Sunday, 1 January 2012

To Inject or not to Inject?

Part of the fun of doing a loco restoration is trying to resolve mysterious features that aren't obvious as to their purpose or function.

Lately, whilst cleaning up and renovating the pair of water tank filter valves, I've been pondering the purpose of a small tube on the left hand side of Sentinel 7109's water tank that is there in some photos but not in others.

Have a look at this:

Sentinel 7109 at Shrewsbury prior to delivery, probably 1927
(Photo courtesy John Hutchings, Industrial Locomotive Society)
Just in front of the cab on the rear filter valve, there is a small tube:
7109 Tube and valve highlighted
But in this later photo, the tube has disappeared:
Missing 7109 Tube
(Photo cropped from C Verrall 1964 original)
Compare these to the similar Radstock Sentinel (7588) which has two tubes to 7109's disappearing one.
Radstock Sentinel 7191 (wks no. 7588) showing its twin tubes
(Photo cropped from R. G. Jarvis original, Richard Dagger collection)
So what was going on, I hear you ask?

Sentinel 7109 was originally supplied with an injector; however, it apparently did not work very well and was soon replaced by a Worthington Simpson type boiler feed pump mounted on the right hand driver's 'window ledge'. Hence the tube was there to feed the injector in the early photo but missing in the later photo.

So if an injector did not work for Sentinel 7109, why was one or more then fitted to the later Radstock Sentinels and left there?

That is the question!

Comments welcome!


  1. They worked out what the problem was an corrected it?

    Would the injectors be a more efficient solution or easier to maintain than the boiler feed pump solution if they got it to work?

  2. ... He'd be surprised if it was for aesthetic reasons (very surprised!).

    7109 was a prototype and many things were probably being experimented with to find the best performance. Injectors are simpler than pumps with fewer moving parts but it may have just been somebody's personal curiosity to try an alternative. Who knows?

  3. Is the Setinel fitted with vacuum breaks enabling for it to be used on passenger carriage services once restored? Please forgive me if the answer is 'all steam locos have a vacuum break' as I am only 14!

  4. Sentinel 7109 has never been fitted with vacuum brakes. It is an industrial loco which was used for shunting heavy loads of loose-coupled wagons which were not vacuum fitted.

    I don't think many Sentinel locos were originally fitted for vacuum braking; however, Isebrook ( now has vacuum and so does William at Elsecar Heritage Railway (

    Sentinel boilers are quite small but quick to react when more power is needed. To release a train's vacuum brakes takes a lot of steam just prior to moving off. So it's possible to end up with the brakes off and no steam to start the train! However, in capable hands and with short trains, the vacuum fitting seems to work reasonably well.

    7109 was built with very low gearing such that it has a top speed of only about 12 miles per hour (great for lengthening the time of a short ride!). So it may be more appropriate for use with shunting and brake van rides rather than adding vacuum for pulling coaches - but you never know!

    It's good to have someone of your age being interested. Keep the questions coming!

  5. Thank You- Ok then, So what do you mean by a Sentinal being two engined, perhaps two boilers? Although I only seem to recall one...

  6. Most of what you are asking should be answered here: (You'll need to cut & paste the link into your browser address bar).

    In the early days (1920s), Sentinel did try out an idea using two 100 HP boilers but later decided to use a bigger 200 HP boiler for twin engines. 7109 has this bigger boiler.

  7. Hi,

    Referring to a 1921 book by Dugold Drummond, he mentions there are quite a few reasons why a feed pump is preferable to using injectors (he recommended injectors should always be fitted as pairs, and the driver used each one in turn). Now, he was referring to main line locos, and the problems with injectors would be increased in the sort of environment that the sentinels would be operated in. If you email me, I can possibly photo copy the pages for you. (You've got my email address from Eddington, today).

    Best wishes,



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