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Discussion in 'Locomotive M.I.C.' started by Steve, Nov 9, 2012.
Steve Part of the furniture Friend
Gentleman of leisure, nowadays
Near Leeds
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Yes I am an active volunteer
When a locomotive is running forwards, the water level will tend to rise in the gauge glass. Conversely, when the locomotive is running backwards, the water level will tend to fall in the gauge glass. To try and illustrate this, let's consider the boiler as a simple cuboid partly filled with water. If it is level, there will be a reading in the gauge glass. If we place it on a gradient, the water level shown in the glass will rise or fall, depending on whether the gradient is uphill or downhill. So far, so good and quite simple. On the level, if we impart motion, a force has to be applied to the water to accelerate it. The force can only be applied to the water by the boiler structure. It follows that the water level at the back end will rise to give accelerating force. That is easy to demonstrate by filling a bowl of water and accelerating it. The water will surge to one end. However, once up to speed and moving at constant speed, the forces on the water are in equilibrium and the water level will be the same as when stationary. All the foregoing is fine and probably easy to understand.
If we now consider the situation of what happens on a gradient running at a constant speed. Like the rest of the locomotive and train, a force has to be imparted to the water to lift it up that gradient. Again the force on the water can only be applied by the boiler structure and, to impart this force, the head of water at the lower end must be higher than that at the upper end. We aren't talking about huge forces, here. 1" of water head is only about 0.04 psi. Hopefully the simple sketches will make this a bit clearer. If the locomotive is going in reverse, all this happens at the opposite end, causing a lowering of the reading in the gauge glass.
This is all associated with the regulator being open because it is the steam in the cylinders that is ultimately providing this force to overcome gravity and to have this force, the reg has to be open. When you close the reg, you remove the applied force, causing the water level to fall going forward and rise going in reverse.
Of course, there is a change in water level around the steam offtake from the boiler but this position does not change relative to the gauge glasses with change in direction so should have no effect on the reading in the gauge glass, everything else being equal. I also know that there is a lot more going on inside the boiler to affect the level shown in the gauge glass.
Steve, Nov 9, 2012
Avonside1563 Member
Bolton's Sidings, just behind the running shed!
Steve, Can you outline the scientific theory behind your reasoning? All I can glean from the above is a description of mass and inertia.
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Avonside1563, Nov 13, 2012
laplace New Member
See the discussion in the 6619 thread.
laplace, Nov 14, 2012
Avonside1563 said: ↑
I tried to explain it in my previous post but seemingly I've failed. I'll have another go!
First, inertia is the rotational equivalent of mass so there is no inertia to consider.
If you fill a tank with water and then move it horizontally, the water will rise at the rear end as it is accelerated. (Try it with a bowl of water.) If it is moving at constant speed, the water level will be constant. I think that everyone is in agreement on these two facts. Going back to the situation of acceleration, think about why the water level rises at the rear end. A force is being applied to the water - the accelerating force. Because water is a fluid, the force cannot simply be transmitted into it. Try pushing your hand through it. Again, you will produce a bow wave in front of your hand. The force can only be applied to the water by having a pressure head difference. Hopefully, you accept all the foregoing.
If the locomotive (and its boiler) is stationary on a gradient, the water surface is level as there is no force trynig to overcome gravity. If it is moving up that gradient at a constant speed, a force has to be applied to overcome gravity. Because it is a fluid, you can only apply the force by having a pressure head difference. Thus there has to be a higher water level at the rear of the vessel.
I hope that this is a better explanation. All this is totally independent of the steam pressure, by the way, and applies whether it is a closed or open tank.
If you totally fill a vessel and fit a differential manometer, it would show a pressure difference under acceleration conditions and being moved at a constant speed on a gradient.
Steve, Nov 14, 2012
RalphW Part of the furniture Staff Member Administrator Friend
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Are you saying that when climbing at a steady rate, the water will be higher at the back as it is under acceleration?
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RalphW, Nov 14, 2012
Sheff Well-Known Member
Retired Engineer & Heritage Volunteer
N Warks
RalphW said: ↑
If you are climbing at a steady rate, then you by definition you aren't accelerating. You'll have a higher level at the rear of the boiler due to the gradient, but the water surface will be 'flat' (ignoring any effects from the steam take off and safety valves as Steve said before).
What we think, or what we know, or what we believe is, in the end, of little consequence. The only consequence is what we do.
