Axle Pump, Donkey Pump, Blower, Blast & Superheated Steam

[joanlluch]

Following in part a recent discussion on Roger's 'speedy' thread I want to raise a subject that has haunted my head for a while (please correct my English in case this is not right as an expression).

I plan to have a donkey steam pump in my loco as opposed to injectors. I know that donkey pumps are not much used in the UK but they are usually fitted in my area so this is what I plan to have. I have also a provision for adding an axle pump.

On a general basis a donkey pump and an axle pump are essentially two versions of the same concept. A donkey pump is a small steam engine that moves a piston pump. An axle pump uses the locomotive engine to move a piston pump.

Differences from the point of view of efficiency that I can think of are the following:

DONKEY PUMP
- Uses saturated steam.
- Works on demand.
- [uses heat power]

AXLE PUMP
- Does not (directly) use steam.
- Steals power from the engine.
- [uses mechanical power]

Compared with an axle pump, a donkey pump will run on demand and will not steal mechanical power from the locomotive, these are positive points. But it will use less efficient saturated steam which will require more water than it would use with dry steam.

Now, we can make the same sort of comparison for the steam blower and blast pipe

STEAM BLOWER
- Uses saturated steam.
- Works on demand.
- [uses heat power]

BLAST PIPE
- Does not (in practice) use steam.
- Steals some power from the engine.
- [uses mechanical power]

Unfortunately, the devices that rely on heat power (donkey pump and blower) will use less efficient saturated steam than the locomotive itself, which made me think that it could be improved.

So the subject I want to raise is about the convenience of having superheated (dry) steam for auxiliary systems. I have never seen a locomotive where this has been attempted and I am seriously considering doing so. On my particular locomotive this can be particularly relevant because the blower will be on all the time (to adjust air intake on the burner), the locomotive will not have a working blast pipe, the locomotive will have a donkey pump, and it will not have injectors.

Is anyone aware of any attempt of having dry steam available for something else than the locomotive engine ?.

My understanding is that this may not have been attempted before because locomotives using injectors require saturated steam (If I get it right) for them to work, and the blower is anyway not on all the time. However, as said before, the above question is still highly relevant for the way my locomotive is intended to work (link) and nonetheless the subject itself is potentially interesting in my opinion. 

Thanks.

[springcrocus]

Hi, Joan

This is not relevant to your particular question but it may be something that will interest you. It follows your statement that you will not be using a blast pipe because of the need to have continually adjustable air for your gas burner.

My thought is that your design lends itself to being able to use condensing apparatus in your loco, enabling you to recover water and, possibly, some sort of heat recovery system rather than just exhaust the steam to space. It could be that you have already researched and reported this in the past and I have missed it, but it may be possible to combine something like this with what you are researching at the moment.

Anyway, fascinated as always with your novel approach.

Steve

[chris vine]

Hi Joan,

Again not directly answering your question, as you have decided not to use injectors.
However, injectors are inherently very efficient (100% almost if not causing overflow): If they use more steam than they should the feed water is just hotter. The only heat lost is external heat losses from the pipes and body of the injector and are minimal.

Yes, the injector still uses steam/energy as it has to pump water against a pressure, but that is simply a part of the cycle of the steam engine. All the heat from the steam ends up back in the boiler.

I wonder if injectors were not much used in Spain because the feed water was often too warm for the injector to work reliably?

Chris.

[RGR 60130]

Hi Joan,

You could maybe take a lesson from marine installations and put the regulator valve after the super-heater. The auxiliaries can then be taken from the dry side of the super-heater which will always be live. Running your donkey pump when stationary will help prevent a build up of wet steam in the super-heater and give you immediate dry steam when moving off.

A bit of background information on this for those that are interested:

On marine, oil fired water-tube boilers a flow of steam has to be maintained through the super-heaters at all times to take the heat away otherwise they will melt. When flashing up these boilers (before any auxiliaries go on line) the steam is vented up the funnel to keep a flow. Once the turbo alternators and feed pumps etc are started the vent valve can be closed off.

