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Saturday, August 12, 2023

Third rail

From Wikipedia, the free encyclopedia

A third rail, also known as a live rail, electric rail or conductor rail, is a method of providing electric power to a railway locomotive or train, through a semi-continuous rigid conductor placed alongside or between the rails of a railway track. It is used typically in a mass transit or rapid transit system, which has alignments in its own corridors, fully or almost fully segregated from the outside environment. Third rail systems are usually supplied from direct current electricity.

Modern tram systems, street-running, avoid the risk of electrocution by the exposed electric rail by implementing a segmented ground-level power supply, where each segment is electrified only while covered by a vehicle which is using its power.

The third-rail system of electrification is not related to the third rail used in dual gauge railways.

Description

Third-rail systems are a means of providing electric traction power to trains using an additional rail (called a "conductor rail") for the purpose. On most systems, the conductor rail is placed on the sleeper ends outside the running rails, but in some systems a central conductor rail is used. The conductor rail is supported on ceramic insulators (known as "pots"), at top contact or insulated brackets, at bottom contact, typically at intervals of around 10 feet (3.0 m).

The trains have metal contact blocks called collector shoes (also known as contact shoes or pickup shoes) which make contact with the conductor rail. The traction current is returned to the generating station through the running rails. In North America, the conductor rail is usually made of high conductivity steel or steel bolted to aluminium to increase the conductivity. Elsewhere in the world, extruded aluminium conductors with stainless steel contact surface or cap, is the preferred technology due to its lower electrical resistance, longer life, and lighter weight. The running rails are electrically connected using wire bonds or other devices, to minimise resistance in the electric circuit. Contact shoes can be positioned below, above, or beside the third rail, depending on the type of third rail used: these third rails are referred to as bottom-contact, top-contact, or side-contact, respectively.

The conductor rails have to be interrupted at level crossings, crossovers, and substation gaps. Tapered rails are provided at the ends of each section, to allow a smooth engagement of the train's contact shoes.

The position of contact between the train and the rail varies: some of the earliest systems used top contact, but later developments use side or bottom contact, which enabled the conductor rail to be covered, protecting track workers from accidental contact and protecting the conductor rail from frost, ice, snow and leaf-fall.

Gallery

Descriptions
Third rail layout. 1:Cover 2:Power Rail 3:Insulator 4:sleeper 5:Rail
Third rail (top) at Bloor-Yonge station (Line 1) on the Toronto subway. Energized at 600 volts DC, the third rail provides electrical power to the power-train, and ancillaries of the subway cars.
Paris Métro. The guiding rails of the rubber-tyred lines also function as current conductors. The horizontal contact shoe is between the pair of rubber wheels.
London Stansted Airport people mover with central rail power feed
London Stansted Airport people mover, showing rail switch

Advantages and disadvantages

Safety

Because third rail systems, which are located close to the ground, present electric shock hazards to people, high voltages (above 1500 V) are not considered safe. A very high current must therefore be used to transfer adequate power to the train, resulting in high resistive losses, and requiring relatively closely spaced feed points (electrical substations).

The electrified rail threatens electrocution of anyone wandering or falling onto the tracks. This can be avoided by using platform screen doors, or the risk can be reduced by placing the conductor rail on the side of the track away from the platform, when allowed by the station layout. The risk can also be reduced by having a coverboard, supported by brackets, to protect the third rail from contact, although many systems do not use one. Where coverboards are used they reduce the structure gauge near the top of rail. This in turn reduces the loading gauge.

There is also a risk of pedestrians walking onto the tracks at level crossings. In the US, a 1992 Supreme Court of Illinois decision affirmed a $1.5 million verdict against the Chicago Transit Authority for failing to stop an intoxicated person from walking onto the tracks at a level crossing in an attempt to urinate.

The end ramps of conductor rails (where they are interrupted, or change sides) present a practical limitation on speed due to the mechanical impact of the shoe, and 161 km/h (100 mph) is considered the upper limit of practical third-rail operation. The world speed record for a third rail train is 175 km/h (109 mph) attained on 11 April 1988 by a British Class 442 EMU.

In the event of a collision with a foreign object, the beveled end ramps of bottom running systems can facilitate the hazard of having the third rail penetrate the interior of a passenger car. This is believed to have contributed to the death of five passengers in the Valhalla train crash of 2015.

Modern systems, such as ground-level power supply (first used in the tramway of Bordeaux in 2003), avoid the safety problem by segmenting the powered rail, with each segment being powered only when fully covered by the vehicle which utilizes its power.

Weather effects

Third rail systems using top contact are prone to accumulations of snow, or ice formed from refrozen snow, and this can interrupt operations. Some systems operate dedicated de-icing trains to deposit an oily fluid or antifreeze (such as propylene glycol) on the conductor rail to prevent the frozen build-up. The third rail can also be heated to alleviate the problem of ice.

Unlike overhead line equipment, third rail systems are not susceptible to strong winds or freezing rain, which can bring down overhead wires and hence disable all trains. Thunderstorms can also disable the power with lightning strikes on systems with overhead wires, disabling trains if there is a power surge or a break in the wires.

Gaps

Depending on train and track geometry, gaps in the conductor rail (e.g., at level crossings and junctions) could allow a train to stop in a position where all of its power pickup shoes are in gaps, so that no traction power is available. The train is then said to be "gapped". Another train must then be brought up behind the stranded train to push it on to the conductor rail, or a jumper cable may be used to supply enough power to the train to get one of its contact shoes back on the live rail. Avoiding this problem requires a minimum length of trains that can be run on a line. Locomotives have either had the backup of an on-board diesel engine system (e.g., British Rail Class 73), or have been connected to shoes on the rolling stock (e.g. Metropolitan Railway).

