How EN 15611 Transforms European Rail Braking & Interoperability

An In-Depth Technical Guide to EN 15611: Relay Valves for Railway Braking Systems

EN 15611 is a European Standard that specifies the requirements for the design, performance, and testing of relay valves used in pneumatic braking systems for railway vehicles. This standard is fundamental to ensuring safety, reliability, and interoperability across the European rail network by defining a common benchmark for these critical components.

A relay valve is a pneumatic device that acts as a pressure amplifier and accelerator within a train’s continuous automatic air brake system. Its primary function is to replicate and amplify the pressure signal from the main brake pipe, using a local air supply from an auxiliary reservoir on each vehicle, to apply or release the brakes quickly and uniformly along the entire length of the train.

The Core Function of a Relay Valve in Railway Braking

In a long freight or passenger train, a pressure signal initiated by the locomotive driver travels down the brake pipe. Due to the pipe’s length and volume, this signal can weaken and slow down, causing a significant delay between the braking action of the first and last vehicles. This can lead to dangerous in-train forces (compression or tension), resulting in rough train handling or even derailment.

The relay valve solves this problem by:

• Sensing: It detects small, precise changes in the control pressure from the brake pipe.
• Amplifying: It uses the high-pressure air stored in the vehicle’s own auxiliary reservoir as its power source.
• Relaying: It delivers a corresponding, but much higher volume and faster-acting, air pressure output directly to the brake cylinders of that specific vehicle.

This ensures that every vehicle brakes almost simultaneously and with the correct force, irrespective of its position in the train, leading to smooth, safe, and efficient braking performance.

Key Technical Requirements of EN 15611

The standard establishes a comprehensive set of criteria that a relay valve must meet. These are verified through rigorous type testing and routine production testing.

Performance Characteristics

Performance is the most critical aspect of the standard, ensuring predictable and reliable operation under all conditions. Key parameters include:

• Sensitivity and Response Threshold: The valve must react to very small and slow pressure changes in the brake pipe (the control signal) but remain stable and not react to minor, insignificant pressure fluctuations.
• Response Time: The standard defines maximum permissible times for the valve to react and deliver pressure to the brake cylinders (application time) and to exhaust it (release time). This is crucial for achieving simultaneous braking across the train.
• Graduability: The valve must allow for finely controlled partial brake applications and releases. The output pressure to the brake cylinders must be directly and consistently proportional to the input control pressure.
• Stability and Hysteresis: The valve must provide a stable output pressure without oscillation or “hunting.” Hysteresis, the difference in control pressure for the same output pressure during application and release, must be within strict limits to ensure consistent performance.
• Pressure Limitation: The valve must incorporate a function to limit the maximum output pressure, protecting the brake rigging and components from over-pressurisation.
• Insensitivity to Supply Pressure: The output pressure of the relay valve must be determined solely by the control signal pressure, not by normal variations in the auxiliary reservoir supply pressure.

Design, Construction, and Materials

EN 15611 also outlines requirements for the physical construction of the valve to guarantee a long service life in the harsh railway environment.

• Materials: Materials used must be resistant to corrosion, oil, ozone, and wear. All non-metallic components, such as seals and diaphragms, must function reliably across a wide temperature range.
• Environmental Conditions: The valve must be designed to operate reliably within the demanding temperature range specified for railway applications (e.g., -25°C to +70°C, with special provisions for lower temperatures). It must also withstand the typical shock and vibration experienced on a railway vehicle.
• Connections: All pneumatic connections must conform to established standards (e.g., ISO standards for pipe threads) to ensure compatibility and leak-free installation.

Testing and Approval Procedures

To achieve compliance with EN 15611, relay valves must undergo a stringent testing regime:

• Type Tests: An exhaustive series of tests performed on a new design or a modified design to validate its full compliance with every requirement of the standard. This includes performance, endurance, climatic, and vibration tests.
• Routine Tests: A simplified set of tests performed on every single valve produced to ensure that manufacturing quality and performance are consistent with the certified type-tested design.

Comparison of Key Performance Parameters in EN 15611

The following table summarizes the key performance parameters defined by the standard and their importance for the overall braking system.

Significance for the Rail Industry

EN 15611 is more than just a technical document; it is a cornerstone of braking safety and interoperability. For component manufacturers, it provides clear design targets and a standardized path to certification. For rolling stock manufacturers and railway operators, it guarantees that a compliant relay valve, regardless of its origin, will perform predictably and can be integrated into any compliant vehicle. This standardization is essential for the seamless cross-border traffic that defines modern European rail transport and is a key element in fulfilling the Technical Specifications for Interoperability (TSI) for rolling stock.