Venting 61bar 515c steam out of a 3" or 4" vent pipe certainly makes a racket. It was common practice on the container ships I often sailed on to have a steam shut-down for maintenance in the terminal outside Sydney, Australia. This provoked a lot of complaints about the noise from local residents. Allegedly one lady was appeased with a bouquet of flowers though the person that shot at the funnel was never found!

Just remember if you use this arrangement you will have to connect the snifting valve after the regulator instead of before the super-heater.

Hopefully this will fuel your thoughts,

Reg

[joanlluch]

Hi Steve, thanks for your input. Indeed all these are things worth to consider, but I suppose I need to stop at some point since this is my first loco and I'm already incorporating a lot of untested stuff. For some time, I was considering to squeeze the last bit of heat from combustion gases by placing a high surface gas/liquid heat exchanger between to boiler and the smokebox to warm up water as it enters the boiler. The basic design would resemble a car radiator or a natural gas house boiler exchanger. House boilers are really efficient because gas combustion does not produce particles which allow exchangers to be designed with great exchange surface. So that was the idea I had for my loco in case I decided to incorporate the same.

Hi Chris, I am aware injectors are the most efficient way to enter water into a boiler, they have no mechanical moving parts and as you point out any heat from steam will be input back to the boiler. Injectors were used in full size locomotives in my country, but also a lot of locomotives incorporated steam pumps. I observed a similar pattern in France and Germany, unless I am wrong. On miniature locos however, for some reason injectors are not fitted. After having talked with local model engineers, my understanding is that injectors are found to be not reliable enough, possibly because water temperature tends to be too high (I know it easily goes above 50 ºC and I'm located at the North of Catalonia). That's possibly worse in central Spain. The result is that virtually all locos use (more expensive) donkey pumps.

Hi Reg, That's interesting. I wonder if this changes a lot of things, such as the regulator must be capable to deal with higher temperatures. My current design uses a regular ball valve with PTFE seals rated at 220 ºC according to the manufacturer. As a venting valve I may simply use the blower, which will have to run all the time anyway specially when the burner is producing power, so this alone may be enough to sort of all possible problems. The only remaining concern is what happens if the regulator valve gets eventually destroyed or locked open due to eventual over-heating and can't be closed. Scary! . Food for thought nonetheless.

[RGR 60130]

Joan,

In a model you don't need worry about keeping a flow through the super-heater. After all, with a conventional arrangement the super-heater doesn't have any flow with the regulator valve closed and the locomotive stationary.

In the event of a regulator valve failure you can notch up into mid gear and bring your locomotive to a stop. I once changed the PTFE? seats on a ball valve to gunmetal and it was quite easy to do.

Reg

[joanlluch]

Hi Reg, I think I understand what you say, but I still think on a case where the burner has been on to raise steam while the locomotive is stopped. With the locomotive stopped the superheaters will quickly become very hot, even red hot if they are radiant type. So the first steam that will arrive at the regulator and cylinders will be very hot, at least for a while until the temperature stabilizes to a certain value. The same will happen with a conventional arrangement, but in such case the regulator will never suffer from extreme temperatures. Said that, your suggestion of replacing PTFE seals by gunmetal in a ball valve is an excellent tip that may eliminate any remaining concerns!

[steam4ian]

Traditionally donkey pumps etc have used steam straight of the boiler range without the complication of superheating. This would firstly be for convenience. Donkey pumps usually have very long cut off close to 100% so there would be little opportunity for expansive working and the economies of superheating would not be realised. Superheated steam makes lubrication more critical on a component for which reliability is essential.

As for the blower superheating might increase the volume of the steam, less evaporation required for a given pressure but from the operation of the jet I doubt it would provide much advantage.

Superheating is not a total cure-all. It has been demonstrated that its main effect is reduction of volume loss during expansion due to condensation on the cylinder walls. This why the calculated tractive effort for a superheated engine is higher than that for a saturated engine (85% vs 75$ factor); the prospective cylinder pressures are the same but the pressure is sustained longer when the steam is superheated.

[jordanleeds]

Will the superheater actually be efficient? I ask as in a lot of cases in small locos they tend to act more like a steam dryer, you seem to be getting advanced of just making a loco and should be applauded for doing so but in my opinion keep it simple and modify as you go along after its finished, you need a blast pipe to direct the exhaust up the chimney and in turn draw hot gasses through the flue tubes regardless

[chris vine]

Hi Jordan,

But Joan is enjoying exploring new territory, that is what he enjoys!!

