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Gas to Flare / Depressurization

Providing isolation vales and control valves for process safety in flare systems

Process

The objective of a flare system is to burn waste gases that are either uneconomical to recover / retain—or released during planned or unplanned over-pressuring of plant equipment. Waste gases are released to the flare via flare headers and burned as they exit the flare stack

One of the major safety issues addressed with utilizing a dependable isolation valve is flashback protection. In this situation, absolute tight shut-off is required. However if de-pressurization is needed, a reliable severe service rotary control valve may be necessary.

In systems where the pressure letdown ratio is between 0.5 and 1, the three elements that need to be controlled to avoid a catastrophic failure are noise, energy absorption and velocity. In any gas flare system the reduction in pressure leads to an increase in volumetric flow rate that needs to be calculated and designed into the system.

Engineered Solutions

MOGAS offers both isolation and control valves for important process safety associated with flare systems.

In the some low to medium pressure letdown systems, fast-operating isolation ball valves are used in conjunction with expanding pipe and manifold layouts to effectively reduce pressure and have allowances for increased volume flow. Flaring is an intermittent operation, so an isolation valve that is ‘robust’ in its design, coupled with the expanding pipes and manifold, can produce an effective flare letdown system that would meet the requirements of the API 521 standard. MOGAS C-Series isolation valves are built to withstand the high stresses that exist in such a system. It manages this through integral actuator mounting flanges, upper and lower steam bearings and an overall design that includes larger design ratios to add mass to the body and ball making it an a low cost of ownership reliable product.

Some flare systems require a ‘controlled’ letdown/blowdown. In these systems controlling the letdown of high pressure to atmosphere requires a control valve that has an ‘intelligent’ pressure control philosophy. MOGAS valves that incorporate FlexStream technology—with its innovative flexible trim element design—allows the trim to have differing numbers of pressure letdown stages across the diameter of the trim. This enables the highest amount of noise attenuation and energy reduction to take place at the start of flaring. As the pressure reduces in line with API 521, the FlexStream trim element will then have steadily increasing flow capacity as it opens, allowing lower pressure higher volumetric flow to pass and affecting a complete controlled letdown. The same features apply to the FlexStream control ball valves that are found in the C-Series isolation with the added actuator control system that controls the flare letdown over a specified time.