The witness, who cannot be named, said he was aged six or seven when one of the nuns at Smyllum Park orphanage went "ballistic" as she physically assaulted him in the Sixties.
The Scottish Child Abuse Inquiry is hearing evidence from witnesses from the child care establishment run by the Daughters of Charity of St Vincent de Paul in Lanark.
Speaking at the inquiry, the witness said the attack left him bloody and with bruises for weeks.
He added: "I distinctly remember there was a boiler and it had a flame coming out of it, a flame thrower... she put my face really close to that.
"I can still remember my hair getting singed, my eyelashes and the smell of singeing stayed with us for quite a while after that."
He said it wasn't until he later saw the publicity surrounding one of television's first lesbian kisses on the soap Brookside, that he realised why the nun had turned "so nasty".
"She was obviously kissing another nun, that's what I think," he said.
Speaking about the impact of the assult, he went on to say in his witness statement: "It was terrible, it was horrendous, really bad.
"I felt threatened all the time, constantly on my toes, just always, always on guard."
The inquiry, chaired by Lady Smith, continues.