Flues and tubes

[tpatrick]

Steam mavens, I need your help. Steam boilers have flues and tubes. I understand that flues carry hot combustion gases from the firebox to the smokebox, provide for draft and help heat the water. But what are tubes? Cutaway drawings I have seen appear to show tubes as the same as flues. Nothing more than a pipe between the firebox and the smokebox. So I must be missing something. Can anyone kindly explain? Or even unkindly?

[jockellis]

I always thought the words flue and tube were interchangeable. The power industry decided to change the name of them to economizer tubes and it got the Georgia Power Company in trouble because when they replaced them in coal fired power plants as part of normal maintenance, they were accused of upgrading plants without proper federal intervention. If GPC had kept the name the same, they probably would have faced no problems. And how do you economize on something using the exact same thing which has been in use for about 200 years?
Jock Ellis

[Anonymous]

Flues are normally any boiler tube larger than 4 inches in diameter. Tubes are smaller than 4 inches in diameter. Tubes and flues both serve the same function: they carry the gases of combustion through the boiler and provide the heat transfer surface which transfers the heat of combustion to the water in the boiler. [:I]

[M636C]

Firstly, Locomotive boilers - while the above descriptions are generally correct for "A" type superheaters, but there are also "E" type superheaters which are different in many respects.

In an A type superheater, there are four pipes in each flue forming a superheater element, connected as a single return loop with three 180 degree bends.

As an example, the AT&SF 3700 class 4-8-2 built in 1918 had 43 5-1/2" flues and 253 2-1/4" tubes, which represents the conventional superheated boiler with an A type superheater.

The E type superheater has only two pipes with a single return bend in each flue, although a single element is made up of the pipes in two flues, giving the same number of pipes altogether as an A type element. There are many more of the smaller flues, and the E type has a higher superheating surface.

An example of the E type superheater is the Santa Fe 3776 class 4-8-4 of 1941, which has 220 3-1/2" Flues and 52 2-1/4" Tubes (almost the reverse of 3700 class regarding flues and tubes). Both boilers were 21 feet between tubeplates, conveniently).

So the tubes are 2-1/4" in both cases (this is related to boiler length and resistance to gas flow). The E type flues are only 3-1/2" diameter and A type 5-1/2" diameter. The correct gas flow resistance is only obtained with the elements in place in the flues.

Now to address Jock Ellis' comments on power stations. Power stations have water tube boilers, where the water is inside the tube and the hot gases outside. In a power station boiler the water passes through three sets of pipes in order, first the "economiser", furtherest from the grate, then the main boiler tubes, closer to the grate, and finally through the superheater closest to the grate. The "Economiser" is equivalent to a locomotive feed water heater.

I hope this clarifies things. I also deny counting staybolts, at least rigid staybolts, but must confess to having counted flexible staybolts, and wondered about the shape of the patterns they form.

Peter

[Modelcar]

..Peter....Please explain what is a flexible staybolt. And where it is used. Staybolt operation and location "holding" two pieces of metal at a certain position with 200 - 300 PSI of steam trying to force the structure apart has always been a mystery to me. The shape of the boiler at that location would seem so vulnerable.

[Overmod]

Be aware that Peter is speaking analogically when he says an economizer is "equivalent" to a locomotive feedwater heater: the two things accomplish similar action -- recovery of heat energy in the feedwater -- but do it by very different physical means. An economizer is different from most styles of locomotive feedwater heater in that the heat transfer is usually directly from combustion gas to the feedwater -- there are 'steaming' and 'nonsteaming' versions depending upon how much heat is passed to the feedwater during its traverse of the heat exchanger. Most locomotive feedwater heater designs either involve closed or open transfer of heat from steam or overcritical water (e.g. in the boiler circuit) to the feedwater -- closed systems like the drum-style Elesco via separate pipes, open systems like the later Worthington mixing the hot and cold "water" flows to get higher heat-transfer efficiency. (An exhaust-steam injector is another approach to utilizing the heat content of exhaust steam for feedwater efficiency).

I think the most familiar example of an 'economizer' in locomotive practice is likely to be the Franco-Crosti system, which in developed form could recover heat from the combustion gas both for feedwater heating and for combustion-air preheating. Traditionally a major problem with these systems was corrosion induced when combustion gas from high-sulfur fuels is cooled below a certain temperature in heat exchangers -- this has kept the use of economizers quite restricted, in US-American locomotive practice at least. It's my opinion that most of the classical difficulties with economizer maintenance can be quite effectively addressed with modern materials science and hard-coatings fabrication...

