Rob Roy lack of steam

[STEVE mirfin 1Participant @stevemirfin1]

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?

[STEVE mirfin 1Participant @stevemirfin1]

[Nigel Graham 2Participant @nigelgraham2]

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?

[STEVE mirfin 1Participant @stevemirfin1]

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.

[Simon CollierParticipant @simoncollier74340]

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.

[Brian Baker 2Participant @brianbaker2]

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

[Nigel Graham 2Participant @nigelgraham2]

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.

[julian atkinsParticipant @julianatkins58923]

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

[Nigel Graham 2Participant @nigelgraham2]

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.

[STEVE mirfin 1Participant @stevemirfin1]

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

[SillyOldDufferModerator @sillyoldduffer]

Posted by Nigel Graham 2 on 26/09/2023 10:42:27:

…

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. …

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!!!)

[Martin Johnson 1Participant @martinjohnson1]

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

[STEVE mirfin 1Participant @stevemirfin1]

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

[julian atkinsParticipant @julianatkins58923]

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.

[Paul KempParticipant @paulkemp46892]

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.

[Nigel Graham 2Participant @nigelgraham2]

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.

[STEVE mirfin 1Participant @stevemirfin1]

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

[Paul KempParticipant @paulkemp46892]

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.

[STEVE mirfin 1Participant @stevemirfin1]

Thank you all again. I have inserted a stainless steel “brick arch” and widened the gaps between the bars, reduced the blast nozzle by .012”and increased the height. This reduced the time to reach 80psi and initially held before slowly dropping. Using paraffin soaked charcoal seemed to give better results.
when warm it starts and runs at 20psi, opening the regulator the pressure drops off.
However after 1.5 hours of repeating to return to 80psi I find the fire dies and the ash pan full to the top of the grate! Which would explain that at that point the fire dies. Using the steam blower seems to use more steam than it generates!

The grate top is 10mm above the sides of the as pan. Does it need to higher to supply more air?

Steve

[Nigel Graham 2Participant @nigelgraham2]

If you are trying to drive the locomotive at only 20psi that would almost certainly drain the boiler’s reserve of steam.

If you’ve made the grate and ashpan as designed I can’t imagine that gives insufficient air; but allowing ash to accumulate to the underside of the firebars is not only bad firing technique but bad for the grate.

The blower should not drain the boiler, but it might if you use it too enthusiastically on poor fuel or too low boiler pressure; or if there is still an air-leak into the smokebox. Test the door, around the pipes through the smokebox wall or possibly around the joint with the boiler.

.

“….. reduced the blast nozzle by .012”and increased the height. This reduced the time to reach 80psi…”

That suggests the boiler is steams very well when the locomotive is in action, because that’s when the blast nozzle is doing its stuff. However, do you notch up when driving? If you drive the whole trip in full-gear the engine will gulp steam at a rate of knots. it’s a bit of a compromise and involves knowing the locomotive and the track.

.

The paraffin-soaked charcoal should only be used for igniting sufficient coal to continue burning just coal, as these machines were designed for; but it may be worth considering the coal you have. A very hard coal such as anthracite is not so good on a very small fire. Also the lumps should not be too large, probably no more than broad-bean size on a small 3.5″ gauge loco with a reasonably deep firebox. Some coals clinker easily too, though possibly less so on a small fire, as it demands a fire hot enough to fuse the silica impurities in the fuel.

.

On firing technique, various model locomotives I’ve driven seem best with the blower used only moderately hard at first, and judiciously eased back as the pressure rises. If the pressure has fallen to around 20psi though, especially on a small boiler, turning it on hard may well be too much, so use a more gentle draught to bring the fire back to condition – it may take a bit longer but be more certain.

The blower should be on when the fire-door is opened on a stationary locomotive, but only just enough to keep the flames going the right way. Opening it hard will pull cold air into the boiler above the fire. (Blow-backs on full-size can fill the cab, occupied by two people, with a sheet of flame…..)

The blower should not be completely closed on a small boiler when the regulator is closed, such as standing in the station. Once the pressure is at full, I watch the smoke. Closing the blower fully normally results in visible fumes struggling to get out of the chimney, so leave it on just enough to keep air through the fire without excessive blowing-off at the safety-valves. The smoke will fade and lose colour, as well as obviously jetting upwards. Once running with the regulator open, the blower is normally closed.

The fire should cover the grate reasonably evenly, but only experience with the individual locomotive will tell you its preferences for lump size, feed-rate according to duty, and fire-bed depth.

This all comes with experience not only at driving generally, but also with the particular locomotive – they can be highly individual, and as above the coal used on the day can affect steaming performance too.

‘

One fault I have often seen, is over-frequent fire-door opening and over-enthusiastic fire-raking – often combined with heavy blower use. This is sometimes a reaction to a very dull fire and low pressure, especially if the water is low in the glass…. and there’s a full train and long queue of public passengers behind you. (Been there…!)  Open the door only to inspect the fire and add coal – usually “little and often” – and use the dart sparingly and gently. It’s to clear the cinders, not the fire, and over-raking can make holes in the fire-bed, while having the blower on hard in the process makes thing worse.

How “little and often”? Trying to recover the fire by stuffing loads of coarse coal in, won’t work. Use small pieces and give each charge chance to ignite. The old British Railways handbooks advised using the smoke as an indicator (yellowish at first, it soon becomes colourless) and although that was about fireboxes the size of a small bathroom, model locomotives with fires the size of the bath-sponge often respond to such treatment too.

