Pressure limiter valve

A pressure relief valve protects the pneumatic braking system against excessive air pressure in air reservoirs, lines and valve blocks. The valve opens as soon as the set opening pressure is exceeded and releases compressed air into the atmosphere. This protects, among other components, the air compressor, air dryer, four-circuit protection valves, brake valves and brake chambers against overloading. Pressure relief valves are used on MAN trucks from the F90, F2000, L2000, TGL, TGM, TGA, TGS, TGX and TG3 ranges, as well as on Lion’s City, Lion’s Coach and Neoplan buses. At Mercedes, they are found in, among others, the SK, Atego, Axor, Actros, Arocs, Econic, Citaro, Tourismo, Vario and Sprinter.

In air brake circuits, these are generally direct-acting, spring-loaded safety valves with a calibrated closing spring, sealing seat and exhaust port. The opening pressure is fixed by design and must correspond to the system pressure and pressure class of the relevant circuit. The valve must not be selected solely on the basis of thread size, appearance or vehicle model. Connections may differ in metric or BSP thread, line diameter, sealing principle, installation position and nominal opening pressure. Therefore, always identify pressure relief valves using the vehicle chassis number; visual comparison or selection based solely on the model name is never sufficient.

On vehicles with engine families such as MAN D08, D20, D26 and D38 or Mercedes OM 906, OM 936, OM 470, OM 471 and OM 473, the version depends on compressor capacity, the design of the compressed-air supply and the auxiliary consumers fitted. These include air suspension, clutch actuation, retarder control or bus doors. A safety valve is not the same as a pressure regulator: the pressure regulator or compressor governor determines the normal cut-in and cut-out pressure of the air compressor; the pressure relief valve acts only as an independent safeguard in the event of an abnormal pressure increase.

A continuously hissing pressure relief valve does not automatically indicate a faulty valve. First, check the actual pressure in the relevant air reservoir with a calibrated pressure gauge and compare it with the vehicle specification. If the valve only releases above its specified opening pressure, the cause is usually a faulty compressor governor, a fault in the air dryer control system or a blocked control line. If it releases at normal operating pressure, a weakened pressure spring, a contaminated sealing seat, corrosion or a damaged diaphragm may be the cause.

First allow the system to become completely depressurised and secure the vehicle against rolling away. Then inspect the line connections, valve housing and exhaust port for oil deposits, rust, ice formation and mechanical damage. Next, build up pressure in a controlled manner and measure the opening and reseating pressures. Insufficient pressure differential between opening and closing, leakage below the nominal pressure or a valve that does not reseal after venting requires replacement. At the same time, inspect the air dryer cartridge and air compressor for excessive oil carry-over: oil and condensate cause rubber seals to swell and moving parts to stick.

Always replace a pressure relief valve as a complete unit when the seat is indented, the housing is corroded or the measured opening pressure falls outside tolerance. Adjusting, blocking or sealing a safety valve is not a safe repair. Before installation, clean the line and connection, use only a sealant approved for compressed air and prevent sealant residue from entering the valve. Observe the specified tightening torque, as excessive installation stress can damage aluminium valve blocks or tapered threads.

In the event of recurring pressure faults, a complete inspection of the air compressor, pressure regulator, air dryer and air reservoirs is necessary. Regularly draining condensate water and servicing the air dryer in time limits corrosion and freezing. A correctly selected pressure relief valve therefore supports the operational reliability of the EBS (Electronic Braking System) and reduces unplanned downtime of trucks, buses and vans.