Gary Kubicek Loco-Notes

Loco-Notes

Small Steam Locomotive Engineering Information

by Gary Kubicek

Part 1
Steam Generation in Small Locomotives

What is steam? Steam is a name usually given to water vapor which is under higher than atmospheric pressure in order that useful work may be accomplished. Water vapor is a colorless transparent gas and is not normally visible except under special conditions. You cannot see steam rising from a lake or escaping from a safety valve. What you do see is a mist of water droplets resulting from the change of the water vapor into liquid form. These very small water droplets refract light making them visible.

Although most people know from an early age that you can change water to steam by heating it, few actually realize that this process (called a phase change or boiling) is conditional on both the temperature of the water as well as the gaseous pressure surrounding the water. For instance, water will boil when it reaches a temperature of 212 degrees Fahrenheit if the surrounding gaseous pressure is equivalent to sea level pressure (14.962 pounds per square inch by scientific definition). When the pressure is decreased, the water will boil at a lower temperature. And no amount of heating will make the water hotter unless you increase the pressure by placing the water in a pressure vessel.

Have you ever tried boiling eggs up in the high mountains? Cooking speed is related to temperature and in the high mountains the boiling point of water may be as low as 160 degrees Fahrenheit (°F) and a 10 minute egg makes a fairly soft boiled-egg.

This variation of boiling point temperature with respect to pressure has been known since the first steam engine was built and is, in fact, the foundation for all boiler design.

In a locomotive boiler, when first filled, closed-off, and then heated, the internal pressure starts at atmospheric level. As the water temperature rises very little change of pressure occurs because virtually no water vapor is generated until some of the water reaches the atmospheric boiling point of approximately 212°F (there can be small changes in atmospheric pressure from day to day). When some of the water reaches a temperature of 212°F, it changes to gas (steam) and as it does causes the pressure inside the boiler to rise. This requires the water temperature to become higher before more steam can be produced so more heating is required.

This process continues with ever increasing boiler pressure until boiler rupture occurs UNLESS the boiler pressure is limited by pressure relief valves (safety vales), or some of the steam is used thereby reducing or maintaining the pressure, or if the heating source is removed (dumping the fire).

To prevent rupture then, a boiler must have safety valves to provide pressure relief during the periods when steam is being generated but not being used and the boiler should have an easy method of removing the heat source.

Why should we sorry about rupture of a boiler? Read ON !

As indicated above, the water temperature required to produce steam is dependent on the pressure inside the boiler containing the water. If the boiler pressure is 100 pounds per square inch, the boiling point or temperature of the water must be approximately 328°F. At 327°F no steam would generated. But note that all of the water could be at 327°F and still no steam would generated. IF, however, the boiler shell ruptures, the pressure on the water would drop to atmospheric pressure and water changes to steam at 212°F for this pressure so that ALL OF THE WATER IN THE BOILER WOULD IMMEDIATELY BECOME STEAM and since steam would occupy approximately 1500 times the volume of the water being converted a fairly large explosion would occur - If the boiler holds 1 cubic foot of water, this would expand to 1500 cubic feet of steam.

With reasonable care in the operation of a small boiler the danger of such an explosion is practically non-existent. Suitable precautions include (1) proper construction when building, (2) adequate safety valve size to handle the steam making capability of the boiler, (3) Always keep the most sensitive part of the boiler covered with water (the fire-box crown-sheet) so that overheating of the metal cannot produce weakening leading to rupture under pressure. It is for this last reason that each engineer should know the positional relationship between crown-sheet and water level. (Ed. See Water gauge)

Since we have talked about pressures it should be understood that there are (in general) two different ways of measuring pressures! The most common way is to measure pressure with respect to atmospheric pressure and is usually referred to as "gage" pressure. Therefore 100 psi (pounds per square inch) gage pressure is really 100 psi above atmospheric pressure. Scientists and in many cases engineers think in terms of "absolute" pressure, psia (pounds per square inch absolute). Atmospheric pressure is then approximately 15 psia and 100 psi (gage) is then 115 psia. Steam tables usually give boiling point temperatures in terms of absolute pressures.

The following table is given as a representative example:

Note that absolute pressures are measured with reference to a theoretical "ZERO" pressure.

So far the discussion has centered on the fact that when you heat water to the proper temperature (dependent on pressure) steam (water vapor) will be produced. The important question then becomes "How does the heat from a fire get to the water in a locomotive boiler?"

Before this process is explained a few remarks about heat conduction in various materials are required.

There is no perfect heat insulator. As long as there is a temperature difference between two sides of any material "heat engergy" will flow from the hotter side to the colder side. The amount of heat energy which flows in a unit of time (usually one hour) is conditional on the type of material and on the temperature difference across the material.

The ability of a material to conduct heat energy is called its thermal conductivity and many engineering texts have tables listing these thermal conductivities for most common materials.

A material with a very low thermal conductivity coefficient is referred to as a thermal insulating material whereas a material with a high value of the thermal conductivity coefficient is referred to as a good heat exchanger.

Wool felt is a good insulaor whereas copper is a good heat exchanger (car radiators). See the table for compartive values.

