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Post by Roger on Feb 8, 2015 11:50:26 GMT
This is being discussed elsewhere but I think it's worthy of its own thread because there seems to be a lot of confusion about the consequences of each method.
Here's my thinking...
1) Bypass valve
a) Only efficient when fully closed
b) Very low losses when just recirculating water (so long as there is little resistance in the return)
c) Inefficient when partly closed because the portion that's returned to the tank had to be raised to boiler pressure. The worst case is when the bypass is only slightly closed. That's when all of the pump volume is raised to boiler pressure but returned to the tank.
d) It's possible to arrange the return so you can monitor it.
e) Smooth operation because the pump volume it always full.
f) There is only a very small suction on the pump so the ram seal only needs to work one way.
g) The most efficient size is when the delivery only just exceeds the maximum required to maintain the level.
2) Throttled inlet
a) High efficiency throughout the range of delivery. This is because the pump volume is not allowed to completely fill when it's throttled. The unfilled space creates a partial vacuum when the ram moves outwards. The energy used in creating that vacuum is regained when the ram returns, ie atmospheric pressure helps it to move inwards.
b) You can't monitor the pump return, there isn't one.
c) Less smooth operation when partially open. The worst case is when it's half full. This is because the ram is at its maximum speed at half stroke when the amount of water that's been allowed into the pump chamber comes into full contact with the ram and is delivered to the output.
d) There is more reverse pressure on the seal so this needs to be taken into account. I don't see it would be a big issue.
e) The size of the pump doesn't affect the efficiency, but a really large ram could cause severe shocks when the valve is half open.
So my assessment is that although the throttled inlet at first sounds more attractive, the smoothness of the bypass version wins the day even though it's less efficient if you use it partially open all the time.
If you want to win IMLEC then use the bypass version either on or off or fit a throttled inlet.
I'm wondering if throttled inlets are popular on Traction Engines because they have a large flywheel and that absorbs the jolts better when the valve it partially open. On a small locomotive I can see that being an issue.
These are just my views based on a though experiment rather than any practical knowledge.
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Post by Deleted on Feb 8, 2015 12:13:30 GMT
I like having the bypass valve...putting aside it's efficiency or lack off it's a good aid for steaming especially when learning. It's reassuring to see water being pumped back into the tank as it confirms that the pump is working and also a good visible aid for judging the rate of flow. Having said that 4472 will have no bypass valve as it will be injector only but then again by the time she's ready to steam I should have much more experience of steaming 4470 and thus hopefully a far better understanding and confidence in what's going on.
Pete
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uuu
Elder Statesman
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Post by uuu on Feb 8, 2015 15:26:01 GMT
On the throttled inlet installation I have used, there was an air bottle on the outlet side, to cushion the pulses.
Wilf
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Post by Roger on Feb 8, 2015 16:33:16 GMT
On the throttled inlet installation I have used, there was an air bottle on the outlet side, to cushion the pulses. Wilf That certainly makes sense, I wonder how common that is. |
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Post by joanlluch on Feb 8, 2015 17:03:03 GMT
Or have anybody tried a variable stroke pump? That would be the ultimate thing as far as pump efficiency is concerned.
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pault
Elder Statesman
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Post by pault on Feb 8, 2015 18:49:28 GMT
One thing to consider is what happens on a throttled inlet installation if you need to turn the pump off. The ram can dry out, it uses water for lubrication.
The pump output will be more variable with changes in speed than a "hydraulically stable" bypass system
Forget the pumps and fit injectors
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Post by Roger on Feb 8, 2015 20:18:27 GMT
One thing to consider is what happens on a throttled inlet installation if you need to turn the pump off. The ram can dry out, it uses water for lubrication. The pump output will be more variable with changes in speed than a "hydraulically stable" bypass system Forget the pumps and fit injectors That's a very interesting point about variability with speed, I hadn't even considered that. Maybe that's another reason they tend to be found on Traction Engines and not Locomotives where the speed range is much greater. I'd fit injectors if I had absolute faith in them, but I don't. I don't like the idea of having to keep water constantly running through them or other fixes. To me, an injector should work without being pampered. |
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Post by Rob on Feb 8, 2015 21:01:08 GMT
I agree with Roger, I'd rather only fit injectors too to be prototypical, but I'd be too worried that they'd not work when needed and I'd be left with no other option.
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jma1009
Elder Statesman
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Post by jma1009 on Feb 8, 2015 23:41:16 GMT
hi rob,
you shouldnt worry re injectors.
i know a little a bit about feed pumps from my interest in Stroudley locos - which all had them fitted.
they all had 'throttled inlets' to use roger's terminology, plus a reservoir on the supply side as referred to by wilf. they also had a 'pet cock' on the delivery side to get rid of air locks. the bypass valve is generally attributed to LBSC, and would have been a much better arrangement than the 'throttled inlet' used in fullsize.
having played around with these things many years ago, there is quite a suction on bypass feed pumps contrary to roger's post.
