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THE FLARE SYSTEM

PRESSURES and PRESSURE DROP

The "back-pressure" within a flare system is self-adjusting because the pressure "backs-up" against the flow. At any point within the system, the total pressure energy at the point may be manifested in various combinations of several, inter-changeable forms of energy. This principle of interchangeability is known as Bernoulli's Theorem and is widely known. When applied to the back-pressure in a flare system, the essential interchangeable components are usually

	static pressure	the pressure which would be measured on a pressure gauge;
	velocity pressure	the energy of motion;
and	static head	the effective "weight" of the contents.

These three components, when added together, form the total pressure energy and the difference in total pressure between any two points within the system is known as the "pressure drop" associated with that part of the system.

In many cases involving flow, the static head is ignored, as it represents an insignificant part of the total energy. For conditions of little or zero flow however, the static head becomes a significant factor.

The velocity pressure component is related to [the square of] the velocity in the pipe and so, for any given flow conditions, the velocity pressure in a small diameter pipe will be greater than the velocity pressure in a large diameter pipe regardless of either the static pressure or the total pressure energy.

Because the constantly reducing feature of back-pressure is actually the total pressure energy, when the velocity pressure component changes with line diameter, so too will the static pressure, because it is simply that component of total pressure which remains after the velocity pressure develops. Flares and flare systems contain elements of many differing equivalent diameters and so the calculated pressure losses within the system must account for the changes in diameter.

For Sonic flows through "choked" jets or orifices, the upstream and downstream velocity pressure components are frequently insignificant and the common calculations appear related only to the upstream and downstream static pressures. Also, most standard calculations make an implicit assumption that the pipe diameter is constant throughout the entire system.
Because of these factors, and due to common perceptions of pressure as static pressure seen on a gauge, the effect of the velocity pressure component is frequently ignored when assessing system pressures.

Truly appropriate calculation methods break down a flare into its various physical and geometrical elements to make these calculations and usually present the results both as total pressure and static pressure at the equipment interface. For interface conditions, the static pressure is usually the significant value as it is that value which would be determined by the standard line pressure loss calculations used by the client for the same interface.

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