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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. This video was done using review code provided by the developer.This is a gameplay and performance review of Inside By Playdead on PC. The game was develope.
A steam drum is a standard feature of a water-tube boiler. It is a reservoir of water/steam at the top end of the water tubes. The drum stores the steam generated in the water tubes and acts as a phase-separator for the steam/water mixture. The difference in densities between hot and cold water helps in the accumulation of the 'hotter'-water/and saturated-steam into the steam-drum.
History[edit]
Initially the boilers were designed with 4 drums and 3 drums like the Stirling boiler. The single drum at the bottom and three drums on the top were connected through a network of tubes which were welded to the drums above and the single drum below. The rational demand of steam in terms of capacity, pressure and temperature resulted in bi drums and single drum boilers.
Working[edit]
The separated steam is drawn out from the top section of the drum and distributed for process. Further heating of the saturated steam will make superheated steam normally used to drive a steam turbine. Saturated steam is drawn off the top of the drum and re-enters the furnace in through a superheater.The steam and water mixture enters the steam drum through riser tubes, drum internals consisting of demister separate the water droplets from the steam producing dry steam. The saturated water at the bottom of the steam drum flows down through the downcomer pipe, normally unheated, to headers and water drum. Its accessories include a safety valve, water-level indicator and level controller. Feed-water of boiler is also fed to the steam drum through a feed pipe extending inside the drum, along the length of the steam drum.
A steam drum is used without or in the company of a mud-drum/feed water drum which is located at a lower level. A boiler with both steam drum and mud/water drum is called a bi-drum boiler and a boiler with only a steam drum is called a mono-drum boiler. The bi-drum boiler construction is normally intended for low pressure-rating boiler while the mono-drum is mostly designed for higher pressure-rating.[1]
On steam locomotives the steam drum is also called a steam dome.
Types of Steam Drums[edit]
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- Three drum/ Four drum boilers - Are the veterans of the normal day boilers, although they are still used in some industries.
- Bi drum boiler - are used for power generation and steam generation both. For power generation they are used now seldom and are replaced by single drum boilers as the bi drum boilers are non-reheat units. So, due to the high heat rate of the plant a single drum boiler or a once through boiler is more feasible. In process steam generation the bi drum boilers are used commonly as they can adapt to the high load fluctuation and respond to load changes.
- Single drum boiler- used mainly for the power plants for power generation. The pressure limit for single drum boilers is higher than that of the bi drum boilers as the stress concentration is reduced to a greater extent. There exists only one drum and the downcomers are welded to it. Single drum boilers are suitable and can adapt to both reheat and non-reheat type of boilers. They can be designed as Corner tube boiler where the frame is not required as the downcomers itself serves the purpose of it and also they are designed as top supported where the whole boiler assembly needs an external frame and supported by top drum.
See also[edit]
- Once-through steam generator, does not have a steam drum
References[edit]
- ^Sathyanathan, Dr V T. 'Bi Drum and Single Drum Boiler Compared'. www.brighthubengineering.com. Retrieved 9 April 2013.CS1 maint: discouraged parameter (link)
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[edit]
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:
- It is difficult to lubricate slide valves adequately in the presence of superheated steam
- With piston valves, the steam passages can be made shorter. This, particularly following the work of André Chapelon, reduces resistance to the flow of steam and improves efficiency
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[edit]
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[edit]
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[edit]
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 valve piston (i.e. the distance the valve needs to move to just begin to uncover the port).[3] Secondly, there is the exhaust lap, which is the amount by which the valve overlaps the port on the exhaust side of the valve piston. 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.
Negative exhaust lap, also commonly known as 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[edit]
Lead 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. For piston diameters and strokes of 75mm lead is not needed to cushion the pistol mass especially when speeds are less than 200 rpm. Engines with pistons of 24 inches plus and masses of over 5 kilos and pressures under 500 psi then cushioning is beneficial. Source P Pellandine; 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[edit]
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[edit]
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:
- Calculate the angle whose cosine is twice the lap divided by the valve travel
- Calculate the angle whose cosine is twice the (lap plus lead), divided by the valve travel
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[edit]
References[edit]
- ^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.
- ^Information plaque on the Swannington engine, National Railway Museum, York.
- ^ abcdefghijklGarratt, C. & Wade-Matthews, M.: The Ultimate Encyclopedia of Steam & Rail (London: Hermes Publishing Company, Ltd., 1998) ISBN1-84038-088-8
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