Flares and Flare Systems

Obviously, any flaring is undesirable and ideally a flare release should only occur during emergency upsets. However, there are often many hundreds of valves connected to the flare system-control valves as well as pressure relief valves-and it can be difficult to keep these leak-free, especially since maintenance access to the flare system is limited. There are two main groups-elevated flares and ground flares. The traditional elevated pipe flare requires a continuous purge as well as permanently lit pilot flames. Fitting a molecular seal greatly reduces the purge rate required. For instance, an open 500 mm flare tip requires ~700nm3h-1 purge rate (depending on gas type) and fitting a molecular seal reduces this to around 20nm3h-1 or less.

Smoke suppression is generally achieved using steam. Often, the steam flow is controlled manually by control room operators watching the flare on closed circuit television. This is wasteful since steam is regularly left at high rate, and often

Ground Flare Header

J-1 Flare gas recovery compressor

Progessive opening ground flare

Figure 8.17

J-1 Flare gas recovery compressor

Progessive opening ground flare

Figure 8.17

Energy efficient flare configuration excessive when actually needed. There are now systems that control steam automatically to the minimum required.

Ground flares are generally 'multi-point' with small individual burners designed to avoid the need for smoke suppression. They are usually surrounded by a wall which limits radiation and noise. Typical design has the burners arranged in banks of progressively larger numbers. Controls based on header pressure ensure that the minimum number of burners necessary are in operation. Since there is always the possibility that the control system could fail, for safety reasons an elevated flare is normally also included, fed from a water seal pot with a deeper seal than the seal pot feeding the ground flare.

For large systems where leakage to flare is difficult to eliminate, a flare gas recovery compressor offers a solution to avoid flaring, generally feeding the fuel gas system. The compressor should be a positive displacement machine due to highly variable gas molecular weight and flow, with spillback control set to maintain the knockout drum pressure below the water seal pressure. Figure 8.17 shows a typical complete system.

8.3.3 Piping

Over the lifetime of a process plant, friction losses in piping systems are a major energy consumer.

8.3.3.1 Capital Cost versus Running Cost

As an example, take 100 m3h-1 of water flowing through a pipe where by using a larger line diameter, the avoidable pressure drop is O.lbar/lOOm. The pumping cost using a 60% efficient pump is -0.27kW per 100m equivalent pipe length. This amounts to -2.3 MWh per year.

Figure 8.18 illustrates the effect of line size on pressure drop for this case. Whereas traditional design practice would almost certainly select 150 mm pipe,

Pipe nominal bore/mm Figure 8.18 Pressure drop vs. pipe diameter.

there may well be a case for choosing 175 mm based on lifetime running cost. Note also that compared with traditional practice, unusual diameter pipe is becoming available so that there is a wider choice with smaller cost increments for increasing line size.

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