Protective Coating Leak Detector

A protective coating leak detector (referred to as a holiday detector) is used to detect the imperfections (holidays) in pipe coatings. The holiday leak detector is an instrument that operates on an electric current. When it is being

Figure 7-15.-Impressed current method of cathodic protection. 7-27

moved along a pipe that is covered by a coating or wrapping, a completed circuit between it and the pipe reveals a holiday and causes a bell to ring or a bulb to light or a buzzer to sound.

MAINTENANCE OF ANODE SYSTEMS

The anode system of cathodic protection requires little maintenance since there is no power source.

Magnesium and zinc anodes used in the anode system sometimes suffer local or self-corrosion that reduces their efficiency. Replace the anode when the efficiency drops to a minimum. Anode life varies from 5 to 30 years, depending upon the type of anode used. It is conservative to figure that about 17 pounds of magnesium or 25 pounds of zinc are wasted away by electrolysis from an anode per ampere year. To detect the effectiveness of cathodic protection, you should install test stations in anode systems.

MAINTENANCE OF IMPRESSED CURRENT SYSTEMS

The impressed system of cathodic protection requires considerably more maintenance than the anode system. This is because an electrical current is used for the operation of the system. The current may come from any alternating current source. When alternating current is not available, you can use other generating sources to furnish the alternating current. The transformerrectifier used in the system requires much less maintenance and servicing than other sources of current. However, systematic maintenance procedures must be used to keep these units in operating condition.

The transformer-rectifier set consists of two units, a transformer and a rectifier. The transformer steps the voltage down to a value of 12 to 40 volts. The rectifier changes the alternating current to direct current. Remember to keep all of the connections on this unit airtight.

The materials most often used for anodes with impressed current are aluminum, high-silicon cast iron, and graphite. Scrap iron and steel may be used for anodes since they waste away at a rate of 20 pounds per ampere year. Replace anodes when they are wasted away. Insulated wire that resists electrolytic action must be used to make the connections between the anodes and the structures to be protected. The insulation on existing current-carrying lines should be checked. Replace the wires if they are deteriorating. Ensure that overhead wiring is fastened securely to the poles and that all connections are tight.

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