The 'flexible staybolts' I'm familiar with aren't really flexible bolts at all -- the bolt itself is quite strong and doesn't 'flex'; it has a round head on one end that fits into a mating socket in a 'sleeve' that threads into the shell from the outside. This allows the staybolt to move relative to the sleeve as a ball-and-socket joint, and accommodate differential thermal expansion better. A cap is secured over the outer portion of the sleeve, in parft to seal the pressure and limit any tendency for high-pressure water or steam to cause erosion or cutting of part of the sleeve, ball head, or staybolt itself.

Note that you are NOT exactly "holding two pieces of metal at a certain position" in a precise sense: what you are doing is providing an essentially radial tension force between two metal surfaces that are being forced in opposite directions by internal pressure. The boiler doesn't 'care' much if the shape of the firebox sheets changes geometry as it heats up, or if that geometry change is quite different from the outer boiler wrapper. It does help, however, to keep the force in the staybolt 'in line' with the shaft of the bolt, and not have it 'bend' or deflect the staybolt or cause stress in the threads or welds that secure the bolt to the metal shells. Having a jointed head even on one end makes the strain relief on the staybolts much easier to accommodate. (Note that it's quite difficult to accommodate effective flexible jointing inside the firebox, where temperatures are high and environment fairly harsh, as opposed to on the outer shell of the boiler...)

[Modelcar]

....Understand what you are saying re: Staybolts...but the harsh invironment of the boiler at the location of the fire box is the most difficult part of a steam engine for me to understand of it's durability. And to be able to be water and {steam}, tight during all of it's operation. And the serious consequences if fire brick are not in proper place...I simply don't undersand how that part of the engine doesn't fail and blow to destruction.

[M636C]

I guess I have a few things to add to Overmod's comments. Firstly, you have to think of heat TRANSFER. That's what the boiler does, takes heat energy from the fire and transfers it to the boiler water where it can be used in the form of pressure in the cylinders. The firebox conditions are generally acceptable as long as the heat is absorbed by the water surrounding the inner firebox. The energy is then carried away by thermal convection, the hot water rises towards the top of the boiler and forms steam at the water steam interface. As long as the water absorbs the heat, the steel or copper that makes up the inner firebox doesn't absorb the heat and reach a temperature that will cause problems. So while the inner firebox reaches a higher temperature than the outer firebox shell (hence the "Flexible" stays) it doesn't reach anything like the fire temperatures, unless something goes wrong.

Boiler explosions generally follow the top of the firebox (the "crown sheet") being uncovered by a low water level in the boiler. This uncovering could be very short in duration, such as caused by emergency braking, and this can result in a very rapid failure. Arch tubes and thermic syphons are there to improve the circulation of water (bringing colder water from the bottom of the boiler) as well as increase the heating surface. But maintaining a minimum water level over the crown sheet is vital, and this is indicated by the gauge glasses (glass tubes showing the water level) in the cab.

Because the staybolts are so important, they usually have holes drilled in the centre at both ends, so that if a crack or corrosion occurs at either end, water will leak through to the hole and appear as a steam leak, drawing attention to the imminent failure. Some wartime German locomotives had tubular staybolts, where the holes in each end met. This makes the stay a source of air for the fire, and this must be taken into account in the firebox design.

While I agree that the "Flexible" stay itself is not flexible, that is the name I've always seen used. My reference to "appearance" was that the cap over the ball joint gave a distinctive appearance to the ball jointed stay, and by looking at the outside of a complete but unlagged firebox, you could see which stays had ball joints. Thus you could tell where the boiler designer thought he would have problems - hence my reference to patterns on the firebox.

In fact, power stations have feed water heaters as well as economiser tubes - my project in the "Steam power" course at university was to detail the feed water heaters in a new power station. I went to the main design office and came away with cutaway drawings and took my (railfan) camera to the power station and photographed the feed water heaters under construction (and gave photos to the rest of the class for their subjects). To my surprise, I topped the class in that subject. These were heat exchanger type, like the locomotive "Elesco" cylindrical type, only MUCH bigger.

I'd agree that the "Franco-Crosti" boiler is the nearest equivalent to an "economiser" in a power station, and the British railways 2-10-0 locomotives so fitted, which also had exhaust steam injectors as form of feed water heater would be a good example to consider.

Late steam locomotives, following on from the work of Skoda in the then Czechoslovakia, had all welded fireboxes, and in these the stays were welded in at each end rather than riveted in. This included the ball jointed stays, which were welded in place both ends, but the outer cap was still screwed on as in the older version. The Chinese QJ locomotives have this design.

