The company plans to shut down its 65,000 bpd crude distillation unit (CDU) 1 as well as the 25,000 bpd fluidised catalytic cracker unit, used to produce gasoline, the company said on Tuesday.
It said the proposal, which workers union Unite Scotland said meant a potential loss of 200 jobs, had been put forward to its employees. The two units were already closed due to the COVID-19 pandemic that has cut fuel demand, it added.
Unite said it has asked the company to use the five-month extension of the British government's furlough scheme to provide time to find an alternative.
Petroineos is a joint venture between Chinese state oil firm PetroChina and privately owned British chemical producer INEOS.
The announcement is the latest in a string of refinery capacity reductions or closures as restrictions to curb the spread of the coronavirus continue to hit global fuel demand.
(Reporting by Ron Bousso; Writing by Julia Payne; Editing by Jan Harvey and Emelia Sithole-Matarise)