"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