Chris.

[joanlluch]

Aug 24, 2016 13:37:18 GMT steam4ian said:

Traditionally donkey pumps etc have used steam straight of the boiler range without the complication of superheating. This would firstly be for convenience. Donkey pumps usually have very long cut off close to 100% so there would be little opportunity for expansive working and the economies of superheating would not be realised. Superheated steam makes lubrication more critical on a component for which reliability is essential.

As for the blower superheating might increase the volume of the steam, less evaporation required for a given pressure but from the operation of the jet I doubt it would provide much advantage.

Superheating is not a total cure-all. It has been demonstrated that its main effect is reduction of volume loss during expansion due to condensation on the cylinder walls. This why the calculated tractive effort for a superheated engine is higher than that for a saturated engine (85% vs 75$ factor); the prospective cylinder pressures are the same but the pressure is sustained longer when the steam is superheated.

Hi Ian,

As you point out, superheated steam has more room for expansion so this also means that any heat looses due to piping dissipation will not cause any significant volume loss. On the contrary, saturated steam will quickly decrease volume as it condensates due to heat loss. So chances that you use steam more effectively at the donkey pump and blower are higher if it has been superheated.

On top of this, superheated steam provides additional volume for the same water use. To show this let's compare saturated steam at 160 C (this would be slightly above 6 bar), with superheated steam at say 210 ºC and the same pressure. If we make the maths we find that their relative volumes are: V1/V0 = (210+273.16)/(160+273.16) = 1.12. So in this particular scenario we'll get 12% more steam if we superheat it.

An additional positive point of superheated steam is enhanced fluidity. Superheated steam is less viscous than saturated one and this should help to decrease energy loses in pipes. (Though to be honest I am not able to compute by how much)

The above considerations mean that even if we are not using steam expansion at the donkey pump, it will anyway use less water to do the same. The extra available volume and reduced viscosity will also improve blower operation. Of course, extra care should be put to assure that proper lubrication is provided to the pump at all times.

[joanlluch]

Aug 24, 2016 17:19:36 GMT jordanleeds said:

Will the superheater actually be efficient? I ask as in a lot of cases in small locos they tend to act more like a steam dryer, you seem to be getting advanced of just making a loco and should be applauded for doing so but in my opinion keep it simple and modify as you go along after its finished, you need a blast pipe to direct the exhaust up the chimney and in turn draw hot gasses through the flue tubes regardless

Jordan,

Thanks for your input. There's no point for me to significantly modify the locomotive "after" it has been finished. In fact I keep modifying it all the time on the computer before it is actually made as new ideas or suggestions come across.

About the blast pipe the actual point is that my loco will have a butane gas burner, not a grate with incandescent coal on it. A gas burner requires an amount of combustion air that is proportional to the gas input. Combustion air thus must be regulated to match gas input. This is very difficult to do with a blast pipe.

So I will rely solely on a blower for this. The idea is to fully optimise the chimney pipe to work as best as possible with a single high pressure blower jet. On the other hand, steam leaving the cylinders will be allowed to blow right into the atmosphere at low pressure without producing any venturi effect.

[joanlluch]

Aug 23, 2016 9:29:19 GMT springcrocus said:

Hi, Joan

My thought is that your design lends itself to being able to use condensing apparatus in your loco, enabling you to recover water and, possibly, some sort of heat recovery system rather than just exhaust the steam to space. It could be that you have already researched and reported this in the past and I have missed it, but it may be possible to combine something like this with what you are researching at the moment.

Anyway, fascinated as always with your novel approach.

Steve

Hi Steve, I just recalled that in a book I read years ago I saw the description of a "feed water heater" using part of the exhaust steam. Its purpose was not to reuse water but to add some extra efficiency by using exhaust steam heat. The book is from Joseph Foster Nelson who is an american model engineer. I took a photo of it for posting here:

Feed water heater by joan lluch, on Flickr

Eventually, I found a similar design in the book about boiler making by K. N. Harris.

