"This safety alert is aimed at haulage contractors, users of tank
containers and road tankers, companies servicing pressure/ vacuum relief
valves and authorised inspection bodies contracted to verify the
functioning of these valves.
Tank containers and road tankers used for flammable, corrosive and
toxic liquids and gases will normally have a valve fitted to prevent
damage to the tank from changes in the internal pressure. Haulage
containers used for less hazardous liquids and gases may also have a
valve.
Following a road incident in April 2020, investigations by Cleveland
Police and HSE found evidence that a pressure/vacuum relief valve,
originally manufactured by Fort Vale Engineering Ltd, had been modified
by a third party. The valve cap had a nut welded to the top, most
likely to allow it to be serviced without the use of a special tool to
remove the valve cap. The unauthorised modification prevented the
valve’s safe operation"
Read the full report here https://www.hse.gov.uk/safetybulletins/failure-road-tanker-pressure.htm