Your Alfa Laval purifier bowl opens fine during testing. Then one day at sea, it does not.
The MOPX series is one of the most common fuel and lube oil separators on ships. Reliable and proven, but after thousands of running hours one part quietly degrades while everyone focuses on the disc stack: the operating slide seal ring.
This O-ring around the outer edge of the operating slide keeps opening water sealed inside the chamber so the bowl can open on command. When it wears or is nicked during reassembly, water leaks past, pressure drops and the bowl will not open.
The tricky part is that the purifier runs normally in purification mode. The fault only shows during the discharge cycle, so the crew thinks the separator is fine until sludge builds up and efficiency drops.
Another part often missed is the sliding ring inside the bowl. Poor water quality pits its inner surface, and the matching square ring eventually slips out of its groove. By then the fix is machining, not a ring swap.
If you are planning a 10,000-hour overhaul, quote the O-ring kit, sliding ring and square rings together with the disc stack and main seal. They cost little compared with the downtime of a bowl that will not close mid-voyage.