When A/E stand by FO heater in use M/E is running the heater fuel inlet v/v open slowly

a. Sudden open of fuel inlet v/v pressurize and damage heater
b. Fuel oil inlet v/v vapour lock inside the heater and allow steam
c. Fuel oil inlet v/v cause drop in M/E fo pressure stand by heater is not pressurzed may by leave sudden change in engine load RPM
d. Heater leaking