Expansion valve (steam engine)

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Cross compound engine, with an expansion valve (top) on the high-pressure cylinder Cross compound engine, plan section (Heat Engines, 1913).jpg
Cross compound engine, with an expansion valve (top) on the high-pressure cylinder

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. [1] This expanding steam is still sufficient to drive the piston, even though its pressure decreases as it expands. [lower-roman 1] 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. [2] The engine (on 1875 figures) may deliver two-thirds of the work, for only one-third of the steam. [2]

Steam engine Heat engine that performs mechanical work using steam as its working fluid

A steam engine is a heat engine that performs mechanical work using steam as its working fluid. The steam engine uses the force produced by steam pressure to push a piston back and forth inside a cylinder. This pushing force is transformed, by a connecting rod and flywheel, into rotational force for work. The term "steam engine" is generally applied only to reciprocating engines as just described, not to the steam turbine.

Valve gear Mechanism for controlling steam flow in a reciprocating steam engine.

The valve gear of a steam engine is the mechanism that operates the inlet and exhaust valves to admit steam into the cylinder and allow exhaust steam to escape, respectively, at the correct points in the cycle. It can also serve as a reversing gear. It is sometimes referred to as the "motion".

Contents

Indicator diagram showing steam pressure with piston movement Indicator diagram steam admission.svg
Indicator diagram showing steam pressure with piston movement

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. [1] They were not used for locomotives, although expansive working was achieved by the use of the later variable expansion valve gears.

Stationary engine

A stationary engine is an engine whose framework does not move. They are used to drive immobile equipment, such as pumps, generators, mills or factory machinery. The term usually refers to large immobile reciprocating engines, principally stationary steam engines and, to some extent, stationary internal combustion engines. Other large immobile power sources, such as steam turbines, gas turbines, and large electric motors, are categorized separately.

Marine steam engine steam engine that is used to power a ship or boat

A marine steam engine is a steam engine that is used to power a ship or boat. This article deals mainly with marine steam engines of the reciprocating type, which were in use from the inception of the steamboat in the early 19th century to their last years of large-scale manufacture during World War II. Reciprocating steam engines were progressively replaced in marine applications during the 20th century by steam turbines and marine diesel engines.

Need for varying expansion

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 [lower-roman 2] 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]

Richard Trevithick British steam and mining engineer

Richard Trevithick was a British inventor and mining engineer from Cornwall, England, UK. The son of a mining captain, and born in the mining heartland of Cornwall, Trevithick was immersed in mining and engineering from an early age. He performed poorly in school, but went on to be an early pioneer of steam-powered road and rail transport. His most significant contribution was the development of the first high-pressure steam engine. He also built the first full-scale working railway steam locomotive. The world's first locomotive-hauled railway journey took place on 21 February 1804, when Trevithick's unnamed steam locomotive hauled a train along the tramway of the Penydarren Ironworks, in Merthyr Tydfil, Wales.

Samuel Homfray was an English industrialist during the Industrial Revolution in Great Britain, associated with the early iron industry in South Wales.

Gridiron expansion valves

Independent gridiron valve, with conventional slide valve Expansion valve (Steam and the Steam Engine, Evers).jpg
Independent gridiron valve, with conventional slide valve
Eccentric and varying ratio linkage to drive a gridiron expansion valve Working of Expansion Valve (Steam and the Steam Engine, Evers).jpg
Eccentric and varying ratio linkage to drive a gridiron expansion valve

The gridiron valve [5] was one of the first forms of expansion valve. [1] 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 [6] or as inlet valves in conjunction with trip- [7] or cam valve gear. [8] [lower-roman 3]

Trip valve gear

Trip valve mechanisms are a class of steam engine valve gear developed to improve efficiency. The trip mechanism allows the inlet valve to be closed rapidly, giving a short, sharp cut-off. The valve itself can be a drop valve or a Corliss valve.

