Air Compressors and Dryers

Three air systems on a rig

• Working air (rig air) at about 7 to 10 bar (100 to 145 psi) runs air tools, air hoists and winches, and pneumatic transfer of barite, bentonite and cement.
• Instrument air comes from the same or dedicated compressors. It passes through dryers and filters to remove water, oil and particles before reaching control valves and pneumatic controls.
• Starting air at about 30 bar (435 psi) comes from two-stage or three-stage piston compressors from makers such as Sauer and Hatlapa. It is stored in receivers sized for a set number of consecutive engine starts.

Configurations

• Oil-injected screw compressors, such as the Atlas Copco GA range, cover most rig air duty and cost less to maintain.
• Oil-free screw compressors, such as the Atlas Copco ZR range, give air with no oil carry-over for instrument and breathing air supply chains.
• Reciprocating piston compressors give the high pressure needed for starting air.
• Refrigerant dryers reach a pressure dew point of about +3°C (37°F). Heatless or heated desiccant dryers reach -40°C (-40°F) or lower.

Key specifications

Standards and what they require

Class rules treat starting air as essential. They require at least two starting air compressors where engines start on air, and receivers sized for a number of consecutive starts. Air receivers, dryer towers and filter housings are pressure vessels. They need certificates to the design code (ASME VIII, EN 13445 or class rules), safety valves and periodic inspection. ISO 8573-1 sets purity classes for particles, water and oil. ISA-7.0.01 recommends an instrument air dew point at least 10°C (18°F) below the lowest temperature any part of the system sees. A compressor in a hazardous area needs ATEX or IECEx certification for its motor and controls, and a non-electrical ignition assessment.

Failure modes and inspection

• Screw airends. Rotor bearing wear shows as rising vibration, noise and discharge temperature. Rotor contact damage follows if it is not caught.
• Oil-injected units. Check the separator element, oil carry-over, and the thermostatic and minimum-pressure valves.
• Piston compressors. Check valve plates and springs, rings, liners, intercooler and aftercooler tubes, and the drain valves.
• Dryers. Check desiccant condition and dusting, switching valves, purge flow, and the dew point sensor calibration.
• Receivers. Internal inspection and thickness gauging, drain condition and safety valve test.
• Controllers. Check the version, sequencing between units and spares availability.

Documentation

• Running hours and airend or block overhaul history
• Oil analysis for larger screw units
• Pressure vessel certificates and the last inspection report for each receiver and tower
• Safety valve test certificates
• Class certificate for starting air compressors
• Dew point and purity test results for instrument air

Integration and compatibility

Check the motor voltage, frequency and starting method against your board. Check the cooling water circuit and materials for seawater-cooled units. Match connection sizes and pressures to the existing ring main and receivers. A new compressor should join the existing sequencing controller or run under its own master control. Bulk transfer demand peaks are short and heavy, so check receiver volume as well as compressor flow.

Overhaul and spares

An airend exchange or overhaul replaces bearings, seals and sometimes rotors to OEM tolerances. Piston compressors are overhauled to OEM wear limits on valves, rings and liners. Unused surplus compressors and dryers from cancelled projects are good value if voltage and frequency match. For starting air, buy only a unit with class documents. The engines it starts are under Main diesel engines, and the bulk tanks it serves under Bulk storage tanks.