Incidentally the full-size books also advised the coal should be the size of a man’s fist, but don’t try to scale that down!.

 

 

[STEVE mirfin 1Participant @stevemirfin1]

Thank you Nigel and all previous contributors for your comments .

I apologise for not writing sooner but was taken ill with some virus which knocked me sideways leaving me unable to enter the workshop or do much else for a month.

I shall to read and reread your comments in order to digest them fully. I should point out that my testing to date has been only on my building frame. My last test a week ago was curtailed by the clack suddenly leaking and steam blowing into the water tank via the bypass valve!! And I have had problems with the hand pump, lubricator and water gauge bow down valve!

I do try to follow the firing advice given as with regards to blower/Fire door opening/raking and coal lump Size.

I am new to coal firing and I did buy on line what was said to be welsh Steam coal if it isn’t not how would I know? So now I will be testing again in the new year.

I wish you all a Merry Christmas and happy abundant steaming New Year. Thank you once again for all your advice, Steve

 

[Nigel Graham 2Participant @nigelgraham2]

Thankyou – a pleasure Steve!

“… said to be Welsh Steam Coal…” Well, it might be but these days it’s going to be a matter of luck what any of us can find.

Welsh Steam Coal is fairly hard, glossy and bituminous, burning with definite flames but not too much smoke; and evolving a lot of heat.

The softer bituminous rocks sometimes called “house coal” burns with long yellow flames until the volatile components have gone. It will work as a boiler fuel but is quite mucky stuff, often with a lot of soot and a distinctly yellow-tinged smoke from high sulphur content.

Anthracite is the hardest natural form of coal, and burns at the highest temperature but is harder to ignite and may need a stronger draught to keep burning fully. Some model-engineers mix it with a softer coal to aid its combustion.

Once all are burning properly though, their individual natures won’t matter much in a miniature locomotive. They will all work.

.

Whatever the coal, the smoke should be visible only when the locomotive is stationary with the blower off, and until the volatile materials have burnt away; but in those mechanical conditions the fire will rapidly die out. Some locomotives have a graduated catch on the fire-door, or a small hole in it, allowing just enough “top air” to help the volatile materials and carbon-monoxide burn. (The CO burns with a very pale blue flame and more importantly, considerable heat, so sending it straight up the chimney is a waste.)

.

You don’t want to put coal-grit on the fire but the lumps can be too big even if they fit through the door. The ideal for the specific engine is found by trial and error, but for 5″ gauge, thinking back to the loco I learnt to drive on*, around broad-bean size is a good start.

Be judicious with the coal hammer though if your source gives you carbon boulders. Some coal will just shatter to a lot of slack if hit too hard, wasting a lot of it. If you look at it, some will have a homogenous structure. Other coal though may show clear bedding – laminations – and is best broken by striking on the bedding edges so it flakes.

 

It is quite likely now that any batch of natural coal we buy, not the “smokeless” brands that are partially coke, will be a mixture from different seams, mines or even regions. Back in the Summer, the coal we were given for our club’s miniature railway at a rally held big lumps of two very different species. One was so hard it took a hammer and chisel to break it. The other was softer but very brittle, too easily shattering into coarse grit. Both species seemed to burn equally well, though.

One piece of the brittle species showed a curious surface pattern thought by fellow geology-club members to have been charring: the original tree had been in a forest fire. Apart from the piece I kept, we completed its combustion some 350 million years later!

.

Your driving tool-kit will need include a tube-brush to suit the fire-tubes; and something like old or cheap paint-brushes for the rest. When cleaning the tubes also brush the blower jets clear, and clean the chimney and petticoat-pipe – place something over the blast-pipe to stop soot falling down it. Sweep out the bottom of the smokebox: some owners use a small portable vacuum-cleaner there.

The state of the muck in the chimney will also very roughly indicate the cylinder-lubricator setting. An oily black sludge suggests over-oiling but that errs on the side of safety: better wasted steam-oil and a gunged-up chimney than ruined cylinders. The chimney holding only a thin coating of dry dirt, may warn of too little oil.

Brush the inside of the smokebox door and the mating face on the smokebox before closing it.

A paint-brush used gently is also ideal for sweeping loose ash and coal-dust from the footplate etc.

”””’

A Merry Christmas and Happy Steaming to you too!

.

*A Maid of Kent, LBSC’s 5″ g interpretation of a Southern Railway 4-4-0, ‘L’ -class I think. Though ours was graced with Maid of Athens – and unusually but welcomed by the drivers’ fingers, the manifold valves’ neat hand-wheels were of ‘Tufnol’ .  I don’t know why this Maid had earned Grecian, proudly on the brass name plates. I have not seen her since around the mid-1970s, but I hope this Athenian still exists somewhere, even better in running order and indeed still in service!

[STEVE mirfin 1Participant @stevemirfin1]

Once again thank you to all those who sent advice and encouragement. It’s been a long time since my last reply but that’s because I have trying various options and having to fix repairs.

i have had success in steaming my Rob Roy. I fixed a super heater manifold leak, removed two bars out the grate, fitted a brick arch and reduce the blast nozzle orifice to 3.5mm diameter and placed it 100mm from the top of the chimney.

All I have to do now is be careful with the water feed and coal firing!!

[GreensandsParticipant @greensands]

<p style=”text-align: right;”>The only times I have problems with the valve seatngs ifsi to sit down properly is when with the engine mounted in its service frame and rotated to gain access to the under</p>

[GreensandsParticipant @greensands]

Apologies for the above mangled message – finger trouble I’m afraid. Can somebody arrange to have it deleted for me.

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