Instrument Air Compressors (Instrument Air Systems)


(Controls & maintenance)


Parts and Component Description.

  • Air Dryers and Filters
  • Compressors
  • Air Coolers
  • Air Receiver
  • Moisture Trap

Operation and Maintenance
Automation and Controls
Rcm & Spare parts.
Problem Case Study.

Basic airflow diagram.
Component Description:

  1. Inlet air filter

The compressor inlet has a filter compartment to prevent any particle from entering into compressor.
Inlet air passes through air cleaner assembly fixed to the filter housing.
There is a “filter service gauge” or vacuum indicator fitted to most of the filter housing. The pressure at compressor suction is actuating the indication.
Should the filter get clogged, the pressure is reduced to below atmospheric due to the compressor suction.
The vacuum indicator goes to red flag if this happens. And this indicates that it is time to change filter element.
The filter is quick detachable. Needs no separate gaskets, and can be replaced easily.
During operation care should be taken that pieces of clothes, leaves, rags, or paper do not obstruct the face of the filter. Do not try to cover inlet during sandstorms.
Do not allow oily, corrosive, toxic or flammable gases to be available at filter inlet.
Filter element change frequency as per ªRCM recommendation. 
Some models incorporate a low suction alarm for air inlet filter instead of the vacuum indicator. The alarms will be annunciated on local panel conventionally or can be included in DCS.
ªRCM = Reliability Centered Maintenance.

  1. 2. Compressors

Prime mover for compressor is electric motor. The vendor in the manual will document all specifications of motor.
On operations point of view, the main attention goes to load of the motor. An ammeter is provided on each motor compressor assembly to indicate the current drawn by motor. The remote control unit (RCU) is provided for local start/stop and safety lock of auto start. The ammeter is mounted on the RCU.
When compressor is loaded, the ammeter shows the full load current and when unloaded, shows idle current.
The idle current will be much lower than the full load current
Compressor casing houses two rotors meshed together.
Oil is injected into compressor for lubrication as well as cooling. There is an oil stop valve on the compressor to stop the compressor from flooding with oil while it is stopped. The discharge pressure of compressor opens the vale. Only and if the discharge pressure at the compressor discharge (before check valve) develops more than oil separator pressure, the oil is allowed to compressor.
The oil circulation is boosted by air pressure.
A malfunction in oil stop valve may cause oil spray to reach air filter in a momentary reverse flow.
Compressor shaft is driving a fan for oil and discharge air coolers.

Compressor Compartment.

  1. 3. Coolers

Compressor discharge air will be hot and cannot be used by instruments. For this reason it requires the cooler that brings down the temperature. Cooler fan is driven by the prime mover and cools the air and oil in two different coolers.
Compressor discharge temperature indicator measures the temperature at compressor air discharge.
Oil temperature is measured at oil receiver.
Cooler performance can be assessed from the trend of these temperatures.

  1. Oil Separator

Lubricating and cooling oil injected to compressor is recovered for recycling by oil receiver. The air/oil mixture enters the separator through the side of the separator to make it flow in a circular path around the element.
The lower part of oil receiver serves as oil storage. Air pressure in receiver forces the oil through cooler, oil filter, and oil stop valve to compressor.
Injected oil, mixed with compressed air leaves the compressor element through discharge and re-enter oil receiver.
Here oil is separated from air.
Primary separation is centrifugal. Air entering through the side of separator swirls around the cylindrical element and the resultant force throws the heavy oil drops to the side of separator.
Secondly, the filter element allows air to pass to outlet; separating any oil mist or fine particle. The oil collected in separator element, moves through a scavenging line to reach compressor.
Separator is equipped with an oil level indicator.
While running on load most of the oil is under circulation and the level indication goes to low operating level. This will return to normal once the compressor is stopped.
A safety valve on separator limits pressurization of separator.
Air leaves the separator through a minimum pressure valve and then goes to the moisture trap.
The separated oil settles down at the bottom of separator and is forced out of separator to the cooler.
Comparatively smaller amount of oil collected at element base is also pushed out of separator through a scavenging line back to compressor.

The drain plug is for use during maintenance.

  1. Moisture Trap

Working principle

Moisture trap is a vessel with a float drain system. A float is keeping the drain- port closed, and when liquid collects in the vessel the float is lifted by liquid to allow it to drain. As soon as the liquid is drained, the float restores its previous position closing the drain.
Compressed air that is hot, enter the trap vessel (comparatively less hot) and moisture in air condenses. This water is collected at the bottom, as level of water increase the float gets lifted opening drain.
Some models have a manual drain too.
The air is fed to an air header where other compressor discharges may join and the header is connected to receiver. On this common header collecting individual compressor discharge a pressure-sensing device is mounted. A set of pressure switches or pressure controllers are common devices found in this service. Load/unload logic receives signal from this.

