The inability of a compressor to reach its target cut-out pressure signals a core system failure that halts production and inflates energy costs. Diagnosing this issue requires systematically determining whether the fault is originating from the generating unit, the distribution network, or the control system.
Why is the Air Compressor Not Building Pressure?
Pressure loss in any system occurs when the volume of air consumed exceeds the volume of air generated. The diagnostic process begins with a simple isolation test that separates the machine from the plant to pinpoint the source of the deficit.
How Do Engineers Use the Isolation Test to Pinpoint Faults?
The isolation test is the definitive first step to check the machine’s mechanical capacity against its own internal volume. The procedure requires closing the main service valve situated directly after the compressor’s air receiver.
- Close the Valve: Shut the manual isolation valve between the air receiver and the main air ring pipework.
- Monitor Pressure: Start the air compressor and monitor the pressure gauge on the receiver.
- Analyse Results: If the machine then rapidly builds pressure to its maximum setpoint and unloads correctly, the fault is demand-side. If the machine still struggles to build pressure, the fault is internal, indicating a supply-side or control-side issue.
How Does Air Demand Affect Pressure Generation?
Many perceived pressure faults are actually demand issues caused by excessive air consumption or restrictions in the pipework downstream of the compressor. Addressing these problems is often the fastest and most cost-effective solution.
What is Artificial Demand?
Artificial demand is the air volume consumed unnecessarily, often caused by leaks, poor regulation, or tools operated at pressures higher than required. Operating a system at a pressure higher than needed significantly increases the mass flow through all leakage points, thereby widening the supply deficit.
- Compressing the air just 1 bar higher than required can waste approximately 7 % of the energy used.
- This excess compression forces the unit to operate longer and harder, which increases wear and reduces the potential life of the unit.
How Significant is Air Leakage?
Air leaks are the single most pervasive cause of low pressure, acting as a constant, parasitic load on the compressor. Estimates suggest 20 % to 30 % of all generated compressed air is lost to leakage before it ever reaches a productive application. Estimated annual energy cost at typical industrial electricity rates.
| Leak Diameter (mm) | Annual Energy Cost (Est. £) |
| 1.6 mm | £500 – £700 |
| 6.4 mm | £7,000 – £9,000 |
| 9.5 mm | £15,000 – £18,000 |
- The economic impact is substantial, forcing the compressor to run unloaded or continuously to compensate for the lost volume.
- Engineers use a Draw-down test to quantify the total leakage volume by measuring how quickly the system pressure decays when all compressors are off. For permanent fixes, consider a Compressed Air Leak Detection survey.
Why Does Pipework Restriction Cause Pressure Drop?
Pressure drop is a loss of force due to friction as air moves through the distribution pipework. This loss is proportional to the square of the air velocity and the roughness of the pipe’s internal surface.
- Traditional galvanised steel piping is prone to internal corrosion over time, which increases the roughness and dramatically restricts flow.
- Choosing modern materials, such as extruded aluminium, creates an ultra-smooth internal surface, allowing laminar flow and minimal pressure drop.
What Mechanical Failures Stop Rotary Screw Compressors from Loading?
In the industrial standard rotary screw technology, a failure to build pressure almost always points to an issue with the air intake modulation or the internal control circuit.
What is the Function of the Inlet Valve?
The inlet valve, or unloader, acts as the machine’s throttle, controlling the flow of atmospheric air into the compression element. If this valve remains mechanically stuck closed, the rotors spin at full speed but are starved for air.
- When starved, the rotors generate a deep vacuum at the inlet instead of compressing mass flow.
- This failure is commonly caused by a solenoid (Y1) failure, pilot air loss, or mechanical seizure due to oil varnish.
How Does the Minimum Pressure Valve (MPV) Affect Output?
The MPV is a check valve located at the outlet of the oil separator tank, immediately prior to the aftercooler. Its primary function is to maintain a minimum internal pressure (typically(typically 3 to 4 bar) necessary for oil circulation.
- If the MPV is stuck open, the internal pressure equals the plant pressure, failing to build the necessary head for proper oil flow.
- If the MPV is stuck closed, the compressor builds pressure only internally until the safety relief valve vents the excess pressure, preventing any air delivery to the plant.
Can Oil Separation Problems Mask Pressure Generation?
Yes, a highly restricted or blocked oil separator element can create a significant pressure drop across the filter medium. The compressor may successfully generate 8 bar internally, but the restriction means only 6 bar reaches the plant outlet.
- A blocked scavenge line, designed to suck pooled oil from the separator element, exacerbates this issue by causing the element to become saturated with oil.
- Furthermore, failure to maintain the correct oil viscosity due to high operating temperatures (thermal thinning) reduces the oil’s ability to seal the rotor clearances, causing internal air leakage.
What Are the Most Common Piston Compressor Pressure Faults?
Reciprocating (piston) compressors rely on mechanical sealing to generate pressure, making them susceptible to component wear that directly affects flow.
Why Do Valve Plates Fail?
Piston compressors use thin, flexible reed or disc valves to direct air into and out of the cylinder. These thin plates are subject to extreme mechanical and thermal fatigue.
- If an intake valve fails, compressed air is pushed back out through the intake filter during the upstroke, significantly reducing the compressor’s volumetric output.