Where gridiron valves are used as secondary valves, they were commonly mounted on the inlet side of the valve chamber for the primary slide valve. They were driven by a separate valve gear, usually a separate eccentric set in advance of the main eccentric. [1] When operating, the additional advance moves the gridiron valve to apply cut-off ahead of the main valve. To vary the expansion they provide, the stroke of the eccentric drive can be varied by an adjustable linkage. When this is adjusted to zero throw, the expansion valve remains fully open and the engine works without expansion. [1] Although the use of a secondary gridiron valve was an early technique, it also remained in service with increasingly sophisticated valves and actuation, throughout the history of stationary engines. 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. [9]

In mechanical engineering, an eccentric is a circular disk solidly fixed to a rotating axle with its centre offset from that of the axle.

McIntosh & Seymour

McIntosh & Seymour was an American manufacturer of steam and internal combustion engines during the late 19th and early 20th centuries.

Gridiron valves were also used on the backs of slide valves, in the manner of the Meyer valve. [7] This was a patent of John Turnbull of Glasgow in 1869. [10]

Meyer expansion valve

Gimson beam engine, fitted with a Meyer expansion valve Expansion valve, Gimson beam engine, Snibston.jpg
Gimson beam engine, fitted with a Meyer expansion valve

The best-known design of expansion valve was the Meyer, the invention of French engineer Jean-Jacques Meyer (1804-1877) who applied for a patent on 20 October 1841. A similar valve was patented by James Morris. [11] A second slide valve rides on the back of an adapted main slide valve and is driven by an additional eccentric. In the Meyer valve, the effective length of the expansion valve [lower-roman 4] can be altered with a handwheel whilst the engine is running. The valve has two heads mounted on left- and right-handed threads on the handwheel's valve rod, so that rotating the wheel moves the heads either together or apart. [12] [13] In this arrangement the cut-off is normally controlled manually. Although automatic control was attempted, it was too slow-acting to be effective.

Engines on display at Snibston Discovery Museum and Coleham Pumping Station have Meyer expansion valves. [14]

Compound engines

Expansion valves were also fitted to compound steam engines. Both techniques are an attempt to achieve greater efficiency, even at the cost of more complexity.

It was usual for expansion valves to be fitted only to the HP (high-pressure) cylinder. Steam supplied to the following LP (low-pressure) cylinder has already been supplied to the engine, so there is little benefit to conserving it. Any early cut-off of the steam inlet to a LP cylinder may also represent throttling the exhaust of the preceding HP cylinder, and a reduction in the efficiency of that 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. Examples existed with four different sets of valves: drop valve HP inlets, Corliss HP exhausts and a LP slide valve with a Meyer expansion valve. [15]

Developments after the separate expansion valve led to more sophisticated valve gears that could achieve the same goal of varying inlet lap with a single valve. The first of these were the link valve gears, particularly the Stephenson link valve gear. This uses a pair of eccentrics with a sliding link mechanism between them that acts as a mechanical adding device. Selecting intermediate positions provides a valve actuation with the effect of increasing cut-off. As such valve gears also provided, and were first developed for, reversing, they were widely used on locomotives. Theoretically, the precise effect is that of reduced valve travel, rather than an earlier cut-off. This has the effect of reducing overall valve opening, reducing initial steam supply and so having the effect of wire-drawing rather than pure expansion. [16] Despite this, the Stephenson gear became one of the two most-widely used gears for locomotives.

Automatic governors

'Automatic' engines, and in turn high-speed engines, operated at increasing speeds and required more precise control of their speed under varying load. This required the coupling of their governor to the expansion valve gear. Earlier engines with Watt's centrifugal governor and throttle valve become inefficient when operating at low power.

The Richardson governor [17] was used for stationary and portable engines produced by his employers, Robey & Co.. [18] This is a simple link valve gear controlled automatically by a centrifugal governor. 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. It usually operated similarly to a Meyer valve, with two valves driven by two eccentrics and the Richardson governor used instead of the Meyer's manual handwheel. [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.

