Superheater behaviour after priming in 5" locos

[joanlluch]

The thread about "Hammer Blow Tests" brought me a couple of questions. I'm sure somebody would be able to reply based on their experience.

I'm fitting concentric pipe type superheaters (similar to Don Young 5" Doncaster design) as opposed to parallel pipe ones (typical LBSC design). My questions are not about performance or efficiency, which was discussed before, but on locomotive behaviour on priming conditions:

(1) I think that concentric type superheaters may be more difficult to control after priming because they may contain more water than parallel pipe ones, but I do not know how this affects a miniature loco or whether this is relevant at all. What's actually the case?

(2) My second question is related to the relative vertical position of superheaters. In full scale locos it it common to have the superheaters at a lower level with respect to the pipe that goes to the cylinder. This allows for water to be trapped in the superheaters which can only get out of by evaporation. On the contrary, all designs I am aware of miniature locos have their pipes arranged in a way that any water entering in the superheaters will flow out of them to the cylinders by just gravity. So my question is whether this an intended design, or it is just a matter of space/convenience? My believing is that it would be safer -for the cylinders- to allow some liquid water to be trapped in the superheaters after slight priming, so that water can only reach the cylinders after it has evaporated. This appears to be the case on full size, but not on miniature as far as I am aware. Am I missing something?

Thanks

[pault]

Hi Joan
I think that the increased volume of the concentric elements may actually help control things a bit. I would suggest the volume relative to the thermal mass of the elements means that rather than a violent release of steam to the cylinders the elements are cooled and the release is relatively controlled. Have a look at
modeleng.proboards.com/thread/7812/7-1-4-dyno-car?page=3
about the 9th post down there is a write up and graph showing the effect of priming on superheat temperature, this is for a coax superheater.

With regard to your second question, I think that the flow of steam through a hair pin style superheater would tend to carry the water through the elements and on to the cylinders. With a coax element if the inner tube is used for the returning steam, water may tend to sit in the outer tube until it is boiled off.

[Roger]

Hi Joan,
This isn't a reply to your question, but have you considered making a regulator arrangement that prevents priming? It seems a simple enough thing to do in principle since it's usually a surge of water reaching the regulator that causes the problem. I would have thought that some kind of valve that's lightly sprung in the open position could be fitted on the inlet to the regulator. When the water, which is much denser than steam, comes up against it, it just closes, and won't open again until the regulator is closed.
I'm stunned that some sore of safeguard wasn't fitted as standard, or a relief valve in the steam line to the cylinders that would allow the driver to vent off the unwanted steam in the event of a runaway.

[joanlluch]

Hi Pault,

Thanks for your reply. I see what you say and it's probably like you describe. Still, I suppose that if the amount of water suddenly reaching a very hot coaxial superheater is small, it may still produce a violent release of steam just like the hair pin superheater would do. Specially if the superheater is prolonged to the firebox to become a "radiant" type. Surely, a coax superheater is better at helping to prevent liquid water to get the cylinders, both because it has more volume and because water will tend to settle in the outer tube.

The thread you mention is a very interesting read. I wasn't aware of it, so thanks for pointing me to it. Such kind of experiments and discussions are to me one of the most attractive aspects of the hobby.

[joanlluch]

Hi Roger,

I definitely have considered the effects of priming and have thought on ways to prevent/minimise it, but most of it is based on taking the steam from the top of a cavity that should have no turbulence, thus allowing any liquid water to settle. I suppose this is the traditional concept of the 'dome'. Problem with this approach is that my loco is meant to look as a main line express and there's not enough height to make this work effectively. For this kind of locos I think that priming may be caused easily by water moving in the boiler at accelerating or braking, not only by sharply opening the regulator.

I understand what you mean about a special normally open, non-return valve, mounted in the reverse way. As an idea, I have thought on it as well, but I do not recall having seen it mentioned so I don't know whether anybody has thought on it or whether it has been tried. 

As you know, I have recently visited Darlington Steam Locomotive Works and I had the opportunity to meet David Elliot who I now regard as a very knowledgeable person of the present time about full size steam locos. I think he could have a design and training role in the making of the "Tornado" A1 locomotive, and he's participating in the building of a the new P2 2-8-2 locomotive at Darlington works. www.p2steam.com  . He's the man at the right of the photo. Unfortunately, my spoken and listening English skills are much worse than my writing and I missed a lot of what he said.

Among many other subjects, He referred to priming and ways to prevent it. Starting by water quality, such as the presence -or lack of- particular salts in the water, up to several design features in the loco to help preventing or dealing with this condition.

It's worth to mention that I recall him mentioning a device to help preventing water entering the regulator, which apparently not all locos incorporated, but unfortunately I do not remember how he named such device because it was of course an English word that I did not write down at the time. Maybe some forum member is able to bring some light into it. That would be nice to know.

