Corrosion technicians conduct routine operational status checks on Cathodic Protection (CP) Systems to ensure that the system is providing adequate levels of protection. Many common systems can include multiple 2-wire, magnesium anode test stations (1 wire for the anode and one wire for measuring Pipe-to-Soil (P/S) potentials) with simple 2-wire P/S test stations between the anode test stations. Normal potential readings should be between -0.950 and -1.150 volts with respect to a CuSO4 reference electrode along the pipeline.
Many materials have been developed to mitigate corrosion in buried and submerged metals (pipelines, storage tanks, etc.) corrosion. The development of corrosion resistant materials i.e., plastic composites, epoxies, urethanes and the like have significantly contributed to the industry. For many applications, the “non-metallic” products are used to simply replace the metal or are applied to the metal as protective coatings. Despite this, metals and their unique properties are still major players in the corrosion control industry.
An “interrupter” is a switch installed on Cathodic Protection (CP) rectifiers to enable the rectifier to be switched OFF and ON in a timed cycle. The interrupter switch can be a conventional mechanical (metallic contact) switch or an electronic, solid-state switch. The purpose of the Interrupter is to enable the CP technician to measure pipe-to-soil potentials in both an ON (CP current applied) & OFF (CP current interrupted) condition. Referred to as an “interrupted survey”, the results of the survey reveal the effectiveness of the CP system within the influence range of the CP rectifier.
Induced AC (Alternating Current) is a common problem that occurs when buried pipelines are installed in a common corridor or near energized high-power transmission lines.
There are two main concerns with induced AC on metallic pipelines:
- Induced voltages can present a shock hazard to technicians who physically touch the pipeline or metallic devices connected to the pipeline.
- Induced AC is known to be the direct cause of soil side corrosion on buried pipelines.
Many cathodic protection (CP) technicians struggle with the proper way to read and interpret shunt measurements or readings.
A shunt is a calibrated resistor of a known value, which is connected in series within an electrical circuit. By measuring the voltage across the shunt (mathematical formula used to calculate the relationship between current, voltage, and resistance: Ohms Law), the magnitude of current flowing within the circuit can be calculated.
Because we cannot physically see volts, amperes or resistance, a comparison to something familiar like a water/hydraulic circuit, is being used to demonstrate the relationship.
EXPERT TIP #8 – HOW NEMA ELECTRICAL RATINGS FOR ENCLOSURES AFFECT CATHODIC PROTECTION DESIGN AND INSTALLATION
The National Electrical Manufactures Association (NEMA) has designated electrical enclosures into different categories or types depending on the level of protection required. These enclosure ratings affect a cathodic protection (CP) design or installation.
Common rectifier problems are often misdiagnosed. Despite zero output current while the rectifier indicates some level of output voltage, it is likely that the rectifier is in working order. A broken or “open” cable or connection in the positive (anode) circuit and/or the negative (structure) output circuit could be the problem.
Determining the Source of the Problem
Failures in the DC output circuit are rare. When they do happen, they can occur suddenly. One day the system is working fine and the next day you have an open circuit. The following are check points:
- With a portable multimeter, confirm that there is some level of output voltage. Rectifier meters can “stick”, providing incorrect information.
- Check the millivolt drop on the rectifier current shunt. The ammeter could be defective while everything else is okay.