Flare headers

The main flare header should not be designed for critical flow at the entrance to the flare stack, or else noise and vibration will result. 

Mak used the same Lapple article to develop a method for designing flare headers. His method has the advantage of starting calculations from the flare tip (atmospheric) end, thus avoiding the trial and error calculations of methods starting at the inlet. 

This chapter describes the basic principles and procedures for the evaluation of overpressure potential in plant equipment, and for the selection, design and specification of appropriate pressure relieving facilities. The design of closed safety valves and flare headers is included in this chapter, but blowdown drums and flares are covered separately. To properly discuss this subject, the reader should become familiar with the following terminology. 

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. 

In offsite locations, thermal expansion PR valves may discharge to a flare header upstream of a knockout drum, if available, or to the equipment (e.g., a tank) on the opposite side of one of the blocking-in valves, or to the atmosphere. Atmospheric discharges must be at grade level in a safe location... 

Flow Meter Orifice Plate - A flow meter orifice plate is permissible in normal process flow pressure reheving path, provided that it can pass the required emergency flow without exceeding pressure limits of the upstream equipment. However, it is not acceptable in PR valve inlets and flare headers. 

This section describes the requirements for the design and installation of pressure relief valve inlet and outlet piping manifolds and valving, including safety valve and flare headers. 

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. 

In adxlition to handling PR valve releases, the flare header is also used to route certain other emergency releases to the blowdown drum. These include drainage from fuel gas, compressor and absorber knockout drums. 

HjS flare headers should be constructed for each of isolation, washing out and dismantling for cleaning. The need for periodic cleaning of the HjS flare header must be recognized, and alternative routing for HjS releases must be provided for such occasions if a shutdown of the HjS sources cannot be tolerated. 

Routing of Flare Header through Process Areas - Flare headers in process areas should be routed to avoid locations of particularly high fire risk, such as over pumps, near furnaces, etc. The headers and subheaders should also be laid out and provided with isolating CSO valves and spectacle blinds, unless prohibited by local codes, such that it is not necessary for flare lines to remain in service in units which are shut down separately. Blowdown and water disengaging drums should be spaced from process areas. 

Thermal Expansion in Flare Header - Sliding-type expansion joints may be used in flare headers as an alternative to piping expansion loops, if required to achieve a reduction in pressure drop or where expansion bends may result in liquid surging, subject to the following conditions ... 

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. 

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 ... 

It is important to note that even if the blowdown is effective in disengaging liquid and vapor, further condensation could occur downstream especially if the vented vapor exits the drum at a temperature above ambient conditions. A proportion of such condensible materials in the blowdown drum vapor release may condense as a result of cooling in the flare header and contact with seal water, and then disengage in the flare seal drum while condensible vapors which are not condensed out at this stage may condense in the flare stack or its inlet line, thus creating the potential for hazardous fallout of burning liquid from the flare. Condensed hydrocarbon in the seal drum can be entrained out with the... 

Lx)cate the blowdown drum (when the non-condensible type is used) at a minimum permissible spacing from the flare, to minimize condensation in the flare header. 

Install a knock-out drum immediately upstream of the flare seal drum, to remove material condensed in the flare header. 

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. 

This chapter discusses some of the criteria for selecting, designing and spacing elevated, burning-pit, and multijet flares. The design of safety valve and flare headers was covered in an earlier chapter, as well as discussions concerning associated blowdown drums, water disengaging drums, etc. 

Flare stack sizing and pressure drop is included with considerations of pressure drop through the safety valve headers, blowdown drums, flare headers, seal drum, etc. Elevated flare tips incorporating various steam injection nozzle configurations are normally sized for a velocity of 120 m/s at maximum flow, as limited by excessive noise and the ability of manufacturers to design tips which will insure flame stability. This velocity is based on the inclusion of steam flow if injected internally, but the steam is not included if added through jets external to the main tip. 

For cases (2) and (3), credit may be taken for heat transfer to the atmosphere from the flare header upstream of the seal drum. 

Y-Leg Seal - The Y-leg seal, which is illustrated in Figure 8, is used for elevated flares in applications where there is no possibility, under any process or ambient conditions, of entrainment or condensation of flammable liquids in the section of the flare header between the blowdown drum and the flare. It is assumed that the blowdown drum is adequately designed to minimize entrainment. 

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... 

Ignition controls shall include upstream (in flare header prior to knockout drum) dual flow sensing equipment which shall start the automatic flare purge, pilot ignition and the flare ignition cycle. N P Refinery will be responsible for the wiring between the flow sensors and the ignition control panel. 

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. 

Main Flare Header and Subheader Pressure Levels... 

Pressure levels at the flare header depend largely on the type and nature of pressure relieving devices that have been specified to protect upstream equipment, as well as the pressure levels of all equipment connected directly to the flare stack. 

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. 

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