- If the discharge valve fails, high-pressure air rushes back into the cylinder from the receiver, rendering the compression stroke ineffective.
What is Piston Ring Blow-by?
Piston rings provide the necessary seal between the moving piston and the cylinder wall. As these rings wear down, air escapes past them and into the crankcase during the compression stroke.
- This phenomenon, known as blow-by, manifests as a compressor that builds pressure quickly at low PSI but slows down drastically and stalls before reaching the maximum setpoint.
- This type of supply-side failure often necessitates a piston replacement or a full rebuild, which can be addressed by a reliable Compressor Repair Service.
How Does the Check Valve Cause Restart Failure?
The tank check valve is the boundary that ensures air flows only into the receiver. If this valve fails and remains stuck open, high-pressure air presses directly against the piston head when the machine stops.
- Upon restart, the electric motor must overcome the “head pressure” of the full tank, which frequently causes the motor to stall, hum, or trip the thermal overload.
- A stuck-open condensate drain valve on the receiver is another common, simple fault that prevents pressure build-up by continuously allowing air to escape.
How Do Electrical and Control Systems Affect Pressure Generation?
Modern units like Atlas Copco Air Compressors rely on sophisticated microprocessor controllers that constantly monitor the system. Faults here can trick the compressor into thinking its job is done.
Can a Faulty Sensor Cause the Compressor to Unload?
Yes, the electronic control system relies on pressure transducers to read the system state. If a transducer drifts, it may report 8 bar to the controller when the actual pressure is only 4 bar.
- The controller logic incorrectly assumes the demand is satisfied and unloads the compressor.
- Engineers must verify the controller’s electronic reading against a calibrated mechanical gauge.
What Instrumentation Errors Are Common?
Sometimes the compressor is working, but the operator receives a false low-pressure indication. This can be caused by a faulty analogue gauge or a clogged sensing line that feeds the gauge or the transducer.
What is the Danger of an Incorrect Phase Sequence?
An electrical supply fault where the phase sequence is incorrect will cause a screw element to rotate in reverse. Screw compressors are designed only to compress when rotating in the correct direction.
- The machine will run at speed but produce negligible flow and pressure, preventing the system from loading.
- Correcting this requires a simple wiring modification to swap two phases on the incoming power supply.
Why is Cold Weather Diagnosis Critical in Scotland?
The operating environment, particularly in a UK climate, directly affects the compressor’s ability to start and build pressure efficiently.
What is Low Ambient Temperature Inhibit?
Atlas Copco controllers include a feature that prevents the machine from starting if the compressor room temperature falls typically below 2 °C. This is a protective measure.
- Cold oil is highly viscous, creating extreme drag on the airend rotors.
- Attempting to start against this viscous drag can cause a high torque spike, potentially tripping overload relays or damaging the coupling.
What is the Risk of Frozen Condensate?
High humidity and cold ambient conditions pose a significant freeze risk to control lines and downstream equipment. Water condensation in the system can prevent the unit from loading.
- Frozen moisture in the control lines can prevent the unloader solenoid valve from actuating.
- Condensate freezing in receivers or pipework can also block the airflow path, causing a restriction that mimics a low-pressure fault.
What UK Regulations Govern Pressure System Safety?
Any diagnosis and repair work involving components that manage system pressure must adhere to strict statutory standards in the UK.
What is the PSSR 2000?
The Pressure Systems Safety Regulations 2000 (PSSR 2000) requires a Written Scheme of Examination (WSE) for any air system with a receiver capacity over 250 bar-litres.
- The receiver and protective devices, like the safety valve, must be formally examined by a Competent Person at statutory intervals.
- The typical statutory inspection interval for an air receiver is every 26 months.
What are the Rules Governing Safety Relief Valves?
The safety relief valve (PRV) is a vital, statutory protective device that vents excess pressure to protect the pressure vessel. The PRV setting cannot be adjusted outside the parameters specified in the WSE.
- Replacing a PRV with one of a higher pressure rating to prevent it from lifting is illegal and constitutes a criminal offence under the Health and Safety at Work Act.
- Design Air helps clients navigate these complex compliance requirements, ensuring safety and legality.
Conclusion: Ensuring Continuous Compressed Air Reliability
Resolving the failure to build pressure requires a detailed, technical approach that systematically rules out demand, supply, and control failures. By focusing on diagnostics like the isolation test and verifying control logic, engineers can avoid costly trial-and-error component replacement. Ultimately, preventative service plans are the most effective defense against pressure issues.
Design Air (Scotland) Ltd provides this technical expertise across the Central Belt from our Airdrie base. We maintain a large inventory of compressor spare parts and provide rapid response for all makes and models.
Act Now: Protect Your Production Uptime
Don’t let low pressure reduce tool performance or inflate your energy bills. Whether you need a comprehensive Compressed Air Energy Audit or immediate repair, our expert engineers offer a 4-hour guaranteed response across the Central Belt of Scotland for emergency support.
- Book Maintenance: Schedule one of our Air Compressor Service Plans to proactively eliminate the common causes of pressure failure.
- Call for Help: If your compressor is currently failing to build pressure, contact Design Air for rapid Compressor Repair Service and compliance guidance.