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Sheff, Nov 14, 2012
Right, that's what I was thinking, but the way Steve has worded it he seems to be saying that due to gravitational force, even at constant speed it will be higher at the back and not 'flat'. :confused1:
I see what you mean, having re-read Steve's piece. It was your use of the word 'acceleration' I was focussing on.
Sheff said: ↑
Yes, that's what I'm saying. it is trynig to explain it via the limits of the Forum that I'm struggling with. essentially, if you apply force to a fluid, there has to be a pressure drop across that fluid as, without a pressure drop, there is no force and to climb a gradient, you need to apply a force! The connection with the regulator being open is that the force is applied by the action of the steam in the cylinders and that only happens when the regulator is open. Thus, close the regulator, - force disappears - water level at the rear drops. If you have a double headed train and one loco is not doing any work, the water level will still rise and fall in that loco.
Taken from the 6619 thread
SE&CR_red_snow said: ↑
Yes, but an individual's experience thus far doesn't seem to cover all the scenarios. If the degree of regulator lift really does vary from nothing at all to "$£#@ where did it go?" then there's potential for coming unstuck. Would be interesting to figure out what the critical factors are, and how they interact.
Things that I know that will affect water level:
Variation in Gradient Fairly obvious and applies whether static or moving. This will be shown in the gauge glass.
The Forces applied to the water, whether it is power on or braking. Acceleration will cause the water to move to the back end of the boiler (back being defined as the end furthest away from the direction of travel) and braking/deceleration will cause the water to move to the front end. As mentioned in my earlier posts, the forces applied to move a loco along a gradient. This will be shown in the gauge glass.
Steam off-takes. this will cause the water level to rise local to the off-take. If the off-take is away from the gauge glass it should not affect the water level shown in the gauge glass. Most regulator take-offs fall into his category. If the off-take is local to the gauge glass, it will have an effect. This is often the case with the steam take-off for the auxiliaries. Turn the injector steam valve on and the water rises on many locos.
Ebullition. You have only to look at the sight glass in a modern electric kettle when it is boiling to see the affect. However, as gauge glasses are remote from the source of heat, this effect is generally nullified. One reason BR standards have the gauge frames mounted on pedestals is to move them further away from the source of heat and give a more accurate reading of the true level.
Foaming due to impurities in the water. Quite frightening when it is bad enough to be seen in the gauge glass.
There are probably others but I can't think of them off the top of my head.
SE&CR_red_snow New Member
That needs putting in context. The debate was whether there is regulator lift when travelling tender / bunker first, and two clear schools of thought had emerged. Those used to relatively big engines and relatively shallow gradients were saying "no", while those used to small engine working on steep gradients were saying "yes". What I was trying to establish was WHICH of the variables matter the most, and WHEN exactly do they start to take effect, because it's clear the relationship isn't uniform otherwise we'd all have seen the same thing.
SE&CR_red_snow, Nov 15, 2012
My experience is that there's negligable regulator lift bunker / tender first, but that's on railways with gradients no steeper than 1-in-60 and averaging 1-in-75 (Bluebell, Bo'ness). If we are stopped at a signal such as HK Up Inner Home when running bunker first there is no drop in water level when the regulator is closed. That means we can run with the water level lower in the glass bunker-first than we can when running chimney first, which makes it much easier to manage the boiler, having enough water to be safe whilst allowing space for station stops and to prevent priming. Clearly from what's been said that's not the case on railways with steeper gradients worked with small engines, where a). the engines are being worked harder, and b). the boiler proportions may well be different to those I'm used to.
This is obviously complicated by the effect of the gradient itself, but at least this can be calculated. Water always finds a level, so without regulator lift one can roughly work this out by simply applying the gradient to the length of the boiler water space. The effect will be split 50/50 between an increase at the firebox end and a decrease at the smokebox end if the loco is stood on a hill facing chimney first, and vice versa for bunker first. The 'true' level is always in the middle. So for example a 12'6" long boiler sitting chimney first on a 1-in-75 hill will have an artificial rise of approximately 1 inch at the firebox end due to the gradient alone, as 12'6" = 150" and (150"/75)/2 = 1".
As that's purely the consequence of the water sitting level while the engine is not, the volume of water space at either end of the boiler makes no difference. If a lake is wider or deeper at one end than the other it doesn't mean the water sits on a slope. With the loco stationary and the regulator closed the only variable is the length of the waterspace - doubling the boiler length to 25' doubles the rise to 2" on the 1-in-75.
