Power Plant Instrument Air System Design

Instrument air system in power plant is one of compressed air system. Instrument air system should be provided for control and instrument equipment. The equipment of instrument air system in power plant as follow:

  • Compressors with oil free screw type and has sufficient capacity.
  • After coolers and separators
  • Pre filters
  • Refrigeration type air dryers.
  • Air receivers
  • All piping, valves and supports in accordance with relevant sections of this specification.
  • All instrumentation including fitting, valves and installation material
  • Acoustic enclosure to house compressors and ancillary equipment
  • All electrical equipment and wiring associated with the above equipment, including local compressors control and selector panel suitable for compressors.

Isolating valve (locked shut) and check valve for connecting to an emergency air supply from the service air system to instrument air system should be provided in the power plant.

From the outlet of each receiver, a common header shall be equipped with the following connections but not limited to:

  • Isolating valve for Boiler
  • Isolating valve for Turbine
  • Isolating valve for CW pump house
  • Isolating valve for Water Pretreatment Plant
  • Isolating valve for Fuel Oil Storage area
  • Isolating valve for WaterTreatment Plant
  • Isolating valve for Chlorinationsystem
  • Isolating valve for Spare connections

The instrument air requirements for the turbine unit shall be taken from the appropriate connections provided on the air receiver common header.

Pipe work shall be sized and route to ensure minimum pressure losses throughout the system. All take off connections from horizontal headers shall be from the top of the main. Cooling water supply (if required) should be provided from closed cooling water system.

After Coolers and Separators

Each compressor shall he supplied with an after cooler and moisture separator complete with all necessary fittings, relief valves and automatic drainage facility.


In each of the three compressed air discharge mains positioned after the moisture separators shall be provided with a removable cartridge type filter. Each filter shall be capable of removing oil, water, dirt and other extraneous material from the compressed air.

Air Dryers

In each of the compressor discharge main shall be provided refrigerant type air dryer after air receivers. The air dryer should be supplied complete with refrigeration system, self- regulating hot refrigerant gas by-pass valve, chiller section, separator automatic condensate discharge trap and complete control system including moisture indicator, power-on light and safety switch. The air dryers shall be completed with by-pass piping and valves to facilitate servicing of each unit.

Air Receivers

The instrument air system can be provided as vertical air receivers. The receivers should be of all welded construction and shall conform to the requirements of ASME Section VIII “Pressure Vessel” Division I. The receivers shall be supplied complete with the outlet connections, manhole, drain connection, pressure gauges, relief valves and instrument connections.


Each compressor shall deliver oil free air and shall be supplied with the following;

  • Water cooled intercooler with safety valve and moisture separator
  • Air intake filter and silencer to reduce noise
  • Unloading mechanism and piping
  • Relief valve
  • All control (local control panel), electrical and protection equipment to ensure safe and reliable operation of the plant.
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