Running rails for power supply

The first idea for feeding electricity to a train from an external source was by using both rails on which a train runs, whereby each rail is a conductor for each polarity, and is insulated by the sleepers. This method is used by most scale model trains, however it does not work so well for large trains as the sleepers are not good insulators. Furthermore, the electric connection requires insulated wheels or insulated axles, but most insulation materials have poor mechanical properties compared with metals used for this purpose, leading to a less stable train vehicle. Nevertheless, it was sometimes used at the beginning of the development of electric trains. The oldest electric railway in Britain, the Volk's Railway in Brighton, England was originally electrified at 50 volts DC using this system (it is now a three rail system). Other railway systems that used it were the Gross-Lichterfelde Tramway and the Ungerer Tramway.

Shoe contact

The third rail is usually located outside the two running rails, but on some systems it is mounted between them. The electricity is transmitted to the train by means of a sliding shoe, which is held in contact with the rail. On many systems, an insulating cover is provided above the third rail to protect employees working near the track; sometimes the shoe is designed to contact the side (called "side running") or bottom (called "bottom running" or "under-running") of the third rail, allowing the protective cover to be mounted directly to its top surface. When the shoe slides along the top surface, it is referred to as "top running". When the shoe slides along the bottom surface, it is less affected by the build-up of snow, ice, or leaves, and reduces the chances of a person being electrocuted by coming in contact with the rail. Examples of systems using under-running third rail include Metro-North in the New York metropolitan area; the SEPTA Market–Frankford Line in Philadelphia; and London's Docklands Light Railway.

Contact shoe gallery

Contact shoes
Contact shoe on Metro-North M8 railcar, designed for both over- and under-running third rail.
A contact shoe for top-contact third rail on SEPTA's Norristown High Speed Line (third rail not visible)
Third rail contact shoe of CTA Chicago 'L' car

Electrical considerations and alternative technologies

Electric traction trains (using electric power generated at a remote power station and transmitted to the trains) are considerably more cost-effective than diesel or steam units, where separate power units must be carried on each train. This advantage is especially marked in urban and rapid transit systems with a high traffic density.

Because of mechanical limitations on the contact to the third rail, trains that use this method of power supply achieve lower speeds than those using overhead electric wires and a pantograph. Nevertheless, they may be preferred inside the cities as there is no need for very high speed and they cause less visual pollution.

The third rail is an alternative to overhead lines that transmit power to trains by means of pantographs attached to the trains. Whereas overhead-wire systems can operate at 25 kV or more, using alternating current (AC), the smaller clearance around a live rail imposes a maximum of about 1200 V, with some systems using 1500 V (Line 4, Guangzhou Metro, Line 5, Guangzhou Metro, Line 3, Shenzhen Metro), and direct current (DC) is used. Trains on some lines or networks use both power supply modes (see § Mixed systems below).

All third rail systems throughout the world are energised with DC supplies. Some of the reasons for this are historical. Early traction engines were DC motors, and the then-available rectifying equipment was large, expensive and impractical to install onboard trains. Also, transmission of the relatively high currents required results in higher losses with AC than DC. Substations for a DC system will have to be (typically) about 2 kilometres (1.2 mi) apart, though the actual spacing depends on the carrying capacity; maximum speed and service frequency of the line.

One method for reducing current losses (and thus increase the spacing of feeder/sub stations, a major cost in third rail electrification) is to use a composite conductor rail of a hybrid aluminium/steel design. The aluminium is a better conductor of electricity, and a running face of stainless steel gives better wear.

There are several ways of attaching the stainless steel to the aluminium. The oldest is a co-extruded method, where the stainless steel is extruded with the aluminium. This method has suffered, in isolated cases, from de-lamination (where the stainless steel separates from the aluminium); this is said to have been eliminated in the latest co-extruded rails. A second method is an aluminium core, upon which two stainless steel sections are fitted as a cap and linear welded along the centre line of the rail. Because aluminium has a higher coefficient of thermal expansion than steel, the aluminium and steel must be positively locked to provide a good current collection interface. A third method rivets aluminium bus strips to the web of the steel rail.

Return current mechanisms

As with overhead wires, the return current usually flows through one or both running rails, and leakage to ground is not considered serious. Where trains run on rubber tyres, as on parts of the Lyon Metro, Paris Métro, Mexico City Metro, Santiago Metro, Sapporo Municipal Subway, and on all of the Montreal Metro and some automated guideway transit systems (e.g. the Astram Line), a live rail must be provided to feed the current. The return is effected through the rails of the conventional track between these guide bars (see rubber-tyred metro).

Another design, with a third rail (current feed, outside the running rails) and fourth rail (current return, midway between the running rails), is used by a few steel-wheel systems; see fourth rail. The London Underground is the largest of these, (see railway electrification in Great Britain). The main reason for using the fourth rail to carry the return current is to avoid this current flowing through the original metal tunnel linings which were never intended to carry current, and which would suffer electrolytic corrosion should such currents flow in them.

Another four-rail system is line M1 of the Milan Metro, where current is drawn by a lateral, flat bar with side contact, with return via a central rail with top contact. Along some sections on the northern part of the line an overhead line is also in place, to allow line M2's trains (that use pantographs and higher voltage, and have no contact shoes) to access a depot located on line M1. In depots, line M1 trains use pantographs because of safety reasons, with transition made near the depots away from revenue tracks.

Aesthetic considerations

Third rail electrification is less visually obtrusive than overhead electrification.

