Flares Sizing

Flare sizing is covered in some detail under a later section in this chapter, as well as sizing of the overall flare system. 

Burning Pit Flare Sizing - The burning-pit area is sized to provide sufficient surface to vaporize and bum liquid at a rate equal to the maximum incoming liquid rate. The calculation procedure is as follows ... 

Figure 6-9. This record of flaring rates shows that a flare sized for 0.2 million scf per day would handle 50% of the flaring and one sized for 1.0 million scf per day would handle 95% of the flaring.

Safety relief systems are verified as part of PSM. This includes the PS Vs themselves and also flare system piping networks. Safety relief valves are covered in Section I—Fluid Flow. A good procedure for sizing the flare system piping is found in Section 19—Safety-Relief Manifolds. This method, first published in the Oil and Gas Journal, has been adopted by APl. I have also used... 

In applying this rule, the capacity of the pressure relief system must also be sized to handle the quantity of fluid released at this pressure (together with other expected loads during this contingency), so that the built-up back pressure will not result in exceeding 1.5 times the design pressure. This additional load need not, however, be considered in calculations of flare and PR valve radiant heat levels. 

Restriction Orifice - In general a restriction orifice should not be used as a means of limiting the capacity of a pressurization path. In special cases, where large incentives apply (such as reducing die size of a flare system), a restriction orifice may be used, provided that all the following conditions are satisfied ... 

Sizing of Flare System and Other Closed Release Systems... 

The factors affecting the sizing of a closed release system to the flare are described below. The same principles apply to the sizing of other closed release systems e.g., PR valve releases which are manifolded into an atmospheric vent. 

Maximum Flow to be Handled - The flare system must be sized to handle... 

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 ... 

Overall sizing and pressure drop in flare systems are covered in a later chapter,... 

Other designs of ground flares suitable for refinery application are available. In some of these cases, noise is appreciable, in comparison with the multijet type, but their compact size, low space requirement, simplicity, and hence low cost, may give an overall advantage. 

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. 

The vapor space is sized to avoid water entraimnent in the flare gas. As a rule, vapor velocities in the drum should not exceed 150 % of critical. This however can be increased to 230 % critical velocity) when considering... 

A flare seal drum may also be used as a sour water disengaging drum, if economically advantageous. In such cases, special care should be given to ensure that the drum is adequately sized to simultaneously meet all design features required for both functions. Also a separate source of makeup water must still be provided to ensure continuity of the seal. 

Odor and pollution problems are often experienced due to incomplete combustion when concentrated HjS is flared. When such considerations are expected to be critical, flare system designs should include a fuel gas connection and equipment sizing sufficient to handle an equal volume of fuel gas when flaring H2S at design rate. Where flaring of HjS is intermittent and the fuel gas diluent is continuous then steam injection at the base of the flare may be needed to reduce smoking. 

The flare tip assembly shall be provided with a sufficient number of adequately sized lifting attachments for proper handling of the assembly. 

A trial and error estimate is made for determining the diameter of the flare header based upon the maximum relieving flare load and considering the back pressure limitation of 10 percent for conventional valves and 40 percent for balanced type valves. Note, however, a single main header in most cases turns out to be too large to be economically feasible. Line sizing procedures are discussed in detail in the next subsection. 

The subheaders in each process area similarly will have two levels of flare headers. The line sizing of each level of subheader in an individual area will depend upon the maximum simultaneous flow in that particular area. Thus the line sizing criterion of a subheader may be the largest single flow due to a blocked outlet condition. This flow may not necessarily be the controlling load for the flare stack. 

When the maximum vapor-relieving requirement of the flare system has been established and the maximum allowable back pressure (as just described) has been defined, line sizing reduces to standard flow calculations. 

The actual mass flow rate through a pipe, G, in lbs per sec per sq ft is a function of critical mass flow G,-i, line resistance, N, and the ratio of downstream to upstream pressure. These relationships are plotted in Figure 19. In the area below the dashed line in Figure 19, the ratio of G to G,. remains constant, which indicates that sonic flow has been established. Thus, in sizing flare headers the plotted point must be above the dashed line. The line resistance, N, is given by the equation ... 

The following steps summarize the procedure for sizing flare headers ... 

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