[4fbuilder]

Hello Joan,

Generally in the operation of miniature (5") gauge locomotives the incidence of priming is rare, almost unknown, most new copper boilers are pickled clean of any residue fluxes etc and following a couple of wash outs with clean water the boilers will if blown down each time following a steam up will probably never prime. Having said that, the use of stored rain water can lead to problems due to organic compounds etc. If in the design of the steam circuit the take off is taken from the highest point in the boiler or failing that up inside the dome the chances of priming are much reduced. The boiler designed highest water level and the steam take off position need really to be a far apart as possiable. Generally the only time our miniature boilers will erupt much like a geyser is probably due to to forgetting to shut off an injector, and with radiant superheaters the effect can be quite exciting if not noticed before opening the regulator! Sometimes you will get a small carry over of water when loading passengers if they tend to shuffle back and forth causing the locomotive and its boiler water to violently move back and forth. I think in our sizes there will not be any difference between spearhead or coaxial type superheaters in the effect of water carry over.

One class I remember in full size that had problems of water carry over, the original BR Britannia class locomotives, that had problems with the locomotive picking up its water at large regulator openings and several mods were made to the design befor a cure was found by simply raising the dome height; water carry over in full size didn't normally affect the superheater performance but it did tend to destroy cylinder covers!

Regards,

Bob

[jma1009]

hi joan,

i would like to echo Bob's excellent post. unless you are a complete idiot driving, a 5" g loco is very unlikely to suffer from priming. in miniature the firebox crown is usually lower (in some cases much lower) than would be to 'scale' and the water gauge shows a longer range as a result. Jim Ewins was firmly against this trend, and ive followed Jim in my own boiler designs, but the smaller range on the gauge glass (typically 5/8" glass showing between the union nuts on my own locos) requires a higher level of attention when driving. you get used to this quite quickly, even with a long boiler.

i dont recall the effects of fullsize priming on miniature locos fitted with superheaters. if by bad judgement or driver error the boiler water level is too high and 'priming' (water carry over occurs to the cylinders) then it is a case of whacking open the cylinder drain cocks and easing off on the regulator till things settle down. you notice a marked reduction in power caused by the water in the cylinders and then the drain cocks being opened, but ive never experienced a sudden burst of power due to water flashing to steam in the superheaters as has been described in fullsize.

so in summary not a problem to worry about.

as to whether you should fit coaxial superheaters that is another subject entirely which i think we discussed in considerable detail last year on here under perhaps Roger's Speedy thread.

cheers,
julian

[steam4ian]

G'day Joan.

On this occasion I agree with Julian that Bob's post is very good but my agreement ends there. Water can be carried over for all kinds of reasons including the operator being an "idiot". Other reasons are poor boiler design and steep grades on sidings or steaming bays. My 7.25" simplex has radiant superheaters and if the water is very high will can some over which will flash if the loco is moved suddenly. Thankfully I am not one of Julian's idiot so I am aware of the potential problem.

Unfortunately some boiler designs in miniature gauges do seem to have the firebox crown too high leaving a very tight steam space; IMHO LBSC's Doris is a potential example of this. I have 5"G Tich derivative with a high fire box crown sheet and the dome at the back of the boiler. Starting on an uphill gradient can be quite tricky because the water is already high and opening the regulator lifts the water even further.

Like Julian I would not worry about the superheater with regard to steam flashing.

I would, however be very concerned with boiler design to make sure there is ample steam space even when the water is at the top nut. One thing I have not heard talked about is the surface area of the water. On round topped fire boxes and in the barrel this area reduces significantly as the water level rises. The steam leaves this area as bubbles and the smaller this area the more likely it is that the bubbles will carry water.

Regards
Ian

[joanlluch]

Hi Bob, Julian and Ian,

Thank you for your input.

About "idiot" drivers, well, I may initially be one of them for a while, but I hope (expect) that after some time things will come naturally to me without having to think. I tend to be fast at learning things, so that does not particularly concern me. I have already a good understanding of almost every single aspect of how a steam loco works, so that's my starting point to learn how to drive it.

So, at present my interest is in the design aspects of boilers. My understanding of steam flashing not usually happening in model locomotives is still based on the common arrangement of the superheaters. It has been said that "nature can not be scaled" and thus behaviour of a full size loco can not compare with a model one, but still there's a significant difference other than scale.

