Injectors - Notes and Problems


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.

Injector drawing
GA drawing of typical injector
Typical Injector
Photo of typical commercial model injector
Typical Injector
Typical injector plumbing on a locomotive

The basics for proper operation require that an injector:-

  1. must have an unrestricted and controllable steam feed
  2. must have an smooth and unrestricted water feed
  3. preferably have a water filter on the inlet side
  4. must be absolutely sealed and air free on the water feed side
  5. should have a 'dry' steam supply from the highest part of the boiler
  6. must have a clear and unrestricted path in the delivery to the boiler
  7. must have a check valve for delivery to the boiler.
  8. must have an overflow open to the atmosphere. (take care during installation that the over flow drips onto the ground and NOT onto the rails or loco parts)
  9. all pipework should be smooth bends rather than elbows, and no sharp corners

Number/Capacity Required

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.

Capacity Required
Enter metric OR English values to find an estimate of required capacity.
Equivalent values shown in grey boxes after calculation
No of Cylinders:
Cylinder Bore: mm ins
Cylinder Stroke: mm ins
Driving Wheel Dia: mm ins
Speed: kmph mph
Wheel RPM: ? RPM
Working Pressure: kpa psi
Cutoff: %
Water Consumption: ? litre/min
or ? UK fluid oz/min
or ? UK pint/min

Note: Simple formulas using +-*/() may be entered if desired,
not just straight plain numbers.
(for example, say 2+(1/4) may be entered for 21/4 [=2.25])

Disclaimer. Note the accuracy of these calculations is not necessarily exact, and are displayed to only 3- or 4-digit accuracy, and figures calculated here may vary from actual results. Results to be used at you own risk. Calculator provided for guidance for your personal use only.

Problems/Fixes - Inlet Side

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:-

  • Check that internal passageways of any steam or water pipes are not reduced in any way by flattened bends, or over generous silver soldering resulting in partial blockage
  • Check for air leaks. The water supply must be air free. This should be interpreted as "there is no chance whatsoever of air getting into the system, from water valve glands, loose fitting rubber pipes, or for any other reason". Try drinking from a glass through a straw with a small hole in it - you don't get much to drink - and neither will the injector, which is also trying to suck water.
  • Check the rubber/plastic feed tubes not being sucked flat.
  • Check that steam is flowing freely though the steam feed pipe.
  • Check for impurities and foreign matter in the water. A water filter must be installed, with a mesh size smaller than the smallest dimension of the combining and delivery cones (typically 0.015"/0.35mm). Recommended mesh size for the filter is less than 0.005"/0.12mm.
  • Check the water filter is of sufficient area to allow full water flow under gravity alone.
  • Check that the water feed pipes are free-flowing. A torturous path, or too many 90° elbows may impede flow, even if the passageways are clear. Ensure that water runs freely through the pipes with the tap open, and not just drip-drip-drip.
  • Check there is no air leaking into the water pipe work.
  • Check there are no air locks in the pipe work, including filter.
  • Check that there is no build-up of scale in the injector from dissolved salts in the water. Clean if necessary.
  • Check that the overflow check valve on the injector itself is operating satisfactorily.
  • Check that water input temperature is 'cool'. Hot water can often stop an injector working.
  • Check that the injector itself is not too hot. If the steam supply is inavertanly left partially on without any water feed, the injector may be too hot to work. Close the steam feed properly, open the water feed and let it run for a period and try again. Running cold water from the steaming bay tap over the injector also helps quickly cool an overheated injector.

Problems/Fixes - Delivery Side

Problems can also occur on the delivery side of the injector. Below is a checklist of possible problems:-

  • Check that internal passageways of any pipes are not reduced in any way by flattened bends, or over generous silver soldering resulting in partial blockage
  • Check the boiler check valve is smoothly operating.
  • Check there is sufficient annular clearance around the ball in the boiler check valve for full flow. Undersize clacks account for more than their fair share of problems.
  • Check that the ball re-seats properly. Remember that the pressure difference is much smaller than boiler pressure, and that the valve is slow acting.
  • Don't use a synthetic ball - use a non-rusting metal ball. They tend to go soft or spall.

Cleaning and Maintenance

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.