VIRTUAL TAPPET IN HYDRAULICALLY ACTUATED AIR-SPRING RETURN EXHAUST VALVES

What is a "virtual tappet"?

In large two-stroke engines with hydraulically actuated, air-spring-return exhaust valves (e.g. MAN B&W MC/ MC-C), the exhaust valve is opened by hydraulic oil pressure (from a high-pressure pump driven by the cam) and closed by an air spring (compressed air acting on the valve piston). The "virtual tappet" is the effective/ equivalent mechanical clearance (lash) in the hydraulic system - the small amount of lost motion between the hydraulic piston and the valve stem that corresponds to the tappet clearance in a mechanical valve train. It is the gap that must be set so that when the valve is closed (seated), there is a small clearance in the hydraulic drive to allow for thermal expansion and to ensure the valve seats fully and is not held open by the hydraulic system.

How it is set:

• The virtual tappet is set by adjusting the position of the hydraulic piston/ the valve stem relative to the valve, so that when the valve is closed there is the specified clearance (the virtual tappet) between the hydraulic piston and the valve stem. This is done by adjusting the valve stem/ the hydraulic piston (e.g. by a threaded adjustment or by shims) to the maker's specified value, measured with a feeler gauge/ dial indicator. The setting ensures the valve seats fully and the hydraulic system does not hold the valve open.

Why damage occurs to the seats of the exhaust valves due to furrowing and cutting:

• Furrowing and cutting of the valve seats occur when the valve is not seating correctly or when there is excessive impact/ hammering of the valve on its seat. Causes include:
• Incorrect virtual tappet (too large a clearance causes the valve to slam onto the seat; too small causes the valve to be held open/ not seat fully).
• Excessive valve lift/ impact velocity.
• Misalignment of the valve/ seat.
• Hard particles (carbon, debris) trapped between the valve and seat.
• The valve rotating/ spinning on the seat, causing a cutting/ furrowing action.
• Overheating/ distortion of the valve or seat.
• These cause the seat to be cut/ furrowed (grooved), leading to leakage, loss of compression, and further damage.

How an incident of "valve drop" leading to extensive damage to running gear can occur:

• "Valve drop" is the failure of the exhaust valve to be held/ returned, so the valve falls into the cylinder (drops) while the engine is running. This can occur if:
• The air spring fails (loss of air pressure), so the valve is not returned/ held closed.
• The hydraulic system fails (loss of oil pressure/ a hydraulic fault), so the valve is not held.
• The valve stem/ piston seizes or the valve breaks.
• The virtual tappet is set incorrectly, allowing the valve to drop.
• When the valve drops into the cylinder, the piston strikes the valve on the upstroke, causing extensive damage to the piston, cylinder head, liner, connecting rod, crosshead and crankshaft (the running gear) - a catastrophic failure.