Mixed systems

Several systems use a third rail for part of the route, and other motive power such as overhead catenary or diesel power for the remainder. These may exist because of the connection of separately-owned railways using the different motive systems, local ordinances, or other historical reasons.

United Kingdom

Several types of British trains have been able to operate on both overhead and third rail systems, including British Rail Class 313, 319, 325, 350, 365, 375/6, 377/2, 377/5, 377/7, 378/2, 387, 373, 395, 700 and 717 EMUs, as well as Class 92 locomotives.

Network Rail claims to run the world's largest third rail network.

On the southern region of British Rail, freight yards had overhead wires to avoid the electrocution hazards of a third rail. The locomotives were fitted with a pantograph as well as pick-up shoes.

Eurostar / High Speed 1

The Class 373 used for international high-speed rail services operated by Eurostar through the Channel Tunnel runs on overhead wires at 25 kV AC for most of its journey, with sections of 3 kV DC on Belgian lines between the Belgian high speed section and Brussels Midi station or 1.5 kV DC on the railway lines in the south of France for seasonal services. As originally delivered, the Class 373 units were additionally fitted with 750 V DC collection shoes, designed for the journey in London via the suburban commuter lines to Waterloo. A switch between third-rail and overhead collection was performed while running at speed, initially at Continental Junction near Folkestone, and later on at Fawkham Junction after the opening of the first section of the Channel Tunnel Rail Link. Between Kensington Olympia railway station and North Pole depot, further switchovers were necessary.

The dual-voltage system did cause some problems. Failure to retract the shoes when entering France caused severe damage to the trackside equipment, causing SNCF to install a pair of concrete blocks at the Calais end of both tunnels to break off the third rail shoes if they had not been retracted. An accident occurred in the UK when a Eurostar driver failed to retract the pantograph before entering the third rail system, damaging a signal gantry and the pantograph.

On 14 November 2007, Eurostar's passenger operations were transferred to St Pancras railway station and maintenance operations to Temple Mills depot, making the 750V DC third rail collection equipment redundant and the third rail shoes were removed. The trains themselves are no longer fitted with a speedometer capable of measuring the speed in miles per hour (the indication used to automatically change when the collector shoes were deployed).

In 2009, Southeastern began operating domestic services over High Speed 1 trackage from St Pancras using its new Class 395 EMUs. These services operate on the High Speed line as far as Ebbsfleet International or Ashford International, before transferring to the main lines to serve north and mid Kent. As a consequence, these trains are dual voltage enabled, as the majority of the routes along which they travel are third rail electrified.

North London Line

In London, the North London Line changes from third rail to overhead electrification between Richmond and Stratford at Acton Central. The entire route originally used third rail, but several technical electrical earthing problems, plus the fact that there are already overhead electric wires on part of the route for freight and Regional Eurostar services, led to the change.

West London Line

Also in London, the West London Line changes power supply between Shepherd's Bush and Willesden Junction, where it meets the North London Line. South of the changeover point, the WLL is third rail electrified, north of there, it is overhead.

Thameslink

The cross-city Thameslink service runs on the Southern Region third rail network from Farringdon southwards and on overhead line northwards to Bedford, Cambridge and Peterborough. The changeover is made whilst stationary at Farringdon when heading southbound, and at City Thameslink when heading northbound.

Northern City

On the Moorgate to Hertford and Welwyn suburban service routes, the East Coast Main Line sections are 25 kV AC, with a changeover to third rail made at Drayton Park railway station. A third rail is still used in the tunnel section of the route, because the size of the tunnels leading to Moorgate station was too small to allow for overhead electrification.

North Downs Line

The North Downs Line is not electrified on those parts of the line where the North Downs service has exclusive use.

The electrified portions of the line are

Redhill to Reigate – Allows Southern Railway services to run to Reigate. This saves having to turn around terminating services at Redhill where due to the station layout, as the reversal would block nearly all the running lines.

Shalford Junction to Aldershot South Junction – line shared with South Western Railway electric Portsmouth and Aldershot services.

Wokingham to Reading – line shared with South Western Railway electric services from Waterloo.

Belgium

The Brussels Metro uses a 900 V DC third rail system, placed laterally, with contact by means of a shoe running under the power rail which has an insulating layer at top and sides.

Finland

The Helsinki Metro uses a 750 V DC third rail system. The section from Vuosaari to Vuosaari harbour is not electrified, as its only purpose is to connect to the Finnish rail network, whose gauge differs only by a couple of millimetres from that of the metro. The route has been previously used by diesel shunting locomotives moving new metro trains to the electrified section of the line.

France

The new tramway in Bordeaux (France) uses a novel system with a third rail in the centre of the track. The third rail is separated into 10 m (32 ft 9+3⁄4 in) long conducting and 3 m (9 ft 10+1⁄8 in) long isolation segments. Each conducting segment is attached to an electronic circuit which will make the segment live once it lies fully beneath the tram (activated by a coded signal sent by the train) and switch it off before it becomes exposed again. This system (called "Alimentation par Sol" (APS), meaning "current supply via ground") is used in various locations around the city but especially in the historic centre: elsewhere the trams use the conventional overhead lines, see also ground-level power supply. In summer 2006 it was announced that two new French tram systems would be using APS over part of their networks. These will be Angers and Reims, with both systems expected to open around 2009–2010.

The French Culoz–Modane railway was electrified with 1500 V DC third rail, later converted to overhead wires at the same voltage. Stations had overhead wires from the beginning.

