Understanding the Key Valves in Screw Compressors — Functions, Failures, and Maintenance

In a rotary screw compressor, valves are the “decision-makers” of the system. They control airflow, maintain pressure, regulate temperature, and ensure the compressor operates safely and efficiently. Although they’re small components, a single faulty valve can cause energy loss, oil carryover, overheating, or even total shutdown.

At OCP Europe, we supply high-quality replacement valves that meet or exceed OEM performance. In this article, we’ll explain how each valve works, the common signs of wear, and how to maintain them properly.

1. Minimum Pressure Valve (MPV)

Function:

The minimum pressure valve ensures a stable pressure inside the compressor’s oil circuit and air-oil separator tank. It only opens when the pressure reaches a preset level (typically around 4–5 bar), allowing air to flow into the downstream system.

If it fails:

• Air escapes prematurely, causing low internal pressure.
• The compressor overheats due to insufficient oil circulation.
• Oil carryover increases.
  Maintenance tip:
  Check for sticking or leakage caused by oil varnish or carbon buildup. Replace the complete valve if cleaning doesn’t restore normal function.

2. Thermostatic Valve (Oil Temperature Control Valve)

Function:

This valve regulates the oil temperature by bypassing the oil cooler until the compressor warms up, then directing flow through the cooler as temperature rises. It ensures proper viscosity and prevents condensation inside the system.

If it fails:

• Compressor runs too cold → moisture and emulsified oil.
• Compressor runs too hot → degraded lubricant, high temperature alarms.
  Maintenance tip:
  Replace the thermostatic element every 2–3 years or sooner in harsh environments. Always ensure correct temperature rating when selecting replacements.

3. Safety Valve (Pressure Relief Valve)

Function:

The safety valve protects the system from overpressure by releasing air when pressure exceeds the maximum safe limit (usually 10–13 bar). It’s a vital safety component.

If it fails:

• Risk of tank or separator rupture if it does not open.
• Air loss if it opens prematurely or leaks.
  Maintenance tip:
  Test regularly during service intervals. Replace immediately if signs of corrosion, deformation, or leakage appear.

4. Bypass Valve (or Oil Stop Valve)

Function:

The bypass valve ensures that oil is properly directed around the cooler or through it depending on system conditions. In some configurations, it works together with the thermostatic valve.

If it fails:

• Incorrect oil flow and poor cooling.
• Unstable temperature readings.
  Maintenance tip:
  Inspect o-rings and seals during every oil change. Replace the assembly if internal wear or spring fatigue is detected.

5. Pressure Regulating / Unloading Valve

Function:

This valve controls the compressor’s load and unload cycles. It opens to release air when demand is low and closes to build pressure when needed, optimizing energy use.

If it fails:

• Compressor stays loaded (wasting energy).
• Frequent cycling or unstable pressure.
  Maintenance tip:
  Clean pilot lines and check solenoid function. Replace seals or the complete valve if response time is slow or erratic.

6. How Valves Interact with Other Components

Valves are closely linked to filters, separators, and oil:

• A clogged oil filter increases backpressure on the thermostatic and bypass valves.
• A dirty separator affects minimum pressure valve operation.
• Poor oil quality accelerates varnish buildup, sticking valves.
  That’s why regular replacement of filters and lubricants is essential to keep valves working correctly.

7. Why Choose OCP Europe for Replacement Valves

OCP Europe offers a complete range of aftermarket valves designed for screw compressors from major brands such as Atlas Copco, Kaeser, Ingersoll Rand, Boge, and Sullair.

Our advantages:

• OEM-level materials and pressure calibration.
• 100% functional testing before shipment.
• Local European stock for fast delivery.
• Technical guidance for identifying equivalent parts.
  Every valve we supply is built to perform reliably under continuous duty, reducing downtime and total cost of ownership.

Conclusion

Valves may be small, but they have a major impact on compressor performance and reliability. Understanding how each one works—and maintaining them with high-quality parts—is the key to preventing costly breakdowns.

For premium replacement valves, filters, separators, and lubricants, explore the OCP Europe Knowledge Portal and get in touch with our technical team to find the right components for your compressor.