Sight glass
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A sight glass or water gauge is a transparent tube through which the operator of a tank or boiler can observe the level of liquid contained within.
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[edit] Liquid in tanks
Simple sight glasses may be just a plastic or glass tube connected to the bottom of the tank at one end and the top of the tank at the other. The level of liquid in the sight glass will be the same as the level of liquid in the tank. Today, however, sophisticated float switches have replaced sight glasses in many such applications.
[edit] Steam boilers
If the liquid is hazardous or under pressure, more sophisticated arrangements must be made. In the case of a boiler, the pressure of the water below and the steam above is equal, so any change in the water level will be seen in the gauge. The transparent tube (the “glass” itself) may be mostly enclosed within a metal or toughened glass shroud to prevent it from being damaged through scratching or impact and offering protection to the operators in the case of breakage. This usually has a patterned backplate to make the magnifying effect of the water in the tube more obvious and so allow for easier reading. In some locomotives where the boiler operated at very high pressures, the tube itself would be made of metal-reinforced toughened glass.[1] It is important to keep the water at the specified level, otherwise the top of the firebox will be exposed, creating an overheat hazard and causing damage and possibly catastrophic failure.
To check that the device is offering a correct reading and the connecting pipes to the boiler a not blocked by scale, the water level needs to be “bobbed” by quickly opening the taps in turn and allowing a brief spurt of water through the drain cock.[2]
[edit] Failure
The gauge glass on a boiler needs to be inspected periodically and replaced if it is seen to have worn thin in the vicinity of the gland nuts, but a failure in service can still occur. Drivers are expected to carry two or three glass tubes, pre-cut to the required length, together with hemp or rubber seals, to replace the tubes on the road.[1] Familiarity with this disquieting occurrence was considered so important that a glass would often be smashed deliberately while a trainee driver was on the footplate, to give them practice in fitting a new tube.[3] Although automatic ball valves are fitted in the mounts to limit the release of steam and scalding water, these can fail through accumulation of limescale. It was standard procedure to hold the coal scoop in front of the face while the other hand, holding the cap for protection, reached to turn off the valves at both ends of the glass.
[edit] Repair of a broken glass
The following is the generally accepted procedure of safely fitting a new tubular glass on a steam boiler in industrial applications. Leather work gloves and a full face shield should be worn while working with the glass; this to prevent burns, cuts, and protect the operator's eyesight.
- Shut both the valves and open the glass drain cock
- Loosen and remove both gland nuts
- Remove any broken glass or other debris from glass valve body
- Place gland nuts and new seals on the precut glass
- Install glass in upper valve first and loosely close nut. Pull glass down to bottom so that the gap in upper and lower valve body is even (this allows for expansion of the glass)
- Tighten both gland nuts hand tight and wrench an additional quarter turn.
- Crack open steam side valve and allow the steam to lightly blow through to warm the glass. This prevents the glass from being subject to sudden thermal shock. (Should the glass be subject to thermal shock the effects may not be noticed immediately. However, the glass may then be much more brittle and even a slight bump might shatter it.)
- Close drain valve and crack open valve on water side of glass
- Observe gland for leakage and tighten as required
- Open both valves fully.
[edit] History
The first locomotive to be fitted with the device was built in 1829 by John Rastrick at his Stourbridge works.[4]
[edit] References
- ^ a b Bell, A.M. (1950). Locomotives. London: Virtue and Company Limited, 38; 284.
- ^ Unidentified author (1957). Handbook for steam locomotive enginemen. London: British Transport Commission.
- ^ Gasson, Harold (1973). Firing Days. Oxford: Oxford Publishing Company, 20. ISBN 0902888250.
- ^ Snell, John B (1971). Mechanical Engineering: Railways. London: Longman.