Notice that the heat energy as measured in units called BTU's.

A BTU, the abbreviation for British Thermal Unit, represents a given quantity of heat and is defined as the amount of heat necessary to raise one pound of water one degree Fahrenheit (at a given water temperature - generally from 39° to 40°F). A 20000 BTU per hour heater could therefore raise 20000 pounds of water one degree Fahrenheit in one hour.

The BTU is used widely to indicate how much heat energy fuel can develop per pound of fuel. Coal can develop between 8000 to 15000 BTU's per pound depending on the type and quality of the coal. Seemingly in the United States the coal quality gets progressively worse the further the mine is located from the East coast of the U.S. With fuel oil, the energy content varies less and is approximately 18500 BTU's per pound.

To release this heat energy from fuel, it is only necessary to provide enough air and to provide an initial heat source which will raise the temperature of the fuel to a value where the burning process becomes self sustaining. This burning is referred to as a combustion process in engineering and the results of this process provide excess heat energy as well as combustion products which are usually exhausted to the atmosphere.

The burning of coal or fuel oil with sufficient air normally produces a flame in the locomotive firebox of up to 2600 degrees Fahrenheit, however, this temperature is seldom reached and even then is not sustained for any length of time. The temperature of the firebed in the case of good quality coal may be maintained at 2000 to 2200 degrees F but the average temperature is more likely to be not more than 1600° to 1800°F.

This value of 1600°F in the firebox and only 354°F in the water (for 125 psi boiler pressure) seems like a very impressive temperature difference from one side of the firebox wall to the other. Remember that the amount of heat energy that flows through the wall is conditional on both type of material and on the temperature difference between the two sides of the firebox wall.

If the firebox wall were made of copper ONLY, generating large quantities of steam in a small boiler would be so easy almost anyone could do it BUT that isn't the way it is. If you had nothing but steel between the flame and the water you would still find steam generation easy but this firebox wall actually has several layers of materials most of which are classified as heat insulating materials. We are very lucky in that the thickness of these unwated layers is very small but these insulating layers do have a significant effect on the transfer of heat from the firebox to the water.

Referring to the temperature diagram - the first layer the heat must penetrate is a layer of realtively cold air which exists next to the inner wall of the firebox. Where this air touches the firebox it will have the inner firebox wall temperature unless it is vigorously agitated. More on this later! In simplified version, the heat energy then penetrates the actualy firebox metal. The next layer encountered by the heat energy is the water side of the firebox encrustation or mud and the next layer is a steam layer along the firebox wall caused by steam being generated at the wall. And finally we come to the actual water we wish to heat.

An examination of the diagrm will show that those layers which have the greatest temperature differential in the layer are really made of some of the heat insulating materials we have previously discussed.

The layers which have the greatest differential are the air layer on the inner wall of the firebox, the encrustation from dirty (dissolved solids) water which gathers on the water side of the firebox wall, and the steam layer next to th encrustation (mud). IF we could reduce or eliminate these layers, heat energy could get to the water much easier thereby making the boiler a more efficient generator of steam.

The air layer inside the firebox next to the wall is there because the burning gases from the fire (without a firebox arch) go directly to the flue tubes and therefore do not provide much wiping action along the firebox walls. This wiping action can be helped by installing a firebox arch which not only gives the combustion gases somewhat better wiping action but also gives the burning gases a long path to travel thereby promoting more complete burning of the gases before they enter the flue tubes (less unburnt gases go out the stack).

The mud inside the boiler can be reduced by periodic cleaning of the boiler as well as using boiler water with little or no dissolved solids (use treated water).

The steam layer next to the deposited solids can be reduced by original design of the boiler to permit very rapid or even violent circulation of the water as it rises along the firebox walls due to its higher temperature compared with the rest of the water in the boiler - note that as the water gets hotter it gets lighter and is forced up by the heavier colder water in other parts of the boiler.

Water circulation in the boiler is a topic of its own and will be presented later, however a diagram of water circulation in the boiler is included.

One last point - Due to the direct exposure of the firebox walls to the burning gases resulting in a high differential of temperature from one side of the firebox wall to the other, the firebox area of the boiler will produce 5 to 7 times more steam per square inch of heating surface than will the flue tubes - the gases entering the flue tubes are at a much lower temperature giving less heat transfer.

TO BE CONTINUED!

Part 1a
Steam Generation in Small Locomotives (continued)

The firebox wall actually receives the heat in two major ways. First, there is the conduction of heat from the burning gases to the firebox wall. Secondly, there is the radiated heat from the burning gases to the firebox wall. In boiler design, both types of heat transfer must be considered. One of the fundamental laws that must be remembered about radiated energy is that it can be reflected, like light, from a polished surface. Various surfaces reflect, in varying degrees, incident radiation and in a boiler firebox, we desire a minimum of reflectivity (a maximum amount of absorption). IN this we are lucky since the combustion of any hydrocarbon fuel usually produces carbon (soot) as a by-product and this is deposited on the adjacent surfaces. And carbon has a low value of reflectance (high energy absorption). A good carbon deposit will reflect less than 5% of the incident heat energy, so that although the carbon deposit hinders slightly the conduction of heat from the burning gases, it more than makes up for it in high absorption of radian heat. (See chart).



