Resistance check on insulation joints

Electrical isolation of one part of a cathodic protected structure from another may be required for a variety of reasons. However the common reason for isolation is to isolate a well coated cathodically protected structure from an uncoated or poorly coated area or structure to which it is connected.
Isolation joints or insulating flanges will be encountered on cathodically protected pipelines. They are used where it is necessary to electrically isolate the underground pipework from pipework etc. which adversely affects the cathodic protection system.
Isolation joints may typically be found at the following locations :
- Above ground installations, e.g. pump stations
- Storage tanks
- Electrical earths
- Between dissimilar metals
- Between sacrificial anode an impressed current systems

Principle of survey

There are two methods of resistance check on insulation joints which can be preformed : the potential change test and the resistance test.

The potential change test
By the use of different potential measurements on both sides of the isolation joint, the joint can be checked on its accurate isolation capacity.

The resistance test
This method is usually used to check insulating flanges, because each part of the insulating connection can be checked individually e.g. each bold separately on its isolation capacity.
By the use of a low resistance Ohmmeter the resistance of each part of the insulating flange is checked.

Site equipment

The potential change test
• Microprocessor controlled data collector/voltmeter with a minimum input impedance of 10 M and an adequate level of AC rejection
• Calibrated Cu/CuSO4 reference electrode
• Current interrupters for switching DC power sources

The resistance check
Low resistance Ohmmeter

Survey procedure

The potential change test
Case 1 : Isolation of a CP-protected structure and a non-CP-protected structure

Potential measurements are carried out on both sides of the isolation joint with the Cu/CuSO4 reference electrode kept on the same place to rule out potential measurements errors because a position change of the reference electrode will measure a different area of the pipeline.

Case 2 : Structure on both sides of isolation are CP-protected by two different systems

To check the isolation capacity of the isolation joint in this case a serie of potential measurements should be carried out with the reference electrode placed alternating above the structure on each side of the isolation joint.
The difference between each measured potential gives an inside of the protection level of the isolation joint.

When in both cases the same potential is measured at each measurement, a cross check should be carried out by switching the CP-current on structure of one side of the isolation joint. By a signal check on the structure on the other side of the isolation joint a bad isolation can be ruled out.

The resistance check

By the use of a low resistance Ohmmeter, which puts a direct current between the two poles, a potential difference is measured between both poles. According to Ohm law the resistance can be calculated. When the poles are placed on the opposite sides of the isolation joint the resistance between both sides. When the poles are placed on particular places on a insulating flanges (e.g. bolts) each part of the flange can be checked, which contributes to a well directed repair.

Data obtained

The potential change test

In both cases a table is presented with the measured potential values. All measurements specification are noted in the same table for each isolation joint.
If a current interrupter is used the on and off potential are noted and the difference between both is calculated.

The resistance check

The resistance measurement are noted in a record data sheet, which also contains the structure specifications and isolation joint location. The resistance measurements measured on each part the insulating flange is noted in the same table.

Processing and presentation of data

Raw data is transferred to computer on completion of each day and the data is checked to verify accuracy and that the equipment was fully functional and to identify any measurement which may require immediate re-survey.

Data collected is normally presented in tabular form.

Manpower and vehicles

A survey could be undertaken by one operator. Only one vehicle is needed to perform this service.

Advantages of the potential change

• Provides an insight of the protective resistance level of a isolation joint on the pipeline
• Quick and accurate method to check isolation joint
• Provides a guidance for future operation and maintenance
• Can easily be combined with GPS-mapping of the isolation joints.

Advantages of the resistance check

• Provides an insight of the protective resitance level of a isolation joint on the pipeline
• Quick and accurate method to check each part of a insulating flange
• Provides a guidance for future operation and maintenance of the insulating flange
• Can easily be combined with GPS-mapping of the insulating flange.

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