Page 49 - Boiler Operator’s Handbook

34 Boiler Operator’s Handbook

something to happen. Voltage is nothing more than a transmitter then there’s a problem elsewhere in the
reference value like steam pressure. The electric com- circuit. I’ll frequently check for a voltage drop between
pany, or you if you’re generating it, produce enough each wire before it is connected to the transmitter termi-
electron flow to keep the voltage up just like you pro- nal and a spot past the screw that holds the wire because
duce enough steam flow to keep the pressure up. Most poor connections are frequently a problem. 24 volts DC
electric flow control is on-off; you close the switch and can’t push current through a loose or corroded connec-
open it to control the flow. You may have a dimmer on tion and corrosion is always a problem in the humid
one or more lights in your home, they control the flow of atmosphere of a boiler plant. I’ve fixed many a faulty
electrons to dim the lights. At other times the equipment circuit by just tightening screws without even checking
is designed to automatically control the flow. the voltage.
I’ve managed over forty years to deal with electric- A voltmeter or even a light bulb in a socket with
ity but I have to admit that I still don’t really understand two wires extended can be used to check the typical 120
what happens with alternating current. I base all my volt control circuit. Just make sure you don’t touch those
operating judgment on principles for direct current and test leads on the light to anything that could be higher
a little understanding of alternating current. I trust you or lower voltage. If the resistance between two points
can do the same, you don’t have to be able to design is zero, or nearly zero, then there’s no voltage and your
electrical systems, only understand how they work and meter or test light will show nothing. If the circuit is
how to operate them. Of course you can troubleshoot open between the two points you put your test leads on
them to a degree if you understand how they work. you will get a reading or the light will shine. The circuit
I even use the basic Ohm’s law on AC circuits to will not operate because the meter or light doesn’t pass
get an idea of what’s going on. I know it isn’t a correct enough electrical current.
analysis but it’s good enough for me. You know Ohm’s In the days of electro-mechanical burner manage-
law, it’s really mother nature’s law, Ohm is just the guy ment systems I added a light to a control panel, down
that realized it. The voltage between any two points in the bottom door, and labeled it “test.” The light was
in a circuit is equal to the value of the current flowing connected to the grounded conductor and a piece of
through the circuit times the resistance of the circuit wire long enough to reach anywhere in the panel was
between the two points. V=IR where V stands for volt- connected to the light and left coiled up in the bottom.
age, I stands for current in amperes, and the R represents All an operator had to do was pick up the coiled wire
resistance in ohms. If you know any two of the values and touch it’s end to any terminal or other wire in the
you can determine the third because current equals volt- panel to find out if the wire or terminal was “hot.” The
age divided by resistance and resistance equals voltage idea was to allow the operator to pick up that lead and
divided by current. troubleshoot the system when he had problem.
Ohm’s law is a lot of help when troubleshooting Most of the time that provision was eliminated
electronic control circuitry. Most of our control circuits from the design after the submittal to the owner. Why?
today use a standard range of four to twenty milliamps It was a combination of Owner management being con-
to represent the measured values. For example, a steam vinced that an electrician was the only one that could
pressure transmitter set at a range of 0 to 150 psig will troubleshoot electrical circuits or they had trade restric-
produce a current of 12 milliamps when the measured tions which required that work be done by an electri-
pressure is 75 psig. If we aren’t getting a 75 psig indica- cian. Frequently it’s assigned to a trade identified as an
tion on the control panel and want to know why we can instrument technician. I’ve discovered, however, that
take a voltmeter and measure voltage at several points most electricians are totally lost in a burner management
in the circuit to see why. Start with the power supply, it control system and few instrument techs understand
should be about 24 volts if it’s a typical one. That gives them. Set up your own test light so you have it when
you a starting point and you can use one side of the you need it.
power supply, whenever possible, to check for voltage The need for troubleshooting burner management
at other points in the circuit. systems has decreased considerably with the introduc-
The voltage drop across the transmitter should be tion of microprocessor based systems. Many of them
more than half that of the power supply because all the include a display that will tell the operator what isn’t
transmitter does is increase or decrease its resistance; to working (failure to make a low fire start switch on start-
control the current so it relates to the measured steam up being a very common one) and they’re simply more
pressure. If there isn’t much voltage drop across the reliable than all those relays and that extensive wiring.