Gary Kubicek Loco-Notes

Loco-Notes #1

by Gary Kubicek

Small Steam Locomotive Engineering Information

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? ReadON!

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: