Fundamentals and Practice of Electrical Measurements
3.3 Potential Measurement
3.3.2 Application of Potential Measurement
3.3.2.2 Potential Measurements on Pipelines and Storage Tanks
Errors can occur in potential measurements on pipelines and storage tanks in soil if foreign voltages such as, for example, ohmic voltage drops in soil, are not taken into account [15]. Figure 3-9 shows the potential curves for a single defect (spherical field) or for several statistically distributed defects (cylindrical field) in the pipeline coating. In general the on potential of the installation to be protected (e.g., a pipeline) is measured, with flowing protection current, against a reference
Fig. 3-6 Electrochemical depolarization after switching off the protection current as a function of the operating time of cathodic protection and the quality of the coating.
Fig. 3-8 Circuit for deter- mining the response time of measuring instruments with internal resistance R.
(explanation in the text).
Fig. 3-7 Electrochemical depolarization after switching off the protection cur- rent for different recording speeds (polarization of steel in artificial soil solution for 200 h).
electrode installed above it on the ground. This on potential contains ohmic volt- age components, according to the data in Section 3.3.1, which must be eliminated when determining the state of polarization. In the method used here, the current must be definitely changed, which is not possible with stray currents caused by dc railways.
For determining the off potentials of cathodically protected pipelines, time relays are built into the cathodic protection station to interrupt the protection cur- rent 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 synchro- nously. The switching of further protection stations can be synchronized in the same manner.
Other timing switch gear works with electronic digital clocks. The 1-s time interval derived from the mains frequency is counted in an integrating counter. The
Fig. 3-9 Ohmic voltage drop at defects in the pipe coating. (1) spherical field;
(2) voltage drop at the soil surface; (3) cylindrical field UIR (s) = voltage drop at the defect; UIR (d) = voltage drop at the grounding resistance.
switching on and switching off times can be set as desired. A completely synchro- nized switching of all protection systems can be effected by a transmitted impulse.
For example, the Bundespost transmitter (Mainflingen) emits the frequency 77.5 kHz on the long wave band. On this frequency a time signal is transmitted every sec- ond, minute, etc. For identification the impulse is omitted on the 59th second. This can be used for synchronizing the switching of cathodic protection stations.
It is recommended that all instruments be provided with an additional time switch, so that the interruption of the protection current supply outside working hours (i.e., during the night) is not maintained. In this way the reduction in the protection current supply is kept as small as possible; this amounts to 10% with 27 s on and 3 s off. This can be significant in the measurement of a long pipeline, which can often take several weeks.
In analyzing the results on a cathodically protected pipeline, the protection current density and coating resistances should be calculated for individual sections of the pipeline in addition to the on and off potentials, the pipe current, and the resistances at insulating points and between the casing and the pipeline. The results should be shown by potential plots to give a good summary [15] (see Fig. 3-20).
In the cathodic protection of storage tanks, potentials should be measured in at least three places, i.e., at each end and at the top of the cover [16]. Widely different polarized areas arise due to the small distance which is normally the case between the impressed current anodes and the tank. Since such tanks are often buried under asphalt, it is recommended that permanent reference electrodes or fixed measuring points (plastic tubes under valve boxes) be installed. These should be located in areas not easily accessible to the cathodic protection current, for example between two tanks or between the tank wall and foundations. Since storage tanks usually have several anodes located near the tank, equalizing currents can flow between the differently loaded anodes on switching off the protection system and thus fal- sify the potential measurement. In such cases the anodes should be separated.
3.3.2.3 Potential Measurement under the Influence of Stray Currents
Where stray currents are involved, several measurements have to be taken that are continually changing with time, simultaneously with each other. A double recorder is most suitable for this. Linear recorders with direct indication of the measurements cannot be used for potential measurements because the torque of the mechanism is too small to overcome the friction of the pen on the paper. Am- plified recorders or potentiometer recorders are used to record potentials. In ampli- fied recorders, as in amplified voltmeters, the measured signal is converted into a load-independent impressed current and transmitted to the measuring mechanism, which consists of a torque motor with a preamplifier. The amplifier results in an
increased torque in order to achieve a response time of 0.5 s with the necessary operating pressure of the recording pen. The energy requirement for amplified re- corders is about 3 W. Technical data on recorders are given in Table 3-2.
