Control valve (railway)
Control valves automatically regulate the flow of compressed air between the main air line , the air supply tank , the brake cylinders and the ambient air in every vehicle of a railroad train equipped with an automatic air brake .
The most common purely pneumatic controls are characterized by:
- Principle of autonomy: When the excess air pressure in the main air line is reduced, e.g. B. by a train separation or derailment - causes the control valve without any action on the part of the driver, that compressed air from the auxiliary air reservoir to the brake cylinder, which brakes the vehicle.
- Principle of the indirect effect: The brake cylinder is not filled directly from the main air line, but indirectly from the auxiliary air reservoir, which is what enables it to operate automatically.
- Principle of inexhaustibility: As long as there is pressure in the main air line, the brake is considered inexhaustible, because when the pressure in the main air line rises it only releases proportionally and at the same time the air reservoir is filled with air again.
history
In 1887, George Westinghouse invented the indirect air brake. With this brake, the compressed air from the locomotive is not fed directly into the brake cylinder, but a control valve on each car controls the brake cylinder pressure at the command of the locomotive driver. With the single-release control valves of the time, the brakes can be applied gradually, but only released all at once.
Most of the relatively short passenger trains on main lines were equipped with air brakes as early as the 19th century. However, these brakes could not be used for long freight trains , so they continued to run with hand brakes . The development of a freight train brake was delayed by the outbreak of the First World War. The International Union of Railways , founded in 1922, took charge of the introduction of freight train brakes. In 1926 the single-release French Westinghouse freight train brake (W) and the multi-release German Kunze-Knorr freight train brake (Kk) were tested and approved internationally.
The Kunze-Knorr-Bremse was further developed by Wilhelm Hildebrand into the multi-part Hildebrand-Knorr-Bremse (Hik), which received international approval in 1932 and was the standard for German railways until the mid-1950s. From other countries, the Drolshammer brake (Dr, Switzerland) and the Bozič brake (Bo, Czechoslovakia ) were approved in 1928 and the Breda brake (Bd, Italy) in 1934 . The abbreviations are written on the car.
The Second World War interrupted development and international cooperation. In the post-war designs of the control valves, the diaphragm control prevailed over the slide control. These control valves are characterized by their lower weight, a modular system and longer maintenance intervals. A number of new registrations were made:
- 1948 Charmilles brake Ch (Switzerland)
- 1950 Oerlikon -Bremse O (Switzerland)
- 1954 Knorr-Bremse with unitary effect KE (Federal Republic of Germany)
- 1955 DAKO DK (Czechoslovakia)
- 1955 Westinghouse France WE
- 1962 Westinghouse Italy WU
- 1969 Westinghouse England WA
These brakes were named after the companies who developed them.
The technical limit of compressed air brakes has been reached with the latest designs. The development no longer concentrated on technical refinements, but on devices that were inexpensive to manufacture and maintain.
The main disadvantage of the purely pneumatic brake is that it has to be around 280 m / s limited penetration speed . Electropneumatic brakes (ep brakes) offer the possibility of reducing the disadvantageous properties of pure air brakes. The indirectly acting ep brake has become established on long-distance passenger trains, as the cars use electrical energy anyway and are equipped with a main air reservoir line . The central control element of every car is (as before) a conventional control valve.
Control valves of the multi-release air brake
The driver can interrupt braking at any time or apply the brakes gradually. The brakes used in Europe today, which can also be released in stages, are referred to as multi-release.
function
Pneumatic control valves usually work according to the so-called three - pressure principle . The main air line pressure (blue), the pressure in the control tank (yellow) and the pressure in the brake cylinder (green) are controlled. The pressure in the auxiliary air reservoir (red) contains the air supply to feed the brake cylinder.
In certain cases, certain control valves can be equipped with a spring instead of the control container (yellow).
Fill and dissolve
Compressed air flows from the main air line 2 via the switch-off valve 3
- into the space (blue) above the control piston 7
- Via the check valve 5 into the auxiliary air tank 1 and into the space (red) above the inlet valve 10
- Via the check valve 6 into the control tank 9 and into the space (yellow) under the control piston 7.
The control piston is somewhat loaded by the spring pressing on the regulating piston 8, consequently the hollow tappet is not in contact with the inlet valve 10 resting on its seat. The brake cylinder is thus connected to the outside via the bore of the tappet.
Brakes
A pressure reduction in the main air line 2 causes the pressure in the room (blue) above the control piston 7 to decrease. At the same time, the check valves 5 and 6 close. This means that compressed air cannot flow back into the main air line either from the auxiliary air tank or from the control tank. As a result of the pressure difference between the control air (yellow) and the main air line air (blue), the control piston rises, the tappet of which lifts the inlet valve from its seat and opens. The connection of the brake cylinder 4 through the bore of the valve tappet is closed and compressed air (green) flows from the auxiliary air reservoir through the open inlet valve 10 into the brake cylinder.
