Page 314 - Boiler_Operators_Handbook,_Second_Edition

Plants and Equipment 299

sign shape had “open” instead of “yield” painted on it. I to help you determine what’s water and what’s steam.
also made sure that, even with the valves open, the shut When the level is within the limits of the glass and you
tab hung a little lower than the open tab so it was easy position yourself opposite the side with the red line you
to grab and pull when the glass broke. A couple of trips will see the narrow red line above the water level but it
under the spray of hot water from a broken glass trying will appear to be much wider below the water level. It
to grab the right chain to close the valve would convince works because the light is bent at the intersection of the
you that my arrangement will pay in the long run. The glass and water but it isn’t at the intersection of glass
idea is you get to be prepared so you won’t have to make and steam. One important consideration is to install the
several passes at those chains. glass with the lines painted on it away from your normal
Locomotive boilers and a few others are fitted with position when viewing it. I saw one job where the opera-
gauge glasses independent of water columns. They nor- tors thought the light shined through the lines somehow
mally have a liquid line that penetrates the boiler with a and put all the glasses in backwards; you couldn’t see
few holes in it to restrict surging flow so the glass level the water level.
is stable. Tubular glasses should be fitted with an additional
Gauge glasses come in many forms but they all glass enclosure, usually wire reinforced, to protect per-
perform the same function. The water level inside the sonnel in case the glass breaks. I don’t think it’s neces-
boiler is repeated in the gauge glass. The water in the sary when the glass is ten feet in the air where you can’t
glass is usually very clear because it’s all condensate. get close enough to it to be hurt by it breaking but the
Steam is constantly condensing in the water column, glasses are used on vessels where you can be right be-
connecting piping and the gauge glass then draining to side them, and those should be guarded. On my first
the bottom of the column and gauge glass and returning ship I blew down a gauge glass on an evaporator to
to the boiler through the connecting piping. Occasional- flush it out so I could see the level and the glass cracked
ly when the water level is fluctuating so the boiler water from the thermal shock. I had to bend over to reach the
is surging into and out of the water column it will mix drain valve and my eyes were about three inches from
with that condensate and any color in the boiler water the bare glass. I had a burn across my left forehead, the
will appear in the bottom of the glass. bridge of my nose, and my right cheek; if I had been
Aboard a ship the entire boiler moved so the wa- a few centimeters to the right or left I probably would
ter was always swinging in and out of the gauge glass. have lost an eye. It’s another reason for red line glasses,
During a storm the determination of water level got I might have been able to see the level through the dirt
very interesting. My last ship had steam drums that ran and not blown the glass down.
thwart ships (that’s left and right as you face the bow or Tubular glasses can’t handle pressure above 150
stern) and we determined drum level during a storm by psig so higher pressure boilers have other products that
the timing between the level rising above the top of the permit viewing the water level. Prismatic gauge glasses
glass then coming back into view compared to the time are heavy steel frames with a groove cut in them to form
it spent out of the bottom of the glass. a tube between the steam and water connections and a
Since the steam and water are both clear it’s dif- special glass bolted to one side. The glass is thick and
ficult to identify the actual water level in some gauge narrow to eliminate the stress associated with the differ-
glasses. The most common one is a simple glass tube like ence in temperature between the water and air sides.
the one in Figure 10-33, twelve to twenty inches tall with A tubular glass tends to expand more on the in-
some paint applied along one side. The paint is applied side where it’s hot and the colder outside of the glass
to form a thin red line along the length of the glass with restrains that expansion resulting in stress that will
a wider white line applied over that and it’s the mini- eventually result in the outer layer cracking, being
mum you should have. pulled apart by the tensile stress. Since the prismatic
I have been in plants where someone decided to glass is narrow the stress is minimized. The glass to steel
save a few bucks and buy plain glass tubes instead of frame joint is sealed by a gasket and the glass is pressed
the red line tubes. In one they also bought a new boiler against the gasket and frame by dogs which are held
because the operators made a wrong decision about against the glass and frame by bolts (Figure 10-34). The
water level. With a plain glass you can’t tell if it’s full notches cast into the glass that produce the sawtooth
of water or completely empty when the level is beyond appearance use the diffraction principle to differentiate
the limits of the glass. The red line glass utilizes the between water and steam. Part of the installation of a
natural diffraction of light through steam and water prismatic glass requires a light shining on it from the