This is the typical design of a full scale loco:




And this is a typical arrangement on a model locomotive with coax superheaters




So if we look at them, I see a significant difference: On the full scale loco, the superheaters can eventually be *completelly* filled with liquid water before reaching the cylinders. On the model loco, the superheaters can only be *partially* filled before water will enter the cylinders.  With hair pin superheaters, the chances of water being trapped in the superheaters is still less. So I wonder if this makes a difference on why some circumstances may happen in a full size loco, which will not happen in a mini loco, and vice-versa (?). This was possibly already answered, but just wanted to make clearer what I meant.

[steam4ian]

Joan

As Julian says, don't get too worried about it.

It does happen but if you are alert for it then it can be managed.

I often experience after lighting up. I have the boiler fairly full to start with and then by the time I have tested the steam pump and the injector and quietened the safeties the level is quite high. On my little loco (5"Tich) the safeties can drop the water level alarmingly so I like to start with it high just in case the injector will not pick up. The surge as I move the loco onto the turntable is enough for some water to pass over and flash in the superheaters and the loco to keep running. The first time it happened was a bit of a shock but now I am waiting for it and it is less likely to happen or at least I can manage it.

Better to spend your worrying time with the boiler design so that you have ample steam space even with the water at the top of the glass.

As for the boiler you showed I would suggest that unless it either has an eccentric tapered barrel or a Belpaire firebox it will be difficult to manage the water level without risking water carry over. Note on the full size boiler the tubes are only in the bottom 5/8th of the barrel; they are proportionately higher in the model boiler so less water/steam space.

Ian

[joanlluch]

Hi Ian,

Thanks for that.

The boiler in the photo above is from the Modelworks britannia kit. It's a belpaire type. See this :www.britanniabuilder.com/kit17.htm

Just a few minutes ago I started a new thread with a description of my boiler. It's not a belpaire type thought. modeleng.proboards.com/thread/10990/gas-fired-boiler

[suctionhose]

The above advice from Steam4ian is spot on. The information here is for entertainment really and a response to Roger's "I can't believe no one invented a valve..."

Even in 5"g a prime when the elements are red hot does cause the engine to take off. Mostly, being constrained by rails and scale, the disaster is more embarrassing than harmful. Though not always!

The picture is the first steaming of my traction engine. As an experiment I fitted two radiant elements. What with gearing, compounding and a sticky reverser it was a heart rending experience watching her tumble off the grass, land upside down on the flywheel (bending the crankshaft) and finally coming to rest without plunging through the timber deck... Don't worry, it was a few years ago now and I am healed! (no condolences necessary)



Roger: A valve to vent pressure...

Most Aussies would be familiar with the Great Central ROD's that came here to Hexham after WW1 for use on J & A Browns colliery lines. The engines were required to shunt decrepit 4 wheel wooden hoppers about and they didn't take kindly to being shoved unexpectedly especially empty. What is not commonly known (I have never seen it in print) is that those ROD's still carrying superheated boilers (late 1960's?) had been fitted with a local device called an "exhauster".

It worked when the throttle closed and vented any remaining pressure in the superheater with a whoosh. It's purpose was to make the superheated engine more controllable when shunting thus saving the odd shunter's life and a mess of hoppers all over the place. The Govt retained saturated engines for shunting duties for the same reasons.

[ejparrott]

I only fill my boilers to a maximum of 1/3rd of a glass when lighting up. This gives room for the expansion, and then allows for testing of injectors, without over filling the boiler.

[joanlluch]

Oct 12, 2015 11:34:55 GMT suctionhose said:

It worked when the throttle closed and vented any remaining pressure in the superheater with a whoosh. It's purpose was to make the superheated engine more controllable when shunting thus saving the odd shunter's life and a mess of hoppers all over the place. The Govt retained saturated engines for shunting duties for the same reasons.

Turbo-charged internal combustion engines usually have a 'venting' valve that connects the two sides of the turbo charger admission turbine. When the driver suddenly releases the throttle after full load, said valve opens and instantly releases pressure from the motor admission pipe to the inlet of the turbo charger. I had once a super-boosted turbo-charged car that I used for racing at local events, which had the output of the venting valve open to the air -as opposed to being connected after the admission filter-, so it was faster to release turbo pressure and as a bonus produced funny loud decompression noises while shifting gears, LOL. (for those who fancy knowing which car it was, it was a genuine first generation Ford Escort Cosworth. Best car I have ever had as fas as handling, if it wasn't for its continuous breakdowns)

So, I wonder why a decompression/venting valve releasing steam chest pressure when the regulator is fully closed was not fitted as standard.

[Roger]

Hi Ross,
What a heart breaking episode you had with your traction engine, it hurts just to see it lying there.
That's an interesting design you describe, and thinking about it further, there doesn't seem to be any reason why full sized locomotives couldn't have easily been fitted with a device that released the pressure in a similar way, but perhaps only actuated by closing the regulator past the closed position to a vent position. I guess it didn't cause enough catastrophes to require action.