The air flowing in increases the pressure in the brake cylinder and in the second chamber of the brake valve from above (green part). This creates a force which presses the valve tappet downwards and closes the inlet valve 10 again. This creates a proportionality between the pressure reduction in the main air line and the pressure increase in the brake cylinder.
The maximum pressure in the brake cylinder is reached when the pressure in the main air line decreases by 1.5 bar is reduced. If the brake cylinder of a traction vehicle was triggered after a first braking operation, the brake cylinder pressure can be reduced by a further decrease in the pressure in the main air line by 0.6 cash can still be built up to a certain value. A maximum pressure limiter on the control valve prevents the maximum permissible pressure in the brake cylinder from being exceeded.
Final position
When the pressure in the brake cylinder 4 and thus the pressure (green) in the space above the regulating piston 8 has risen so far that the downward force of the regulating piston is able to cancel out or slightly outweighs the upward force of the control piston 7, the control piston becomes and with it the hollow tappet is moved downward so far that the inlet valve 10 is pressed onto its seat by its spring and closed. The further air supply from the auxiliary air reservoir after the brake cylinder is thus completed. Because the hollow valve tappet remains in contact with the inlet valve, no air can escape from the brake cylinder to the open air. Every further pressure reduction in the main air line results in a corresponding increase in the pressure in the brake cylinder, with each pressure stage being completed in the same way. The full braking effect is achieved when the pressure in the main air line has been reduced to such an extent that there is pressure compensation between the auxiliary air reservoir and the brake cylinder.
Gradual release
If compressed air is let into the main air line again through the driver's brake valve, the pressure in the space (blue) above the control piston increases. This disturbs the state of equilibrium of the control piston and it moves down until its valve tappet loses contact with the inlet valve. The compressed air (green) can now escape from the brake cylinder through the bore in the valve tappet into the open, but only as long as the downward force is sufficient to keep the control piston in the release position and thus the bore of the valve tappet open. With the decrease in the brake cylinder pressure, the pressure acting on the regulating piston 8 from above also becomes weaker, so that the control piston moves upwards under the influence of the control pressure (yellow) until the valve tappet closes the air outlet from the brake cylinder. If the pressure in the main air line increases further, the release process is repeated.
Complete loosening
The brake is only fully released when the original, i.e. H. the pressure that was present before the first braking is reached again and the auxiliary air reservoir is filled again.
The brake cylinder is completely released as soon as the pressure in the main air line drops to 0.2 bar has risen below normal pressure. The loosening limit is set to this value in order to avoid loosening problems, i. H. All braking devices, especially those at the end of the train, should be released reliably when the main air line is increased to normal pressure, even with long trains.
Depending on the type of braking
The brake cylinder filling and release times depend on the braking position. In the braking position P (passenger train brake ) the air passages in the control valve are less restricted than in the slower acting position G (freight train brake ).
In the case of long freight trains must prevent the rear pulling member runs onto the front already fully braked pulling part and to buffer overlapping elements and Zugtrennungen comes. Therefore, when braking, the individual brakes must respond as quickly as possible, but the braking force must then be built up relatively slowly. The rapid start of the braking force (injection) at the start of braking is generated in the control valve with the so-called minimum pressure valve. This allows compressed air from the auxiliary air reservoir to flow directly into the brake cylinder, up to a pressure of 0.8 bar is reached. Then the connection is closed and the further increase in pressure in the brake cylinder proceeds slowly because the compressed air has to flow through a throttle bore with a small diameter.
Control valves of the earlier single-release air brake
Because this brake is applied gradually but can only be released all at once, it is referred to as a single brake. If a release process that has not yet been completed is interrupted by braking again, the auxiliary air tanks are not yet completely filled with compressed air. When the brakes are applied again, the full brake cylinder pressure is not reached again and the braking effect decreases. If the brakes are released and put on repeatedly, there is a risk that the brakes that have been released will be completely exhausted and the braking effect will be lost.
Single control valves have not been used in international traffic in Europe since 1988.
function
Single control valves work according to the two-pressure principle . The air pressure in the main air line (blue) and in the auxiliary air reservoir (red) are involved in controlling the air brake.
To fill
After the pressure in the main air line 2 has been increased, compressed air flows into the space (blue) under the control piston 5, which presses the control piston into the release position. As a result, the filling groove 6 (red) is opened and the auxiliary air tank 1 is filled slowly and throttled to the pressure of the main air line.
The complete filling of the auxiliary air reservoir of a train after an emergency braking to 5 bar pressure lasts up to 2 minutes.
Brakes
When the main air duct air (blue) is reduced significantly below the pressure (red) in the auxiliary air tank 1, the control piston 5 with the graduated valve 7 moves into the braking position and prevents the compressed air from flowing back from the auxiliary air tank. Compressed air (green) flows through the opened slide 8 from the auxiliary air tank into the brake cylinder 4.
