Rob Roy lack of steam

I have made a 3.5in Rob Roy and it has passed the boiler test with fittings. On air it runs easily on 15 psi. However when on steam I open the regulator at 80psi the boiler pressure drops away quickly. I have put springs under the valve nuts and raised the blast nozzle. Has anyone got suggestions to solve this problem?

Springs under the valve nuts? To do what?

Is this on the track, hauling you on a driving-truck, or under test on a rolling-road?

Some drop in pressure on pulling away, I would expect, but it should stabilise and even recover with normal firing and driving. What happens if you notch up a bit?

Altering the blast-pipe suggests having to correct poor steam-raising. An air leak in the smokebox?

Thanks Nigel for your comments, The springs are to keep he valves on the port face. Pressure loss is when loco on blocks, notching up does slow down the pressure loss.

Make sure the smokebox door seals and there are no air leaks anywhere in the smokebox. The blast produces a slight vacuum in the smokebox which draws the fire. Any gaps around door, pipes, boiler joint etc., will prevent satisfactory steaming.

Greetings Steve and congratulations on completing your Rob Roy build.

You must remember that this is a more than 50 yr old design and the loco was designed for Welsh steam coal whilst most people use anthracite beans, which need a little more draft. I suggest that apart from the excellent check points mentioned above by Simon, you consider making a blast nozzle about 15 thou smaller than the one shown on the plans.

Plenty of these locos are about running well, although Martin himself suggested that a single radiant superheater, and more tubes might give better results. I have built 2 of these, years back, and had the same trouble as yourself with the first one. Smaller blast pipe helped.

Finally, I think you should try it on the track, get a good fire, plenty of water in the gauge, and with full pressure get the loco moving well, notch up and try to keep going.

Regards

Brian B

Steve -

I don't know the Rob Roy deign myself but I do know it is a successful, popular design.

Do the drawings call for springs on the valves? They are not usually necessary because the steam pressure holds the valves against the port faces. Some engines have something like that called "balanced slide valves" but it is not a simple spring, and is to reduce the load on the valve to reduce the engine's internal power loss.

Were the valves leaking directly to exhaust? That will lose steam all the time. It should show as a blow up the chimney between the exhaust beats, possibly stronger if the regulator is opened in mid-gear with the drain-cocks open.

One cause, assuming the valve and port faces are as they should be, is the valve sticking on its spindle. I don't know if this locomotive uses cross-nuts or bridles to connect the valves to their spindles, but either way ensure the valves can "float" without end-play.

The same leak would occur of course when testing the locomotive on air, but probably masked by the compressor easily keeping up with it.

Notching up is unlikely to reduce the leak particularly, but as it reduces the amount of steam entering the cylinders at each revolution it will reduce the combined steam demand.

Hello Steve,

As your Rob Roy runs ok on air at 15 psi, I would look elsewhere. These things are always difficult to diagnose.

I don't think it would be the draughting per se ie blast nozzle to petticoat pipe. The Rob Roy draughting can be tweaked. What you describe suggests something more problematic.

It could be firing technique or fuel problems.

However, I would suggest you have a look at the wet header and hot header and superheaters for leaks. Also the blower internal pipe where it connects with the smokebox tubeplate. The regulator pipe similarly. Blank off the steam pipes to the steam chests and do a hydraulic test of the steam circuit from boiler to the end of the steam pipes including the headers and superheaters.

My hunch is that you have a faulty joint somewhere such as the notoriously difficult superheater return bends that would fit your description of the sudden loss of pressure when the regulator is opened under steam.

Cheers,

Julian

That fault would be easy to diagnose initially, though the leak might be harder to trace, by air or steam; without dismantling anything yet.

Open the smokebox door, and watch or listen to what happens in there when you open the regulator (mid-gear, etc.!) To do this by air, connect the boiler to a suitable pump or compressor. You won't need full working pressure.

If a leak is audible but not giving a visible sign (as by air) hold a slip of paper or a feather in front of each superheater flue or by each accessible joint in turn. Or gently push a wad against each flue in turn and note which if any alters the sound. Testing cold by air, allows you to plug the flues' firebox ends with cloth or tissue-paper to enhance the effects in the smokebox.

If that reveals something leaking, you can then apply more sophisticated tests to identify where, needing disconnecting and blanking fittings.

Thank you Simon, Nigel, Brian , Julian for your comments I will reread and study them all and take them all into account.

I put springs under the valve nuts (after reading a comment on a forum) because I was experiencing blow through the chimney at the same time I lapped the valves onto the port face to best of my ability!

I suspect Julian may be right as the pressure loss is dramatic and continues to fall but runs on steam at 15/20psi.

Steve

A sensitive variant of this is burning a Joss Stick and watching what happens to the smoke.

I'd have thought the source of a rapid loss of steam at 80psi would be fairly obvious, in this case is it all blowing up the chimney? Mention of springs makes me suspicious of the valves. D-valves are supposed to be held down by steam pressure. Could there something be wrong with the assembly allowing them to lift, such as being fitted upside down. (A wild guess!!!)

I had trouble with valves blowing through on my RR. It was the cross piece that drives the valves being slightly cock eyed and too good a fit in the valve. That made the valves sit cock eyed lifting one end off the port face. Make sure the valve is a free sliding fit on the cross piece and that the valve can sit flat on the face. I hope that is clear. Good luck,

Martin

Thanks Martin, I never thought of that, I was so concerned about lapping the surfaces. I did think I left sufficient clearance but i will bear that in made if I take them apart yet again.

Steve

I don’t think that Steve’s slide valves are lifting otherwise the loco would not run on air or as he has said on low pressure steam.

I would myself hesitate to suggest checking for leaks on joints on superheaters or the wet or hot header whilst the loco is in steam. It seems to me to be a dangerous option. Far better and safer and the results more definitive using a hydraulic test.

Fundamental first question; is the satisfactory running on 15psi of air achieved by pressurising the boiler and using all the plumbing, or was it the bare chassis that was tested so? If the former it points to a small leak in the smokebox that under steam is destroying the draft. It doesn’t take a big steam leak in the front end to kill the draft and deaden the fire. OP doesn’t state what the fire looks like as the pressure is dropping, is it bright and looks like it is drawing well or is it dull / black? First thing I would do is look at what the fire is doing, is it producing enough steam? Sounds likely to me to be a leak that is killing the fire rather than an issue with the running gear.

Paul.

Good point but the leak could be of air into the smoke-box. I'd start with that before thinking about a steam leak, which is harder to trace.

I'd also take those springs out of the valve-chests. Two reasons:

- They won't help seal a leaky valve face, but will add extra load to the motion-work which seems to have been behaving as it should, by the air-test.

- They don't appear to have solved the problem, and were a solution to a problem not yet diagnosed anyway.

Thanks Paul, Yes it runs via air supplied via S/V boss. Taking advice previously given this morning i checked the with soapy water (pressurised air not steam) and found the super heater header flange leaking when opening the regulator and suspect a little air leak at the bottom of the boiler/ smoke box ring judging by the lack of sealant in one spot. Would this explain the fire was dull and seem a long time to get pressure up?

So now I will have to reassemble and try again. I will of course keep you informed. Thanks again, Steve

Steve,

I would say you have found your problem. Yes, would explain the poor fire when regulator open and also the air leak would not help raising steam in the first place.

Paul.