Boiler Feedwater
Aim - This page is dedicated to describing the operation of boiler feedwater systems.
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Index
Introduction
For safety reasons it was important to make sure that the boiler water level is always maintained high enough to cover the crown of the boiler, as a low water level could lead to a boiler explosion or a fusible plug failure with loss of pressure. Over the years the following feedwater devices were used to feed water into the boiler from the tender.
- Water Motion Pump - were pumps run off the wheel motion to pump water into the boiler. Typically used on early locomotives prior to the 1860s.
- Steam Injectors - used steam to inject water into the boiler, and were typically used after the 1860s.
- Feedwater Heating Pumps - preheated the feedwater prior to pumping it into the boiler. Typically fitted to locomotives from the 1930s.
Steam locomotives were normally provided with two independent feedwater devices from the above devices to supply water to the boiler.
A more detailed description of these devices is provided below.
Boiler Water Level
The locomotive crew on a steam locomotive relied on the water glass to determine the level of water in the boiler. The diagram below shows a typical water glass gauge.
Typically, as indicated in the diagram above, the water gauge has three cocks: A (Top Steam Valve), B (Bottom Water Cock) and C (Drain Cock).
To check that the water gauge is working:
- First close the top steam cock ( A ) and bottom water cock ( B ) and open the drain cock ( C ); this should cause the water in the glass to disappear.
- Then open the top steam cock ( A ) and close it again to blow steam through the gauge.
- Then open the bottom water cock ( B ) and close it again to blow water through.
- Next close the drain cock ( C ) and open both the top cock ( A ) and bottom cock ( B ) and the water in the glass should rise to show the correct level.
Maintaining the correct water level in the boiler was critical for safe operation of the boiler. Operating over rising and falling gradients can result in major variations in the water levels in the water glass gauge. The diagram briefly shows what happens to the water in the boiler of a locomotive when ascending or descending a gradient.
For a more detailed explanation see as shown by "Fundamentals of Steam Locomotive Boiler Water Level Management"
It should be noted that the water level will appear differently in the water glass depending upon the type of boiler orientation as described below.
- Normal Orientation - cab at rear of the locomotive, water levels as shown in the above diagram.
- Centre Cab - Typically for Camel locomotives the cab will be in the centre of the boiler. Water levels, depending upon the cab location along the boiler, will not be impacted by the track gradient.
- Cab Forward - cab is at front of the locomotive, water levels will be inversed compared to the diagram above.
- Vertical - boiler is vertical, water levels will be the same as the diagram above, but as the diameter of the boiler must be less than the width of the locomotive, change of gradient will have very little impact on the observed water level.
- Sloping - Typically for a Rack Railway steam locomotive, the boiler is sloped at an additional angle to the horizontal so that the water level will be "level" on the predominant gradient. Water levels will be the same as the diagram above, with an additional correction due to the boiler sloping.
It is important at all times for the locomotive crew to maintain a sufficient depth of water over both the firebox crown at the rear of the boiler and the boiler tubes at the front end of the boiler. If the firebox crown is not covered then the fusible plugs will melt.
The firebox crown includes one or more fusible plugs made of brass, sometimes with a lead core. If the crown of the firebox becomes uncovered then the plugs will melt and steam will enter the firebox. If this happens then the locomotive crew should put both injectors on and immediately deaden or drop the fire. Damage to the firebox crown or boiler tubes due to overheating can lead to a boiler explosion, hence it is essential that the crew check that the water gauge(s) are working and that there is sufficient water in the boiler whenever they join a locomotive.
Harvey in the "Handbook for Steam Locomotive Enginemen" explains that:
When working under normal conditions the water should be kept in sight in the top half of the glass, and before descending severe gradients or working over curves with a large amount of superelevation a higher water level should be carried.
However running with too high a water level can also reduce locomotive performance as water may be carried over into the cylinders or superheater. This is known as priming.
Motion Water Pumps
Early steam locomotives were generally fitted with two motion pumps, either driven from the crosshead or from some other part of the valve gear. In USA, pumps driven from the crossheads were preferred. These were generally single action pumps 1.75 inches in internal diameter. Such pumps continued to be fitted to new locomotives in USA until the 1880s. In Europe very early locomotives had similar pumps, but by the 1840s, short stroke pumps driven from the valve gear were being used. Such mechanical pumps depended on the movement of the locomotive with the amount of water delivered to the boiler depending on the speed. This had the disadvantages that water could not be delivered when the locomotive was stationary and that the amount of water delivered when climbing gradients slowly might not be enough to meet the needs of the boiler.
The driver could turn the flow of water from the tender to the pump(s) on or off by means of a cock fitted to the pipe from the tender.
The following guidance for their uses was given in "Practical Rules for the Management of a Locomotive Engine" (1841):
"The most favourable time for allowing the feed-pumps to act, is when the steam is blowing off with force from the safety-valve, and the fire strong; and the least favourable time is when the steam and fire are low: indeed the Engine-man should manage that it may never be necessary in the latter case, as the addition of water rapidly lowers the steam."
Weale (1856) explains:
"For this reason, the action of the feed-pump is generally suspended when the engine is ascending an inclination, and requires its greatest power, and the supply of water is made up by working both pumps on a succeeding level or descent."
Some locomotives were also provided with a hand operated pump, the use of which is described by Tredgold (1850):
"Whenever, through any accident, an engine has to stand with steam up, the hand-pump must be worked to keep up the water in the boiler, the fire-door opened, the damper shut, and other means used to prevent, as far as practicable, the generation of steam."
