In the hydraulically actuated air-spring return exhaust valve, valve opening is effected by hydraulic oil acting on a piston block in the valve, while closing is by compressed air acting as an air spring below/around the piston block. Because the hydraulic oil is incompressible and the valve actuator has no rigid mechanical link to a cam (no tappet screw), the effective clearance between the actuating piston and the valve spindle stem is determined by the geometry and by the oil volume/position of the piston when the valve is closed. The "virtual tappet" is the controlled "clearance" (cushion) built into the hydraulic system - it is the small axial gap/over-travel between the hydraulic actuating piston and the valve spindle when the valve is closed, which ensures the valve opens fully and closes evenly. It is set by adjusting the length of the valve spindle extension / the position of the actuating piston relative to the valve spindle, so that when the hydraulic pressure pushes the piston down it contacts the spindle and opens the valve, and when the pressure is released the air spring closes it. The setting is done by measuring the distance between the piston lower face and the valve spindle upper face with the valve in the closed position and adjusting shims/spacers or the spindle to give the specified clearance, then verifying the valve opens to the full lift.

The exhaust valve and seat ring (valve seat) are subject to high temperatures (up to 500 deg C on the gas side of the hot valve) and severe thermal and mechanical loading. Damage occurs because:

• Deposits of combustion products (carbon, ash, sodium-sulpho-vanadium compounds from HFO) and partially-burned fuel accumulate on the valve seat. When the valve closes onto its seat, these hard deposits are trapped between the tapered seating faces.
• The high seating velocity and the cyclic hammering of the valve force the hard deposit particles into the softer seat material, gouging "furrows" (score lines) and "cutting" (grooving) the faces.
• Thermal distortion of the valve and seat ring (from uneven heating, cooling by the fuel, and the gas) prevents uniform seating, allowing gas blow-by which erodes and burns the seat face and the valve margin.
• Corrosion by lead/vanadium salts at high temperature and poor atomization of fuel erode the seat.

These combine to pit, furrow, cut and burn the seating surfaces, losing sealing (valve blow), causing further overheating and damage.

"Valve drop" is a catastrophic failure where the exhaust valve separates/falls and its head enters the combustion space uncontrolled while the piston is moving, or the valve becomes stuck open/in the wrong position. It happens if: (1) the valve spindle breaks (e.g. at a notch, at the collets/retainer, or from fatigue/corrosion), (2) the collets/keepers or the valve retainer fail, (3) the hydraulic actuation/air spring fails such that the valve is thrown open and falls, or (4) the valve becomes detached from the actuating mechanism. If the valve head drops into the cylinder, the piston travelling up at high speed strikes it. The result is catastrophic: the valve head or its fragments are driven into the piston crown, bending/distorting the connecting rod, damaging the crosshead, breaking the gudgeon pin, cracking the liner and the cylinder head, damaging the guides/slippers and the crankshaft/bedplate - i.e. extensive damage to the running gear and main engine, often requiring a complete engine rebuild.