The French branch line which serves Chamonix and the Mont Blanc region (Saint-Gervais-le-Fayet to Vallorcine) is third rail (top contact) and metre gauge. It continues in Switzerland, partly with the same third rail system, partly with an overhead line.

The 63 km (39 mi) long Train Jaune line in the Pyrenees also features a third rail.

Many suburban lines that ran out of the Paris Saint Lazare station used third rail (bottom contact) feed.

Netherlands

To mitigate investment costs, the Rotterdam Metro, basically a third-rail-powered system, has been given some outlying branches built on surface as light rail (called Sneltram in Dutch), with numerous level crossings protected with barriers and traffic lights. These branches have overhead wires. In most recent developments, the RandstadRail project also requires Rotterdam Metro trains to run under wires on their way along the former mainline railways to The Hague and Hook of Holland.

Similarly, in Amsterdam one "Sneltram" route went on Metro tracks and passed to surface alignment in the suburbs, which it shared with standard trams. Sneltram is operated by Gemeentelijk Vervoerbedrijf in Amsterdam lightrail with third rail and switching to overhead on the traditional tramway shared with Trams in Amsterdam. Line 51 to Amstelveen ran metro service between Amsterdam Centraal and Station Zuid. At Amsterdam Zuid it switched from third rail to pantograph and catenary wires. From there to Amstelveen Centrum it shared its tracks with tram line 5. The light rail vehicles on this line were capable of using both 600 V DC and 750 V DC. As of March 2019 this metro line has been decommissioned, partly because of issues regarding switching between third rail and overhead wires. Its line number 51 has been assigned to a new metro line running partly the same route from Amsterdam Centraal railway station to Station Zuid and then following the same route as metro line 50 to Amsterdam Sloterdijk railway station.

Russian Federation and former Soviet Union

In all the subways of post-Soviet countries, the contact rail is made to the same standard.

Third rail schema
Third rail schema: two brackets and third rail for bottom contact
1) brackets
2) The third rail
3) The contact surface
Bottom contact third rail in the sheath insulator
Two contact rails at the Rechnoy Vokzal station of the Novosibirsk Metro a station with left-hand platforms.

United States

In New York City, the New Haven Line of Metro–North Railroad operates electric trains out of Grand Central Terminal that use third rail on the former New York Central Railroad but switch to overhead lines in Pelham to operate out onto the former New York, New Haven and Hartford Railroad. The switch is made "on the fly" (at speed), and controlled from the engineer's position.

The main two stations in New York City – Grand Central and Pennsylvania Station – do not permit diesel locomotives to operate in their tunnels due to the health hazard resulting from the exhaust. As such, diesel service on Metro-North, Long Island Rail Road, and Amtrak use dual-mode/electro-diesel locomotives (the P32AC-DM and the DM30AC) that are able to make use of the third rail power in the stations and approaches. When under third rail operation, these locomotives are less powerful, so on open-air (non-tunnel) trackage the engines typically run in diesel mode, even where third rail power is available. New Jersey Transit also makes use of ALP-45DP dual mode locomotives for operation into Penn Station alongside their normal electric fleet. However, their dual mode locomotives make use of the overhead power supply instead, as it is available elsewhere on much of their network.

In New York City (on most of the island of Manhattan) and in Washington, D.C., local ordinances once required electrified street railways to draw current from a third rail and return the current to a fourth rail, both installed in a continuous vault underneath the street and accessed by means of a collector that passed through a slot between the running rails. When streetcars on such systems entered territory where overhead lines were allowed, they stopped over a pit where a man detached the collector (plow) and the motorman placed a trolley pole on the overhead. In the US, all these conduit feed powered systems have been discontinued, and either replaced or abandoned altogether.

Some sections of the former London tram system also used the conduit current collection system, also with some tramcars that could collect power from both overhead and under-road sources.

The Blue Line of Boston's MBTA uses third rail electrification from the start of the line downtown to Airport station, where it switches to overhead catenary for the remainder of the line to Wonderland station. The outermost section of the Blue Line runs very close to the Atlantic Ocean, and there were concerns about possible snow and ice buildup on a third rail so near to the water. Overhead catenary is not used in the underground section, because of tight clearances in the 1904 tunnel under Boston Harbor. The MBTA Orange Line's Hawker Siddeley 01200 series rapid transit cars (essentially a longer version of the Blue Line's 0600's) recently had their pantograph mounting points removed during a maintenance program; these mounts would have been used for pantographs which would have been installed had the Orange Line been extended north of its current terminus.

Dual power supply method was also used on some US interurban railways that made use of newer third rail in suburban areas, and existing overhead streetcar (trolley) infrastructure to reach downtown, for example the Skokie Swift in Chicago.

Simultaneous use with overhead wire

A railway can be electrified with an overhead wire and a third rail at the same time. This was the case, for example, on the Hamburg S-Bahn between 1940 and 1955. A modern example is Birkenwerder Railway Station near Berlin, which has third rails on both sides and overhead wires. Most of the Penn Station complex in New York City is also electrified with both systems.

Non-standard voltages

Some high third rail voltages (1000 volts and more) include:

Hamburg S-Bahn: 1200 V, since 1940
Manchester–Bury, England: 1200 V (side contact) (Until Metrolink conversion in 1991)
Culoz–Modane railway, France: 1500 V, 1925–1976
Guangzhou Metro Lines 4 and 5: 1500 V
Bay Area Rapid Transit, San Francisco, 1000 V

In Nazi Germany, a railway system with a 3,000 mm (9 ft 10+1⁄8 in) gauge width was planned. For this Breitspurbahn railway system, electrification with a voltage of 100 kV taken from a third rail was considered, in order to avoid damage to overhead wires from oversize rail-mounted anti-aircraft guns. However such a power system would not have worked, as it is not possible to insulate a third rail for such high voltages in close proximity to the rails. The whole project did not progress any further owing to the onset of World War II.

