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Injectors are precision engineered devices and can be a very reliable way of feeding water to boilers. However, care must be taken in the installation and operation. When there are problems, it is more likely due to installation or operational considerations which is the cause, not the injector itself.
GA drawing of typical injector
Photo of typical commercial model injector
The basics for proper operation require that an injector:-
It is usual practice to fit 2 injectors to a model steam locomotive, for redundancy reasons more so that capacity issues, and 2 injectors are mandatory if they are the sole and only means of feeding water to the boiler. This partly due to capacity, but the most important reason is redundancy as it is critical to have working boiler feeds. Too large a capacity injector can be detrimental to performance as it uses too much steam (and may exceed boiler steam production capacity on smaller boilers, or steam delivery capacity of the piping) leading to poor performance, and it also puts too much 'cold' water into the boiler at once, cooling the boiler water and subsequent lowering steam pressure. Too small an injector may not be able to keep pace with consumption.
Fortunately, most commercially available miniature injectors have more than sufficient capacity. Sometimes a smaller capacity injector is used in conjunction with a larger capacity one to give a better range of available capacity.
One question commonly asked is "What size injector do I need?" In some ways this is like saying "How long is a piece of string?", but it is possible to get a rough idea by estimating steam consumption of the loco, and therefore the water consumption of the boiler - which is the amount of water the injector is needed to pump.
The calculator at the right can estimate the amount steam used by the loco. But note that this does not take into account the amount of steam used by the injector itself, not other steam losses from safety valves, whistles, or other miscellaneous leaks.
The calculations look at cylinder size, speed, working pressure etc to caclulate the total swept volume of all cylinders at that speed (=steam used), and looks up specific volume of steam in the steam tables to estimate the corrresponding water consumption.
Injector efficiency and other losses (such as safety valve, whistle and miscellaneous pipe leaks loss etc) all add to the water consumption, and are difficult to measure or estimate, but probably 30%~50% would be a reasonable number. So an injector with say double the capacity calculated would be about right.
While injectors are extremely reliable, for steam powered models it is wise to have some form of emergency mechanical back-up water feed system, such as a hand pump or axle pump.
If there are problems with an injector, some people look at the injector in isolation, whereas it is part of the complete water feed system - from the water tank or boiler through the piping, valves and injector itself and on through the delivery pipes/valve and into the boiler. The problem could be in any part of the total path.
Below is a checklist of possible problems:-
Problems can also occur on the delivery side of the injector. Below is a checklist of possible problems:-
If the whole injector requires cleaning, disconnect it and soak overnight in kitchen vinegar (weak acetic acid). This is a mild non-corrosive acid and does a reasonable job. Using this method with citric acid would also work, but it can't be left unattended and a close eye should be kept on the job to monitor cleaning progress.
Another way which can be done while the injector is still mounted, and steam in the boiler, is to disconnect the water feed and replace with a temporary rubber tube feed into a jar of citric acid solution. The steam is turned on, but only enough to pull the water through the injector and out of the overflow, but not into the boiler. The mixture is caught at the overflow and re-used until the fluid is quite hot. This will make sure the injectors' sensitive bits get their fair share of acid in the working areas. This method is easier if the injector is of the lifting type.
Use citric acid at between 25gms (1oz) and 50gms (2ozs) per litre (1.75 pints) of water. Do not simply drop in and leave, but remove and clear liquid from inside the injector and replace in the acid from time to time in order to replenish the exhausted acid from the interior. Citric acid is kinder to the metal than some others. Some injectors have been naively put into some quite ferocious brews and the result is not pretty (some popular fizzy drinks fall into this category but are better drunk with an alcoholic flavouring!). Acetic or citric acid are better propositions.
[Updated: 17 Sep 2015]
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