Gas systems specialist Wärtsilä Hamworthy attributes its recent success in securing significant flare gas recovery (FGR) system contracts with Petrobras, Statoil, BP, ConocoPhillips and other operators to three key elements in its FGR system: flare closure, recovery equipment and a reliable ignition system.
The company has a list of the do’s and don’ts of flare gas recovery. In the first instance, flare recovery, the flare is ‘tight shut-off’ during normal operations though in some instances it may be partially shut off if there are any constraints in the process system limiting the recovery rate.
In regard to recovery equipment, the allowable operation pressure in the flare system and the process tie-in pressure determine the recovery solution. A simple low cost crossover line may be used to recover the flare gas if the flare system can tolerate increased operation pressure and the process tie-in pressure is relatively low. Boosting equipment is required if this is not possible, including a blower, an ejector or a compressor, to boost the flare gas from the flare system pressure to the process tie-in pressure.
When introducing FGR, it is also important that the flare gas is ignited by a reliable ignition system when reverting from recovery to flaring mode. An ignition system excluding the need for lit pilot burners is preferred. The ignition system should be initiated automatically upon demand.
After installation of an FGR, the gas previously flared would be available as fuel gas or export gas and would generate extra income. When closing the flare line during normal operation the flare tip wear would significantly decrease, usually lasting the lifetime of the field.
A conventional flare system is used both for normal process releases and emergency releases. Emergency streams, such as those from pressure relief valves and depressurising systems must always have flow paths to the flare available at all times. The flare closing arrangement should always consist of a primary and a secondary path to the flare in order to maximise the safety aspect.
The primary closing item should be a highly reliable, leak tight, quick opening fail open valve, normally referred to as a fast opening valve (FOV). An alternative to the FOV might be a water seal drum, but this solution is normally not recommended because of its narrow control range and the possibility for liquid freezing and liquid carry over.
The FOV will open on high-pressure detection, as a result of compressor trips or in the event of any other given process and emergency shutdown signals from the control system. The flare isolation valve has a ‘fail open’ design to secure a free path to the flare at eventual failure such as loss of instrument air. The FOV is normally equipped with limit switches for both open and closed position for monitoring of the valve position.
A secondary pressure protection device is by-passing the FOV providing a free path to the flare in the event that the FOV should fail to open on demand. The secondary closing items may be non re-closing items as bursting discs or buckling pin relief valve or pilot pressure safety valves.
The pressure in the flare system including the flare kick-off drum is controlled to a predetermined operating pressure with the introduction of FGR. The flare system will operate at a constant pressure during flare gas recovery mode. The recovery equipment will, in general, be able to handle gases with fluctuations in composition, temperature and flow rates. FGR equipment, typically, is not designed for emergency flare loads. Normal rate to flare will typically vary widely and the recovery equipment will typically be designed to operate over a wide range of dynamically changing loads.
Wärtsilä Hamworthy’s pellet type ignition system was introduced in the North Sea in 1995. The installation of an FGR system would result in the flare normally being unlit - the consequence being that the ignition system had to be completely reliable. A review of systems at the time led to the conclusion that such reliability was unattainable and the first ignition system was developed to establish the required high reliability.
The business has developed two generations of ignition systems.
guided launching of ignition pellets by high pressure nitrogen;
guided launching of ignition pellets by low pressure air.
The flare ignition system, protected by international patents, guarantees safe ignition of the flare whenever required. It is designed to ignite the flare gas exiting the flare tips in a reliable manner. The reliability of the flare ignition system is high enough to obviate the need for pilot burners which might be considered as an extra precaution for flares with a high H2S content. The fragment collector, located next to the flare tips is designed to withstand the maximum heat emitted at the maximum flaring case.
Significantly, the system does not require any sensitive equipment such as instruments, cables or other rigorous ignition or detection items near the flare tips.
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