A flare FOR SAFETY

Flare and vent design has historically been based on the need for safe disposal of gas for routine or emergency purposes. These requirements have been determined largely as a consequence of the main plant design and are considered to be 'add on' features.

Plants are designed largely for economy of scale. Nowadays, they are more visible and more highly automated, incorporating fail-safe features which often lead to increased flaring or venting of gas. There are ever increasing environmental pressures and controls on hydrocarbon and CO2 emissions.

Flare and vent systems, therefore, need to be planned as an integral part of a process plant; changing circumstances demand a new approach. The systems themselves are often the source of hazard, particularly during maintenance operations, and under emergency high discharge conditions.

They both generate noise. Flares also produce heat and light. Noise is becoming a bigger issue offshore as the 1989 Noise Regulations are this year being applied offshore. Heat radiation is also a problem, requiring sterile areas, refuges, radiation shields and restricted access, depending on location. Light and smoke create a nuisance onshore with local communities. Perversely, it seems that the less frequent the vent or flare operates the greater may be the nuisance and the resulting complaints.

Overall, flaring is preferable to venting as CO2 has less global warming potential compared to hydrocarbons. Environmental considerations should not compromise safety, but the application of emission reduction strategies should cut waste and contain hazardous gases.

It is essential during plant commissioning of any facility that particular attention is given to the flare or vent system. Once the plant goes 'live' then interventions cannot normally be made without complete plant shutdown. Start-up is the first opportunity to damage the flare or vent system by internal explosion. This can arise from inadequate purging, air ingress from a vacuum caused by gas density differentials, the weather or the 'chimney effect'.

In normal operation, continuous purges and excess gas have been identified as the major emissions from flares and vents. But, normal operating conditions may be significantly different from the design or emergency condition for which the flare was selected. Flare manufacturers complain that they often have insufficient information on the operating range of the flare and gas to be burned, and have to quote for a single, often, emergency operating condition. It is not surprising that turndown may be a problem in operation. Measurement of gas waste to the vent or flare needs to be part of the original design.

Maintenance operations, such as the removal of a relief valve or other component, may lead to the introduction of air into the flare system, with resulting flame back or explosion down the header. Moreover, performance tests may lead to overpressure of the system.

Whilst the flare or vent is conceived as a safe place for the disposal of plant contents in an emergency, incorrect design can result in the flare or vent system becoming the emergency itself. Typical causes of such 'own goals' are: liquid collection due to the header sloping the wrong way, vibration causing cracks leading to leaks, overpressure, flare blowout and ignition of the vent.

A novel Approach

Safety and environmental considerations need to be integrated with plant performance at the design stage, and through the life-cycle to abandonment including the flare and vent systems.

There are three main considerations in the optimum design of flare and vent systems: size; simplicity and reliability. Minimal inventory will require smaller flares and vents. Containment rather than blowdown should be possible particularly if the system is designed for full pressure. High integrity overpressure protection systems (HIOPPS) can reduce full flow relief requirements. As for simplicity, what you don't have requires no design, costs no money and can't go wrong. Thirdly, we need to eliminate unnecessary fail-safe devices and improve the redundancy of the rest to avoid unnecessary shutdowns. Overall, we need to design plant to operate safely not to fail safe.

Much has been achieved in improving gas processing efficiency and safety. The extension of a traditional approach to flare and vent design is no longer adequate, however, to meet the safety and environmental challenges of the future. Environmental discharge restrictions are already in place and in some countries taxes are levied. As a result some 'normally zero emission' flare systems have been designed. The diligent use of control valves and bursting discs allows full emergency flare but with no or minimal normal flare or purge loss.

We need to adopt a new approach to integrate safety, health and environmental considerations into conceptual design, through to abandonment and to be able to demonstrate through Risk Assessment, BATNEEC and BPEO that this has been achieved.

David Henson is Safety, Health and Environment Manager at Total Oil Marine plc, Aberdeen. Tel. 01224 858000.