Analytical Instrumentation & Environmental Monitoring


Flare stack gas analysis

April 2013 Analytical Instrumentation & Environmental Monitoring

A flare stack or gas flare is an elevated vertical stack or chimney found on oil wells and oil rigs and in refineries, chemical plants and landfills used for burning unwanted gas and liquids. It is primarily a safety device through which a sudden excess of gas can be removed and burnt in an environmentally friendly manner.

During processing, hydrocarbons are often put under pressure into storage vessels, when this pressure becomes too high it has to find an outlet through a safety valve. These valves are connected to a flare stack by large piping runs, known as flare or vent headers. Inside the flare stack combustible gases mix with oxygen and ignite by contact with a pilot flame at the top of the stack. In order to keep the flare stack functional a small amount of waste gas is continuously burned, so it is ready for its primary function as an over pressure safety system.

Some flares are used to burn off waste flammable gases or by-products. This practice is being reduced around the world in favour of gas recovery systems that reduce the waste and emissions, though flare stacks still fulfil an important role in many plants.

The correct mix of stream and hydrocarbons guarantees clean combustion, a clear flame and no harmful emissions to the atmosphere. The analysis of flare stack gases generally falls into two categories:

1. The measurement of oxygen to prevent the build up of a potentially explosive mixture of flammable gas and oxygen building up. Such a mixture could ignite at the burner causing a flash-back of burning gases into the plant. Typically, alarms are generated by the analysis equipment when the oxygen level exceeds about 1%.

2. The measurement of hydrocarbon components. The variance of gases to be destroyed in the flare may generate instability within the flame and only be partially burnt. In certain types of chemical plants, it is necessary to know the composition of the bulk gas mixture being fed to the burner.

Servomex paramagnetic, laser and infrared analysers allow these measurements to be performed close to the flare stack. The ServoToughE SpectraExact 2500 is used to measure hydrocarbons or total hydrocarbons in the header to the flare stack to optimise and guarantee combustion, ie, control of the steam. Excess steam is wasting money. Not enough steam means incomplete combustion. The SpectraExact can be used to help prevent both of these conditions.

The Servomex 2500 series of infrared analysers can be configured to analyse many of the components present in gas feeds to flare stack, as well as solvents which may also be vaporised and flared. Typical gasses measured are ethylene, whilst solvents include acetone and isoproponal. The infrared analyser can also be incorporated into the sampling system used with the oxygen analyser, further reducing installation and overhead costs.

The analyser employs a single beam, dual wavelength measurement technique, which gives increased measurement sensitivity and a high degree of stability. This technique is virtually unaffected by contamination on the sample cell windows. Maintenance requirements are low and the cells can easily be changed or cleaned. The analyser utilises a rugged construction and is certified for use in hazardous areas.

For more information contact Chantelle Conradie, Elemental Analytics, +27 (0)11 918 6994, info@e-analytics.co.za, www.e-analytics.co.za



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