Last reviewed 16 March 2021

Following a road incident in April 2020, investigations by Cleveland Police and the Health and Safety Executive (HSE) found evidence that a pressure/vacuum relief valve 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 it. However, the unauthorised modification prevented the valve's safe operation.

During what should have been a momentary operation to vent the road tanker, the vacuum relief element of the valve became stuck in an open position allowing an uncontrolled escape of hazardous vapour from the nitric acid being carried.

Some valves fitted to other road tankers operated by the same haulage contractor were found to have the same potentially dangerous modification.

HSE has now issued a Safety Alert to users of tank containers (iso-tanks) and road tankers used for carriage of dangerous liquids and gases.

This can be found on the HSE website and 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.

It states that users should visually inspect the pressure/vacuum relief valves fitted to tank containers and road tankers. The inspection should establish if there have been any unauthorised modifications made to the valve, particularly items added to the valve to allow for easier dismantling.

It is normally not necessary to remove or dismantle the valve to carry out a visual external inspection, the Alert notes, although safe access for working at height may be required if not already fitted to the tanker.

Where valves have both a pressure and vacuum relief function, then both the over-pressure and under-pressure safety functions should be verified during the pressure test.