I do think it's a little unfair to anyone to assume that it takes an 'idiot' to prime a steam engine. Inexperience does not make someone an idiot, and forgetting that you've left an injector on when distracted is something I bet everyone has done.

[vulcanbomber]

Oct 12, 2015 13:24:00 GMT joanlluch said:

Oct 12, 2015 11:34:55 GMT suctionhose said:

It worked when the throttle closed and vented any remaining pressure in the superheater with a whoosh. It's purpose was to make the superheated engine more controllable when shunting thus saving the odd shunter's life and a mess of hoppers all over the place. The Govt retained saturated engines for shunting duties for the same reasons.

Turbo-charged internal combustion engines usually have a 'venting' valve that connects the two sides of the turbo charger admission turbine. When the driver suddenly releases the throttle after full load, said valve opens and instantly releases pressure from the motor admission pipe to the inlet of the turbo charger. I had once a super-boosted turbo-charged car that I used for racing at local events, which had the output of the venting valve open to the air -as opposed to being connected after the admission filter-, so it was faster to release turbo pressure and as a bonus produced funny loud decompression noises while shifting gears, LOL. (for those who fancy knowing which car it was, it was a genuine first generation Ford Escort Cosworth. Best car I have ever had as fas as handling, if it wasn't for its continuous breakdowns)

So, I wonder why a decompression/venting valve releasing steam chest pressure when the regulator is fully closed was not fitted as standard.

that only applies to Petrol Engines Joan

[joanlluch]

Oct 12, 2015 18:18:27 GMT vulcanbomber said:

that only applies to Petrol Engines Joan

Yes, sure. Thanks for pointing that out. Of course my old Escort Cosworth sucked an unbelievably big amount of petrol that would be obscene for any current standards.

[jma1009]

hi roger,

i have trained up many tyro drivers in my time. some have bags of enthusiasm and no sense, and the only way to impress upon them the importance of maintaining the correct water level is by resort to threats of heinous crimes with the coal hammer, throwing large lumps of coal at them, or telling them they are not fit to drive a GWR loco.

the first rule of footplate work whether fullsize or miniature, is to maintain the correct water level. in your case, you did not have to be told or threatened! but others need a bit more of a forthright approach! letting the water level get too high is just as bad (but with perhaps less disastrous consequences, except in the case of Blue Peter) as letting the water level get too low.

if Joan would like to send me a PM with his email address i will send him Jim Ewin's paper on firebox crown height, and his objections to co-axial superheaters.

cheers,
julian

[joanlluch]

Julian, I am interested on this, but nothing forbids to share this with everybody. I have already given my email address to you some time ago, but for some reason you only seem to reply occasionally to my PM's. I'm sending it again in a new PM. Thanks for that. Regards.

[Deleted]

Oct 12, 2015 13:24:00 GMT joanlluch said:

Oct 12, 2015 11:34:55 GMT suctionhose said:

It worked when the throttle closed and vented any remaining pressure in the superheater with a whoosh. It's purpose was to make the superheated engine more controllable when shunting thus saving the odd shunter's life and a mess of hoppers all over the place. The Govt retained saturated engines for shunting duties for the same reasons.

Turbo-charged internal combustion engines usually have a 'venting' valve that connects the two sides of the turbo charger admission turbine. When the driver suddenly releases the throttle after full load, said valve opens and instantly releases pressure from the motor admission pipe to the inlet of the turbo charger. I had once a super-boosted turbo-charged car that I used for racing at local events, which had the output of the venting valve open to the air -as opposed to being connected after the admission filter-, so it was faster to release turbo pressure and as a bonus produced funny loud decompression noises while shifting gears, LOL. (for those who fancy knowing which car it was, it was a genuine first generation Ford Escort Cosworth. Best car I have ever had as fas as handling, if it wasn't for its continuous breakdowns)

So, I wonder why a decompression/venting valve releasing steam chest pressure when the regulator is fully closed was not fitted as standard.

ahhh...cars? Joan....you are describing what we here call a 'dump valve' where the turbo boost is dumped to 'atmosphere' whereas the other type you mention is a 'recirculating' valve or BOV where the boost is redirected back through the inlet which is supposed to help keep the turbo spooling amongst other things. Problem is when you are going for high boost you need a clean fuel mix with a good spark, recirculating the boost dirties the mixture hence why some tuners dump to atmosphere, I also dump to atmosphere on my car for that very reason. BTW my second eldest is in the middle of rebuilding an Escort Cosworth... chassis is nearly ready for paint ...all my son's are fast Ford men, followed their dad I guess...trouble is I'm into Porsche's these days...still trying to convert my boys....costs put them off but considering how many times they break down perhaps they would find changing to Porsche would save them money.....

Pete