However as a result of this debate I think the water space dimensions may well make a difference when discussing regulator lift. Firstly, as Steve has already said, the proximity of the steam take off points to the gauge glasses obviously makes a difference. If regulator lift is far more pronounced on small engines on steep gradients, this could be due not only to the volume of steam being used (i.e. engine being worked harder relative to its size, so more water being 'drawn' towards the dome) but also the fact that on a smaller engine everything is that much closer together, so the effect is far more pronounced in the gauge glass readings.
So far so good. However the relative capacity of the boiler water / steam space along its length may also make a difference to the apparent variation in regulator lift in the gauge glasses depending on the direction of travel. My maths and science aren't really up to this, but here are two questions for starters:
1. Can we assume that the steam and water drawn towards the dome when the regulator is opened come from all round the boiler in equal proportion, i.e. so much steam per square inch of evaporative surface area / heating area?
2. If so, is it then reasonable to assume that on a gradient, the variation in apparent 'draw' on the water level at the firebox end when running forwards or backwards will be more or less pronounced depending on the difference in evaporative surface area / heating area between one end of the boiler and the other?
[I.e. if the boiler is angled on a slope so most of the water is at the front of a sharply-tapered barrel, will it make a big difference compared to tipping it the other way so it sits around and over a wide, deep and very warm firebox. And conversely would it make far less difference if the boiler had a much smaller variation between evaporative surface area / heating area / temperature for a given amount of water at either end?]
It is a question that it will probably impossible to answer definitively. For a fluid to flow, there must be a pressure drop in the direction of that flow but this may be of negligible proportions. Thus, the pressure of steam immediately below the off-take has to be lower than the pressure at the extremities of the boiler. This is the reason that the water level lifts at this point and the greater the pressure drop, the greater the lift. The density of the water also has an effect so foaming water, which is less dense, will lift more. Density of the water adds another variable and complicates it even more. The water circulates in the boiler and for this to happen there has to be a temperature and density difference in different parts of the boiler. The water is hottest at the firebox and therefore must be less dense than that at the smokebox. If it is less dense it will require a greater head for the same pressure; therefore the water level at the firebox is going to be higher than at the smokebox due to this. We are talking very small variations here. As I've said before, 1" of water head equates to 0.036 lbf/sq in so the smallest variation in pressure from front to rear of the boiler for whatever reason will show at the glass.
olly5764 Member
Normally in a brake van somewhere
The regulator will lift the water in the glass no matter which way you are running, however, when running in back gear, this will be counteracted to some degree by the fact you are accelerating backwards, which will pull your water down, however, don't forget, the ammount the water will lift in the glass will also be affected by how clean the water is. The reverse osmosis water we use will barely lift at all, however, I do know of one occasion, when the R.O. plant had failed and we had a loco that was running on "Town water" and due a wash out, and it was lifting the water almost the full length of the glass!
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olly5764, Feb 28, 2013
olly5764 said: ↑
Opening the regulator will cause the water level to rise local to the steam take off but will not generally provide any indication in the gauge glass, which is usually remote from the steam take off. Water in the gauge glass is cooled by the air around the glass and is usually not boiling so it is denser than that in the boiler, one reason why back plates on protectors are usually perforated is to provide cooling and why BR standard gauge glasses are mounted on a pedestal and not directly on the boiler (copying GWR practice but without the tri-cocks). The water level in the gauge glass is always slightly lower than the true boiler water level because of this slightly greater density.
What happens in a boiler is very complex and variation in water level is a consequence. If the injector is on and the feed is to the front of the boiler, the water in this area will be cooler and therefore more dense. Water at the rear, over the firebox is going to be marginally hotter and less dense so to balance the level at the rear is going to be slightly higher than at the point of water feed. GWR locos generally take their auxiliary feed from above the rear of the firebox so turning on such things as injectors and steam heating will also have an effect and these are sufficiently near the gauge glass for it to be noticed.
Foaming due to impurities adds a whole new set of rules to the subject but it still comes down to the density of the water and head of water required to produce all the necessary forces necessary for flow and circulation. See my posts above.
Stu in Torbay New Member
GPS Navigation Engineer
Goodrington Bank, Paignton
In therms of a sustained lift at one end or the other while moving, surely if velocity is constant the level will equal at both ends. Analogy is your cup of coffee (glass of Merlot in 1st class!) on the table of a speeding train. The fluid in that is not lifted on the side of the glass facing the rear of the train?