We now know how heat gets to the water in the firebox area to make steam and we know that the amount of heat that gets through the firebox walls is dependent on the temperature of the burning fuel.

The same information applies to the flue tubes (firetubes). We are faced with several problems however in the flue tube area.

First, the heat absorbed by the firebox walls reduces the temperature of the burning gasses to 1000°F or less. This then means that the temperature difference from water to hot gases is much lower than in the firebox area and therefore much less heat is transferred in the flue tubes per square inch of surface. The low temperature is also insufficient to maintain further combustion so that the radiated energy drops to a very low value. For these two reasons the flue tube is about 1/5 to 1/7 as effective a heat absorber when compared to the firebox area. This condition is correct by using many tubes to provide as large a heating surface as is reasonable with respect to blockage from cinders or soot deposits.

One important thing to remember is that the temperature in the flue tubes continues to drop so that in usual practice, the smoke box gas temperature may be from 500°F to 700°F depending on how effective the flue tubes are.

On some small locomotives with quite a few tubes, I have measured stack temperatures in the 500°F range while on others with a few large tubes, the temperatures have approached 800°F. This is quite a waste of heat energy (fuel) but both types operated satisfactorily. The one with large tubes burned the fuel at a higher rate to make up for the fuel going out the stack!

The low gas temperature at the smoke box end of the flue tubes produces very little heat transfer in this area, and referring back to the circulation diagram, the water in the boiler at the smoke box end is much colder than that at the firebox end.

That is why boiler feedwater devices are connected at the smoke box end. The smaller difference in temperatures between boiler water and feedwater creates fewer stresses in the metal reducing cracking caused by such stresses.

It must be understood that most feedwater on small locomotives is colder than the water in the boiler even though the feedwater comes from injectors. IN a few small locomotives, feedwater is preheated by installing a coil of tubing in the front of the smokebox with inlet and outlet placed low on the smokebox behind the saddle. This type of feedwater heater is particularly effective for steam operated cold water pumps. The hotter the feedwater the less problems created at the front tube sheet.

We have discussed how steam is generated and how pressure of the steam is proportional to the boiling point of temperature at which steam is generated. Before continuing our discussion of steam generation we will draw a few conclusions.

First, we know that as the pressure in a boiler gets higher, so does the temperature of the water in the boiler. If this pressure is suddenly decreased, the water erupts or violently changes to steam. This sudden decrease in pressure occurs immediately under a safety valve each time the safety valve releases and, if the water is carried high in the boiler, the violent change of water to steam carries some water out the safety valve giving quite a shower - this is referred to as priming. This is not foaming which results from some chemicals in the water which create a form of suds when the water is agitated.



Secondly, and for the same reason, the boiler outlet to the cylinders must be placed high in the boiler (in the steam dome) to prevent priming. Water carried over through the throttle may loosen a cylinder cover through hydraulic pressure. Superheaters on small locomotives tend to prevent this since the water carried into the dry pipe by priming is changed to steam in the superheater tubes. In some cases a sieve (drilled plate) in the bottom of the steam dome also helps keep water out of the dry pipe as well as providing support for the dry pipe.

Thirdly, the rapid agitation of the water in the firebox area will directly affect the reading in the water glass. The upper connection to the water glass must either have an anti-surge tube extending into the boiler (simple to make) or in many cases an anti-surge column is mounted behind the water glass to reduce water glass fluctuations and thereby provide more accurate readings.

The anti-surge tube is simply a small diameter piece of tubing with some small drilled holes in the top. The tube extends several inches into the boiler.

The anti-surge water column, which is more easily mounted on completed boilers, is a piece of tubing with an internal diameter about twice that of the opening in the water glass. The water glass is mounted on the surge tube with the upper and lower ends of the surge tube connected to the boiler. The surge tube is generally slightly longer than the water glass.



There is one more fact that should be discussed concerning steam generation in boilers. We have engineering knowledge which tells us how much heat energy is developed when burning a pound of fuel. We must be able to analyze where this heat goes and what it does. For that reason the following explanation is given.

One of the things that the average person doesn't know about steam is that steam has what is referred to as both internal energy and external energy. What this means is that when liquid is converted to water vapor (steam) an amount of heat energy is absorbed in the process and cannot be used to produce work. This is referred to as the internal energy and the only way to get this energy back is to re-convert the steam back to water. This internal energy must be considered whenever a heat balance is calculated to determine how much steam can be obtained when burning a given amount of fuel. The value of energy involved in the conversion of water to steam is roughly 800 BTU's per pound of water. The actual value is given in the table since the actual value is dependent on the temperature of the steam. This simply means that all of the heat energy developed by burning fuel to generate steam is not usable to do work.

This fact is important if the heat balance of the boiler is to be determined. A consideration of this information will fall more properly in the area of heat energy calculations in the combustion process and how this effects locomotive performance - A topic for a later discussion.