Cathodic protection station

The terms protection current and protection current densities refer to any values of total cathodic currents that meet the criterion in Eq. (2-40). However, in the field, and for designing cathodic protection stations, another term is of interest, the protection current requirement. This term is concerned with the lowest value of the protection current that fulfills the criteria in Eqs. (2-39) or (2-40). Since with an extended object having a surface S the polarization varies locally, only the current density for the region with the most positive potential has the value J. In other regions 17. 1 > 7. . For this reason, the protection current requirement 4 is given by ... 

For determining the off potentials of cathodically protected pipelines, time relays are built into the cathodic protection station to intermpt the protection current synchronously with neighboring protection stations for 3 s every 30 s. The synchronous on and off switching of the protection stations is achieved with a synchronous motor activated by a cam-operated switch. The synchronization of the protection station is achieved as follows a time switch is built into the first protection station. An interruption of the protection current is detectable at the next protection station as a change in the pipe/soil potential. Since the switching time is known, the time switch of the second protection station can be activated synchronously. The switching of further protection stations can be synchronized in the same manner. 

The variation in the on and off potentials or the potential difference along the pipeline will usually indicate faults that prevent the attainment of complete cathodic protection. The protection current requirement of the pipeline may be estimated from experience if the age and type of pipeline is known (see Fig. 5-3). Figure 3-20 shows the variation in the on and off potentials of a 9-km pipeline section DN 800 with 10-mm wall thickness. At the end of the pipeline, at 31.84 km, an insulating unit is built in. The cathodic protection station is situated at 22.99 km. Between this and the end of the pipeline there are four pipe current measuring points. The applied protection current densities and coating resistances of individual pipeline sections are calculated from Eqs. (3-40) and (3-41). In the upper diagram the values of... 

Fig. 8-1 Construction of an impressed current cathodic protection station.

Cathodic protection stations frequently operate under conditions that are continually changing. These include ... 

Impressed Current Equipment and Transformer-Rectifiers 239 Table 8-2 Troubleshooting at cathodic protection stations... 

On inspection, the built-in measurements and the counter readings should be read off and recorded. Any breakdown should be reported to the authorized headquarters of the cathodic protection organization. Table 8-1 illustrates a form for use in the supervision of cathodic protection stations. Causes of trouble and remedial measures are given in Table 8-2. 

Fig. 10-8 Pipe/soil potentials and protection currents for a pipeline. Drainage test x-x after 1 year o-o. P = potential test point R = pipe current test point LA = cathodic protection station / = insulating joint SP = pipe casing potential test point.

Deviations from the protection criterion determined by the detailed measurements must be rectified either by digging up the pipe and repairing the coating or by resetting the cathodic protection station or constructing an additional station. The detailed measurements should then be repeated and recorded. 

According to Ref. 32, the functioning of impressed current cathodic protection stations should be monitored every 2 months, and the stray current protection station every 1 month. If protection installations are provided with measuring instruments for current and potential, this supervision can be carried out by operating staff, so that the readings are recorded and sent to the technical department for... 

For commissioning and monitoring of cathodic protection stations, the advice in Refs. 1 and 2 is relevant. For potential measurement, the explanations in Section 3.3 are valid. 

A direct current flows in the installations during operation of cathodic protection stations therefore, the transformer-rectifier must be switched off when pipes are out or other work on the fuel installation is carried out, and the separated areas must be bridged with large cross-section cables before the work is started in order to avoid sparking that could come from the current network. 

For normal operation, the breakdown fuse is not connected because if it is called upon to act, the cell would be short circuited, which could lead to its destruction. When there is work being done on the cable and the cathodic protection station is switched off, the fuse is connected by closing the switch. The cell (2) can be isolated by removing the connection (4). Finally, by closing the connection (1), direct grounding is established. In installing the cell, the process is carried out in the reverse direction. 

The charge state of the cell must be maintained in operation to have a cell voltage of 0.9 to 1.2 V [6]. Overcharging the cell is to be avoided due to electrolytic decomposition of water and evolution of gas. The cell voltage should therefore not exceed 1.4 V. Cathodic protection stations should be operated so that the cell voltage lies in the desired range. 

When commissioning the cathodic protection station for the well casing, a protection current about 10% higher than that determined in the measurements as in Section 18.3 is supplied. In the case of a separate protection current supply for the flow lines, the pipe/soil potential should be set at a value of f/cu.cuso4 =... 

Table 22-1 Installation costs of a cathodic protection station with three magnesium anodes for = 100 mA, r = 30 W m and service life of 36 years.

On the other hand, the costs for an average cathodic protection station for 6 A come to = 40,000 DM according to Table 22-2. For very small installations as, for example, the external cathodic protection of a tank, the costs of an impressed current system where a current supply is already available without cost, with lower current output, can be reduced to about 4000 DM. With larger tanks and greater soil resistivity, the following considerations point to the increased suitability of an impressed current system. 

The effective costs of a cathodic protection station depend very much on the local conditions, particularly on the costs of the power supply and the extent of the anode bed. Table 22-2 contains the costs of a protection station with four silicon iron anodes. 

The structural costs of impressed current protection of harbor and coastal structures are about 1.5 to 2.5% of the total cost of the object to be protected. As an example, for the installation of a cathodic protection station in a tanker discharge jetty, the construction costs amounted to 2.2% of the total costs. The annual cost of current, maintenance, testing, and repairs amounted to 5% of the construction costs of the cathodic protection [22]. 

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