1. Evaluation Techniques
CHAPTER 5
Part II
Survey Methods and
Evaluation Techniques

2. Typical Applications
There are several current measurements commonly made in cathodic protection surveys:
Current output of a galvanic anode system
Rectifier current output
Test current for determining current requirement of a structure
Current flow on a structure (this is a voltage measurement, and current is calculated)
Current flow across a bond

3. Galvanic Anode Output
An ammeter can be inserted directly into the galvanic anode circuit.
This is not difficult in a test station head where the anode and structure wires are connected by a shorting strap.
In many test stations, however, the connection is made with a split bolt connector and then taped.
Opening this connection takes time, and after testing, the connection must be made again.
As mentioned earlier, even the low resistance of the ammeter can cause the instrument to read a current lower than the actual anode output.

4. Galvanic Anode Output
Consequently, a shunt in the circuit is the preferred method of current measurement.
The voltmeter used for measuring current across the shunt in a galvanic anode installation needs to have a very low scale to measure small current outputs.
Full scale deflections as low as 2 mV is sometimes necessary.
With a shunt in place, the measurement can be made without having to open the circuit.
This not only saves time, but also yields a more accurate reading than that obtained with an ammeter.

5. Rectifier Current Output
Most rectifiers have a shunt on the panel.
Usually the shunt rating data are stamped on the shunt.
You can determine the rectifier current output by measuring the voltage drop across the shunt.
The reading can then be compared with that shown on the rectifier ammeter, this serves as a check of the accuracy of the rectifier meter.
Many rectifiers have large outputs, however, so it is necessary to be certain you have an ammeter capable of handling the current.
There is also a safety hazard in working with higher currents.
If you do test the output this way, be certain the rectifier is turned off when the circuit is opened and when it is closed again.

6. Current Requirement Tests
You may be asked to assist in making current requirement tests.
In these tests, a current is impressed on a structure to be protected and the potential changes brought about by that current are measured.
From the data, the amount of current required for protection can be determined.
The current in the test circuit can be determined by inserting an ammeter into the circuit or by the use of a shunt.
Here again, if an ammeter is used, it must have sufficient capacity to handle the test current.

7. 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.

8. 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. 1-igure 5.11 shows the test setup. Table 5.3
on page 20 provides some resistance values for common pipe sizes.

9. 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.

10. 2-wire Line Current Test
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 lbs./ft ( kg/m) is V,
then current flow is calculated as follows:
Pipe resistance/ft = 2.44 µΩ/ft = Ω /ft
Total resistance = 200ft x Ω /ft = Ω
Measured voltage drop = V
Current (I) = E/R = V/ = 348 rnA or A
Note as shown in the sketch, the meter is showing a positive indication.
This means the current is entering the meter on the positive terminal.

11. 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.

12. 4-wire Line Current Test
The test procedure is as follows:
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/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.

13. 4-wire Line Current Test
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 current.
Calculate current flow by multiplying the calibration factor by the voltage drop measured above:
I (A) = K (A/mV) x mV drop
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.

14. 4-wire Line Current Test

15. 4-wire Line Current Test
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.
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.

16. Bond Current
Resistance bonds (cables) are placed between structures to connect them for cathodic protection or to drain a stray current back to its source.
Bonds usually have a shunt in them to permit measurement of the magnitude and direction of current flow.
The procedure for reading these shunts is the same as discussed earlier in the section on "Shunts."

17. Measuring Resistance Typical Measurements
There are several types of resistance measurements made in corrosion control work.
Among these are:
across isolating fittings
casing-to-pipe
structure continuity
structure-to-structure
structure-to-anode
structure-to-earth
anode-to-earth .
Measurements are made using Ohm's Law and sometime an ohmmeter.
These two methods are discussed below.

18. Using Ohm's Law
Using Ohm's Law is one of the best ways to determine resistance between structures buried or immersed in an electrolyte.
This procedure has been discussed in relation to the 2-wire method of measuring current flow on a pipeline.
It will be used again in measuring resistance across an isolating fitting.
Under this application, a known current and voltage are used and the resistance is calculated.

19. Using an Ohmmeter An ohmmeter is the resistance meter in a multimeter.
The meter determines resistance by providing a small current or voltage across an internal resistor and comparing its value to the external resistor.
The test voltage of the meter is DC voltage and is suitable for metallic elements.
This instrument is not suitable for electrochemical paths because the DC voltage causes polarization and a change in resistance.
Using an ohmmeter to check effectiveness of an isolation joint in service is not reliable because of the parallel resistance paths through the soil as illustrated in the sketch.

