Page 25 - Boiler Operator’s Handbook

10 Boiler Operator’s Handbook

air as it compresses it and that hot air will cool off while the plumbing trade which we call pressure, normally
it sits in the tank and the pressure will drop. We need measured in pounds per square inch. Occasionally we
to know the pressure and temperature of a gas to deter- confuse everyone by calling it “head.”
mine the density. The steam tables list the specific vol- We normally describe the rate that we make steam
ume (cubic feet per pound) of steam at saturation and as pounds per hour and use that as a unit of rate ab-
some superheat temperatures. Specific volume is equal breviated “pph.” The typical boiler plant can generate
to one divided by the density. To determine density, di- thousands of pounds of steam per hour so the numbers
vide one by the specific volume. get large and we’ll identify the quantity in thousands or
Liquids are normally considered non-compressible millions of pounds of steam. A problem arises in using
so we only need to know their temperature to determine the abbreviations for large quantities because we’re not
the density. The specific volume of water is also shown consistent and use a multitude of symbols.
on the steam tables for each saturation temperature. Wa- We’ll use “kpph” to mean thousands of pounds of
ter at that temperature occupies the volume indicated steam per hour but use “MBtuh” to describe a thousand
regardless of the pressure. Btu’s per hour. Most of the time we avoid using “mpph”
We also use pounds to measure force. Just like a both because it looks too much like a typo of miles per
weight of, say ten pounds, can bear down on a table hour and because many people wonder if we mean one
when we set the weight down we can tip the table up thousand or one million. A measure of a million Btu’s
with its feet against a wall and push on it to produce a per hour can be labeled “MMBtuh” sort of like saying a
force of ten pounds with the same effect. Weights can thousand thousand or use a large “M” with a line over
only act down, toward the center of the earth, but a force it which is also meant to represent one million. I’ve also
can be applied in any direction. Just like we can measure seen a thousand Btu’s per hour abbreviated “MBH.” The
a weight with a scale we can put the scale (if it’s a spring ASME is trying to be consistent in using only lower case
loaded type) in any position and measure force; they’re letters for the units. It will be some time before that’s ac-
both measured in pounds. cepted. This book uses the publisher’s choice.
Rates are invariably one of the measures of dis- Pressure exists in fluids, gases and liquids, and has
tance, area, volume, weight or mass traversed, painted, an equivalent called “stress” in solid materials. Most of
filled, or moved per unit of time. Common measure- the time we measure both in pounds per square inch but
ments for a rate are feet per minute, feet per second, there are occasions when we’ll use pounds per square
inches per hour, feet per day, gallons per minute, cubic foot. Pounds per square inch is abbreviated psi. The
feet per hour, miles per hour and its equivalent of knots units mean we are measuring force per unit area. It isn’t
(which is nautical miles per hour, but let’s not make this hard to imagine a square inch. It’s an area measuring
any worse than it already is). Take any quantity and any one inch wide by one inch long. Then, if we piled one
time frame to determine a rate. Which one you use is pound of water on top of that area the pressure on that
normally determined according to the trade discussing surface would be one pound per square inch. If we pile
it or the size of the number. We normally drive at sixty the water up until there was one hundred pounds of
miles per hour although it’s also correct to say we’re water over each square inch, the pressure on the surface
traveling at 88 feet per second. We wouldn’t say we’re would be 100 psi. It isn’t necessary for the fluid to be on
going at 316,800 feet per hour. Be conscious of the units top of the area because the pressure is exerted in every
used in trade magazines and by various workmen to direction, a square inch on the side of a tank or pipe cen-
learn which units are appropriate to use. You can always tered so there’s one hundred pounds of water on top of
convert the values to units that are more meaningful to every square inch above it sees a pressure of 100 psi. The
you. The appendix contains a list of common conver- air in a compressed air storage tank is pushing down, up
sions. and out on the sides of the tank with a force, measured
There are common units of measure used in oper- in pounds, against each square inch of the inside of the
ating boiler plants. Depending on what we’re measuring tank and we call that pressure.
we’ll use units of pounds or cubic feet or gallons when When we’re dealing with very low pressures, like
discussing volumes of water. We measure steam gener- the pressure of the wind on the side of a building, we
ated in pounds (mass) per hour but feed the water to the might talk about pounds per square foot but it’s more
boiler in gallons per minute. We burn oil in gallons per common to use inches of water. A manometer with
hour, gas in thousands of cubic feet per hour, and coal one side connected to the outside of the building and
in tons per hour. We use a measure that’s shared with another to the inside would show two different levels