Control system designed to cope with offshore conditions

Midland-ACS, working closely alongside Citech (formerly Alstom Energy Systems SHG Ltd), has developed a control system that was chosen by the gas turbine heat recovery specialist to control its innovative adjustable cylindrical damper design. The Citech heat exchangers are for Qatar's Al Shaheen field offshore facilities, operated by Maersk Oil, as part of a $5bn project to increase the output to 525,000 barrels per day from its current level of 240,000 bpd.

More usually found in oil refineries, a total of nine large-bore cylinders, complete with three control panels, were provided to Citech to lift the cylindrical damper on the heat exchangers.

Integral linear transducers

Manufactured to be Zone 1 (Eexd) compliant with ATEX-approved electrics, the cylinders have a bore of 160mm and a stroke of 916mm, lifting vertically to control each damper. Made from an epoxy-coated steel barrel and incorporating a hollow 316 stainless steel piston rod that houses a linear transducer, the cylinders can operate in temperatures ranging from -20 to +80degC. Each bank of three cylinders is controlled by a separate control panel. The linear transducers contained within the cylinders manage the stroke in the damper cylinder to control the heat exchange process. Boosters used in conjunction with 'smart' positioners (with Hart compatibility) give a fast speed of operation.

The Hart (Highway Addressable Remote Transducer) protocol was chosen for its 1200 baud frequency shift keying (FSK) process to superimpose high-resolution digital information onto a conventional 4-20mA analogue signal. This allows both intelligent digital field devices and current generating physical actuation signals to integrate seamlessly in a wider automation system.