Functional safety of gas mixing systems with hydrogen-nitrogen mixtures
1. introduction
Forming gas, a mixture of hydrogen (H₂) and nitrogen (N₂), is used in numerous industrial processes – including soldering, annealing, heat treatment and metal processing. Due to the proportion of hydrogen, a highly flammable gas, there are often increased requirements for process safety. The concept of functional safety is becoming increasingly important, especially in safety-critical applications.
The Safety Integrity Level (SIL) in accordance with the IEC 61508 and IEC 61511 safety standards can be a benchmark for this. This article looks at the relevance of SIL in gas mixing systems for forming gas, the technical challenges and possible solutions.
2 Basics: What is SIL?
SIL (Safety Integrity Level) is a measure of the reliability of systems. https://en.wikipedia.org/wiki/Safety_integrity_level
The operator determines the safety integrity level for the component or system as part of a risk assessment (HAZOP). As part of the study, potential hazards are identified and eliminated before they can lead to an accident. The study results in safety requirements at the appropriate level. There is a level classification from 1 (lowest) to 4 (highest), which is a measure of the reliability of the system in
Gas mixers for forming gas (forming gas is a gas mixture of hydrogen and nitrogen) have also been operated without SIL for more than five decades. However, the trend towards increased safety and more comprehensive risk management with a safety concept now also extends to this type of gas mixer and gas mixing systems (also known as hydrogen mixers). The safety of systems (system safety) for forming gas production can therefore be increased, for example, with a SIL 1 hydrogen gas analyser, SIL differential pressure monitoring and a SIL 1 shutdown. This reliably monitors and prevents the unauthorised accumulation of hydrogen in the process gas.
3 Forming gas: properties and risks
Typically, forming gas consists of 3-10 % hydrogen in nitrogen. In rare cases, gas mixtures with significantly higher hydrogen contents are also produced. The lower explosion limit (LEL) of hydrogen in air is around 4% by volume and the upper explosion limit (UEL) is 77% by volume. This means that hydrogen can only explode in air within a certain concentration range.
Hydrogen has the following properties:
- Extremely light and diffusive: Increased risk of leaks.
- Low ignition energy (~0.02 mJ): Even small sparks can trigger explosions.
- Wide explosion spectrum (4-75% by volume in air): Requires precise process control.
Sources of risk include:
- Hydrogen enrichment due to uncontrolled mixing ratios.
- Leaks in gas mixing technology or pipework.
- Malfunctions in pressure control or mixing ratio.
If the hydrogen content in the gas mixture exceeds the set limit value, an explosive atmosphere can occur in the process when mixed with air. Gas analysers with shutdown of the hydrogen line are used as standard in forming gas mixers to reliably detect this accumulation at an early stage and subsequently prevent it.
These risks are managed by safety solutions such as gas analysers, differential pressure monitoring and emergency shutdown with a validated SIL level.
4 SIL requirements for forming gas mixers
The safety-related design of a forming gas mixer with SIL comprises various subsystems. The following safety-related components are used:
4.1 Gas analysis
SIL 1 Hydrogen gas analyser: Continuously measures the hydrogen concentration.
Function: Precise analysis to regulate the mixing ratio. Documentation. Limit value monitoring
Advantage: Prevents unauthorised hydrogen enrichment
4.2 Differential pressure monitoring
SIL 1 Differential pressure sensor: Monitors the pressure of the gas flows (H₂ and N₂).
Function: Detects uncontrolled changes in a gas flow
Advantage: Stable process control and minimisation of explosion risks.
4.3 Safety valves and actuators
SIL-certified components: Tested shut-off valves in accordance with ISO 13849 or IEC 61508.
Function: Safe separation of gas supplies, automatic reset or bypass switchover.
Advantage: Increased system safety thanks to reliable actuators.
5 Special features of SIL integration in gas mixing systems
The integration of a SIL circuit in gas mixing systems requires a holistic approach that includes sensors, logic and actuators.
5.1 Holistic SIL circuit
Components: Gas analyser (sensors), safety relay (logic), shut-off valves (actuators).
Assessment: The safety function is validated in the overall context, not just on a component basis.
Standards: Compliance with IEC 61508 and IEC 61511 for safety standards.
5.2 Calculation and verification
PFDavg calculation: Determination of the average probability of failure (Probability of Failure on Demand).
Redundancies: Consideration of redundant systems and test intervals to minimise risk.
Documentation: Safety documentation with SIL validation, proof of function and commissioning checklists.
5.3 Challenges
Hydrogen behaviour: High diffusivity requires dense systems and precise hydrogen detection.
Calibration: The sensor system must remain easy to maintain under process conditions.
Reaction speed: Protective functions must react quickly to deviations.
6 Practical example: Moulding gas mixer with SIL 1
Application: Heat treatment process with 5 % H₂ in N₂
Requirements:
Process stability
Explosion protection
Documentation of the safety functions
Solution by LT GASETECHNIK:
H₂ gas analyser (SIL 1) with 4-20 mA signal
Differential pressure monitoring between gas flows
Emergency shutdown via safety relay
Safety valves in accordance with ISO 13849-1 / IEC 61508
Result:
Successful acceptance by operators and experts
Smooth operation over several years
Maintenance-friendly design with test logs
7. Conclusion
The use of Safety Integrity Level (SIL) for forming gas mixers represents an opportunity to increase process safety. Due to the flammable properties of hydrogen, a systematic safety assessment is essential. Despite higher acquisition and operating costs (annual SIL circuit test), there are several advantages:
Increased operational safety by minimising explosion risks
Protection of personnel and plant technology
Transparency: complete traceability through documentation
LT GASETECHNIK is one of the few suppliers that can supply complete gas mixers with SIL circuits and validated components for forming gas. With over 40 realised systems, the company offers proven expertise in the planning, calculation and implementation of SIL-compliant gas mixing systems. LT GASETECHNIK is available as an experienced partner for questions regarding the implementation of SIL requirements in gas mixing systems.
Contact us: mail@lt-gasetechnik.com