V measured R shunt
5.5.2 Rectifier Current Output
5.5.2.2 Current Flow on a Pipeline or Cable
A length of pipeline, cable, or similar long, thin metal structure can serve as a shunt. Line current measurement is useful in determining the distribution of current along a cathodically protected structure, solving stray current problems, and in locating areas of poor coating or perhaps a short.
In this section we will explore two techniques for determining current flow along such a structure:
• 2-wire test points • 4-wire test points
Connections to the structure should be made by permanent test wires; probe rods are sometimes used on bare or poorly coated piping.
5.5.2.3 2-wire Line Current Test
Where 2-wire test points span a known length of a structure such as a pipeline and the diameter and wall thickness or the weight per foot are known, current flow across the span can be calculated. This is done by measuring the voltage drop across the span, determining the resistance of the span from a pipe table, and using Ohm’s Law as you would with a shunt. Figure 5.11 shows the test setup. Table 5.3 on page 20 provides some resistance values for common pipe sizes.
Figure 5.11 2-wire Line Current Test
Because the voltage drop across a pipe span is relatively small, with earlier instruments it was necessary to correct for the voltage drop in the test leads caused by the current drawn by the meter. However, with the high-input resistance meters available today, this correction is not necessary.
Using a high-input impedance meter, the procedure involves simply measuring the voltage drop between the test leads. For example, if the voltage drop across a 200-ft span of 30-in. (76.2 cm) pipe weighing 118.7 lbs./ft (176.65 kg/m) is 0.00017 V, then current flow is calculated as follows:
Pipe resistance/ft = 2.44 /ft = 0.00000244 /ft
Total resistance = 200 ft x 0.00000244 /ft = 0.000488
Pipe Span in Feet Wires must be color coded
0.17 mV
+ _
Pipe size and wall thickness or weight per foot must be known
Pipeline
N
West East
Pipe Span in Feet Wires must be color coded
0.17 mV
+ _
Pipe size and wall thickness or weight per foot must be known
Pipeline
N
Pipe Span in Feet Wires must be color coded
0.17 mV
+ _
Pipe size and wall thickness or weight per foot must be known
Pipeline Pipe Span in Feet
Wires must be color coded
0.17 mV
+ _
Pipe size and wall thickness or weight per foot must be known
Pipeline
N
Measured voltage drop = 0. 00017 V
Current (I) = E/R =0.00017 V/0.000488 = 348 mA or 0.348 A Refer again to Figure 5.11. Note the meter is showing a positive indication. This means the current is entering the meter on the positive terminal. The positive terminal is connected to the west end of the span. Since the meter is in parallel with the span, current flow on the pipe is from west to east.
The accuracy of this test method depends greatly on accurate knowledge of the dimensions of the pipe. Should there be an odd- sized joint within the span, or some appurtenance such as a valve, the calculated resistance will not be correct. The 4-wire test method overcomes these difficulties.
Table 5.3: Table of Pipe Resistances
Steel Pipe Resistance*
(A)(B)*Conversions: 1 in. = 2.54 cm 1 ft = 0.3048 m
(A) Based on steel density of 489 lbs/ft3 (7832 kg/m3) and steel resistivity of 18 microhm-cm.
(B) R = 16.061 x resistivity in microhm-cm = Resistance of 1 ft of Weight per foot pipe, microhms
5.5.2.4 4-wire Line Current Test
A 4-wire current test station can be used to determine the line current even if the dimensions of the pipeline are unknown or there are anomalies within the test span. Figure 5.12 shows the set-up.
Pipe Size
Outside Diameter
Wall Thickness Weight Resistance in. in. Cm in. cm lb / ft kg / m ohms/ft ohms/m
2 2.35 5.97 0.154 0.39 3.65 5.43 76.2 256.84 4 4.5 11.43 0.237 0.60 10.8 16.07 26.8 87.93 6 6.62 16.81 0.280 0.71 16.0 28.28 15.2 46.87 8 8.62 21.89 0.322 0.82 28.6 42.56 10.1 33.14 10 10.75 27.31 0.365 0.93 40.5 60.27 7.13 23.39 12 12.75 32.38 0.375 0.95 46.6 73.81 5.82 16.09 14 14.00 35.56 0.375 0.95 54.6 81.26 5.29 17.36 16 16.00 40.64 0.375 0.95 62.6 93.16 4.61 15.12 18 18.00 45.72 0.375 0.95 70.6 105.07 4.09 13.42 20 20.00 50.80 0.375 0.95 78.6 116.97 3.68 12.07 22 22.00 55.88 0.375 0.95 86.6 128.88 3.34 10.96 24 24.00 60.96 0.375 0.95 94.6 140.78 3.06 10.04 26 26.00 66.04 0.375 0.95 102.6 152.69 2.82 6.25 28 28.00 71.12 0.375 0.95 110.6 164.59 2.62 8.60 30 30.00 76.20 0.375 0.95 118.7 176.65 2.44 8.01 32 32.00 81.28 0.375 0.95 126.6 188.41 2.28 7.48 34 34.00 86.36 0.375 0.95 134.6 200.31 2.15 7.05 36 36.00 91.44 0.375 0.95 142.6 212.22 2.03 6.66
Figure 5.12 4-wire Line Current Test
The test procedure is as follows:
1. Calibrate the span by passing a known amount of battery current between the outside leads and measuring the change in voltage drop across the span (E) using the inside leads. Divide the current flow in amperes (I) by the change in voltage drop in millivolts to express the calibration factor (K) in “amperes per millivolt.” The calibration factor is calculated as follows:
K = I / E = I / E with current applied – E with no current applied
Normally, if the pipeline operating temperature is stable, this can be done only once as the calibration factor may be recorded for subsequent tests at the same location. On pipelines where the temperature of the pipe changes considerably (with accompanying changes in resistance), more frequent calibration may be necessary.
2. Measure the voltage drop in millivolts across the measuring span (without the battery current) using the inside potential measurement leads. This voltage drop is due to normal pipeline
Pipe Span for Measuring Current
Wires must be color coded 0.17 mV + _ Pipeline Power Source + _ Current Interrupter _ + AMPS VOLTS
3. Calculate current flow by multiplying the calibration factor by the voltage drop measured above:
I (A)= K (A/mV) x mV drop
4. Note the sign of the voltage drop to determine the direction of current flow. If the voltage drop reading is positive, then the direction of current flow is from the positive to the negative terminal of the voltmeter. If the reading is negative, then the direction of current flow is from the negative to the positive terminal.
Based on Figure 5.12 if 10 A of battery current passes from the east to the west outside leads, resulting in voltage drop change (E) of :
On = 5.08 mV Off = 0.17 mV E = 4.91 mV
Then the calibration factor will be:
Without test current applied, the voltage drop across the span between the inner leads is 0.17 mV. Therefore, current flow is:
Referring again to Figure 5.12, note that the positive lead of the digital meter is attached to the east inner lead. The meter indicates a positive display, showing that current is entering the meter on the positive terminal. Again, the meter is in parallel with the pipe span, so current flow on the pipe is from east to west.
mV A mV A or mV A K 2.04 / 17 . 0 08 . 5 10 91 . 4 10 mA 347 A 347 . 0 mV mV 17 . x A 04 . 2 drop mV x K I
Most digital meters will not read below 0.1 mV. If readings below 0.1 mV are anticipated, or if a zero reading is obtained during a test, a more sensitive meter must be used.