A flair for problem solving at Grangemouth
15 Jan 2000
The most striking feature of many chemical plants and refineries are the flares that burn off excess gases. They are also among the most safety-sensitive. The gas flare system at BP’s Grangemouth complex is no exception.
The flare system operates on the pipelines that bring crude oil to the complex from the North Sea Forties field. The oil also contains tonnes of gas, which is used at the complex. The flow of oil cannot be stopped, even if the volume of available gas exceeds the demand. In this case, the gas is sent to four large ground flare boxes, each containing internal burner rails to mix the gas with air, ensuring smokeless combustion. If this system fails, the gas is forced up a 300ft tower to an elevated flare. There is no margin for error, so BP installed a control system that simply shut the whole process down in the event of power interruptions or other emergencies.
However, explains Ewan MacAllister, the technical director of system integrator Elite Control Systems, this means that some systems that worked perfectly well were being shut down. Elite redesigned and reconfigured the system to introduce `passivation groups.’ Using Siemens S5-115F PLCs, Elite designed a new system whereby the PLCs keep the gas valves to the various units of the flare system open, and simultaneously keep closed valves to a nitrogen supply in the flare boxes. In the event of a failure, the gas valves shut and the nitrogen valves open, cutting off the combustion supply and smothering any chance of a fire. `The original system was configured in such a way that, in the event of a failure, it simply shut down the whole system - everything. Now, if a fault were to develop, that group in which the fault lies, and that group only, would be shut down,’ says MacAllister.
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