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Optimal Flare Control Valve Bypass Solution

By Fike’s Roger Bours, Director Pressure Relief, and Michael Hunt, Business Development Manager

A gas flare, also known as a flare stack, is a gas combustion device used in industrial plants such as petroleum refineries, chemical plants and natural gas processing plants. They are also common at oil or gas extraction sites having oil wells, gas wells, offshore oil and gas rigs and landfills. In industrial plants, flare stacks are used for burning off flammable gas released by pressure relief devices during unplanned over-pressurizing of plant equipment. Flare stacks are also used during (partial) plant startups and shutdowns for planned combustion of process gases over short periods. At oil and gas extraction sites, flares are similarly used for a variety of startup, maintenance, testing, safety, and emergency purposes.

A flare system collects and discharges gas from process components to the atmosphere at a safe location for release during abnormal conditions and emergency relief. Flare systems generally have an ignition device for the gas exiting the system, as the discharge may be either continuous or intermittent. A flare system from a pressurized source may include a control valve, collection piping, flashback protection, and a gas outlet. A scrubbing vessel removes any liquid droplets that carry over with the gas relief sent to the flare. A plant relief system often has multiple flares to treat the various sources for waste gases.

A flare system is used for normal process releases and for emergency releases. Emergency streams, such as those from pressure relief valves and depressurizing systems must have flow paths to the flare available at all times.

The piping arrangement typically consist of a primary and a secondary path to the actual flare in order to maximize the safety aspect. The primary closing item is a leak tight, quick opening valve, normally referred to as a Fast Opening Valve (FOV) or Main Flare Valve. This device will open on high-pressure detection, created by compressor trips or in the event of other process and emergency shutdown signal from the control system. The flare isolation valve has a ‘fail open’ design to secure a free path to the flare at failures such as loss of instrument air or activation energy. Typically, butterfly-style valves are selected for this application with an estimated opening time of some seconds. Once activated, the FOV remains open until reset after normal operating conditions are reestablished.

To ensure that in the event of the FOV failing to open on demand, a secondary pressure protection device is bypassing the FOV, providing an alternative path to the flare. For reasons of redundancy, the secondary closing items are non-re-closing items such as Rupture Discs (RD) or Buckling (or rupture) Pin Valve (BPV). This design concept is specifically recommended by ISO 25457, a leading guide for flare system design in the petroleum and petrochemical industry.

The alternative buckling or pin valve is essentially an offset butterfly valve, which could potentially fail to open for the same reasons as the primary FOV fails. This can be due to plugging, corrosion, Iron Fluoride build up or many other possibilities that prevent a mechanical device from operating properly. A rupture disc is not dependant on a specific sequence of mechanical actions as required by the FOV and is by far the faster acting mechanical device. Most engineers consider a Rupture Disc to be the most reliable secondary protection, designed to operate in the event that the FOV fails to open.

System designers and users should be aware that failure mode risks determined for the FOV may also apply to the BPV due to design concept similarities. This would seriously question the redundant character of the secondary relief path, especially taking into account the complexity of design and number of critical components used in BPV compared to RD.

The RD should be set as high as possible to prevent activation and simultaneously avoid overpressure of the closed part of the flare system. Continuous purge, normally by nitrogen, is required downstream of the FOV when it is closed to prevent air ingress into the main flare stack, and also to remove any residual, unburned hydrocarbon gas.

Furthermore, manufacturers of BPV devices will specify that the seals used in the unit must be replaced after a specified number of activations, requiring the BPV to be removed from the process line. Such action will require skilled technicians and lifting equipment, whilst rendering the installation out of function for up to 24 hours. RDs are of a much simpler design where replacement after activation can typically be done in a matter of 1 or 2 hours.

Today, system designers are challenged to apply highest safety whilst keeping cost under control. Cost is defined as total cost, including investment cost, maintenance cost and downtime cost. When looking into the total cost a bypass system using RD instead of BPV could not only be the undisputed winner but also would provide maximum safety.

The RD should be set as high as possible to prevent activation and simultaneously avoid overpressure of the closed part of the flare system. Continuous purge, normally by nitrogen, is required downstream of the FOV when it is closed to prevent air ingress into the main flare stack, and also to remove any residual, unburned hydrocarbon gas.

The recent developments and use of state-of-the-art technology has made Rupture Discs the best when it comes to reliability and user-friendliness. Fike’s Axius and Atlas Reverse Acting Rupture Discs with precise opening pressures, elevated backpressure resistance and unsurpassed pressure cycling capability are widely accepted to offer best protection of flare bypass systems. Their low need for maintenance and easy installation will add to the low cost of ownership whilst elevating the reliability of the system to the highest possible level.