Flare elevation

With the flare tip and flare seal pressure drop and flare elevation fixed, the flare stack, headers and laterials are sized for the largest release, while not exceeding the maximum allowable operating pressure on the associated blowdown drums and water disengaging drums. These maximum allowable operating pressures are in turn determined by ... 

Flare elevation and spacing must be such that permissible radiant heat densities for persoimel at grade are not exceeded under conditions of maximum heat release. The appropriate calculation procedures and personnel exposure criteria are described later. In some special cases, flare elevation and spacing may be governed by radiant heat exposure of certain vulnerable items of equipment, rather than personnel. 

Preferably, the HjS flare system should consist of a segregated header and separate line routed up the side of a conventional elevated flare stack, sharing the same structure, pilots and igniters. However, the HjS header may be tied into the regular flare seal drum if there are special mechanical design problems associated with the separate stack e.g., in the case of a flare which is to be dismantled for overhaul. Flare elevation must be sufficient to meet atmospheric pollution and ground level concentration requirements for the sulfur dioxide produced. 

Vents and flares are intended to take contaminants released from safety valves away from work areas. However, if an elevated vent is at the level of an occupiable platform on the same or an adjacent unit, a worker may, under certain wind conditions, be subject to the nearly undiluted effluent of a vent. Whereas such elevated platforms may rarely be occupied, a heavy exposure from a vent could incapacitate a worker or cause a fall. Tanks that vent only when being filled are common causes of this concern. The usual solution is to raise the vent above any occupiable platform or, at greater cost, to scmb the vent effluent. 

Flare noise (roar of combustion) is the most serious because it is elevated and the sound carries. The flare can be located at a remote distance from the operating unit or surrounding community. Noise of steam injection into the burner can be reduced by using multiple no22les. Furnace noise from air intake, fuel systems, and combustion blower forced draft/induced draft (FD/ID) fans can be reduced by acoustics. The plot plan should be evaluated for noise generation and to find the means of alleviating or moving noise to a less sensitive area. 

This low viscosity resin permits cure at low (70°C) temperatures and rapidly develops excellent elevated temperature properties. Used to increase heat resistance and cure speed of bisphenol A epoxy resins, it has utihty in such diverse appHcations as adhesives, tooling compounds, and laminating systems. A moleculady distilled version is used as a binder for soHd propellants (see Explosives and propellants) and for military flares (see Pyrotechnics). Its chief uses depend on properties of low viscosity and low temperature reactivity, particularly with carboxy-terminated mbbers. 

Elevated Flares See Flares for a general definition. The elevated flare, by the use of steam injection and effective tip design, operates as a smokeless combustion device. Flaring generally is of low luminosity up to about 20 % of maximum flaring load. Steam injection tends to introduce a source of noise to the operation, and a compromise between smoke elimination and noise is usually necessary. When adequately elevated (by means of a stack) this type of flare displays the best dispersion characteristics for malodorous and toxic combustion products. Visual and noise pollution often creates nuisance problems. Capital and operating costs tend to be high, and an appreciable plant area can be rendered unavailable for plant operations and equipment because of excessive radiant heat. 

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. 

Three types of flare systems are commonly used the elevated flare, the ground flare, and the burning-pit flare. Although the three basic designs differ considerably in required capital and operating costs, selection is based primarily on pollution/public relations considerations such as smoke, luminosity, air pollution, noise and spacing factors. Table 1 summarizes the advantages and... 

Three types of stack for elevated flares are used ... 

We shall first consider the design of elevated flare systems. Sizing of flare systems is a function of maximum allowable back pressure on safety valves and other sources of release into the emergency systems. 

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. 

Elevated Flare Location, Spacing and Height - Ijocation, spacing and height of elevated flares are a function of permissible radiant heat densities, possible burning liquid fall-out, and pollution considerations. Design requirements are as follows ... 

In the consideration of elevated flare pilots and igniters, proprietary flare tips are normally provided with the manufacturer s recommended igniter and pilot system. Usually, one to four pilots are used depending on the flare tip type and diameter. The forced air supply type of igniter system (described below) is normally preferred. Controls should be located at a distance from the base of the... 

Normally an overcapacity line to an elevated flare is provided to handle the excess flow when the flaring rate exceeds the capacity of the multijet flare. The overcapacity flare is usually not equipped with steam injection, and smoke formation is accepted during infrequent operations. The overcapacity line and flare is designed to handle the entire maximum flow so that it can spare the multijet flare when the latter is shut down for maintenance. 

Clearance from the elevated overcapacity flare must comply with radiant heat spacing requirements for elevated flares, considering persoimel exposure when maintenance work is being performed on the multijet flare and the... 

Figure 4. Typical flare seal drum arrangement. For use on multijet or staged elevated flares.

All flares must be provided with continuous pilots to ensure combustion of any releases discharged to them, and to prevent flame-out from occuring. Various designs of pilot burner are available, and proprietary tips for elevated flares are normally provided complete with pilots. 

The Seal Drum - A typical flare seal drum for an elevated flare stack is illustrated in Figure 7. A baffle maintains the normal water level, and the vapor inlet is submerged 75 mm to 100 mm. Drum dimensions are designed such that a 3 m slug of water is pressured back into the vertical inlet piping in the event of... 

Figure 6. Typical elevated flare pilot and igniter.

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. 

Figure 11. Continued - Other typical smokeless elevated flare tip designs.

The following calculation procedure is used to predict the heat flux K incident on a surface normal to the direction of radiation, at any distance from the flare stack and any elevation above grade. The information required for this procedure is as follows ... 

Maximum value of K at design flare release at any location where personnel have access e.g., at grade below the flare, or on a service platform on a nearby tower. Exposure must be limited to a few (approx, six) seconds, sufficient for escape only. On towers or other elevated strucmres, ladders must be provided on the side away from the flare, so that the tower or strucmre can provide some degree of shielding. 

The elevated flare shall meet the noise level as specified in the Noise Control Specification ME-0- JD002. 

---