initial heating of this trapped air is what causes the false pressure spike on a
pressure gauge fitted locomotive. Once
this hot-trapped air is released to the atmosphere the steam pressure gauge will
again read zero and the next movement of the gauge will reflect the real raising
of steam pressure.
is conveniently ignored is that water expands in volume when it is heated (or
freezes) and so the 20% of "free space" over the boiler water level is
now compressed to something less as the level of boiler water rises. This fact is most easily observed in model locomotives like
the Aster/LGB Frank S. and the Hyde Shay both of which have "port hole" water
gauges on the back head tube sheet.
you fill a Frank S. boiler to halfway up the porthole you will have a ¾ full
reading after the boiler pressure reaches 2kg/cm2 (28psig).
It is more difficult to see this change on model locomotives fitted with
5mm, or less, diameter sight glasses.
now visualize what is happening in the model boiler as it reaches operating
pressure. The water level is well
above the top of the fire tube (gas flue), and as the water expands, due to
being heated, it is compressing the trapped air at the top of the boiler into a
space less than the original 20% of volume.
trapped air may or may not be vented off by the operator, but at some point, as
the temperature of the of the heated water exceeds 212F, steam will begin to be
present in the space above the boiler's water level.
The boiler's pressure will continue to rise until the pressure relief
device opens. At this point
locomotive is ready to go!
the operator opens the throttle the steam delivery line and the cylinders will
fill with condensate (hot water) until they warm up.
Once the cylinders are cleared of condensate the locomotive will start to
run on its own. At this point one
will notice that the exhaust steam appears to be very wet.
is caused by the surface of the boiling water effervescing water into the steam
in the top of the boiler as the water boils and steam bubbles to the surface.
Some of this hot water is carried over (its called carryover in full
scale practice) with the steam flowing to the cylinders.
water cools as it flows through the steam line and the cylinders.
It does not "flash" into steam, and it degrades the performance
of the locomotive until the boiler water level drops low enough so that
many small-scale locomotives the steam turret is located at the top of the back
head. This is an additional source
of carryover because every time the locomotive bumps itself running with a full
water level some of the water surges rearward and is forced up into the throttle
valve passage and hence to the cylinders.
This is one reason that piston valve engines seem to run better in reverse.
rapid and explosive boiling action of the water in a model boiler has been
documented both by myself and Larry Bangham as we both worked on perfecting
boiler water level electrical sensors that would tell a pump when to turn on and
off to maintain a constant water level irrespective of the locomotive's speed
the sensors were designed to cancel out the chaotic and unpredictable boiler
water surface conditions that are prevalent in these tiny boilers. This rapid and unpredictable boiling can be observed by
looking at the porthole type water gauge on the back head of a Frank S.
commences at the point that the boiler water level falls low enough that the
potential carryover caused by bursting steam bubbles is no longer a problem.
At this point the boiler/burner is delivering the "driest"
saturated steam to the cylinders that the combination can.
locomotive will then continue to "zip" right along, running real well,
until the water level drops low enough to start to uncover the top of the fire
tube. At this point the locomotive
will start to act "tired" as the fire tube’s effective steam
generation area starts to fall as the water level goes down.
The locomotive will continue to "tire" until it finally stops when there
is no more water to boil.
now we can see the three phases of model locomotive boiler operation. First up is so/so running with a full boiler that promotes
carryover and priming,
followed by strong, "zippy", running promoted by an abundance of relatively dry
saturated steam, And finally "tiredness" creeping in as the boiler water level
drops low enough to uncover the fire tube’s steam generation area.
personally only operate in the "zippy" area.
Each one of my locomotives that has a way to add boiler water while under
pressure is only initially filled with enough distilled water to cover the top
of the highest fire tube plus 1/8" to 3/16".
do this by using a sealed squeeze bottle with a silicone tube and a short piece
of brass tubing affixed to the end. The
brass tube has a notch filled through the side of it about 5/32" from the
open end of the tube.
fill the bottle with distilled water, seal it tight, insert it into the fill
opening on the top of the boiler until the end of the brass tubing contacts the
top of the fire tube, and then squeeze the water through the silicone and brass
tubing into the boiler until it overflows.
I release my grip on the bottle and evacuate the excess water.
When the water level in the boiler drops down to the notch in the side of
the brass tube air is admitted to the squeeze bottle and evacuation ends.
What is left in the boiler is a water-covered flue that will be even more
water covered as the water heats up and expands.
this point I fire up the burner, come up to operating pressure, clear the
cylinders (my Ruby has cylinder drains), and "zip" right along as I
add small measures of boiler makeup water every circuit or so while I remain
alert for any signs of "tiredness".