History

Third-rail electrification systems are, apart from on-board batteries, the oldest means of supplying electric power to trains on railways using their own corridors, particularly in cities. Overhead power supply was initially almost exclusively used on tramway-like railways, though it also appeared slowly on mainline systems.

An experimental electric train using this method of power supply was developed by the German firm of Siemens & Halske and shown at the Berlin Industrial Exposition of 1879, with its third rail between the running rails. Some early electric railways used the running rails as the current conductor, as with the 1883-opened Volk's Electric Railway in Brighton. It was given an additional power rail in 1886, and is still operating. The Giant's Causeway Tramway followed, equipped with an elevated outside third rail in 1883, later converted to overhead wire. The first railway to use the central third rail was the Bessbrook and Newry Tramway in Ireland, opened in 1885 but now, like the Giant's Causeway line, closed.

Also in the 1880s, third-rail systems began to be used in public urban transport. Trams were first to benefit from it: they used conductors in conduit below the road surface (see Conduit current collection), usually on selected parts of the networks. This was first tried in Cleveland (1884) and in Denver (1885) and later spread to many big tram networks (e.g. New York, Chicago, Washington DC, London, Paris, all of which are closed) and Berlin (the third rail system in the city was abandoned in the first years of the 20th century after heavy snowfall.) The system was tried in the beachside resort of Blackpool, UK but was soon abandoned as sand and saltwater was found to enter the conduit and cause breakdowns, and there was a problem with voltage drop. Some sections of tramway track still have the slot rails visible.

A third rail supplied power to the world's first electric underground railway, the City & South London Railway, which opened in 1890 (now part of the Northern line of the London Underground). In 1893, the world's second third-rail powered city railway opened in Britain, the Liverpool Overhead Railway (closed 1956 and dismantled). The first US third-rail powered city railway in revenue use was the 1895 Metropolitan West Side Elevated, which soon became part of the Chicago 'L'. In 1901, Granville Woods, a prominent African-American inventor, was granted a U.S. Patent 687,098, covering various proposed improvements to third rail systems. This has been cited to claim that he invented the third rail system of current distribution. However, by that time there had been numerous other patents for electrified third-rail systems, including Thomas Edison's U.S. Patent 263,132 of 1882, and third rails had been in successful use for over a decade, in installations including the rest of Chicago 'elevateds', as well as those used in Brooklyn Rapid Transit Company, not to mention the development outside the US.

In Paris, a third rail appeared in 1900 in the main-line tunnel connecting the Gare d'Orsay to the rest of the CF Paris-Orléans network. Main-line third-rail electrification was later expanded to some suburban services.

The Woodford haulage system was used on industrial tramways, specifically in quarries and strip mines in the early decades of the 20th century. This used a 250 Volt center third rail to power remotely-controlled self-propelled side dump cars. The remote control system was operated like a model railroad, with the third rail divided into multiple blocks that could be set to power, coast, or brake by switches in the control center.

Top contact or gravity type third rail seems to be the oldest form of power collection. Railways pioneering in using less hazardous types of third rail were the New York Central Railroad on the approach to New York's Grand Central Terminal (1907 – another case of a third-rail mainline electrification), Philadelphia's Market–Frankford Line (1907), and the Hochbahn in Hamburg (1912) each had bottom contact rail, also known as the Wilgus-Sprague system. However, the Manchester-Bury Line of the Lancashire & Yorkshire Railway tried side contact rail in 1917. These technologies appeared in wider use only at the turn of the 1920s and in the 1930s on, e.g., large-profile lines of the Berlin U-Bahn, the Berlin S-Bahn and the Moscow Metro. The Hamburg S-Bahn has used a side contact third rail at 1200 V DC since 1939.

In 1956, the world's first rubber-tyred railway line, Line 11 of Paris Metro, opened. The conductor rail evolved into a pair of guiding rails required to keep the bogie in proper position on the new type of track. This solution was modified on the 1971 Namboku Line of Sapporo Subway, where a centrally placed guiding/return rail was used plus one power rail placed laterally as on conventional railways.

In 2004, the third-rail technology at street tram lines was in the new system of Bordeaux (2004). This is a completely new technology (see below).

Third-rail systems are not considered obsolete. There are, however, countries (particularly Japan, South Korea, Spain) more eager to adopt overhead wiring for their urban railways. But at the same time, there were (and still are) many new third rail systems built elsewhere, including technologically advanced countries (e.g. Copenhagen Metro, Taipei Metro, Wuhan Metro). Bottom powered railways (it may be too specific to use the term 'third rail') are also usually used with systems having rubber-tyred trains, whether it is a heavy metro (except two other lines of Sapporo Subway) or a small capacity people mover (PM). New electrified railway systems tend to use overhead for regional and long-distance systems. Third-rail systems using lower voltages than overhead systems still require many more supply points.

Chemical plant

From Wikipedia, the free encyclopedia
https://en.wikipedia.org/wiki/Chemical_plant

A chemical plant is an industrial process plant that manufactures (or otherwise processes) chemicals, usually on a large scale. The general objective of a chemical plant is to create new material wealth via the chemical or biological transformation and or separation of materials. Chemical plants use specialized equipment, units, and technology in the manufacturing process. Other kinds of plants, such as polymer, pharmaceutical, food, and some beverage production facilities, power plants, oil refineries or other refineries, natural gas processing and biochemical plants, water and wastewater treatment, and pollution control equipment use many technologies that have similarities to chemical plant technology such as fluid systems and chemical reactor systems. Some would consider an oil refinery or a pharmaceutical or polymer manufacturer to be effectively a chemical plant.