As shown in Fig. 3-10, in potentiometer recorders with a servomotor, the aux- iliary current Ik is supplied by a measuring bridge. The voltage to be measured Ux is compared with the compensation voltage Uk. The voltage difference is converted to an alternating voltage, amplified 106-fold and fed to the control winding of a servomotor. This moves the slider s of the measuring bridge to the left until the voltage difference Ux - Uk is zero. The position of the slider indicates the value of the voltage to be measured, which is compensated and indicated by the recorder.
Potentiometer recorders have a relatively short response time—up to 0.1 s—and a high accuracy of 2.5% of the maximum scale reading.
The chart movement of the recorder is driven by a synchronous motor with a rating of 2 to 5 W. With rapidly varying readings, a chart speed of 600 mm rr1 is needed, but for stray current fluctuations, usually 300 mm tr1 is sufficient. For recording over several hours, chart speeds of 120 or 100 mm tr1 are advantageous.
Optical evaluation of recorder traces is sufficient to obtain average values as well as extreme values important for corrosion protection measurements. In general, recordings at a test point are not made for more than 0.5 to 1 h. Occasional extreme values or potentials at night are not usually recorded. The frequency and length of time that the protection potential is below the required level can be determined with a limiting value counter.
Fig. 3-10 Principle of a compensation circuit for recording potentials (explanation in the text).
Where there are stray currents, the switching method described in Section 3.3.1 cannot be used. Stray current protection stations are usually installed where the pipeline has the most positive pipe/soil potential. When the stray current drainage is cut off, a too-positive stray current exit potential that is not 7/?-free is quickly established. In distant areas a too-negative stray current entry potential that is not IR-free will be measured. The determination of the 7/?-free pipe/soil potential is only possible in stray current areas when the origin of the stray current is not oper-
ating [17]. In order to avoid more positive potentials than the protection potential, the pipe/soil potential in stray current areas is set much more negative for safety reasons than in installations where there is no stray current. From recordings it can be established at which places the 7/?-free pipe/soil potential should be measured when the stray current source is not active. If the potential measured at these points is more negative than the protection potential, it can be assumed that there is suffi- cient cathodic protection. To estimate the 7/?-free potential of cathodically pro- tected pipelines during the operation of the stray current generator, the polarization state can be obtained with the help of potential test probes (see Section 3.3.3.2).
3.3.2.4 Potential Measurement under the Influence of Alternating Currents
In cathodically protected pipelines that are affected by high tension transmis- sion lines or electrified railway lines, the pipe/soil potential has superimposed on it an induced alternating voltage. This can considerably falsify the potential mea- surement if, for example, the induced alternating voltage of some 10 V at 50 or 162/3 Hz is superimposed on the measured voltage, which is on the order of 1 V [18]. The insensitivity to alternating voltage depends on the mechanism in ampli- fied voltmeters and on their circuitry. If it is insufficient, an RC filter must be included in the circuit. The magnitude of the resistance, /?, and capacitor, C, can be calculated to a sufficient approximation from the following equations [19]:
and
where R{ is the resistance of the measuring instrument, F is the allowable measur- ing error, A is the attenuation factor, R is the resistance of the RC filter, and/is the frequency.
Filters have a time constant i = Rv x C which increases the damping of the measuring instrument. The time constant depends on the required attenuation and the interfering frequency, but not on the internal resistance of the measuring in- strument. The time constants of the shielding filter are in the same range as those of the electrochemical polarization, so that errors in the off potential are increased.
Since the time constants of attenuation filters connected in tandem are added, but the attenuation factors are multiplied, it is better to have several small filters con- nected in series rather than one large filter.
In contrast to direct voltage, alternating voltage can be measured using a ground- ing rod as reference electrode. The grounding resistance of the rod is considerably lower than that of the reference electrodes in Table 3-1 but can still be too high for measurements with soft iron, moving iron or electrodynamic measuring instru- ments. It is therefore also recommended that amplified voltmeters or amplified recorders be used which have high internal resistances, great accuracy and a linear scale. Attention to frequency and shape of curve is important in the technique of ac measurements. In general, measuring instruments are gauged for effective values for 50 Hz and sine curves. They can therefore give false readings for diverse fre- quencies and waveforms (phase control). Measurement errors due to different wave- forms can be recognized by the fact that they give different results in different measurement regions.