No one was hurt in the explosion, which destroyed the boiler house at the firm’s Aspatria Creamery, Station Road, Aspatria on July 29, 2010.
The force of the blast blew the roof off the building and blew out part of two ground floor walls.
Carlisle Magistrates Court heard that an investigation by the Health and Safety Executive (HSE) found that a blocked vent on the calorifier ‒ a water heating and storage system ‒ had caused it to explode.
The calorifier could hold up to 9,000 litres of water and was used to produce hot water for washing the dairy.
The HSE investigation revealed that maintenance engineers at the plant had been manually controlling the steam supply valve to the calorifier after the automatic control broke.
But the vent pipe on the tank, which should have allowed water to escape if it became too hot, had become completely blocked. The blockage was caused by a layer of calcium carbonate, which had accumulated possibly over several years.
The blockage meant that pressure inside the tank continued to rise. The temperature of the water reached nearly 150 deg C, before it eventually exploded.
HSE inspector Michael Griffiths said after the hearing: “This ultimately led to the explosion after the tank could not cope with the increase in pressure when the temperature of the water reached almost 150 deg C.
Act as a warning
“I hope this prosecution will act as a warning to any other company that uses hot water and steam systems to make sure they are properly maintained so that incidents like this don't happen in the future.”
The First Milk Cheese Company pleaded guilty to a breach of the Pressure Systems Safety Regulations 2000 by failing to ensure the vent pipe on the calorifier was kept clear.
The firm was fined £20,000 and ordered to pay £36,064 in prosecution costs.
A spokeswoman for First Milk said: “We pleaded guilty at the earliest opportunity to the case against us in relation to an incident that took place in 2010 at our Lake District Creamery.
"We recognise the seriousness of the incident and throughout the process of investigation, we have worked very closely with the Health and Safety Executive.”
More information from the HSE on the safe use of calorifiers, boilers and other pressure systems is available at www.hse.gov.uk/pressure-systems.