The main stop valve, code P51315-03, serves as a vital safety and control element in the fuel or air supply system of the MAN L21/31 engine. Designed to quickly isolate flow in emergency or shutdown conditions, it helps protect the engine and surrounding systems from damage or uncontrolled operation. Its robust design and fast-acting functionality ensure operational integrity, making it indispensable for secure and compliant engine operation in marine and power plant environments.