Fundamentals and Practice of Electrical Measurements
3.4 Current Measurement
rion 7). Potential test probes are only efficient if they are located in the same soil as the pipeline.
measuring rods. In damp conditions, a cell voltage acting in parallel can falsify the measured value if there is no clean, low-resistance contact. The resistances of mea- sured stretches of the pipeline that are not part of a permanent measurement setup should therefore be checked before every measurement. Today it is usual to install solid measuring connections by cable and pole clamps at test points so that a per- fect contact can be quickly achieved (see Section 10.3.2). In excavated pipelines or fittings, good contacts can be made with metal rods, and in the case of small- diameter pipes, with battery or metal clamps. A safer contact can be made above ground with the help of magnets [29].
3.4.2 Pipe Current Measurement
Besides potential measurement, the measurement of currents in the pipeline is of considerable importance, not only for investigating the causes of corrosion, but also for detecting failures in cathodic protection. The current flowing in the pipe- line cannot be measured directly because of its low resistance (1 km of pipeline DN* 700, 8-mm wall thickness -10 mQ) even if, for example, a measuring instru- ment can be switched into the electrically interrupted pipeline at insulating sockets or by disconnecting the pipeline. The internal resistance of a low-resistance 60-mV shunt would amount to between 1 and 10 mQ and therefore would be on the same scale as the resistance of the circuit to be measured. Cable sheathing and pipeline currents are therefore determined indirectly by Ohm's Law from the voltage drop over a standard resistance.
S
The resistance per unit length can be calculated from the resistance formula of the linear conductor if the outer diameter da, the wall thickness s, and wall cross- section S = ns (da - s) are known:
A further measurement of current can be obtained from the pipe mass per meter, given in the standards, m = mil = S ps
where ps is the specific gravity of steel.
With ps = 1.7 x 10~5 Q cm and ps = 7.85 g cnr3 for steel, it follows from Eqs. (3-37) and (3-38):
The specific electrical resistances usually depend on the material and the tempera- ture [31]. For the most important pipe materials these are (in 10~5 Q cm):
Steel St 34: 1.7; gray cast iron [32]: 8 to 10, Steel St 60: 1.8; ductile cast iron: 7.
The values calculated using Eqs. (3-36) and (3-37) are only true for welded pipe- lines. Extension joints, fittings, and screwed or caulked joints can raise the longitu- dinal resistance of a pipeline considerably and therefore must be bridged over for cathodic protection.
The usual geometric length for measurements on pipes of 30 m has for DN 700 a resistance of about 0.3 m£2. This allows, with an easily measurable voltage of 0.1 mV, a current of > 0.3 A to be determined with sufficient accuracy. For a measured distance of pipe current over DN 700, 50 m is sufficient. Since with unwelded steel tubes the wall thickness can vary by 10%, and with welded pipes by 5%, and often the specific conductivity of the steel is not definitely known, it is recommended that long pipelines have built-in calibration stretches. The current measuring sec- tions for the pipe current described in Section 11.2 can be somewhat varied. Sepa- rate leads for injecting current and measuring voltage are necessary for accurate calibration of the measuring sections for pipe current. On roads where measure- ment connections can easily be made above ground, two measuring points with 1 m between measuring connections should be provided for each calibration mea- surement section on longer distances (about 100 m).
Figure 3-12 shows the current flow and potential distribution on a DN 80 pipe of wall thickness 3.5 mm with a current input of 68 A. Since the pipe was relatively short, no pipe current could flow to the right. At a distance of 15 cm to the left, practically no deviation from the linearly decreasing voltage drop in the pipeline could be noted. Two pipe diameters were therefore a satisfactory distance for sepa- rating measuring cables for current and voltage measurements. With short excava- tions, an improvement can be achieved by alternating the connections at 45° to the right and to the left [33].
3.4.3 Measurement of Current Density and Coating Resistance
The protective cathodic current demand can only be ascertained under stable conditions, that is, on objects that have been cathodically protected over long peri- ods. If two cathodic protection stations are acting on the section to be measured, both stations must be switched off from time to time by using current interrupters. Be- sides the protection current density, the ohmic voltage drop in the soil at holidays must also be determined. From this the apparent coating resistance of the pipeline can be determined. This corresponds to the sum of the parallel grounding resistances of the holidays (see Section 5.2.1.2). By plotting the pipe current along its length, contacts with foreign lines can often be recognized (see Section 3.6 and Fig. 3-20).
The procedure for determining the protection current density and the average coating resistance is explained in Fig. 3-13. At the current drainage point, the cur- rent 2 70 is fed into the anode of the cathodic protection stations or an auxiliary ground electrode and periodically interrupted. With a symmetrical current distri- bution, the current 70 flows back from each side of the pipeline. On account of the minimal length resistance of welded pipelines with a good coating, the pipe/soil potential decreases very slowly. Mean values can be linearly approximated ac- cording to recommendations by NACE [33,34]. This applies especially if the test point spacings, /t, /2 and /3 are small in comparison with the length of the protected range, L. The current 7j, 72,73 ..., In flowing in the pipeline is measured at measur- ing stations with a spacing A / = 1 to 2 km, and the current that enters each section between test points is:
This entering current causes ohmic voltage drops in the soil at holidays in the pipe coating, which are specified as Af/j, A£/2, A£/3..., AUn from which the average value
is given for every measured length /. From this follows the specific coating resis- tance that corresponds to an average defect resistance according to Section 5.2.1.2:
where S is the surface area of the pipeline in the measured section. The sheathing resistance is not only determined by the size and number of holidays in the coating, but also by the specific soil resistivity.
Fig. 3-12 Current and voltage distribution for a DN 80 pipeline with current drainage at point / = 24 cm; ohmic voltage drop in the lower figure: upper surface of the pipe (—), middle ( —), underside (—).
Fig. 3-13 Determination of the protection current density and coating resistance of a pipeline (explanation in the text).