A gas canister expert estimated the container, weighing 65kg when empty, had been three-quarters full before it flew off.
Opening the prosecution’s case Nigel Wray said: "These two defendants were scrapping gas cylinders, cylinders they had found, and they had no regard whether they were full or empty.
"It is a strange and peculiar type of manslaughter which relies on the gross negligence of these two defendants in failing to see the obvious risk of death in what they were doing."
A jury of four women and eight men heard Dean had driven home in Jennings’ van after the accident while Jennings went to the hospital with Mr Donaghue.
The court was told Dean later drove the van into a field near his home.
When fire crews discovered the contents of the van, which included acetylene, oxygen, butane and propane canisters, they cordoned the area off from a distance of 100 metres, the court was told.
Mr Wray said: "Here both their actions were grossly negligent.
"The Crown say they owed Timothy Donaghue a duty of care which they breached causing his death.
"It was grossly negligent and a clear indifference to the risk of death."