Peter

[jockellis]

Peter, thanks for the information about boiler flues, tubes and economizer tubes. Now could you answer me this? In a book on live steaming, it pointed out that the expansion coefficient of a 300 series stainless steel and copper are almost identical. Has anyone ever made a stainless steel boiler with a copper firebox and boiler tubes?
Jock Ellis

[Anonymous]

I'm not a metallurgist but I'm also not sure why you would want to (I've personally never seen a copper firebox). My tables show the melting point of Cu (1082 deg C +/-, or about 1981 deg F) is about 800 deg C (1472 deg F) below that of SS, and the oxidation characteristics for SS are much hardier, which should allow for better performance in a direct flame environment such as a firebox.

Other Eng. opinions?

[M636C]

Jock,

No I've never heard of a stainless steel boiler, with or without a copper firebox. At least, not a boiler made from anything CALLED stainless steel. In the late 1930s, however, steel with high nickel content was used for boilers, particularly in Germany and to some extent in the USA. This was used because of its high strength and thus the ability to make the locomotive lighter. The ATSF 3776 was an example of this, high strength steel being used for both the boiler and the roller bearing equipped connecting and coupling rods. When the wartime 2900 class was built to the same design, the nickel steel was not available, being required for other war related purposes, and mild steel was used for the boiler and the rods. I mention the rods because they were (A) visible and (B) huge. The rods were replaced with high strength steel post war, not suffering from heat related difficulties. The mild steel in the boilers ended up being an advantage, because the nickel steel suffered from (I believe) hardening and cracking resulting from the high temperatures and continuing contraction and expansion in use. I'm sorry I can't explain it better, but most of the explanations I've seen are in German, because they had the same problem with nickel steel. The Germans also stopped using it for boilers during the war, and in Post War West Germany, the engineers didn't like the class 52 "War Locomotives" which had chassis simplifications and welded coupling and connecting rods. But the nickel steel boilers on the otherwise similar class 50 were rapidly failing. The solution chosen was to transfer the mild steel boilers from the class 52 to the class 50 chassis and scrap the class 52 chassis and class 50 boilers. New boilers were built for many other locomotives that had the nickel steel boilers, both in East and West Germany. This gave them the opportunity to improve the design, but it must have been very costly.

I think this experience slowed down any experimentation post WWII with stainless steel or other high strength steel for boilers, and with the rapid introduction of diesel locomotives, it became just a matter of academic interest (like us here!).

On a nearly related topic, in Australia from the mid 1920s, boilers were built with round top fireboxes, copper inner fireboxes and radial staying, as used in the USA on steel fireboxes. I'm not sure exactly why, but new boilers with steel inner fireboxes and direct staying were substituted within ten years.

Radial staying uses stays at an angle to the crown sheet, looking a little like spokes in a wheel (which are truly radial). Direct staying has all the stays vertical, but with a set of cross stays taking the stress in the transverse direction.

My guess is that the different expansion coefficients of steel and copper, combined with the angular radial stays created greater stresses in the copper inner fireboxes causing failure. This problem occurred on three major Australian locomotive classes in NSW and Victoria between 1925 and 1940, by which time all had been refitted with steel inner fireboxes. Somehow this isn't mentioned in any of the railfan histories.

Now I know I haven't answered your question directly, but I think I've covered some of the problems that could be encountered.

Peter

[Anonymous]

I've read that some of the later Northern Pacific A class Northerns had problems with nickel steel boilers cracking as well. They were difficult and expensive to repair as welded boiler seams weren't allowed(?) until during or after WWII. If I remember right, some of the Challengers(NP and UP) had the problem as well.

[jchnhtfd]

The New York Central Niagaras had nickel steel boilers and trouble with cracking, as well as the Northern Pacific A class.

Nice discussion on the niceties of nomenclature in boilers, though, guys!

[CBQ_Guy]

When I was working with stationery boilers about 35 years ago, I was told that up to 3 inch diameter (or was it 4"), they were called tubes. A diameter above that was called a flue.

[jockellis]

Thanks for the info. I haven't herd of a stainless steel boiler with copper fire box, either although live steamers do use totally copper boilers. I just said that I read in a book on live steam where the author said it would seem theoretically possible since the co-efficients of expansion were almost identical. Why would you want stainless steel? It has an awesome insulation property because it doesn't transfer heat. We wa***he big turbine blades in 150 degree water and sometimes the top 4-5 inches will stick out of the water. After several minutes in the hot bath, the area out of the water is still cold. As a non destructive testing inspector in a gas turbine blade machine shop, I see plenty of stainless steel but ,only rarely, cracks. I guess our manufacturing process from forging to polishing has been going long enough that they know how to keep them from cracking. Of course, when one does let go in a spinning turbine, it can really ruin a utility company's day.
They probably follow operating procedures like railroads did and let the metal cool slowly so that tears will not result. If the insulating property of stainless could be successfully used in a boiler with a metal such as copper which really trtansmits heat, it would raise the efficiency of steam locomotives dramatically.
Jock Ellis