A recent HSE investigation has found that piston actuators fitted to safety critical foot valves on LPG road tankers may not operate on demand at low temperatures created by a loss of containment.
Subsequent low temperature tests on similar valves showed that two types of piston actuators also failed to move a valve to the closed position. In one case, moisture in the actuator cylinder froze preventing movement of the piston. In the second case, neoprene bellows froze, the bellows shattered and remaining pieces of the bellows prevented actuator movement.
Other types of actuator used for safety critical isolation on road tankers and fixed vessel installations may be vulnerable to failure at low temperatures.
Other liquefied gases either transported by road or stored in fixed vessels on COMAH sites may create low temperatures on release.
A road tanker carrying liquid propane suffered a flange failure that allowed boiling propane at approximately minus 40oC to come into contact with the pneumatic actuator that controls the opening and closing of the tanker foot valve. The foot valve isolates the tanker discharge pipe work and is considered safety critical in preventing a loss of containment. Upon failure of the flange, the tanker driver activated the emergency stop system but the actuator was frozen and would not fully close off the foot valve. This led to a loss of containment of the tanker’s entire inventory.
The incident has been replicated in tests undertaken by The Health and Safety Laboratory using a 5kg carbon dioxide cylinder in a damp environment. The carbon dioxide was released over a seventeen-second period to create cold temperatures. The actuators tested did not function as required.
Two causes of failure were observed;
Photograph 1: Actuator frozen and holding valve partially open.
- The first was caused by ingress of moisture to the actuator cylinder. The moisture froze to prevent the piston moving within the cylinder. The ingress of moisture was caused by a worn bellows allowing road spray and rainwater to enter the actuator cylinder. Photograph 1 shows the actuator frozen partially open.
- The second failure resulted from the use of an unsuitable bellows material that froze then shattered. The remaining pieces of the bellows prevented actuator movement. Photograph 2 shows the frozen bellows and actuator. It was found that the original bellows material was polyisoprene rubber. The actuator had been reconditioned using polychloroprene rubber (neoprene) bellows, which are not suitable for low temperature operation.
Photograph 2: neoprene bellows failure during low temperature test.
During normal operation, the actuators were capable of performing the required duty of opening and closing the safety critical foot valve. On the valve that was involved in the incident, there was wear in the linkages and that may have led to poor valve opening performance, but was unlikely to have held the valve open. The valve had undergone regular maintenance but an assumption had been made that the actuator was suitable if it moved through its stroke on demand during normal operating conditions. The condition of the components had not been considered or their operation during low temperatures caused by a loss of containment. The bellows should have been inspected for wear and damage to avoid moisture ingress.
Operators of vehicles, which fall within the scope of the Carriage of Dangerous Goods and Use of Transportable Pressure Equipment Regulations 2009, CDG, and the operators of COMAH establishments who store or transport substances that have the potential to create low temperatures during a loss of containment incident are recommended to ensure that:
- They have undertaken a suitable and sufficient risk assessment to identify any hazards and associated risks to persons using the gates. This should include consideration of the following;
- 2. Where the actuators are fitted with bellows;
- The bellows material is checked to ensure they are compatible with low temperature operation.
- The bellows are regularly inspected to ensure they are maintained in a condition that will prevent moisture and other contaminants ingress into the moving parts of the actuator.
- Similar checks are recommended where low temperatures could be created by the release of other stored substances and there is the potential for this to prevent the operation of safety critical, actuated valves.
- Control of Major Accident Hazards Regulations, 1999, Regulation 4.
- Carriage of Dangerous Goods and Use of Transportable Pressure Equipment Regulations 2009, Regulation 5.
- Provision and use of Work Equipment Regulations, 1998, Regulation 4 and 5.
Health and Safety Executive
Hazardous Installations Directorate
Cardiff CF14 5SH
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