  1. Air Receiver

Receivers are volume vessels storing compressed air. This introduces capacitance in air system to reduce sudden pressure fluctuations. The storage provides a time lag in pressure drop.

  1. Filters

Filters are screens made of fiber web. Webs of micro fiber are placed one above the other to make filter body. Cylindrical filter is mounted in a casing fixed to the base block. The casing and filter element can be removed for service.
Air filter assembly comprises of two filter units in parallel.
Compressed air is directed into the inside of filter element. Air must pass through the element from inside to outside. Solid particles remain in the fiber web and clean air flows out through multi stage matrix of micro fiber sandwich.
After extended service life, the fiber web begins to clog with the solid particles that are remaining on it due to filtration. ΔP (differential pressure) across filter increases.

The element needs replacement when Δp increases more than recommended value. Usually >1Bar.


  1. Air Dryers

Each skid invariably is equipped with one dryer unit. The dryer is a set of two columns packed with desiccant (activated alumina or silica jell).
One column (one in drying mode) allows air in at the bottom and air travels up wards to exit dryer.
While flowing through desiccant, water particles adhere to surface of desiccant granules. Water is retained and dry air leaves the column.
After few minutes the desiccant surface is saturated with water and it cannot dry the air further. Then this column is isolated and the other column is put in service.
The first column containing granules coated with water can be made ready for drying again by removing water. This process (regeneration) is achieved by allowing dry hot air to flow from top to bottom (reverse of drying flow) in higher velocity.
Bottom side is opened to atmosphere, column depressurized and air is allowed to flow from top.
After regeneration time is elapsed, column is restored to drying mode and the drying one is taken for regeneration.
Column change over is done automatically by control logic.

Control functions related to each component is given below. Some are mandatory and some found only in certain models. Refer pertaining manual before attempting study.

  • Air filter.
  1. Vacuum indicator.

This is a transparent flag indicator made of polyurethane like material. It senses the pressure at the point to which it is connected and a red flag (strip) is made visible if the pressure crosses the set value.

  1. Low suction pressure switch.

A low range pressure switch that changes over a set of contacts if the pressure it sense is crossing the set value. The contacts are wired to alarm panel.
Either of these is to show the suction pressure of air compressor. While the compressor is drawing air from the atmosphere any blocking in inlet filter will cause a pressure drop at compressor suction. The pressure at space between after filter and before compressor suction is reduced below ambient. An alarm / indication is initiated.

  • Compressor


  1. Load / unload control.

Compressor maintains the air pressure in receiver by adding pressure whenever the pressure goes down.
Therefore, two set points are required to allow compressor to add pressure and stop adding.
One setpoint where the compressor starts adding pressure, and another where it stops adding, so that pressure in receiver is maintained within those limits.
The lower set point is called loading pressure. Set points are from pressure sensing devices mounted on header entering receiver.
Pressure in receiver is consumed by station instrumentation and begins to reduce. When it falls to loading pressure, a signal is sent to logic circuit. The logic circuit opens the inlet valve to compressor allowing air. Now compressor is loaded and starts to add pressure to receiver. Pressure in receiver gradually increases.
When it reaches second set point the unloading pressure, a signal is sent to logic thereby closing the inlet to compressor.
Air inlet is stopped and compressor is unloaded so that adding pressure to receiver is stopped.
The pressure in receiver is maintained within loading and unloading pressure setpoints.

  1. Oil stop valve

Oil injected into compressor for lubrication as well as cooling enters the compressor through an oil stop valve.

Demand for oil for compressors while loaded, and while unloaded are not same.

More oil is required while compressor is loaded. On loading, the compressor develops pressure at discharge and this pressure opens the oil stop valve to allow oil to compressor.


  1. Check valve

The compressor does not develop pressure while unloaded. Header after discharge of one compressor is connected to system and to avoid system pressure flow back to compressor, a one way valve (check valve) is installed on compressor discharge.
Malfunction of oil stop valve may bring oil spray back to inlet air filter. This happens when loading valve opens and compressor begins to develop pressure. Immediately after valve opening, during the moments the compressor build pressure, separator pressure is higher and oil may splash back to filter.

·         Coolers

  1. Oil filter bypass

A built in valve on oil filter will open on excessive pressure drop across filter.

  1. Cooler bypass

When oil temperature is rising more than 70˚C a built in temperature-sensing valve directs the oil flow through cooler.

