The purpose of this Safety Alert is to highlight the risks associated with compromised pressure relief circuits on Cryogenic Tank Containers.

Background

Cryogenic Tank Containers are commonly used for transportation and/or storage of cryogenic liquids including, but not limited to Nitrogen, Oxygen and Argon.

A large explosion had occurred at a cryogenic gas transport trailer containing 25,000 litres of liquid nitrogen cryogenic tank container at a cryogenic gas transport depot in Tamworth NSW., destroying the container, trailer, and damaging nearby structures. The worker on-site at the time suffered minor injuries from a fall. Debris was scattered over an area of approximately 5,000 square metres.

A white cryogenic tank container mounted on a trailer in an open field, displaying multiple danger warning stickers.
Cryogenic Tank Container
A cryogenic tank container and skel trailer after a rupture, with the tank shattered into multiple pieces and lying on its side, resembling an explosion. A person wearing a helmet and high-visibility vest is inspecting the wreckage.
Cryogenic Tank Container & Skel Trailer Post Rupture
                                                                                                                                        

Safety information

In this incident, the cryogenic tank container filled with liquid nitrogen catastrophically ruptured after the pressure increased beyond maximum allowable working pressure of 1,000 kPa and pressure safety valves failed to operate.

The cryogenic tank container had two pressure safety valves fitted on a common pipe, both set at 1,000 kPa. The pressure safety valves had been periodically inspected and tested.

Investigation concluded that the root cause was a blockage in the pipe feeding the two pressure safety valves, highly likely to be water ice.

The design of the internal pipework leading to the pressure safety valves on this tank was more susceptible to a blockage than other cryogenic tanks. This was due to a low point, in which moisture could accumulate, and the fact that the pipe leading to the pressure safety valves was independent of all other lines, limiting the ability to purse it dry.

Diagram illustrating a cryogenic tank system with labeled safety valves, gas space, and cryogenic liquid. It shows how moisture in the line accumulates at a low point, freezes, and blocks the pipe, creating a dangerous condition for the tank.
Figure - Diagrammatic representation of the internal pipe leading to the two safety valves
                                                                                                                                        

Action required

Work Health and Safety legislation requires the person with management or control of plant at a workplace must ensure, so far as is reasonably practicable, that the plants are without risks to the health and safety of any person.

Specific control measures

The following risk control measures are recommended for all Cryogenic Tank Containers.

Acceptance testing to be carried out for Cryogenic Tank Containers

Acceptance testing needs to be performed when a newly purchased or leased Cryogenic tank is put into service in Australia. It is not adequate to simply rely upon manufacturers' test certificates. Pressure relief circuits need to be tested, for example, by increasing pressure in a vessel under controlled conditions, until the pressure safety valves lift, to demonstrate that:

  • The pressure safety valves work.
  • Pressure safety valve pipework is clear and full flow is possible.

Purge procedures (checking / purging every outlet pipe, especially pipework leading to pressure safety valves)

Purge procedures for Cryogenic Tank Containers must ensure that pipework leading to pressure safety valves is specifically purged and checked for moisture. This may require specific procedures to be developed to ensure every section of pipework is adequately purged and checked for moisture, especially pipework leading to pressure safety valves.

Maintenance requirements for Cryogenic Tank Containers

Maintenance teams must be trained and made aware of the following: Potential issues associated with moisture accumulation and the need to prevent or minimise moisture ingress when containers are opened to atmosphere.

  1. The need to continuously purge all outlet points, especially those associated with pressure safety valves once a container is opened to atmosphere.
  2. need to test pipework leading to pressure safety valves for moisture following maintenance.
  3. Examples of specific purge procedures are:
    • Purge times, temperatures and quantities.
    • Maintaining plant under slight dry nitrogen positive pressure when opened to atmosphere.
    • Where practical, using a borescope to confirm no water accumulation in relief pipework.

Pressure safety valves best practices:

Procedures relating to pressure safety valves must ensure the following:

  • Every pressure safety valve should be checked closely against its Safety Valve Test Certificate prior to installation on a tank (especially serial number, set pressure, next test due date).
  • A pressure safety valve should not be installed on a tank if any irregularity around performance is noted on its corresponding Safety Valve Test Certificate (lift and/or reseat pressure for example).
  • A check must be made prior to installing any pressure safety valve to ensure that the locking wire is intact, sealed, and secured.
  • Any pressure safety valve found in service without a properly sealed locking wire should be treated as potentially inoperable.
  • When removing for maintenance, seal off the pressure safety valve connection opening as soon as practical to minimise moisture ingress or maintain plant under slight positive dry nitrogen pressure.

Remote monitoring of Cryogenic Tank Container pressure via telemetry (recommendation):

Since remote monitoring of Cryogenic Tank Container levels and pressures via telemetry has become much more robust and affordable than it used to be, it is recommended that remote telemetry systems should be installed on Cryogenic Tank Containers that are not subject to regular visual checks.

Any such telemetry solution should incorporate alarm points with automatic notifications to internal emergency response teams when normal operating parameters are breached. In virtually all circumstances, this would allow sufficient time for manual intervention before a critical threshold is reached.

Further information and guidance materials

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