* Refinery units to be restarted Tuesday
* Length of time for start-up process not known
CALGARY, Alberta, March 16 (Reuters) - Suncor Energy Inc (SU.TO) said it plans to restart units at its 135,000 barrel a day Edmonton, Alberta, refinery later on Tuesday after a mechanical problem forced a partial shutdown the day before.
The plant suffered a "process upset" on Monday morning, prompting a controlled shutdown of some processing units. The plant kept producing some fuels after the outage, Suncor, Canada's largest oil company, said.
"We are planning for start-up later today," spokeswoman Sneh Seetal said.
She said it was not immediately known how long the start-up process would take. The company did not disclose which units were affected by the outage. (Reporting by Jeffrey Jones; editing by Rob Wilson)