·         Oil separator

  1. Differential pressure

The differential pressure across oil separator is measured, indicated or alarmed. This is for monitoring separator element performance. Element clogging will cause a higher-pressure drop.

  1. Minimum pressure.

The oil separator during normal running is not allowed to depressurize fully. A pressure of 4 Bar is retained in it by minimum pressure valve. This is to ensure that the oil has enough pressure to lubricate compressor.

  1. Safety valve

To prevent over pressurization of separator, a relief valve is provided.

  • Moisture trap

Level control of moisture trap is by operating a float automatically.

  • Air filters

Differential pressure across filters is indicated using Δp gauges or transmitters.
There is a purge air filter in the air supply line to solenoids.

  • Air Receiver
  1. Pressure control

Pressure control of receiver is by loading/unloading of compressor. There are different methods and devices used to attain control.
Refer manual of the correct make and model of compressor in use.

  1. Two pressure switches are set at loading and unloading pressure settings. The contacts of switches change over when pressure crosses the set value. 24 V dc power from control logic circuit is switched “on” and “off” by the pressure switches. The “make” or “break” of switch contacts are signals that are going to logic circuit. Correspondingly, logic gives an electric power output signal to a solenoid. (Solenoid is a valve, opening or closing when electric supply is given/removed). The solenoid opens a small airline to the loading / unloading valve piston to operate it.
  2. Two pressure transmitter-controllers set at load/unload values give pneumatic output that operate pressure switches and give signal to logic.
  3. One differential gap controller operating pressure switches to give signal to logic.


  1. Liquid level.
  2. A sight glass is provided for the receiver to see water level if any occurs.
  3. Auto drain and manual drain are used to remove liquid.


  1. Contingency

Limit of pressurization is achieved by relief valve.

  • Air Dryers

Dryers need change over for full drying capability. Change over is done by closing/opening the inlet and outlet of dryers. Four piston-operated valves do this job corresponding to signal from a timer.
Timer change over a set of contacts at regular intervals that energizes and de-energizes two solenoids. Solenoids give air supply to piston valves in inlet and outlet of dryers.
A transmitter measures differential pressure across the dryer. Dryer bypass will open if differential pressure increases more than set value.

  • Standby control

In two compressors skid one takes the load while the other is standby. In case the running one fails (reset is gone due to any trip alarm) the other starts automatically. In case of header pressure falls further below a setpoint set at a pressure switch on header, the standby cuts-in.
Duty/standby selection is provided for each compressor separately and any one can be selected as duty/standby.
Start / stop is controlled by logic circuit. Logic circuit is powered by a 24-volt direct current supply. This is denoted as “24 V dc”.
The control logic also incorporates Duty Standby assignment to compressors as appropriate to selection. Instrument relay panel holds the control buttons, and establishes interface between electrical circuit breakers and push buttons.
Trips caused by malfunction can be “reset” after attending and rectifying the defect.
In three compressors skid, standby selection is called “sequence” selection. Sequences are labeled as a) “1-2-3”, b) “2-3-1” and c) “3-1-2”.
This means that in sequence “a”, number one compressor will run continuously supplying the base load, the second compressor runs loading/unloading and the third compressor stays as standby.  Selecting to b) position, the second compressor starts running as main base load duty, the third compressor runs as loading/unloading and first compressor standby. Selecting to c) position, the third compressor starts running as main base load duty, the first compressor runs as loading/unloading and second compressor standby.

  1. Compressor is protected against high discharge temperature by a temperature switch (for trip) mounted on compressor exit pipe.
  2. Oil separator pressure and compressor discharge may rise with out control if the separator element is clogged. For protection against this a differential pressure switch stops the compressor when pressure difference across element goes above setpoint.
  3. Over pressurization protection of separator is by a relief valve that is mounted on it.
  4. Receiver is protected against over pressurization by relief valve.
  5. Header low-pressure shut down and high-pressure shut down on the skid may be incorporated to avoid excessive load or no consumption.
  6. Compressors are started only after “reset” to ensure they do not auto start after a break down.

Refer relevant manual for details of selections and protections of your compressor. These are not unique.

