Page 26 - Boiler Operator’s Handbook

Operating Wisely 11

of water and the pressure difference between the inside there’s atmospheric pressure on the inside and outside
and outside of the building is identified in inches of of the gage’s sensing element. When the gage is connect-
water, the difference in the water level. It’s our favorite ed to a pipe or vessel containing a fluid at pressure the
measure for air pressures in the air and flue gas passages gage is indicating the difference between atmospheric
of the boiler and the differential of flow measuring in- pressure and the pressure in the pipe or vessel. Absolute
struments. pressure is a combination of the pressure in the pipe or
There is another measure of pressure we use; vessel and atmospheric pressure. Add 15 to gage pres-
“head” is the height of a column of liquid that can be sure to get absolute pressure, the pressure in the vessel
supported by a pressure. I have a system for remem- above absolutely no pressure. If you would like to be
bering it, well… actually I mean calculating it. I can more precise use 14.696 instead of 15. Atmospheric pres-
remember that a cubic foot of water weighs 62.4 pounds. sure varies a lot anyway so there’s not a lot of reason to
A cubic foot being 12 inches by 12 inches by 12 inches be really precise.
means a column of water one foot high will bear down Later we’ll also cover stress, the equivalent of pres-
on one square foot at a pressure of 62.4 pounds per sure inside solid material, under strength of materials.
square foot. Divide that by 144 square inches per square Viscosity is a measurement of the resistance of a
foot to get 0.433 pounds in a column of water one inch fluid to flowing. All fluids, gases and liquids have a
square and one foot high so one foot of water produces viscosity that varies with their temperature. Normally
a pressure of 0.433 psi. Divide that number into one and a fluid’s viscosity decreases with increasing tempera-
you get a column of water 2.31 feet tall to produce a ture. You’re familiar with the term “slow as molasses in
pressure of one psi. The reason we use head is because January?” Cold molasses has a high viscosity because
pumps produce a differential pressure, which is a func- it takes a long time for it to flow through a standard
tion of the density of the liquid being pumped, see the tube, what’s called a viscometer. The normal measure of
chapter on pumps and fans. viscosity is the time it takes a certain volume of fluid to
Head in feet and inches of water (abbreviated “in. flow through the viscometer and that’s why you’ll hear
W.C.” for inches of water column) are both head mea- the viscosity described in terms of seconds. A chart for
surements even though a value for head is normally conversion of viscosities is included in the appendix
understood to mean feet. along with the viscosity of some typical fluids found in
Okay, now we’ve got pressure equal to psi, why do a boiler plant. More on viscosity when we discuss fuel
we see units of psig and psia? They stand for pounds per oils.
square inch gage and pounds per square inch absolute. It’s only fair to mention, while we’re discussing
The difference is related to what we call atmospheric measurements, that there is something called dimen-
pressure. The air around us has weight and there’s a sional analysis. Formulas that engineers use are checked
column of air on top of us that’s over thirty miles high. for units matching on both sides of the equation to en-
That may sound like a lot but if you wanted to simulate sure the formula is correct in its dimensions (measure-
the atmosphere on a globe (one of those balls with a ments). It ensures that we use inches on both sides of an
map of the earth wrapped around it) the best way is to equation, not feet on one side and inches on the other.
pour some water on it. After the excess has run off the Since I promised you at the beginning of the book that
wet layer that remains is about right for the thickness you wouldn’t be exposed to anything more complicated
of the atmosphere, about three one-hundredths of an than simple math (add, subtract, multiply and divide)
inch on an eight inch globe. Anyway, that air piled up I can’t get any more specific than that. Just remember
over us has weight. The column of air over any square that you have to be consistent in your use of units when
inch of the earth’s surface, located at sea level, is about you’re making calculations.
15 pounds. Therefore, the atmosphere exerts a pressure Not a real measurement but a value used in boiler
of 15 pounds per square inch on the earth at sea level plants is “turndown.” Turndown is another way of de-
under normal conditions. (The actual standard value is scribing the operating range of a piece of equipment or
14.696 psi but 15 is close enough for what we do most system. Instead of saying the boiler will operate between
of the time) If you were to take all the air away we 25% and 100% of capacity we say it has a four to one
wouldn’t have any pressure, it would be zero. turndown. The full capacity of the equipment or system
A pressure gage actually compares the pressure in is described as multiples of the minimum rate it will
the connected pipe or vessel and atmospheric pressure. operate at. Unless you run into someone that uses some
When the gage is connected to nothing it reads zero, idealistic measurement (anybody that says a boiler has a