Page 30 - Boiler_Operators_Handbook,_Second_Edition

Operating Wisely 15

is greatest. There are many activities that can have an tank opens as the level drops to increase water flow and
effect on load that you’ll have to pick out to understand closes to decrease flow as the level rises; that’s modula-
your operation. In addition to the ones mentioned here tion. There is, of course, more to know and understand
there’s the effect of wind, solar gain (sunny or cloudy), about these two methods of control but they’ll be ad-
unusual activities such as a conference that increases use dressed in the chapter on controls; we need to learn a lot
of doors, hot water heating and such things as kitchens, more about flow itself right now.
laundries, and other heat users. Accepting the premise that all we can control is
flow makes it a lot simpler to understand the operation
of a boiler plant. Every pound of steam that leaves the
FLOW boiler plant must be matched by a pound of water en-
tering it or the levels in the plant will have to change.
Here’s a concept that always raises eyebrows: You Water wasted in blowdown and other uses like softener
can’t control pressure; you can’t control temperature; regeneration must also be replaced by water entering
you can’t control level; the only thing you can control is the plant.
flow. Before you say I’m crazy, think about it. You main- The energy in the steam leaving the boiler plant
tain the pressure or temperature in a boiler by control- requires energy enter the plant in the form of fuel flow. If
ling the flow of fuel and air. You maintain the level by the steam leaving contains more energy than is supplied
controlling the flow of feedwater. Pressure, temperature, by the fuel entering then the steam pressure will fall.
level, and other measures will increase or decrease only Some of the energy in the fuel ends up in the flue gases
with a change in flow. An increase in flow will increase going up the stack so the energy in the fuel has to match
or decrease the value we’re measuring depending on the the sum of the energy lost up the stack and leaving in
direction of the flow. the steam. The sum of everything flowing into the boiler
That’s usually my first statement in response to op- plant has to match what is flowing out or plant condi-
erators’ questions about their particular problem in main- tions will change. An operator is something of a juggler.
taining a pressure, temperature or level. It always brings You are always performing a balancing act controlling
a frown to the operator’s face and I continue relating it to flows into the plant to match what’s going out.
their specific problem until that frown turns into a bright A boiler operator basically controls the flow of
smile. They don’t get an answer to their problem from fluids. The energy added to heat water or make steam
me; they get an introduction to the concept of flow and comes from the fuel and you control the amount of
how it affects the particular measure they are concerned energy released in the boiler by controlling the flow of
with so they can see for themselves what is causing their the fuel. Gas and oil are both fluids because they flow
problem. It’s a fundamental that, once grasped, will al- naturally. Operators in coal fired plants could argue
ways serve an operator in determining the cause of, and they are controlling the flow of a solid but when they
solution to, a problem with control. look at it they’ll realize that they’re treating that coal
If you don’t buy it you simply have to think about the same way they would a fluid. The only other flow
it for a while. Read that first paragraph again and think an operator controls is the flow of electrons in electrical
about your boiler operation and you’ll eventually un- circuits, another subject for another chapter—electricity.
derstand it. There’s absolutely no way for you to grab Controlling those flows requires you understand what
a pressure, temperature, or level and change it. Any makes them flow and how the flow affects the pressures
description you can come up with for changing those and temperatures you thought you were controlling.
measures always involves a change in flow. All fluids have mass. Fuel oil normally weighs
Now that you have the concept in hand, let’s talk less than water. Natural gas weighs less than air but it
about how you control flow to maintain all those desir- still has mass. We can treat them all the same in general
able conditions in the boiler plant. You have two means terms because what happens when they flow is about
for controlling flow. You can turn it on and off or you the same. Gas and air are a little more complicated be-
can vary the flow rate. When you’re changing the flow cause they are compressible, their volume changes with
rate we call it “modulating” and the method is called pressure. In practice the relationship of flow and pres-
“modulation.” To restore the level in a chemical feed sure drop are consistent regardless of the fluid so we’ll
tank you open a valve, shut it when the level is near the cover the basics first.
top, and you add chemicals to restore the concentration; Flow metering using differential pressure is based
that’s on-off control. A float valve on a make-up water on the Bernoulli principle. Bernoulli discovered the