Meeting Standards

All components contribute to meeting standards.
Pressure:         Shall be not less than 690kpa (ISA)
This will be taken care by loading/unloading pressure setting.
Dew Point:      Pressure dew point as measured at the drier out let shall be
at least 10°C below the minimum temperature to which
any part of the air system is exposed.
Moisture traps and dryers do this job.
Filter:               Maximum particle size allowable in the air system is
5 microns.
Lubricant:        Where lubrication oil is used in air compressors, the header
air shall contain traces of lubricant less than ” one ppm.v/v.”
Filters are provided considering this. Filter specification is such that particle size allowed will be less than 5 microns.
Receiver:         Shall be provided with a relief valve and an auto drain (ISA).
Pressure pulses due to loading or process loads and over pressurization is the function of receiver.
Important:       The air intake shall be free of engine exhaust, dust, rust particles, chemical particles and corrosive or hazardous gases.
RCM & Spares
The operations people ensure that the instrument air supply is uninterrupted and adequate.
Routine maintenance according to station plan is carried out

  1. Check and make sure duty/standby selection.
  2. Checks the air pressure trend.
  3. Checks for any abnormal sound in compressor skid.
  4. Checks oil level in air / oil separator.
  5. Checks all condensate traps, drains.(assess oil carryover)
  6. Checks pressure drop across filters.
  7. Check the dryer changeover cycle is normal.
  8. Check the loading/unloading is proper.
  9. Check the vacuum indicator on compressor air intake filter is OK.
  10. Checks for any leak on airlines on skid and afterwards in plant.
  11. Check the gas compressor air intake air blast cleaner is normal.


  • Air filters after receiver shall be changed when differential pressure across filter exceeds 1 Bar. Elements are stock item.
  • Maintain adequate compressor oil reserve in the station.
  • Desiccant is a stock item, which maintenance crew procures when needed.
  • Purge control air filter requires cleaning during every major servicing.
  • Replacement gauges are stock item.

Problems: Case Studies


Compressor is running loaded but header pressure goes low.
Possible causes

  1. Air leaks somewhere in header. Air consumption exceeds capacity of compressor.
  2. Blocked air-filter at air intake.
  3. Solenoid valves (load/unload) system faulty (on air to unload type compressors).
  4. Loading/unloading valve on compressor does not open fully.
  5. Relief valve leaking.
  6. Compressor element damaged.


  1. Check and arrest leak. Reduce operational demand.
  2. Remove and check filter.
  3. © Remove and test valve.
  4. © Remove valve and inspect.
  5. © Test valve setting.
  6. © Check all possible causes before removing element for inspection.

© = Call maintenance
Compressor overheating and/or compressor shutdown by air temperature switch.
Possible cause

  1. Insufficient compressor cooling.
  2. Oil cooler clogged externally.
  3. Oil level in separator is too low.
  4. Compressor temperature shutdown switch incorrectly set.
  5. Fan blades broken.
  6. Oil stop valve (Vs) stuck in closed position.


  1. Improve the ventilation.
  2. © Clean the cooler.
  3. © Refill to correct level.
  4. © Adjust switch to trip at recommended specified temperature.
  5. © Replace fan.
  6. © Remove and inspect

Excessive consumption of oil, oil carry-over through discharge line.
Possible causes

  1. Oil level is too high.
  2. Nozzle (Restrictor) in oil scavenging line clogged.
  3. Incorrect oil is causing foam.
  4. Oil-separator element (OS), defective.


  1. Check oil level, drain off to correct level if overfilled.
  2. © Clean the nozzle.
  3. Change to correct oil. Send sample for analysis.
  4. © Check element.

Excessive oil flow through filter after stopping.
 Possible cause
 Check valve (CV) leaking or oil-stop valve (Vs) stuck open.
 © Remove the valve and fix the defect, then replace air filter.
Dryer is switched on, but does not operate and the absorbers are not change over correctly.
Possible causes

  1. Fuses blown.
  2. ‘Timer-motor’ out of order.
  3. Dryer changeover solenoid valves faulty.
  4. Discharge silencer’s clogged.


  1. © Check and replace fuses
  2. © If timer cams do not turn, replace timer.
  3. © Remove and inspect
  4. Remove silencer and re-start dryer. If dryer functions correctly, clean and reinstall silencer.



Dryers remain pressurized
Possible causes

  1. Purge air discharge silencers clogged.
  2. Solenoid valves out of order or leaking.


  1. Remove the silencer and clean it.
  2. Remove and inspect.

Safety Precautions: 
When operating this compressor unit, the operator is expected to employ safe working practices and to observe all related local work safety requirements and ordinances.
The owner is responsible for maintaining the compressor in a safe operating condition. Compressor parts and accessories shall be replaced if unsuitable for safe operation.
Authorized, trained and competent personnel shall only perform installation, operation, maintenance and repair.
Critical ratings (pressures, temperatures, time-settings etc) shall be durably marked and maintained.
Apart from general engineering practice conform with the regulations of local authority, the following directives are specially stressed;

  1. Any blanking flanges, plugs or caps as well as eventual desiccant bags shall be removed before connecting up the pipes.

