Expansion valve (steam engine)

An expansion valve is a device in steam engine valve gear that improves engine efficiency. It operates by closing off the supply of steam early, before the piston has travelled through its full stroke. This cut-off allows the steam to then expand within the cylinder. This expanding steam is still sufficient to drive the piston, even though its pressure decreases as it expands. As less steam is supplied in the shorter time for which the valve is open, use of the expansion valve reduces the steam consumed and thus the fuel required. The engine (on 1875 figures) may deliver two-thirds of the work, for only one-third of the steam.

An expansion valve is a secondary valve within a steam engine. They represent an intermediate step between steam engines with non-expansive working and later valve gears that could provide for expansion by controlling the motion of a single valve.

Expansion valves were used for stationary engines and marine engines. They were not used for locomotives, although expansive working was achieved by the use of the later variable expansion valve gears.

The pressure of the expanded steam is less than that of steam supplied directly from the boiler. An engine working with an expansion valve set to an early cut-off is thus less powerful than with the valve fully open. Accordingly, the engine must now be driven, so that the valve is manually adjusted as the load on the engine changes. An engine running under light load may be operated efficiently with an early cut-off, an engine under heavy load may require a longer cut-off and the cost of more steam consumption.

When Trevithick supplied his 1801 engine for a rolling mill at Tredegar Iron Works, the engine was more powerful when worked without expansion and Samuel Homfray, the ironmaster, preferred to use the extra power despite the potential saving in coal costs

The gridiron valve was one of the first forms of expansion valve. The gridiron valve is an arrangement of two plates with overlapping slats. One plate can move so that its slats overlap either the slats of the other plate, or the slots between them, to thus be either open or closed. It has the advantages of a relatively large opening (up to half of the total area) and a rapid opening, needing to be moved by only one slat width to change from fully open to fully closed. Its disadvantage is that they do not seal particularly well. Because of the short actuating distance for a gridiron valve their valve timing would be relatively imprecise if used with an eccentric or similar. Some large steam engines later used them as primary valves, either as exhaust valves for LP cylinders or as inlet valves in conjunction with trip- or cam valve gear.

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Wikipedia