Expansion valve (steam engine)

It operates by closing off the supply of steam early, before the piston has travelled through its full stroke.

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[ii] for a rolling mill at Tredegar Iron Works,[3] 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[4] The gridiron valve[5] was one of the first forms of expansion valve.

McIntosh and Seymour engines used one driven by a cam and toggle arrangement that moved intermittently and stood still when open, giving precise timing, and independent adjustment of each valve movement.

Engines on display at Snibston Discovery Museum and Coleham Pumping Station have Meyer expansion valves.

It was usual for expansion valves to be fitted only to the HP (high-pressure) cylinder.

Later compound mill engines with sophisticated valve gears often fitted the complex gear to the HP cylinder whilst retaining a simpler traditional slide valve for the LP cylinder.

Selecting intermediate positions provides a valve actuation with the effect of increasing cut-off.

Theoretically, the precise effect is that of reduced valve travel, rather than an earlier cut-off.

Earlier engines with Watt's centrifugal governor and throttle valve become inefficient when operating at low power.

Rather than the Stephenson's manual control of the die-block position within the oscillating link, the Richardson governor adjusts this according to engine speed.

[19] This avoided the wire-drawing problem of the Stephenson's reduced valve travel and improved efficiency for stationary engines that might run at low power for long periods.

[20] This design was unsuccessful, owing to mechanical problems with differential thermal expansion of the two valve materials.