There have been three recent failures in brass non-return valves in separate high pressure gas point installations across CERN. Whilst each was in a different gas service, the visual nature of the failure has been similar.
In all three cases, these components were connected to stainless steel flexible connections and stainless steel pipework.
From the metallurgical investigation of the failed component, it appears that the failure is linked to uncontrolled tightening, leading to a localised weakening resulting in premature failure when subjected to pressure.
Lead levels in the examined components appear to be a contributing factor to the reduction in ductility but are not identified as the root cause. It has also not been possible to attribute failure to a particular batch of material.
The Occupational Health & Safety and Environmental Protection Unit prescribes the following actions to be taken, aligned with the CERN Safety Rules:
- Verification of all brass non-return valves (prioritising on those used within stainless steel systems):
- Visual examination of general installation condition and visual aspect, including markings;
- If there is any evidence of damage or deterioration, or a lack of traceability, then the non-return valve is to be appropriately disposed of and replaced with a new component;
- Where there is reasonable doubt on the condition or integrity of the component due to, for example, the age of the installation, then the non-return valve is to be appropriately disposed of and replaced with a new component;
- Replacement non-return valves in stainless steel systems are to be stainless steel, to minimise leak-tightness issues due to a mismatch in materials properties.
- Verification of the state of high pressure flexible elements on gas point installations:
- Visual examination of general condition;
- Assurance of the fixture of the anti-whip cables, since these are safety elements in case of failure;
- Visual examination of the condition of the sealing joints...
... with replacement as necessary if there is evidence of damage or deterioration.
- When working on gas point installations, provision and use of personal protective equipment against splashing/spills, as well as the use of hearing protection in case of sudden pressure releases resulting in high proximity noise levels.
- All tightening activities on gas point installations are to use only correct and controlled torque values, using appropriate torque wrenches or equivalent. The use of ‘flogging’ (e.g. using pipe extensions or hammering on spanners) is to be totally excluded.
- New gas point installations are to be designed, manufactured, procured, installed, accepted and commissioned and used in accordance with the CERN Safety Rules including Specific Safety Instruction SSI-M-2-4 Metallic Pressurised Piping. The use of brass fittings (e.g. non-return valves) in stainless steel systems is to be avoided.