Pipe Size Puzzle.

[Nigel Graham 2Participant @nigelgraham2]

Problem: Injector very hard to start flowing, with a very delicate touch needed, and when it does run it puts as much water on the ground as in the boiler.

Locomitive: 7-1/4″g NG outline,, the Ken Swan interpretation of the Kerr-Stuart ‘Wren’ so fairly large boiler.

Injector fitted but not prototypically, with its steam feed from the dome. Instead previous (indeed, “past”) club members gave it a manifold above the backhead, but with a rather weeny looking steam valve for the injector.

Injector: commercial but no maker’s name. Figure ‘5’ stamped in the top cap.

All three spouts 3/8 X 32 ME threads (for 1/4″ od pipe unions).

The steam pipe is only 3/16″ o.d. and to complicate things has a kink on one of its wriggles (I would use “bends”, but…). The injector end has a 1/4″ union, the gap between nipple and pipe filled with silver-solder; the manifold end has a 5/16″-pipe union to fit the valve.

 

Water feed: Controlled by standard 15mm plumbing ball-valve on outlet of a tank in the driving-truck. The bore through the ball in these is 8mm, severely cramping the flow in domestic plumbing but more than adequate for a small injector.

 

Operating: water full on. Steam full on. Slowly turn the water back until it is on the verge of stopping, and there seems a “sweet spot” where the injector suddenly picks up and puts some of the water in the boiler. It is very narrowly-defined: not quite there produces copious hot water from the overflow; slightest touch further stops the water.

 

I have searched both Evans and Brown. Neither book is very forthcoming about miniature injector sizes and pipe diameters. Nor do they explain the devices’ rather vague specifications and their selection; but Brown goes by cone diameters, relevant only if you are making them as his book instructs. I even had to use a cookery-book to find units conversions, but that didn’t help much more.

DAG Brown gives the main cause of water-dropping as incorrect steam-cone depthing, and states he finds this common on commercially-made injectors; but I assume with all the rest of the system correct. Obviously I am not going to try modifying the injector’s tiny, very delicate parts to within 0.001″. Not on a massive great Myford ML7. (Read the book to see why). They should be made the right size from the start.

 

So am I right making the steam-pipe the prime suspect: about 18 inches of undersize tube reducing the steam volume and throttling it even before it finds the kinky bit? (And undersize manifold valve too, of course.)

And that all three pipes should be 1/4″ o.d., with control valves to match?

I think the water and delivery pipes actually may be that size, but what happens if the delivery is also 3/16″ od.?

What is the number on the injector cap? 5 of… pints? litres? per minute? Some maker’s design-number?

 

We do have the drawings but the ad-hoc modifications to make the loco a bit easier to drive rather negate those. I suspect whoever fitted the injector simply used some pipe he happened to have in stock!

(On the full-size ‘Wren’ the driver has to dismount and reach up to operate the blower and injector valves, unless he can reach them, Twizzle-like, through the spectacle-plate apertures.)

[noel shelleyParticipant @noelshelley55608]

No 5 would indicate 2.66 pints/min. You need ample hot steam and  plenty of cold water. For this size 1/4″ pipe is required. Until these conditions are met all bets are off. 18″ of steam pipe – I would lag the pipe. I’m no expert and others will no doubt add to the above.  Good luck.  Noel.

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