Petrochemical plants (plants using chemicals from petroleum as a raw material or feedstock) are usually located adjacent to an oil refinery to minimize transportation costs for the feedstocks produced by the refinery. Speciality chemical and fine chemical plants are usually much smaller and not as sensitive to location. Tools have been developed for converting a base project cost from one geographic location to another.

Chemical processes

Chemical plants use chemical processes, which are detailed industrial-scale methods, to transform feedstock chemicals into products. The same chemical process can be used at more than one chemical plant, with possibly differently scaled capacities at each plant. Also, a chemical plant at a site may be constructed to utilize more than one chemical process, for instance to produce multiple products.

A chemical plant commonly has usually large vessels or sections called units or lines that are interconnected by piping or other material-moving equipment which can carry streams of material. Such material streams can include fluids (gas or liquid carried in piping) or sometimes solids or mixtures such as slurries. An overall chemical process is commonly made up of steps called unit operations which occur in the individual units. A raw material going into a chemical process or plant as input to be converted into a product is commonly called a feedstock, or simply feed. In addition to feedstocks for the plant as a whole, an input stream of material to be processed in a particular unit can similarly be considered feed for that unit. Output streams from the plant as a whole are final products and sometimes output streams from individual units may be considered intermediate products for their units. However, final products from one plant may be intermediate chemicals used as feedstock in another plant for further processing. For example, some products from an oil refinery may used as feedstock in petrochemical plants, which may in turn produce feedstocks for pharmaceutical plants.

Either the feedstock(s), the product(s), or both may be individual compounds or mixtures. It is often not worthwhile separating the components in these mixtures completely; specific levels of purity depend on product requirements and process economics.

Operations

Chemical processes may be run in continuous or batch operation.

Batch operation

In batch operation, production occurs in time-sequential steps in discrete batches. A batch of feedstock(s) is fed (or charged) into a process or unit, then the chemical process takes place, then the product(s) and any other outputs are removed. Such batch production may be repeated over again and again with new batches of feedstock. Batch operation is commonly used in smaller scale plants such as pharmaceutical or specialty chemicals production, for purposes of improved traceability as well as flexibility. Continuous plants are usually used to manufacture commodity or petrochemicals while batch plants are more common in speciality and fine chemical production as well as pharmaceutical active ingredient (API) manufacture.

Continuous operation

In continuous operation, all steps are ongoing continuously in time. During usual continuous operation, the feeding and product removal are ongoing streams of moving material, which together with the process itself, all take place simultaneously and continuously. Chemical plants or units in continuous operation are usually in a steady state or approximate steady state. Steady state means that quantities related to the process do not change as time passes during operation. Such constant quantities include stream flow rates, heating or cooling rates, temperatures, pressures, and chemical compositions at any given point (location). Continuous operation is more efficient in many large scale operations like petroleum refineries. It is possible for some units to operate continuously and others be in batch operation in a chemical plant; for example, see Continuous distillation and Batch distillation. The amount of primary feedstock or product per unit of time which a plant or unit can process is referred to as the capacity of that plant or unit. For examples: the capacity of an oil refinery may be given in terms of barrels of crude oil refined per day; alternatively chemical plant capacity may be given in tons of product produced per day. In actual daily operation, a plant (or unit) will operate at a percentage of its full capacity. Engineers typically assume 90% operating time for plants which work primarily with fluids, and 80% uptime for plants which primarily work with solids.

Units and fluid systems

Specific unit operations are conducted in specific kinds of units. Although some units may operate at ambient temperature or pressure, many units operate at higher or lower temperatures or pressures. Vessels in chemical plants are often cylindrical with rounded ends, a shape which can be suited to hold either high pressure or vacuum. Chemical reactions can convert certain kinds of compounds into other compounds in chemical reactors. Chemical reactors may be packed beds and may have solid heterogeneous catalysts which stay in the reactors as fluids move through, or may simply be stirred vessels in which reactions occur. Since the surface of solid heterogeneous catalysts may sometimes become "poisoned" from deposits such as coke, regeneration of catalysts may be necessary. Fluidized beds may also be used in some cases to ensure good mixing. There can also be units (or subunits) for mixing (including dissolving), separation, heating, cooling, or some combination of these. For example, chemical reactors often have stirring for mixing and heating or cooling to maintain temperature. When designing plants on a large scale, heat produced or absorbed by chemical reactions must be considered. Some plants may have units with organism cultures for biochemical processes such as fermentation or enzyme production.

Separation processes include filtration, settling (sedimentation), extraction or leaching, distillation, recrystallization or precipitation (followed by filtration or settling), reverse osmosis, drying, and adsorption. Heat exchangers are often used for heating or cooling, including boiling or condensation, often in conjunction with other units such as distillation towers. There may also be storage tanks for storing feedstock, intermediate or final products, or waste. Storage tanks commonly have level indicators to show how full they are. There may be structures holding or supporting sometimes massive units and their associated equipment. There are often stairs, ladders, or other steps for personnel to reach points in the units for sampling, inspection, or maintenance. An area of a plant or facility with numerous storage tanks is sometimes called a tank farm, especially at an oil depot.

