Page 600 - Corrosion Engineering Principles and Practice

562 C h a p t e r 1 3 C a t h o d i c P r o t e c t i o n 563

The efficiency of a CP rectifier may be determined by measuring
the AC power input with a watt meter, measuring the DC volts and
amperes output, and calculating the efficiency as follows:
(
% Efficiency = DC voltage (V) × DC current (A) × 100 (13.2)
AC power (W)
For example, a rectifier operating at 20 A and 32 V DC output is
found to have an AC input of 940 W. The efficiency will then be (20 ×
32 × 100)/940 = 68 percent.
The power loss in the rectification process becomes heat and
cooling is necessary to prevent the rectifying components from
becoming too hot and eventually fail. One cooling method utilizes an
air-cooled assembly with the rectifier cabinet arranged so that there
will be a natural draft of air through the rectifier components. In
another method, the rectifier components are immersed in electrical
insulating oil in a large steel case. The rectifying elements give up
their excess heat to the oil that is in turn cooled by radiation from fins
on outer surfaces of the case.

13.5.4 Other Current Sources
Commercial AC power lines may not always be available. In such
instances, alternate DC power sources may provide the necessary
power [14]. Engine-generator sets, for example, may be used to
provide the electrical energy for CP rectifiers if a large power source
is needed. In such cases, gas or a petroleum product suitable for
engine fuel taken directly from a pipeline can be used to power the
engine of an ICCP system. Otherwise, fuel must be brought to the
generator station periodically. Engine-generator installations must be
designed with reliable equipment that will operate unattended for
several weeks. The cost of operating such an installation will be
relatively high, particularly if engine fuel must be brought in.
Closed cycle vapor turbogenerators (CCVTs) are commercially
available as power sources for remote CP systems. Modern CCVT
systems can supply up to 5 kW and 100 V. The system consists of a
Rankine cycle turbine and an alternator. A burner heats an organic
liquid that vaporizes and expands. The vapor is directed through the
rotating turbine wheel, providing power to the alternator. Standard
thermoelectric generator units are also available at power outputs up
to 600 W and voltages up to 48 V (Fig. 13.30). Since thermoelectric
generators contain no moving parts, maintenance is minimal. Annually,
it is necessary to replace the fuel filter and clean the fuel orifice.
In areas where sunlight is abundant, a combination of solar
cells and storage batteries may be a convenient power source to
provide a continuous flow of current to a CP installation. Systems
are now available that operate at power outputs of up to 1 kW,
voltages up to 20 V, and currents up to 50 A. Battery storage
capacities up to 3200 Ah (at 12 V) are available. Such a battery