Piston valve (steam engine)

For other applications of piston valves, see Piston valve.
Diagram of cylinder and piston valve. The valve is next opened by moving it to the right, allowing the clear space in the middle of the valve to align with the channel in the cylinder above it.

Piston valves are one form of valve used to control the flow of steam within a steam engine or locomotive. They control the admission of steam into the cylinders and its subsequent exhausting, enabling a locomotive to move under its own power. The valve consists of two piston heads on a common spindle moving inside a steam chest, which is essentially a mini-cylinder located either above or below the main cylinders of the locomotive.

Overview

In the 19th century, steam locomotives used slide valves to control the flow of steam into and out of the cylinders. In the 20th century, slide valves were gradually superseded by piston valves, particularly in engines using superheated steam. There were two reasons for this:

The usual locomotive valve gears such as Stephenson, Walschaerts, and Baker valve gear, can be used with either slide valves or piston valves. Where poppet valves are used, a different gear, such as Caprotti valve gear may be used, though standard gears as mentioned above were used as well, by Chapelon and others.

Most piston valves are of the "inside admission" type, where fresh steam is introduced from the boiler via the space between the two piston heads of the valve, and exhaust steam leaves via the space between a piston head and the end of the steam chest. The advantage of this arrangement is that leakage, via the gland which seals the steam chest from the operating rod of the valve gear, is much less of a problem when the gland is subjected to low exhaust pressure rather than full boiler pressure. However, some locomotives, like Bulleid's SR Merchant Navy class, used "outside admission" where the reverse was true, in Bulleid's case because of the unusual chain-driven valve gear arrangement.

Examples

The Swannington incline winding engine of 1833 incorporated a piston valve

The Swannington incline winding engine on the Leicester and Swannington Railway, manufactured by The Horsely Coal & Iron Company in 1833, shows a very early use of the piston valve.[1] Piston valves had been used a year or two previously in the horizontal engines manufactured by Taylor & Martineau of London, but did not become general for stationary or locomotive engines until the end of the 19th century.[2]

Design principles

When on the move, a steam locomotive requires steam to enter the cylinder at a controlled rate.[3] This entails controlling the admission and exhaustion of steam to and from the cylinders.[3] Steam enters and leaves the valve through a steam port, usually at the middle position of the piston valve.[3] Where the valve is in contact with the steam ports, a consideration of the "lap" and "lead" is required.

Lap

The "Lap" is the amount by which the valve overlaps each steam port at the middle position of each valve.[3] However, there are two different types of "Lap."

The first kind is the "steam lap," which is the amount by which the valve overlaps the port on the live steam side of the cylinder.[3] Secondly, there is the "exhaust lap," which is the amount by which the valve overlaps the port on the exhaust side of the cylinder. "Exhaust lap" is generally given to slow-running locomotives.[3] This is because it allows the steam to remain in the cylinder for the longest possible amount of time before being expended as exhaust, therefore increasing efficiency.[3] shunter locomotives tended to be equipped with this addition.

The "Negative exhaust lap", also commonly termed "exhaust clearance," is the amount the port is open to exhaust when the valve is in mid-position, and this is used on many fast-running locomotives to give a free exhaust.[3] The amount seldom exceeds 1/16 in. when exhaust clearance is given; the cylinder on both sides of the piston is open to exhaust at the same time when the valve is passing through the mid-position, which is only momentary when running.[3]

Lead

The "lead" of the valve is the amount by which the steam port is open when the piston is static at front or back dead centre.[3] Pre-admission of steam fills the clearance space between the cylinder and piston and ensures maximum cylinder pressure at the commencement of the stroke.[3] The design criteria is to both cushion or assist the mass of the piston slow down and change direction and to reach a maximum pressure of the same value as the incoming steam. At slow speeds no lead is ideal. Source Pelland Engineering. "Lead" is particularly necessary on locomotives designed for high speeds, under which conditions the valve events are taking place in rapid succession.[3]

Valve travel

Long-travel piston valves allow the use of large steam ports to ease the flow of steam into, and out of, the cylinder.

Calculating valve events

Given the valve's lap, lead, and travel, at what point in the piston's stroke does the valve open and close, to steam and to exhaust?

Calculating an exact answer to that question before computers was too much work. The easy approximation (used in Zeuner's and Realeaux's diagrams) is to pretend that both the valve and the piston have a sine-wave motion (as they would, if the main rod were infinitely long). Then, for instance, to calculate the percent of the piston's stroke at which steam admission is cut off:

Add the two angles and take the cosine of their sum; subtract 1 from that cosine and multiply the result by -50.

As built the Pennsylvania's I1s 2-10-0 had lap 2 inches, lead 1/4 inch and valve travel 6 inches in full gear. In full gear the two angles are 48.19 deg and 41.41 deg and the maximum cutoff comes out 49.65% of the piston stroke.

See also

References

  1. ↑ Clinker, C.R. (1977) The Leicester & Swannington Railway Bristol: Avon Anglia Publications & Services. Reprinted from the Transactions of the Leicestershire Archaeological Society Volume XXX, 1954.
  2. ↑ Information plaque on the Swannington engine, National Railway Museum, York.
  3. 1 2 3 4 5 6 7 8 9 10 11 12 Garratt, C. & Wade-Matthews, M.: The Ultimate Encyclopedia of Steam & Rail (London: Hermes Publishing Company, Ltd., 1998) ISBN 1-84038-088-8
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