Fluid systems for carrying liquids and gases include piping and tubing of various diameter sizes, various types of valves for controlling or stopping flow, pumps for moving or pressurizing liquid, and compressors for pressurizing or moving gases. Vessels, piping, tubing, and sometimes other equipment at high or very low temperature are commonly covered with insulation for personnel safety and to maintain temperature inside. Fluid systems and units commonly have instrumentation such as temperature and pressure sensors and flow measuring devices at select locations in a plant. Online analyzers for chemical or physical property analysis have become more common. Solvents can sometimes be used to dissolve reactants or materials such as solids for extraction or leaching, to provide a suitable medium for certain chemical reactions to run, or so they can otherwise be treated as fluids.

Chemical plant design

Today, the fundamental aspects of designing chemical plants are done by chemical engineers. Historically, this was not always the case and many chemical plants were constructed in a haphazard way before the discipline of chemical engineering became established. Chemical engineering was first established as a profession in the United Kingdom when the first chemical engineering course was given at the University of Manchester in 1887 by George E. Davis in the form of twelve lectures covering various aspects of industrial chemical practice. As a consequence George E. Davis is regarded as the world's first chemical engineer. Today chemical engineering is a profession and those professional chemical engineers with experience can gain "Chartered" engineer status through the Institution of Chemical Engineers.

In plant design, typically less than 1 percent of ideas for new designs ever become commercialized. During this solution process, typically, cost studies are used as an initial screening to eliminate unprofitable designs. If a process appears profitable, then other factors are considered, such as safety, environmental constraints, controllability, etc. The general goal in plant design, is to construct or synthesize “optimum designs” in the neighborhood of the desired constraints.

Many times chemists research chemical reactions or other chemical principles in a laboratory, commonly on a small scale in a "batch-type" experiment. Chemistry information obtained is then used by chemical engineers, along with expertise of their own, to convert to a chemical process and scale up the batch size or capacity. Commonly, a small chemical plant called a pilot plant is built to provide design and operating information before construction of a large plant. From data and operating experience obtained from the pilot plant, a scaled-up plant can be designed for higher or full capacity. After the fundamental aspects of a plant design are determined, mechanical or electrical engineers may become involved with mechanical or electrical details, respectively. Structural engineers may become involved in the plant design to ensure the structures can support the weight of the units, piping, and other equipment.

The units, streams, and fluid systems of chemical plants or processes can be represented by block flow diagrams which are very simplified diagrams, or process flow diagrams which are somewhat more detailed. The streams and other piping are shown as lines with arrow heads showing usual direction of material flow. In block diagrams, units are often simply shown as blocks. Process flow diagrams may use more detailed symbols and show pumps, compressors, and major valves. Likely values or ranges of material flow rates for the various streams are determined based on desired plant capacity using material balance calculations. Energy balances are also done based on heats of reaction, heat capacities, expected temperatures and pressures at various points to calculate amounts of heating and cooling needed in various places and to size heat exchangers. Chemical plant design can be shown in fuller detail in a piping and instrumentation diagram (P&ID) which shows all piping, tubing, valves, and instrumentation, typically with special symbols. Showing a full plant is often complicated in a P&ID, so often only individual units or specific fluid systems are shown in a single P&ID.

In the plant design, the units are sized for the maximum capacity each may have to handle. Similarly, sizes for pipes, pumps, compressors, and associated equipment are chosen for the flow capacity they have to handle. Utility systems such as electric power and water supply should also be included in the plant design. Additional piping lines for non-routine or alternate operating procedures, such as plant or unit startups and shutdowns, may have to be included. Fluid systems design commonly includes isolation valves around various units or parts of a plant so that a section of a plant could be isolated in case of a problem such as a leak in a unit. If pneumatically or hydraulically actuated valves are used, a system of pressurizing lines to the actuators is needed. Any points where process samples may have to be taken should have sampling lines, valves, and access to them included in the detailed design. If necessary, provisions should be made for reducing high pressure or temperature of a sampling stream, such including a pressure reducing valve or sample cooler.

Units and fluid systems in the plant including all vessels, piping, tubing, valves, pumps, compressors, and other equipment must be rated or designed to be able to withstand the entire range of pressures, temperatures, and other conditions which they could possibly encounter, including any appropriate safety factors. All such units and equipment should also be checked for materials compatibility to ensure they can withstand long-term exposure to the chemicals they will come in contact with. Any closed system in a plant which has a means of pressurizing possibly beyond the rating of its equipment, such as heating, exothermic reactions, or certain pumps or compressors, should have an appropriately sized pressure relief valve included to prevent overpressurization for safety. Frequently all of these parameters (temperatures, pressures, flow, etc.) are exhaustively analyzed in combination through a Hazop or fault tree analysis, to ensure that the plant has no known risk of serious hazard.

Within any constraints the plant is subject to, design parameters are optimized for good economic performance while ensuring safety and welfare of personnel and the surrounding community. For flexibility, a plant may be designed to operate in a range around some optimal design parameters in case feedstock or economic conditions change and re-optimization is desirable. In more modern times, computer simulations or other computer calculations have been used to help in chemical plant design or optimization.

Plant operation

Process control

In process control, information gathered automatically from various sensors or other devices in the plant is used to control various equipment for running the plant, thereby controlling operation of the plant. Instruments receiving such information signals and sending out control signals to perform this function automatically are process controllers. Previously, pneumatic controls were sometimes used. Electrical controls are now common. A plant often has a control room with displays of parameters such as key temperatures, pressures, fluid flow rates and levels, operating positions of key valves, pumps and other equipment, etc. In addition, operators in the control room can control various aspects of the plant operation, often including overriding automatic control. Process control with a computer represents more modern technology. Based on possible changing feedstock composition, changing products requirements or economics, or other changes in constraints, operating conditions may be re-optimized to maximize profit.