cheers,
julian
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Post by Roger on Feb 9, 2015 7:11:03 GMT
hi rob, you shouldnt worry re injectors. i know a little a bit about feed pumps from my interest in Stroudley locos - which all had them fitted. they all had 'throttled inlets' to use roger's terminology, plus a reservoir on the supply side as referred to by wilf. they also had a 'pet cock' on the delivery side to get rid of air locks. the bypass valve is generally attributed to LBSC, and would have been a much better arrangement than the 'throttled inlet' used in fullsize. having played around with these things many years ago, there is quite a suction on bypass feed pumps contrary to roger's post. cheers, julian Hi Julian, I think we may be talking at crossed purposes... there's a potential for a lot of suction, but with the inlets connected straight to the supplies, there should be very little compared to when you try to stop it drawing water. |
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Post by Shawki Shlemon on Feb 9, 2015 8:29:46 GMT
I use axle/cross head pumps on all of my engines , I also have a hand pump on every engine even that are seldom or never used except for hydro test but provide me peace of mind as last resort if needed , I also have an injector on most of my engines and on some I even have steam operated pumps as well , I like to make sure that I can put water in the boiler at any time other than not having water on board . Now to the argument , one gets nothing for nothing , some energy in some form is required to pump the water , axle pumps are not very efficient because of the by passing not required water but for that we have water pumped whenever the engine is moving ( this may also be a disadvantage as no water when stationary ) , the by pass is very good for controlling water supply and also a mean to check if pumps are in working order before steaming , two smaller pumps with opposite stroke are smoother than one large if possible or double acting pump , I have done that on few engines , Injectors are great when working , they need cold water supply and no air leaks , steam pumps suit some engines that had air pumps and these are moody devices , lubrication is critical and in my experience they are not reliable as main water supply , secondary or when stuck in traffic and axle pump not working . In my opinion there is no right or wrong its a matter of what one likes also what can or can not be fitted on the engine .
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Post by Roger on Feb 9, 2015 8:38:16 GMT
Hi Shawki,
I completely agree with all of that, peace of mind is important, especially if you're a beginner. On the point of efficiency of axle pumps, I think the problem is worse when the size of the pump is too big and it's used all the time with the bypass open. That would be very wasteful. I've downsized SPEEDY's pump to what I think ought to cope with the largest delivery required so even if it's left on all the time with a little being bypassed it won't be too bad. It does make me think that it might be better to fit a quick action valve on the bypass though, and to use it either on or off. How do you use yours?
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Post by ejparrott on Feb 9, 2015 10:16:17 GMT
The Hunslet just has throttled suction and a bleed valve, no air bottle.
The Manning Wardle has throttled suction and a bleed valve and an air bottle.
The Kerr Stuart has an air bottle but I cannot remember whether it was built as a throttled suction or a bypass, I'd be inclined to think it was throttled suction if I'm honest. The model has a bypass.
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Post by Roger on Feb 9, 2015 10:33:37 GMT
The Hunslet just has throttled suction and a bleed valve, no air bottle. The Manning Wardle has throttled suction and a bleed valve and an air bottle. The Kerr Stuart has an air bottle but I cannot remember whether it was built as a throttled suction or a bypass, I'd be inclined to think it was throttled suction if I'm honest. The model has a bypass. Thanks for that Ed, it's interesting to know what's out there with each system. My guess is that the Hunslet, being a very sturdy locomotive, doesn't show the effects of the harsh pulses as much as other locomotives. I can't see any reason to have an air bottle if there isn't a suction valve because it will always have a smooth delivery. |
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Post by chris vine on Feb 9, 2015 10:56:38 GMT
Hi Roger,
the biggest inefficiency with a feedpump (axle) is that it is being powered by the steam engine, which is about 5% efficient (steam energy to mechanical work) if you are lucky. So trying to make it a bit more efficient is a bit like shutting to door after the horse(power) has bolted.
With an injector, at least if it uses too much steam (compared to the minimum in a perfect injector), your feed water has been heated some more so the energy isn't lost. I guess you could say it is 100% efficient, as long as it does not dribble hot water/steam from the overflow.
Just a thought for you...
Chris.
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Post by Roger on Feb 9, 2015 11:00:51 GMT
Hi Roger, the biggest inefficiency with a feedpump (axle) is that it is being powered by the steam engine, which is about 5% efficient (steam energy to mechanical work) if you are lucky. So trying to make it a bit more efficient is a bit like shutting to door after the horse(power) has bolted. With an injector, at least if it uses too much steam (compared to the minimum in a perfect injector), your feed water has been heated some more so the energy isn't lost. I guess you could say it is 100% efficient, as long as it does not dribble hot water/steam from the overflow. Just a thought for you... Chris. And a very good thought it is indeed! If they were totally dependable, I wouldn't be fitting a mechanical pump, but from what I've read they're far from being that. |
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Post by joanlluch on Feb 9, 2015 11:19:39 GMT
Just a thought for you... Chris. And a very good thought it is indeed! If they were totally dependable, I wouldn't be fitting a mechanical pump, but from what I've read they're far from being that. I'm with Roger on this. If only I could make sure injectors would run reliably in my club track in Summer time, I would just forget about the axle pump. On a side note, the axle pump also has the advantage of self regulating the required amount of water. The more the locomotive speed (same as strokes per second in the main cylinder) the more water is injected in, just what it is required. |
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Post by ejparrott on Feb 9, 2015 11:20:15 GMT
They can de totally dependable, but I always suggest learner drivers have a pump as they hardly ever fail, and it's a confidence booster, when injector faults are quite often operator error.
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Post by ejparrott on Feb 9, 2015 11:21:51 GMT
And a very good thought it is indeed! If they were totally dependable, I wouldn't be fitting a mechanical pump, but from what I've read they're far from being that. I'm with Roger on this. If only I could make sure injectors would run reliably in my club track in Summer time, I would just forget about the axle pump. On a side note, the axle pump also has the advantage of self regulating the required amount of water. The more the locomotive speed (same as strokes per second in the main cylinder) the more water is injected in, just what it is required. Not strictly true. A locomotive can obtain speed from heavy train pushing it downhill. Steam is not being used so water is not being used, so water is not required. |
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Post by joanlluch on Feb 9, 2015 13:04:51 GMT
Not strictly true. A locomotive can obtain speed from heavy train pushing it downhill. Steam is not being used so water is not being used, so water is not required. Fair enough! That's the difference between someone having experience on actively driving locos compared with someone with only a theoretical background. I certainly didn't think on that :-) |
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