Isolation Joints
Isolation joints create an intentional boundary in the metallic path so cathodic protection current stays on the structure it is intended to protect.
Quick Definition
An isolation joint is a dielectric separation between two metallic systems. Common examples include insulating flange kits and factory-fabricated monolithic isolation joints.
Why Isolation Is Used
A buried pipeline may need CP current, while the piping inside a pumphouse, meter station, or facility may be electrically grounded for safety. If those systems are metallically continuous, the grounded side can become a large unintended current drain.
Isolation does not make the facility safer by itself and it does not replace grounding. It separates the CP design problem from the facility grounding problem so each can be handled correctly.
What Can Defeat Isolation?
- failed or contaminated insulating gasket material
- incorrectly installed flange hardware
- bonding cables or instrument tubing around the joint
- pipe supports or structural contact
- OVPs, SSDs, polarization cells, or grounding devices that are conducting
- water, debris, or conductive contamination around the isolation assembly
System Behavior
The test question is not only “is the flange isolated?” The better question is: where can current flow right now? A pipeline may have a good insulating gasket and still be bypassed through another conductive path.