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Stu in Torbay, Feb 28, 2013
KeithH New Member
Maintenance engineer in a combined heat and power
Have a look at the Spirax Sarco website they have filmed inside an operating boiler you will see the turmoil inside a boiler producing steam the things that cause priming foaming and carryover are well documented they are a mixture of Total dissolved solids, Alkalinity of the water and load also including operating at correct design pressure. other contaminants like oil and grease can also cause problems in boiler water as does oxygen. Reverse Osmosis water is good in boilers as it has so few dissolved solids but will require pH balancing with caustic which means that you need to consider the possibility of caustic enbrittlement. If you use a water softener (Base Exchange) this softens the water but does not reduce T.D.S. and makes the water alkaline so if not careful priming will result. Realistically when you draw steam from a boiler steam bubbles released in the process will give an rise in water level in the boiler but this may not always be seen in the gauge glass. When you add feed water you slow even stop steam production for a while the pressure drops ad you may even run on flash steam for a short while. Dropping the boiler pressure will increase the steam bubble size and turbulence in the boiler water as more steam is required to do the same work as the energy in the steam drops the driver lets out the reverser creating a larger steam volume flow of steam increasing the likely hood of priming or carry over.
KeithH, Mar 1, 2013
Stu in Torbay said: ↑
I've tried to explain what happens previously but I'll try again with an example. Firstly, I think that everyone accepts that, when you move a container, there is a surge of water to one end, the end opposite the direction of movement. this is due to the force applied to accelerate the liquid. Secondly, if the container of liquid is moving at a constant speed on the level, thee is no force applied to it and the level is constant across the container. Again, I think everyone accepts this and it is a fundamental law of motion. Let us now consider what happens when the container is moving at a constant speed up a gradient. As an example, take a container 4ft wide and 15 ft long which is filled to a level 4ft high. That, very conveniently, works out at 1500 gallons and weighs 15000 lb. If we move a 15000 lb load up a 1 in 50 gradient we have to exert a force of 15000 ÷ 50 = 300 lbf. This applies whether it is a solid or a fluid. To get this 300 lbf transmitted into the water, the force on one end of the tank must be 300 lbf greater than the force at the other end. The head of water when static is 4ft, which is equal to 1.73 lbf/in2. The pressure distribution thus varies linearly from zero at the surface to 1.73lnf/in2 at the bottom. 300lbf across the area of the tank end is equal to 300 ÷ (4 x 4 x 144) = 0.1302 lbf/in2 The pressure at the bottom of the tank will thus become 1.73 + 0.1302 = 1.86 lbf/in2. To achieve this pressure at the bottom, the head of water must be 48 (in) x 1.86 ÷1.73 - 51.6 in. therefore the increase in water level is 3.6 in. However, we have not added any more water to the tank so to maintain the volume, actual rise in water level at the end of the tank is only half that. i.e. 1.8 in.
The same principle applies to a loco boiler but is much more complex due to its shape. A Black 5 boiler just happens to also hold 1500 gallons. The area of the backplate covered in water is somewhat greater than 4 x 4 at about 27 ft2. That same 300 lbf is thus spread over a much larger area and results in a pressure rise of only 0.077 lbf/in2. If it were a simple rectangular tank, it would result in a total head difference of 2.08 in or 1.04 inches show in the gauge glass. If the loco is going in reverse uphill the water would fall by a similar amount in the gauge glass. However, as I've said, the geometry of the boiler makes it far more complex than this but the principle is the same. The reason why the water level rises/falls with opening the regulator is that the ultimate source of the 300 lbf is the cylinders and they are only producing this force when the regulator is open. If you were to drag an out of steam loco up a gradient with another loco the water would behave in exactly the same way.
Q.E.D. I hope :smile:
Steve, Mar 4, 2013
ZBmer New Member
Ah. I may not understand the figures but now I get the principle. Thanks Steve. On my narrow-gauge footplate, the effects of running bunker first are basically win/win as far as water levels are concerned. Water level raised by the regulator uphill; water level raised at the firebox end downhill, with my safety margins thereby increased all round. And any enthusiast passengers in the first coach are right up against the smokebox; the better to appreciate some quality chuff :cool1:.
ZBmer, Mar 6, 2013
ZBmer said: ↑
Ditto, now clear as steam! thanks Steve
Stu in Torbay, Mar 6, 2013