Feed water heater by joan lluch, on Flickr

These were apparently mostly used in combination with donkey steam pumps in full size american locomotives and I found most miniature ones from american model engineers to have them fitted as well.

In France, I'm aware that the so called "economisers" were used in full scale. The diference is that they used the last remaining bit of heat from combustion gases rather than heat from exhaust steam.

Just out of curiosity, I was surprised that I didn't find any equivalent systems being applied on UK locomotives, so I wonder whether these systems were not used in the Island at all for some reason or I'm just missing the right documents (?)

[uuu]

Have a look here:

en.wikipedia.org/wiki/BR_Standard_Class_9F_92020-9

This is one that uses the combustion gases in a pre-heater - passing back from the smokebox at the front to a second chimney near the cab.

Wilf

[joanlluch]

Aug 26, 2016 16:52:40 GMT uuu said:

Have a look here:

en.wikipedia.org/wiki/BR_Standard_Class_9F_92020-9

This is one that uses the combustion gases in a pre-heater - passing back from the smokebox at the front to a second chimney near the cab.

Wilf

Ah yes!. That's the Franco-Crosti boiler. But I think they were difficult to maintain and possibly not too successful. No to mention that the side-mounted chimney looked awful!

André Chapelon in France used a more simplistic "economizer" approach, quite clever for its lack of sophistication. He just would part off the front part of the boiler with a steel baffle and a water overflow above the boiler water level. The baffle simply serving to retain the colder incoming water to give it time to pick temperature from the last section of the fire-tubes before it "overflows" into the evaporative (rear) section of the boiler. Something as simple as that apparently proved to be very effective to increase available locomotive power, and it would definitely be very easy to apply to our miniature boilers.

[joanlluch]

For your interest, I found a description of the Chapelon "Economiser" in English. Quoted from here

"
Chapelon-type economizer: This is a partition at the front of the barrel formed by an intermediate tubeplate and in free communication (by an overflow hole(s)) with the rest of the boiler. This free communication means zero pressure difference between the two and there may therefore be no need for pressure-tight joints where the boiler tubes pass through the intermediate tubeplate a good fit of the tubes in the holes to stop gross flow of water between the two sections may suffice, e.g., for parallel tubes, to be achieved by drilling all tubeplates to the same template and aligning them up with some dummy tubes in place when welding to the barrel. The boiler clack valves are situated in the economizer section, all incoming feedwater entering the boiler at the front where the combustion gasses are at their lowest temperature, and therefore acting to give the highest temperature difference, and consequently highest heat transfer, in this part of the boiler, improving its absorption efficiency.
"

Quite simple, isn't' it?

[eightpot]

German locos usually combined a feed pump with a feed water heater, this latter being usually mounted transversely on the top of the smokebox forward of the chimney. This was the normally used system of feeding the boiler with an injector as the secondary means.

The LNER here fitted a number of locos with such a system, many of the ex GER B12 class 4-6-0s had it for a time in the 1920s and 30s, also the pioneer P2 'Cock O' The North in its original form. However, any savings in efficiency compared with an exhaust steam injector (the norm in the UK), were more than offset by the maintenance costs, so all eventually had the pumps etc. removed and replaced by the regular exhaust and live steam injector arrangement.

[joanlluch]

It's interesting that an injector will run on exhaust steam. I just did a little research but I didn't know before about these devices. They don't seem to exist for model locomotives though.

[chris vine]

Yes, Joan. It is a miracle that they worked at all! When you think that the back pressure on the exhaust was only a few PSI and they were pumping against boiler pressure, it is quite some performance.

I think they usually had a high pressure (live) steam jet too, but I don't think this was needed to operate once they were running. Others will know more than me.

The thing which helped them to work was that at the core of the injector there is a vacuum because the steam is condensed. so the operating pressure for the steam had an extra 15 psi added to the back pressure. IE probably more like a pressure drop of 20 psi. Again, others will know better: Julian??

Chris.

[joanlluch]

Hi Chris,

Yes, all the majic (they seem to defy physics laws) of injectors is that they create an extra pressure drop due to steam condensation in them. Whoever invented then was quite clever! But as you say the same made to work with exaust steam it's just amazing.