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Pilot and Ignition Systems

With regard to flare maintenance, operational efficiency and environmental emission assessment, pilots and ignition systems must be separated. The functions of each are independent. The pilot system requires pilot gas as a utility under pressure to inspire air to mix in the nozzle with the raw gas. The combustible mixture burns at the pilot head. The function of the pilot is to maintain flare stability, re-igniting the flare if required. The pilot, however, is not an ignition system. In the experience established in Norway of zero flare systems, pilots in general are removed as the flare is only called upon to operate under high flow conditions. In practice, however, many pilot systems operational in the North Sea areas fail after only a few years of service and in many cases, if replaced by a more reliable means of igniting the flare, can be safely removed.

The ignition system is a system used to light either the pilot, or the flare direct, under any weather condition, as and when required. The ignition system must be highly reliable. Some ignition systems require a pilot. Such systems light the pilot first, and require the pilot to be constantly lit. Other ignition systems light the flare itself and as such are independent of the pilot. Such systems can and often are used without pilots.

Figure 4: A Typical Flare Installation in the North Sea

Electronic Ignition Systems

There are two basic forms of these systems using either high-energy or high voltage. Both forms of electronic ignition systems are, in the main, pilotbased systems that are used to light the pilot. Overall, all spark igniters of this category do rely on the spark generation equipment being located in close proximity to the pilot. Thermal damage to the pilot, the igniter ceramic rods, or electric cables are all frequent causes of failure. In addition, frost and vibration caused by high flowrate flaring can cause damage to the ignition system ceramic rods. Similar systems that may be independent of pilots also suffer frequent failure due to flame impingement as well as vibration during flare operation.

Flame Front Generator

The most widely used flare pilot ignition system is the compressed air flame front generator. The popularity of this system is generally due to the historical fact that these systems were the first to be developed and remained as the only ‘reliable’ ignition system until the 1980s. With this system, compressed air (generally instrument or plant air) and fuel gas are metered through control valves into a mixing chamber located on a panel in a safe area. The principal advantage of the compressed air flame front generator is that the flow controls and the sparking device are in a safe area and can be serviced while the flare is in operation. However, difficulty in operation and requirements for further system utilities and piping on the flare risers have made these systems less popular. Also, these systems are dependent on the pilot system reliability.

Pellet-based Ignition Systems

As part of the zero flaring developments in Norway, Statoil and ABB GT have developed an alternative ignition system designed to offer increased reliability. The system, known as the USIS 2000 ignition system, utilises a launching panel located in a safe area that will either manually or automatically (on demand) launch a pellet. The pellet is pushed through a guide pipe to the edge of the flare deck. When the pellet leaves the confines of the guide pipe, it is triggered to ignite, burning thousands of particles of zirconium. The system lights the flare direct and is independent of the pilot and as such, may be used with or without pilots.

Conclusions

To conclude, the flare system is an essential safety part of any offshore or onshore petroleum facility and is able to relieve gas from the process during upset or emergency situations. There is, however, a marked difference between the operation of flare systems in Norway and the UK. In the UK, flare systems have contributed consistently in the region of 20% of petroleum operations’ CO₂ emissions, whereas in Norway the figure is reduced to 10%. In addition, the Norwegian petroleum operations are achieving lower maintenance cost from their flare systems by introducing zero flaring on many installations. Pilot-based ignition systems have been replaced by ballistic ignition systems eliminating a high cost maintenance area of operation. The drive behind zero flaring in Norway in earlier years was the introduction of CO₂ taxation. However, the level of maintenance benefit and improvement in technology has led to many new installations worldwide to specify zero flare systems. It is now time for all existing operational systems to reevaluate the potential maintenance benefits in adopting at least the lower cost ballistic ignition systems, or the full zero flare solutions. This change, if widely adopted in the UK, will cut CO₂ emissions from 20% to 10% of petroleum operations’ total emissions and will hence reduce the UK total CO₂ emission by a small but significant level, whilst reducing operational cost.

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