Successor valve types

In the fully developed forms of the high-speed engine (from around 1900) though, expansion was controlled by governing the timing [lower-roman 5] of a single valve, rather than a separate expansion valve. These led to further complex valve types such as poppet valves, often driven by cam-based valve gears rather than linkages. [lower-roman 6]

Increasing use of superheating encouraged the replacement of slide valves with piston valves, as these were easier to lubricate at the increased operating temperatures. They also made it impractical to use secondary valves like the Meyer, running on the backs of the primary valves. Possibly the last new design to use a secondary valve as an expansion valve was the Midland Railway Paget locomotive, that used bronze sleeves as expansion valves around its cast iron rotary valves. [20] This design was unsuccessful, owing to mechanical problems with differential thermal expansion of the two valve materials. [21]

Footnotes

  1. This fall of pressure with expansion is inevitable, according to Boyle's law. [2]
  2. This engine was one of the first to employ any cut-off and deliberate expansion of steam. However this cut-off was fixed and could not be varied whilst the engine was working.
  3. A desmodromic cam-driven valve gear with very fast-acting gridiron valves driven at half crankshaft speed was a key feature of Ferranti's high-speed cross compound vertical generating engines. [8]
  4. This length, relative to the port spacing, controls the inlet lap of the valve.
  5. As this was the timing of the valve that was controlled, not its stroke, it avoided some of the Stephenson link's throttling drawback.
  6. Link and radius valve gears are simple to make but have performance limitations. Arbitrarily-shaped cams can offer valve control tailored closer to an ideal operation, although they were difficult to manufacture accurately and their performance worsened dramatically as they wore. Improvements to machining techniques, metallurgy and lubrication increasingly favoured the cams.

Related Research Articles

Poppet valve valve typically used to control the timing and quantity of gas or vapour flow into an engine

A poppet valve is a valve typically used to control the timing and quantity of gas or vapor flow into an engine.

Variable valve timing process of altering the timing of a valve lift event

In internal combustion engines, variable valve timing (VVT) is the process of altering the timing of a valve lift event, and is often used to improve performance, fuel economy or emissions. It is increasingly being used in combination with variable valve lift systems. There are many ways in which this can be achieved, ranging from mechanical devices to electro-hydraulic and camless systems. Increasingly strict emissions regulations are causing many automotive manufacturers to use VVT systems.

Walschaerts valve gear

The Walschaerts valve gear is a type of valve gear invented by Belgian railway mechanical engineer Egide Walschaerts in 1844 used to regulate the flow of steam to the pistons in steam engines. The gear is sometimes named without the final "s", since it was incorrectly patented under that name. It was extensively used in steam locomotives from the late 19th century until the end of the steam era.

Automatic lubricator

An automatic lubricator, is a device fitted to a steam engine to supply lubricating oil to the cylinders and, sometimes, the bearings and axle box mountings as well. There are various types of automatic lubricator, which include various designs of displacement, hydrostatic and mechanical lubricators.

Stephenson valve gear

The Stephenson valve gear or Stephenson link or shifting link is a simple design of valve gear that was widely used throughout the world for all kinds of steam engines. It is named after Robert Stephenson but was invented by his employees.

In a steam engine, cutoff is the point in the piston stroke at which the inlet valve is closed. On a steam locomotive, the cutoff is controlled by the reversing gear.

Corliss steam engine Type of steam engine using rotary steam valves

A Corliss steam engine is a steam engine, fitted with rotary valves and with variable valve timing patented in 1849, invented by and named after the American engineer George Henry Corliss of Providence, Rhode Island.

Southern valve gear

Southern valve gear was briefly popular on steam locomotives in the United States. It combines elements of the Walschaerts and Baker patterns.

Mastodon (steam locomotive) American 4-8-0 locomotive (1882–1935) nicknamed “Mastadon”

Mastodon was the unofficial name of the Central Pacific Railroad's number 229, the world's first successful 4-8-0 steam locomotive.