If the auxiliary air reservoir does not fully reach the control pressure of 5 bar, only a reduced braking effect occurs.
Final position
The engine driver can interrupt braking at any time or brake in stages. As soon as the pressure in the brake cylinder 4 (green) is able to cancel the upward force of the graduated valve 7, the graduated valve moves upwards and prevents further air supply from the auxiliary air reservoir 1 to the brake cylinder. Because the slide 8 is still in the braking position, no air can escape from the brake cylinder to the outside and the regulated brake cylinder pressure is maintained.
To solve
If the locomotive driver initiates the release of the brakes with the driver's brake valve by increasing the pressure in the main air line 2 again, the pressure in the space (blue) above the control piston 5 increases. The control piston with the graduated valve 7 is pressed into a seat, see above that through the opened filling groove 6 (red) and the auxiliary air tank 1 is gradually filled with compressed air. The graduated valve moves the slide 8 into the release position, whereby the compressed air (green) can escape from the brake cylinder 4 into the open.
The single-release brake can be applied in stages, but only released all at once. If possible, the brake should remain released until the auxiliary air reservoirs are completely filled. If it is applied again beforehand, the full brake cylinder pressure cannot be achieved due to the lower pressure in the auxiliary air reservoir. This decrease in the braking effect when the brake is operated (too) often is referred to as exhaustibility.
Vacuum control valve
Suction air or vacuum brakes are used on some narrow gauge networks in different countries. In Switzerland, the vacuum brake is used on the Rhaetian Railway , the Matterhorn-Gotthard-Bahn , the Montreux-Berner-Oberland-Bahn and the Transports publics fribourgeois . In the past, the suction air brake was also used in regular and wide-gauge networks, for example in Great Britain, Spain, Argentina and in the South African Cape Gauge Network. Due to the two-chamber effect, indirectly acting suction air brakes were multi-release right from the start, which made them particularly suitable for mountain railways with long descents.
The vacuum-controlled air brake has the advantage over the pure suction air brake that it can be equipped with anti-slip protection. The combination with a maneuvering brake and a skid brake is possible for traction vehicles . However, the vehicle requires compressed air, which is either supplied by a compressor or from the feed line .
The vacuum-controlled compressed air brake is equipped with a vacuum control valve in addition to the pure vacuum brake. This has the same function as the air brake control valve. The brake cylinder is supplied with compressed air via the feed line and the vacuum control valve.
When filling and loosening, the main air line 2 and the control container 6 are evacuated to 52 cm Hg by the vacuum pump of the locomotive. The piston of the control valve moves down and the brake cylinder 4 is vented.
When braking, the destruction of the negative pressure in the main air line causes the piston of the control valve to move upwards. The reference vacuum of 52 remains in the control container 6 cm Hg obtained by closing the check valve. Compressed air flows from the feed line 1 through the open inlet valve 9 into the chamber above the regulating piston 8 and into the brake cylinder.
The increase in pressure in the chamber above the regulating piston causes the piston of the control valve to be pushed down and the inlet valve to be closed.
Due to the negative pressure in the main air line, the negative pressure in the control tank and the brake cylinder pressure, the control valve works according to the three-pressure principle. This makes it possible to regulate the brake cylinder pressure in stages.
The requirement for the reliable functionality of the vacuum-controlled compressed air brake is the supply of compressed air via the feed line. With the minimum pressure monitoring 10, the pressure of the feed line is monitored. If this falls below 5.5 bar, the main air line is forcibly ventilated.
Conditions on a control valve
The control valve gives the brake the following important properties:
- Inexhaustible with proper brake operation
- good regulating ability, i.e. high sensitivity when braking and releasing
- high penetration speed
- low sensitivity to overpressure in the main air line
- Insensitivity to small pressure fluctuations
- automatic replacement of the air losses in the brake cylinders
- rapid response of the brake and continuous pressure curve
- Reliability in any weather.
Good regulating ability is important both when braking and when releasing. The brakes can always be regulated in stages when braking and, with the multi-release brake, also when releasing.
To increase the breakdown speed, the control valve is usually equipped with an acceleration valve. It ensures that a certain amount of compressed air flows out of the main air line on the spot when braking. As a result, the penetration speed can range from approx. 90 to 180 m / s to approx. 250 to 280 m / s can be increased.
literature
- Hans Schneeberger: The control valve as the heart of the brake . In: Swiss Railway Review . No. 6/1984 . Minirex, ISSN 1022-7113 , p. 210-214 .
Web links
- The brake booth - what is that?
- Federal Office of Transport FOT Driving Service Regulations (FDV)
- Annexes to the 4th edition of the leaflet: Brakes - compressed air brakes for freight and passenger trains. (PDF; 44 kB) In: Appendices to UIC Code 540. UIC International Union of Railways, March 1, 2005, archived from the original on October 24, 2014 .
Individual evidence
- ↑ The brake booth - the control valve KE. Retrieved January 19, 2019 .