20. Using an Ohmmeter

21. Electrical Continuity
All parts of a structure receiving cathodic protection from a single source, galvanic anodes or impressed current, must have electrical (metal) continuity.
Welded structures by nature of the fabrication have electrical continuity.
If electrical continuity does not exist, bonds (cables) or some other means must be used to establish continuity.
Among items requiring bonds are:
couplings and other compression fittings, screw joints, bell and spigot joints, lap joint flanges, pile groups, and reinforcing steel in concrete.
Heat exchanger water boxes and tube sheets are often protected by anodes in the water box.
The water box is connected to the exchanger shell through hinges and bolts.
It is important that electrical continuity between the water box and the shell be maintained in order to protect the tube sheet.

22. Electrical Isolation Purpose and Usage
The purpose of electrical isolation is to confine the protective current to the structure being protected.
If, for example, a production well casing is being protected, and it is electrically connected to unprotected structures such as gathering or other pipe, building grounds, or other underground structures, some of the protective current will be lost to those structures.
As another example, when galvanic anodes are used to protect well-coated piping or tanks, loss of current to other structures often means that the piping or tanks do not receive adequate current for protection.
Isolating fittings may also be used for stray current control.
Isolation strategically placed in a piping network, for example, can increase the longitudinal resistance sufficiently to minimize the pick up of stray current.

23. Electrical Isolation Purpose and Usage
In heat exchangers where the tube sheet is perhaps Monel metal and the water box is cast iron, it is often desirable to protect only the water box.
This can be achieved by electrically isolating the water box from the shell.
There are times when it may not be desirable or even practical to isolate protected from unprotected structures.
Examples are refineries, industrial plants, large tank farms, and similar complex facilities.
There are many connections to the underground structures in such places and each would require a dielectric fitting.
Maintenance of such a large number of fittings becomes prohibitive; also, one short circuit could cause loss of protection on a large number of structures.

24. Electrical Isolation Purpose and Usage
Occasionally it is desirable to provide joint cathodic protection for a number of pipelines belonging to different companies, usually all running in the same right of way.
In cases like this, the pipelines can be connected together and protected by a series of rectifiers, with each company assuming responsibility for a portion of the system.
Isolation (Insulating) Joints
Commercial fittings available for providing electrical isolation include:
Flanges, couplings, unions, monolithic isolating pipe joints, nonmetallic pipe and structural members, and swivels (meter sets).

25. Accidental Contacts
Some of the locations where electrical isolation can be compromised are:
Crossing structures.
Attachments.
Casings.
Grounds.
Methods for testing an isolation fitting include:
Measure the pipe-to-soil potential on each side of the isolation fitting with the reference electrode in a stationary position.
If the difference in potential is approximately 100 mV or greater, the isolation fitting is effective.
If less than 100 m V, further testing may be necessary.

26. Accidental Contacts Methods for testing an isolation fitting include:
If the cathodic protection current can be interrupted "on" and "off" pipe-to-soil potentials can be recorded on each side of the isolation fitting with the reference cell in the same location.
Similar "on" and "off" potentials on opposite sides of the fitting indicate a short.
If the "on" potential was depressed on the side of the isolation away from the current drain, the fitting is effective.
If there is no cathodic protection current to interrupt or if it is difficult to do so, a temporary current drain could be established and "on" and "off" readings recorded as described above.

27. Testing Resistance Between a Pipe and Casing
The procedure is the same as used to test the resistance across an isolating fitting.
The test current is set up between the pipe wires and those on the casing.
If the casing has only one wire, the vent is used as the other one.

28. Measuring Structure Continuity
There are several ways to measure structure continuity.
We will discuss only the fixed cell, moving ground method.
In this test, one reference electrode is placed at a fixed location and the positive lead from the multimeter is placed in contact with various parts of the structure.
If essentially the same structure-to-electrolyte potential is read from each contact continuity is indicated.
It is possible that an isolated portion of the structure could have the same potential as the rest of the structure.
If you suspect this is the case, other testing, beyond the scope of this course, will be required.

29. Diode Bias This is a multimeter operated in the diode bias mode.
A functioning diode will typically display a meter value from 0.3 V to 0.9 V in the forward bias; positive lead to anode, negative lead to cathode.
In the reverse bias condition, positive lead to cathode and negative lead to anode, a functioning diode will display "OL" (overload or out of limits).
For shorted diodes, the meter will display some low voltage value in
both toward and reverse bias configuration.
In the case of an open circuit diode, the meter will display "OL‘’ in both forward and reverse bias.
To correctly verify diode operation, at least one lead must be disconnected from the circuit.
Diodes cannot be properly checked while in the circuit or with power on.