Workers

As in any industrial setting, there are a variety of workers working throughout a chemical plant facility, often organized into departments, sections, or other work groups. Such workers typically include engineers, plant operators, and maintenance technicians. Other personnel at the site could include chemists, management/administration and office workers. Types of engineers involved in operations or maintenance may include chemical process engineers, mechanical engineers for maintaining mechanical equipment, and electrical/computer engineers for electrical or computer equipment.

Transport

Large quantities of fluid feedstock or product may enter or leave a plant by pipeline, railroad tank car, or tanker truck. For example, petroleum commonly comes to a refinery by pipeline. Pipelines can also carry petrochemical feedstock from a refinery to a nearby petrochemical plant. Natural gas is a product which comes all the way from a natural gas processing plant to final consumers by pipeline or tubing. Large quantities of liquid feedstock are typically pumped into process units. Smaller quantities of feedstock or product may be shipped to or from a plant in drums. Use of drums about 55 gallons in capacity is common for packaging industrial quantities of chemicals. Smaller batches of feedstock may be added from drums or other containers to process units by workers.

Maintenance

In addition to feeding and operating the plant, and packaging or preparing the product for shipping, plant workers are needed for taking samples for routine and troubleshooting analysis and for performing routine and non-routine maintenance. Routine maintenance can include periodic inspections and replacement of worn catalyst, analyzer reagents, various sensors, or mechanical parts. Non-routine maintenance can include investigating problems and then fixing them, such as leaks, failure to meet feed or product specifications, mechanical failures of valves, pumps, compressors, sensors, etc.

Statutory and regulatory compliance

When working with chemicals, safety is a concern in order to avoid problems such as chemical accidents. In the United States, the law requires that employers provide workers working with chemicals with access to a Material Safety Data Sheet (MSDS) for every kind of chemical they work with. An MSDS for a certain chemical is prepared and provided by the supplier to whoever buys the chemical. Other laws covering chemical safety, hazardous waste, and pollution must be observed, including statutes such as the Resource Conservation and Recovery Act (RCRA) and the Toxic Substances Control Act (TSCA), and regulations such as the Chemical Facility Anti-Terrorism Standards in the United States. Hazmat (hazardous materials) teams are trained to deal with chemical leaks or spills. Process Hazard Analysis (PHA) is used to assess potential hazards in chemical plants. In 1998, the U. S. Chemical Safety and Hazard Investigation Board has become operational.

Plant facilities

The actual production or process part of a plant may be indoors, outdoors, or a combination of the two. It may be a traditional stick-built plant or a modular skid. Large modular skids are especially impressive feats of engineering. A modular skid is built including all of the modular equipment needed to do the same job a traditional stick-build plant may perform. However, the modular skid is built within a structural steel frame, allowing it to be shipped to the onsite location without needing to be rebuilt onsite. A modular skid build results in a higher functioning end product, as less hands are required in the onsite setup of the modular skid process unit, resulting in minimized risk for mishaps. The actual production section of a facility usually has the appearance of a rather industrial environment. Hard hats and work shoes are commonly worn. Floors and stairs are often made of metal grating, and there is practically no decoration. There may also be pollution control or waste treatment facilities or equipment. Sometimes existing plants may be expanded or modified based on changing economics, feedstock, or product needs. As in other production facilities, there may be shipping and receiving, and storage facilities. In addition, there are usually certain other facilities, typically indoors, to support production at the site.

Although some simple sample analysis may be able to be done by operations technicians in the plant area, a chemical plant typically has a laboratory where chemists analyze samples taken from the plant. Such analysis can include chemical analysis or determination of physical properties. Sample analysis can include routine quality control on feedstock coming into the plant, intermediate and final products to ensure quality specifications are met. Non-routine samples may be taken and analyzed for investigating plant process problems also. A larger chemical company often has a research laboratory for developing and testing products and processes where there may be pilot plants, but such a laboratory may be located at a site separate from the production plants.

A plant may also have a workshop or maintenance facility for repairs or keeping maintenance equipment. There is also typically some office space for engineers, management or administration, and perhaps for receiving visitors. The decorum there is commonly more typical of an office environment.

Clustering of commodity chemical plants

Chemical Plants used particularly for commodity chemical and petrochemical manufacture, are located in relatively few manufacturing locations around the world largely due to infrastructural needs. This is less important for speciality or fine chemical batch plants. Not all commodity/petrochemicals are produced in any one location but groups of related materials often are, to induce industrial symbiosis as well as material, energy and utility efficiency and other economies of scale. These manufacturing locations often have business clusters of units called chemical plants that share utilities and large scale infrastructure such as power stations, port facilities, road and rail terminals. In the United Kingdom for example there are four main locations for commodity chemical manufacture: near the River Mersey in Northwest England, on the Humber on the East coast of Yorkshire, in Grangemouth near the Firth of Forth in Scotland and on Teesside as part of the Northeast of England Process Industry Cluster (NEPIC). Approximately 50% of the UK's petrochemicals, which are also commodity chemicals, are produced by the industry cluster companies on Teesside at the mouth of the River Tees on three large chemical parks at Wilton, Billingham and Seal Sands.

Corrosion and use of new materials

Corrosion in chemical process plants is a major issue that consumes billions of dollars yearly. Electrochemical corrosion of metals is pronounced in chemical process plants due to the presence of acid fumes and other electrolytic interactions. Recently, FRP (Fibre-reinforced plastic) is used as a material of construction. The British standard specification BS4994 is widely used for design and construction of the vessels, tanks, etc.

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