Water Injector
In the 1850s Henri Giffard invented an injector that could be used to feed water into steam boilers. The condensation of a jet of steam accelerated through a converging nozzle and condensed by cold feed water produces a jet of water that has sufficient energy to overcome the boiler pressure.
From the 1860s locomotives began to be fitted with injectors in place of mechanical pumps. Early injectors were not particularly robust and to begin with some locomotives were fitted with one injector and one mechanical pump. A frequent problem with early injectors was that bubbles of air or water vapour, often caused by movement of the train, would stop the injector from working. The introduction of self-acting injectors, which restarted automatically after any interruption overcame this problem. Locomotives were fitted with two injectors and in older locomotives (and some newer designs) both injectors used "live" steam taken from the boiler. When operating such locomotives the fireman would normally use each injector alternately to make sure that both were kept in good working order.
Drummond in "Lectures on the Working of Locomotive Engines" (1908) explains that:
Two injectors are always provided, and they should be worked in turn, as it is a bad practice to continue using one until it fails; in this way the other may and has been found unworkable at the very time it is wanted.
There was some variation in operation, between different live steam injectors, including the ability to moderate the steam pressure in certain injectors such as the Nathan injector. However the vast majority of injectors were operated in the same way. Firstly the steam valve would be opened, admitting saturated steam at boiler pressure. Then the water valve was opened gradually until the injector began to operate. To begin with water would spill out from the overflow, until the injector "picked up" and the output had sufficient energy to overcome the boiler pressure. At least one fireman was killed as a result of leaning over the side of the cab to check if the injector was working. Some injectors later provided an indication in the cab that would show that the injector was operating correctly.
Some injectors included a "tell-tale" pipe inside the cab. If the injector should ever stop working, the tell-tale pipe will start dripping water, alerting the crew.
In the late 1870s some exhaust steam injectors were produced. Exhaust steam taken from the base of the blast pipe was used to replace some or all of the live steam from the boiler. This arrangement improved economy as it reduced the amount of live steam required by the injector and allowed some of the heat contained in the exhaust steam to be used to raise the temperature of the feed water. In the 1880s the first injectors which could use either exhaust steam or live steam were produced. Early examples were really two separate injectors in the same housing each of which needed to be independently controlled. By the end of the 1880s there were injectors that could automatically change from using exhaust steam when this was available to using live steam when the regulator was closed or the locomotive was drifting. Injectors of this type did not become widespread until the 1920s. British Railways standard locomotives and many pre-nationalisation locomotives were fitted with one exhaust steam injector and one live steam injector. In normal running the exhaust steam injector would be used the majority of the time, with the live steam injector providing additional capacity when required. In instances where the exhaust steam was not present, (ie regulator closed) most injectors reverted to a "live" steam operation, until exhaust steam was available again.
Typical Working Instructions for Davies & Metcalfe Type "J" exhaust injector
To start the injector, open the water valve by means of the water regulator sector handle in the cab. Then open fully the injector steam valve on boiler. The injector then commences to work and the amount of feed can be regulated by the water regulator sector handle. The steam valve must always be kept fully open and not regulated. The injector then works with exhaust steam or live steam according as the regulator is open or shut.
To stop the injector, shut the injector steam valve on boiler, then shut the water valve by turning the water regulator sector handle to mark "shut."
Note 1. - The water regulator when in the " shut " position cuts off the injector water supply, so that the tender feed cock need not be touched, and should be left in the open position.
Note 2. - So far as is possible the injector should be kept constantly at work while the engine is running. A constant water level should be maintained by regulating the water supply. Intermittent feeding should be avoided. The regulation of capacity of the injector is large, so that a constant feed can be maintained under all conditions, whether the engine is worked heavily or lightly. Filling the boiler at stops or when running down hill should, if possible, be avoided as the maximum fuel saving is only made while the exhaust injector is working with exhaust steam. It is necessary, if the best results are to be obtained, that this method of boiler feeding (which is exactly the reverse to that of the live steam injector) should be adopted, otherwise a proportion of the economy will be lost, owing to the boiler being fed with live steam instead of exhaust steam.
Instead of an overflow cab indicator, the Elesco exhaust steam injector included a dial injector gauge. This gauge has three pointers, one of which is moved from the outside like the pointer of a barometer.
- Black pointer shows exhaust pressure m the injector body when working on exhaust steam, and auxiliary pressure at the same point when working as a live steam injector.
- Red pointer shows overflow pressure in injector.
- Brass pointer is set at the pressure which, when reached by the red hand, indicates spilling at the overflow. Since the pressure at which spilling occurs varies with the different boiler pressures. this hand is set on the engine with full working pressure in the boiler.
The performance of injectors is influenced by the pressure of the steam supplied to them and also by the temperature of the feed water. Depending on the construction of the injector the maximum amount of water that can be delivered increases until the optimum steam pressure is reached and then decreases as steam pressure continues to increase. The minimum amount of water that can be delivered also increases as steam pressure increases, such that in some circumstances the minimum delivery is little different to the maximum. Injectors work most efficiently with colder water, as this is able to condense a greater mass of incoming steam. As the temperature of the water supply increases, the performance of the injector decreases. Some companies manufactured specially designed injectors for high pressure steam or hot feed water.
In many parts of the world feed water heaters were used to extract heat energy from the exhaust steam, with the resulting hot water being fed into the boiler by a steam driven feed pump.
Test results for both Exhaust and Live steam injectors can be found in the Useful References, below
Useful References
Water Motion Pumps
Injector Tests
Injector Manuals
General Information