Bagnall–Price valve gear is a type of steam engine valve gear developed at locomotive manufacturer W.G. Bagnall as an alternative to the more common Walschaerts valve gear. The gear was patented in 1903 by W.G. Bagnall and T. S. Price, the manager of the works.

Cylinder (locomotive)

The cylinder is the power-producing element of the steam engine powering a steam locomotive. The cylinder is made pressure-tight with end covers and a piston; a valve distributes the steam to the ends of the cylinder. Cylinders were cast in cast iron and later in steel. The cylinder casting includes other features such as valve ports and mounting feet. The last big American locomotives incorporated the cylinders as part of huge one-piece steel castings that were the main frame of the locomotive. Renewable wearing surfaces were needed inside the cylinders and provided by cast-iron bushings.

Piston valve (steam engine) form of valve within a steam engine or locomotive

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.

The EDWARD BURY to PFEIL series of early German locomotives were tender engines operated by the Leipzig–Dresden Railway Company.

A Johnson Bar is a control lever on a steam locomotive, used to control the timing of the admission of steam into the locomotive's pistons. By controlling this timing, the amount of power delivered to the wheels is regulated, as is the direction that the wheels rotate, giving the lever the alternate name of the reversing lever. This is the term employed in British English, while the term 'Johnson Bar' is the norm in the United States.

South African Class MA 2-6-6-0 class of 1 South African Mallet locomotive

The South African Railways Class MA 2-6-6-0 of 1909 was a steam locomotive from the pre-Union era in the Natal Colony.

Gab valve gear

Gab valve gear was an early form of valve gear used on steam engines. Its simplest form allowed an engine to be stopped and started. A double form, mostly used on steam locomotives, allowed easy reversing.

High-speed steam engine

High-speed steam engines were one of the final developments of the stationary steam engine. They ran at a high speed, of several hundred rpm, which was needed by tasks such as electricity generation.

Steeple compound engine

A steeple compound engine is a form of tandem compound steam engine that is constructed as an inverted vertical engine. Because of their great height, they became known as "steeple" engines.

References

  1. 1 2 3 4 5 Evers, Henry (1875). Steam and the Steam Engine: Land and Marine. Glasgow: Williams Collins. pp. 78–81.
  2. 1 2 3 Evers (1875), pp. 51–53.
  3. Hills, Power from Steam , p. 102
  4. Trevithick, Francis (1872). Life of Richard Trevithick with an Account of his Inventions. Vol. II. p. 132.
  5. Evers (1875), pp. 73–74.
  6. Southworth (1986) , p. 26
  7. 1 2 Hills & Power from Steam , p. 188
  8. 1 2 Hills, Power from Steam , pp. 226–227
  9. Hawkins, Nehemiah (1897). New Catechism of the Steam Engine. New York: Theo Audel. pp. 97–99.
  10. GB 3207
  11. GB 9571,James Morris
  12. Southworth, P.J.M. (1986). Some Early Robey Steam Engines. P.J.M. Southworth. pp. 4, 21–22, 24. ISBN   0-9511856-0-8.
  13. "The expansion slide valve and governors". Old Engine House. Archived from the original on 2012-02-04. Retrieved 2012-03-30.
  14. Hills, Richard L. (1989). Power from Steam. Cambridge University Press. p. 174. ISBN   0-521-45834-X.
  15. Robey & Co. engine of 1887 Southworth (1986) , pp. 21–22,24
  16. Evers (1875) , p. 78
  17. GB 14753,John Richardson
  18. "The Richardson Governor". Old Engine House. Archived from the original on 2011-09-19. Retrieved 2012-03-30.
  19. Southworth (1986) , p. 20
  20. Ahrons, E.L. (1966). The British Steam Railway Locomotive. I, to 1925. Ian Allan. p. 345.
  21. Self, Douglas (8 February 2004). "The Paget Locomotive". Loco Loco gallery.