A male employee was taken by ambulance to the Saint John Regional Hospital to be treated for minor injuries, Sgt. Jay Henderson said in a release.
The 60-year-old worker was not burned or struck, according to the Saint John Fire Department's Acting Platoon Chief Brian Wilson.
It appears the worker was thrown by the force of the compression and suffered superficial scrapes, Wilson said.
It is not known if the worker has since been released from hospital.
The incident occurred at about 11:52 a.m. AT during scheduled hydrogen plant turnaround work, company spokeswoman Carolyn Van der Veen said in a short statement that was issued about two hours after emergency crews were called.
A carbonate tank undergoing maintenance work was overpressurized, she said.
Van der Veen did not refer to the incident as an explosion.
The employee was taken to hospital as a "precaution" and there was no other damage, Van der Veen said.
The refinery issued the all-clear to resume normal operations by 12:40 p.m., she said.
"Due to the planned turnaround work taking place, we do not anticipate any production impacts."
WorkSafe New Brunswick is investigating.
Blast felt by residents
Emergency crews were called to the scene at about 11:55 a.m. AT for a medical call, Staff Sgt. Mike King told CBC News.
In responding, crews confirmed a small explosion on site, he said.
Police and fire officials left the scene shortly after 1 p.m.
Sixteen firefighters and uniform police officers responded.
Ann Hickey, who lives in nearby Champlain Heights, said she felt the explosion.
"I had just come out and sat on the couch and the house shook," she said.
"I got up and went to the back of the house and looked over to where the refinery would be but you couldn't see anything."
Clean air activist Gordon Dalzell, who also lives in the area, said a refinery official described the accident to him as an "operational incident" and said there is no danger to the public.
Shasta Collins, who works at Atlantic Label on Grandview Avenue, across from the massive refinery tanks, said she heard a "great big bang" at about 11:45 a.m. and the two-storey building shook.
"We thought that a car had lost control or something and slammed into the building," she said.
"We thought that was the only thing that could have made such a loud bang and make our building shake like that."
When Collins went outside to see what had happened, she saw an Irving employee wearing coveralls with an Irving logo on them, and he asked to use the phone.
Refinery largest in Canada
Collins said the employee told her there was one man down, who had taken the brunt of the explosion.
"I assume he was probably calling home to let his family know he was OK, but yeah, it was kinda scary," she said.
Collins said she did not see any unusual smoke.
Several Irving vehicles were pulled over along the side of the road and several people were standing around the gates into the facility, she added.
The refinery, located on Loch Lomond Road on the city's east side, is the largest in Canada, capable of producing more than 300,000 barrels of product per day.
The sprawling site is about 316 hectares.
One Irving Oil employee was killed and two others were injured in an explosion and fire on June 9, 1998.
That explosion occurred in an area known as the hydro cracker unit, where crude oil is superheated into diesel and furnace oil.
The blast sent flames and a huge cloud of black smoke over the facility.