Page 231 - Boiler_Operators_Handbook,_Second_Edition

216 Boiler Operator’s Handbook

other electrical devices, including lighting. Prior to each EDDY CURRENT TESTING
annual shutdown, if you have one, you should perform
voltage balance checks and check all large motors for Eddy current testing has become a standard prac-
current imbalance. tice for large chillers on their 15 year overhaul and is
Checking for voltage imbalance is relatively easy, gaining acceptance as a method for checking the condi-
is only applicable to three phase power, and only takes tion of boiler tubes. A technician (because the interpre-
a voltmeter and some PE (Protective equipment includ- tation of the readings requires someone skilled with
ing face shield, leather jacket and rubber gloves suitable it) has a helper insert a probe connected to a cable into
for the voltage). The PE is required to protect you from each tube and pass it through the tube while watching a
accidentally contacting hot conductors and arc flashes. monitor for indications. If you ask nicely the technician
Read the phase to phase voltage for the three phases at will show you a sample of a tube (that matches yours in
each three phase motor starter. This requires opening size and material) with holes drilled in it and sections
the starter cabinet, which usually requires opening the where the metal is cut to be thinner that must be detect-
disconnect, then restoring power by closing the discon- able with the equipment at setup before beginning the
nect with the door open. That usually requires manually testing of your tubes. The testing will not go well if your
over-riding the interlock that proves the door is shut. tubes are scaled up, especially if there’s so much scale
After recording the voltages, calculate the average volt- that the probe can’t be pushed through the tube. This
age by adding the three values and dividing the result by technology is relatively new and improving daily so you
three. Then calculate the voltage imbalance by dividing can expect to see it in the near future.
the maximum difference between the average voltage
and each of the three phases by the average voltage and
multiply by 100 to get a voltage imbalance in percent. MISCELLANEOUS
Do the same thing with the motor operating. Op-
erating a motor with a voltage imbalance of more than 2 As mentioned in the section above on electrical
to 3% is not recommended. If voltage readings vary con- equipment, painting is a maintenance activity that can
siderably between the motor off and motor running val- create problems. In many plants painting seems to be
ues the wiring to the motor should be checked. Voltage the only form of maintenance. If it’s necessary to paint
imbalance can be caused by a lack of balance of single then make sure nameplates, gauge faces, and other
phase loads in the facility or, rarely, a problem with the items that shouldn’t be painted are adequately masked
utility feed. before the painting process begins.
Current imbalance in a three phase motor can be Keep in mind that multiple layers of paint are in-
caused by voltage imbalance (which should be corrected sulation and can shorten the life of electrical equipment.
first) shorted motor windings, or a high resistance con- Paint can block tiny openings that are required for prop-
nection. For current imbalance you’ll also need an am- er operation of self contained control valves and other
probe to measure the current on each leg of the three equipment. Regular painting of screens and narrow lou-
phase motor. Current imbalance is calculated in the vers can reduce the free opening to reduce air flow with
same manner as voltage imbalance and a motor should possible hazardous or damaging consequences.
never be operated with a current imbalance of more I dislike inspecting a plant where I have to scrape
than 10%. When you detect a current imbalance of mea- several layers of paint off nameplates in order to get the
surable value then it’s advisable to check to determine if information and I consider painting a poor excuse for
the problem is in the source or the motor. To determine maintenance. Instead of painting the plant, try cleaning
the source of the problem shut down the motor, isolate it. Proper use of cleaners, soap and water can restore the
the power and rotate the leads then take another set of condition of a plant at a lower cost and with less harm
readings. Note that rotating the leads is not the same than painting. It will look good when it’s done and some
as switching the leads; to change a three phase motor’s people will think you painted. As far as I’m concerned
rotation you switch any two leads. Rotate the leads by the only things that should need regular painting are
connecting L1 to M2, L2 to M3, and L3 to M1 where L floors and handrails because they are exposed to wear.
denotes the line and M denotes the motor lead. If the ASME CSD-1 and the NFPA 85 Codes are adopted
readings at the line match then the problem is in the by law in many states and contain requirements for
power supply. If the readings at the line show different maintenance. Factory Mutual and other insurance un-
values for each phase the problem is with the motor. derwriters also have their own requirements for testing