Overpressure is excess pressure in a process equipment or pipe of its maximum allowable working pressure.
Overpressure can be caused by inflow exceeds outflow, which occurs if upstream flowrate control device fails or it there is blockage in the downstream or if there is condition of gas blow-by from upstream component.
Overpressure can also be caused by thermal expansion of fluids if heat is added while the inlet and outlet are closed.