Control Air System
Control Air System
A branched airline through a pressure-reducing valve is supplied to the Control Airline. Pneumatic control equipment is sensitive to contaminants that may be in compressed air. Viscous oil and water emulsions can cause moving parts in control equipment and control valves to stick and produce general deterioration of diaphragms, spools, and other parts made of rubber.
Water can cause rust build-up which may also result in parts sticking or being damaged by these rust particles. Metallic wear and other small particles can cause damage by abrasion. Any solids mixed with oil and water emulsions can conspire to block small orifices.
Clean and dry control air is thus essential for the trouble-free operation of control-air systems. When the source of control and instrument air is the main air compressors and the main air reservoir itself, then a special provision is necessary to ensure that air quality is high.
The pressure-reducing valve which brings the main air pressure to the 7 or 8 bar required by the control air system, can be affected by emulsion carryover and can require frequent cleaning to stop air contamination.
Large amounts of free moisture and oil emulsion carry-over in the air can be removed by special control-air membrane filters installed in the control air line.
Control Air Filter
A typical control air filter arrangement consists of an oil & moisture collecting filter followed by a membrane air dryer filter. The treatment of air through these membrane filters results in the air being filtered and dried in order to remove virtually all traces of oil, moisture, and air impurities.
A simple line air filter is provided with a small plastic float and auto drain arrangement. The filter can also be drained manually if the vessel enters a highly humid environment and frequent draining is required. The filter dryer unit is made up of a primary filter, a secondary filter, and membrane hollow fibre elements.
Air Flow Through The Control Air Dryer
The control air enters the dryer chamber through the line filter located in the lower part of the dryer unit. In the dryer unit, the primary filter removes the coarse rust particles, dust and other larger impurities. The secondary filter acts like a coalescer, separating water droplets and oil mists up to 0.3 microns.
A differential pressure gauge indicates the condition of the primary and secondary filters. A higher differential pressure indicates a dirty membrane filter. The membrane elements are to be renewed as per the ship’s PMS.
Piping
The high-pressure air piping from the air compressor to the receiver should be as smooth as possible without any bends in the pipeline so as to allow air to flow freely to the receiver without restrictions. Bends in the piping can create backpressure in the line in case of accumulated moisture or oil emulsion in the line.