Distribution pipes and connections shall be of correct size and suitable for the working pressure.

  1. Place the compressor where the ambient air is as cool and clean as possible. If necessary install a suction duct.

Never obstruct the air inlet. Care shall be taken to minimize the entry of moisture with the inlet air.

  1. The aspirated air shall be free from flammable fumes or vapours e.g. paint solvents that can lead to internal fire or explosion.
  2. Air-cooled compressors shall be installed in such a way that an adequate flow of cooling air is available and that the outlet does not re-circulate to the inlet.
  3. If remote control is installed, the compressor shall bear an obvious sign reading;

DANGER:     This compressor is remotely controlled and may start without warning.
As a further safeguard, persons switching on remotely controlled, compressors shall take adequate precautions to ensure that there is no one checking or working on the compressor.  To this end, a suitably worded notice shall be affixed to the start equipment.

  1. In multiple compressor systems manual valves shall be installed to isolate each compressor. Check valves shall not be relied upon for isolating pressure systems.
  2. Never remove or tamper with safety devices, guards or insulation fitted on the compressor unit. A pressure-relieving device shall protect every pressure vessel or auxiliary, installed outside he unit to contain air above atmospheric pressure or devices as required.
  3. Pipework or other parts with a temperature in excess of 80 deg. C and which may be accidentally touched by personnel in normal operation shall be guarded or insulated. Other high temperature pipework shall be clearly marked.
  4. If the ground is not level or can be subject to variable inclination, consult Atlas Copco.
  5. The electrical connections shall correspond to the local codes. The units shall be grounded and protected against short circuits by fuses.


  1. Hoses shall be of correct size and suitable for the working pressure.

Never use frayed, damaged or deteriorated hoses.  Use only the correct
type and size of hose end fittings and connections.
When blowing through a hose or airline, ensure that the open end is held securely.
A free end will whip and cause injury.
        Never play with compressed air.
        Do not direct air stream at personnel.
Never use it to clean dirt from your clothes.

2.      Never operate the compressor where there is a possibility of taking in flammable or toxic fumes.
3.      Never operate the compressor at pressure below or in excess of its rating as indicated on the Principal Data Sheet
4.        All canopy doors shall be shut during operation.
5.      People staying in compressor rooms where the sound pressure level exceeds 90-dB (A) shall use ear-protectors.

  1. Periodically check that:

All guards are in place and secure.
All hoses and / or pipes are in good condition, secure and not rubbing.
There are no leaks.
All fasteners are tight.
All electrical leads are secure and in good order.
Dirt or paint does not obstruct pressure relief devices.
Adequately trained personnel shall only carry out maintenance and repair work; if required, under supervision of someone qualified for the job.

  1. Use only the correct tools for maintenance repair work
  2. Use only genuine spare parts
  3. All maintenance works, other than routine attention, shall only be undertaken when the compressor is stopped and the mains are switched off. Ensure that the unit cannot be started inadvertently.
  4. Before removing any pressurized component, effectively isolate the unit from all sources of pressure and relieve the entire system from pressure.
  5. Never use flammable solvents or carbon tetrachloride for cleaning parts.

Take safety precautions against toxic vapours of cleaning liquids.

  1. Observe scrupulous cleanliness during maintenance and repair. Keep dirt away by covering the parts exposed openings with a clean cloth, paper or tape.
  2. Never weld or perform any other operation involving heat near the oil-system. Oil tanks must be completely purged, e.g. by steam cleaning, before carrying out such operations.

Never weld or in any way modify any pressure vessel.

  1. Make sure that no tools, loose parts or rags are left in or on the compressor, the prime mover or the driving gear.
  2. Before clearing the unit for use after maintenance or overhaul, check that operating pressures temperatures and time-settings are correct and that the control and shutdown devices function correctly.
  3. Every six months examine the discharge pipe and discharge pulsation damper for carbon deposits; if excessive, the deposits should be removed.
  4. Protect the Motor, electrical and regulating components, etc. to prevent moisture from penetrating these parts, e.g. when steam cleaning.
  5. Keep the sound pressure level as specified. Never remove any sound damping material.
  6. Never use caustic solvents or synthetic oils which can damage materials of the air net, e.g. polycarbonate bowls.
  7. The following safety precautions are stressed when handling refrigerant R22, R12 etc.


  1. Never inhale refrigerant vapours.

Check that the working area is adequately ventilated.

  1. Always wear special gloves.

In case of refrigerant contact with the skin, rinse the skin with water.
On no account may clothing be removed.

  1. Always wear safety glasses.