarbon separator is bubbled through a layer of caustic soda solution in a neutralizing drum and is then routed to the flare header. To avoid corrosion in the special acid blowdown system, no releases which may contain water or alkaline solutions are routed into it. 

Relieving vapors from various pressure-relief and depressuring valves in the system must be collected in individual flare headers that should be appropriately located near each process area. Subheaders must be interconnected to a main flare header which feeds to a knock- out drum and disposal system. Condensates that are carried over by vapors are separated in the knock-out drum. The vapors that exit the vessel go to the flare stack where they are burned. 

The number of flare headers and individual subheaders connected to them depends upon the type of vapors handled, process temperature conditions, and the available back-up pressure or limitations of the pressure receiving devices specified for the system. This section reviews some of the important design criteria and considerations for the headers and subheaders, which is an integral part of the overall flare system design. 

The type of safety valves employed (either conventional or others) in a specific collection system dictates the level of back pressure in that system. In flare headers where multiple discharges exist, each safety valve must be checked so that it does not exceed its allowable back pressure. 

The number of flare headers required depends upon an economic evaluation of system combinations that will result in the minimum piping cost. The following steps outline the procedure for comparative estimations ... 

For a high-temperature system, a separate subheader may be run up to the point where the temperature drops down to the allowable limit of a less expensive material. It may then be connected to the main flare header (either low pressure or high pressure).To properly evaluate this a heat loss calculation is needed. As a rule of thumb a heat loss of 10 BTU/hr/ft may be assumed for a quick estimate for bare pipe. Consideration should also be given to the need for expansion joints. Main flare headers may be as large as 36 to 42 inches in diameter for a large-capacity plant. Expansion joints of such magnitudes may be so expensive as to call for a separate small header for the hot flare system. 

Some vessels are provided with two full-size relief valves so that one can be changed with the plant on line. On the plant side of the relief valves, isolation valves are usually provided below each relief valve, interlocked so that one relief valve is always open to the plant (Figure 10-2). If the relief valves discharge into a flare system, it is not usual to provide such valves on the flare side. Instead the relief valve is simply removed and a blank fitted quickly over the end of the flare header before enough air is sucked in to cause an explosion. Later the blank is removed and the relief valve replaced. 

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. 

Where relief valves are provided on liquid storage tanks or vessels, where there is a possibility of liquid release, i.e., liquid slug, careful evaluation of the release disposal system (e.g., flare header) needs to be undertaken. In some cases, a liquid slug may block a header from releasing pressure and defeat the purpose of the pressure release system. 

All waste hydrocarbon gases (vents, relief valves, and blowdowns) should be routed to a flare or returned to the process through a closed header system. Release of vapors to atmosphere may produce a vapor cloud, and even through the release may be remote from the facility it may drift or the effects of ignition (i.e., blast overpressure) of the cloud will be felt at the facility. 

Onshore, the icing risk is often mitigated by connecting the exhaust of all these SRVs to a dry flare header or a recovery system. Only dehydrated hydrocarbon services are connected to a dry flare. In doing so correctly, the SRV system... 

The design of a flare system includes the sizing of safety and relief valves, inlet and discharge piping, and flare header. All these should be adequately sized to prevent overpressuring of equipment in case of operational failure, such as fire, inlet or outlet blockage, reflux failure, power failure or instrument failure. 

The flare header, which collects the vapors from the safety valves for safe discharge to the knockout drum and the flare stack, is sized for the largest vapor load caused by a single failure. This vapor load is obtained from a tabulation of relief loads from safety valves connected to the flare system. The loads which may occur simultaneously as a result of fire, cooling water failure, etc., are summed up. From these summations the largest load is determined. 

Changes to passive safety systems such as the firewater and flare headers can lead to covert safety problems. As additional equipment is installed, these systems can be overloaded. The capacity of the flare system should be checked, but, if it is not, and since such systems are passive, there are no indications that they have become overloaded until they are called upon to operate. 

Relief valves are designed to respond automatically to sudden increases in pressure. A relief valve opens at a predetermined pressure. In a relief valve, a disc is held in place by a spring that will not open until system pressure exceeds its operating limits. Tremendous pressures can be generated in process units. When a system overpressurizes, safety valves respond to allow excess pressure to be vented to the flare header or atmosphere. This prevents damage to equipment and personnel. Relief valves are designed to open slowly, and thus are best for pressurized liquid service. They do not respond well in gas service, where quicker pressure reduction is needed. 

Flare system—safely burns excess hydrocarbons. A flare system is composed of a flare, knockout drum, flare header, fan optional, steam line and steam ring, fuel line, and burner. 

Flare systems are part of a plant s safety system. Most process units are aligned to safety relief valves that lift when specified pressures are exceeded. These safety valves discharge into the flare header. Unexpected process upsets are dumped to the flare system as a last course of action. A typical flare system includes ... 

It is not uncommon for my clients to maintain a positive pressure in the flare system by continuously purging the most distant ends of the flare header piping with fuel gas, natural gas, or even propane. This is an expensive alternate to the water seal method 1 described above. It eliminates the need for the water seal and also the annoying sight of the pulsating flare flame. But to me it seems like a wasteful and, hence, dumb alternate. 

The open bleed/vent valve represents a low-pressure spot in the system. Therefore, if there is a release through the closed block valve, the hazardous materials will bleed off before entering the equipment being worked on. If the discharge from the bleed/vent line could itself be hazardous, it can be routed to a safe location, such as a flare header. 

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