Page 403 - Boiler_Operators_Handbook,_Second_Edition

388 Boiler Operator’s Handbook

steam flow recorder that will change. Normally steam on line in automatic.
flow recorders require a constant pressure for accuracy Modern digital controllers can use digital (on or
(see boiler plant instrumentation). off) inputs to determine which boilers are in automatic
When we want to keep producing steam at the same and calculate what the response will be to a control-
pressure we need an integral controller. In the early days ler action so a good digital control system shouldn’t
of controls one of those controllers was an expensive be affected by the number of boilers in automatic or
item so we chose to use one to control all the boilers in what size they are. The need for that degree of control
a plant and called it the plant master pressure controller. sophistication isn’t enough to justify a full explanation
It sensed the pressure in the common steam header so in this book. If you’re constantly changing the number
it wouldn’t be affected by shutting a boiler down and it or combined capacity of boilers operating on automatic
was close enough to the steam flow elements that it main- control and find the response of the master is never that
tained a reasonably constant pressure at them for accu- good, there is a solution for it.
racy in recording. Those rules still apply today but lower
costs for instruments and controls have made it possible
to have a controller at every boiler if desired. FLUID TEMPERATURE MAINTENANCE
A plant master pressure controller produces an
output signal that is used by each set of boiler controls to Controls for heating fluids require special consid-
adjust their firing rate so they produce steam to satisfy eration that’s not necessary for pressure controls. The
the requirements of the facility while maintaining the largest single problem is making sure that the device that
steam pressure in the header at the setpoint. We’ll be senses the temperature you are using as a process vari-
discussing the several types of boiler firing rate control able is representative of the heat flow you are really con-
systems later but they all change the flow of steam out trolling. Always be aware that the sensor may be shielded
of their boiler proportional to the change in the plant by such things as air trapped above the fluid or scale or
master signal. other material coating the sensor so it can’t detect the
You should always tune your plant master with temperature properly. It may be necessary to locate the
the normal number of boilers on automatic. Most plants sensor where it can’t detect changes in temperature when
with multiple boilers in service and a plant master run flow is interrupted; additional sensors and controls (like
one boiler on automatic and the rest on hand so the plant a flow switch) may be necessary to prevent hazardous
master will operate properly regardless of the number of operation under those circumstances.
boilers in operation. Note that this works only when all This chapter is dedicated to boiler plant controls,
the boilers are the same size. If that’s the case and you particularly hot water boilers for hydronic heating and
try operating two boilers on automatic you’ll find the similar applications. The control of boilers for service
pressure will jump around a bit when there’s a change water heating (domestic hot water) is described in the
in the steam load. chapter on water heating.
Under those conditions the two boilers change Most hot water boilers are supplied with a pro-
their steam output but the master controller expected portional control similar to that described for steam
the change in output to alter the steam flow the same as boilers. The only difference is the temperature control
the output of one boiler. With two boilers in automatic switch and modulating controller sense boiler water
the response to a controller action is doubled. If you run temperature, not pressure. In many hydronic systems
two boilers on automatic most of the time and one on oc- the quantity of water in the boiler is large enough that it
casion it’s better to tune the master for two boiler opera- can operate much like a steam boiler, using temperature
tion and live with the slower response when one boiler control instead of pressure. Simply convert the pres-
is on. Plants with boilers of different sizes will also see a sure values in the previous figures to the corresponding
different response out of the plant master. steam saturation temperature from the steam table in
Prior to the days of digital controls it wasn’t practi- Apprndix A and you have it.
cal to deal with that situation in the controls, the plant The decisions for setting the start, stop and modu-
operator had to adjust the tuning of the master control- lating range for fluid temperature control are based on
ler if the operation was erratic. Some plants added de- several considerations. The fluid has to be hot enough
rivative control to help account for it. I created a number when it reaches the using equipment to transfer all the
of complicated logic systems that adjusted the gain of heat required. The fluid cannot be so cold that acids in
the master controller according to the number of boilers the boiler flue gas condense on the boiler surfaces and