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testing injectors.

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RRMBK10/06/2018 15:49:12
118 forum posts
15 photos

Good day all

Does anyone know or have experience of whether it is feasible to test an injector off line against a pressure head by using compressed air? Obviously there is not the same amount of expansive energy available, but will the inlet flow being gas rather than wet steam stop it from working altogether?

Nigel Bennett10/06/2018 17:29:10
256 forum posts
6 photos

No, I'm afraid that you need a supply of steam to make a steam injector work! The whole principle of its operation is that the water feed condenses the steam, which gives up its latent heat of vaporisation, thereby imparting sufficient energy to the water to enable it to be fed into the boiler. Air has no latent heat and the injector will not work. Yes, there will be changes of velocity and pressure with an air/water arrangement due to the changes in the bore of the cone sections, but not enough to enable it to feed against boiler pressure.

You'll need a bespoke Test Boiler to do a lot of testing; DAG Brown gives some hints and tips in his book, which is well worth a read.

Clive Brown 110/06/2018 20:38:46
157 forum posts
5 photos

Laurie Lawrence, in his series on injectors in ME 1986, mentions an injector testing pressure valve designed by Bill Carter, but I have no reference for it. It enables testing against boiler pressure, but with the water delivery going to waste instead of filling the boiler. Could be useful, has anyone got more details?

Andrew Johnston10/06/2018 22:17:27
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4083 forum posts
502 photos

I expect the injector to not work with compressed air. In the combining cone conservation of momentum rules. I don't see how the latent heat of vaporisation creates pressure energy in the water? A fast moving stream of steam combines with more or less stationary water. Of course the steam condenses so the combined stream of only water, with the same total momentum, can then be slowed in the delivery cone to create pressure. Therein lies the problem with compressed air, the stream entering the delivery cone will be a mix of air and water and so won't produce the same pressure.

Of course the latent heat released when the steam condenses has to go somewhere. it goes to heat the feed water. If there is sufficient heat to reach boiling point then the injector will stop working. Hence there is a limit on the temperature of the feed water for an injector to work. That temperature vaies with the ratio of steam to water and with initial steam pressure.

Andrew

Chris Gunn12/06/2018 14:07:59
251 forum posts
16 photos

RR, if you are trying to find out if the injector is working, if it is already on an engine, just loosen the tube nut on the clack inlet slightly and try it, and you will see if water is being forced through as far as the clack. If it is working but water is not getting into the boiler then the clack is the most likely reason.

Chris Gunn

Balljoint12/06/2018 16:27:37
27 forum posts
9 photos

Have a look at this page where a simplified Bill Carter test rig is described.

http://www.modeng.johnbaguley.info/injectors/injectors2.htm.

SMEE has the original Bill Carter test rig in its collection. 3 of us spent an afternoon using it to test some homemade injectors, although we could not get it to work properly, probably operator error.

Colin

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