Isolating couplings or flanges for pipelines need to be suitable for the operating conditions encountered on the particular installation, e.g. pressure and temperature, and the product.


Isolating joints


The isolating joint shall be welded into the pipeline at the specified location so that the joint is not subject to bending moments and stress.

Welding shall be carried out so that the heat generated does not damage the insulating material. Manufacturer’s instructions with respect to welding shall be strictly followed.

Testing prior to installation

Following delivery of the isolating joint, the Contractor shall measure the electrical resistance with a 1000 V insulation tester and the resistance shall be greater than 1 MW in air.

Testing After installation

  • After installation and with both sides of the joint earthed, the joint shall be tested by applying a DC current in the pipeline on one side of the joint. This may be achieved by the use of a permanent cathodic protection system or by a temporary groundbed and DC supply.
  • A Swain meter shall be used to confirm zero current flow across the isolating joint.
  • As an alternative to this test a potential measurement may be performed. Upon applying the current to one side of the joint, the ON/OFF potentials on both sides shall be recorded.
  • Test results shall be reported to the Principal for review and acceptance.
  • If more than one isolating device is installed in parallel, or when earthing or instrument cables are installed in parallel with the isolating joint, the failure may also be caused by another device and the test using the “Swain” type meter is then required to locate the fault.


Isolating joints are factory assembled and repairs shall not be performed by the Contractor. If a unit is found to be faulty, it shall be replaced.

Isolating Flanges


  • Isolating flanges shall be welded into the pipe at the required locations specifically for the purpose of cathodic protection isolation. Isolating flanges that may have to be dismantled for the servicing of other equipment (valves, spading off) are not recommended.
  • For new construction, isolating flanges shall be assembled before being welded into the pipeline or piping system to ensure correct alignment and to avoid stresses during installation. Before installation, the assembled isolating flange set shall be tested in accordance with the procedure for isolating joints and the resistance shall be at least 1 MW.
  • When insulation is installed on existing flanges, it is essential for the bolt holes and flanges to be exactly aligned to prevent damage to the insulating material. Oversized bolts or dowel pins shall be fitted at equal spacing around the flange to maintain alignment during installation.
  • When insulating existing flanges that are not exactly aligned, smaller diameter bolts of a higher strength shall not be used unless approved by the Principal.
  • Approved gaskets designed for isolating flanges shall be installed without the use of graphite filled lubricants or greases. Information concerning the use and application of gaskets.
  • The insulating sleeves shall be inserted into the bolt holes and checked for length and clearance. The bolts shall then be passed through a steel washer, an insulating washer and then the sleeve in the flange. Insulating washers shall then be fitted over the bolts next to the opposite flange, then a steel washer and the nuts shall be installed.
  • The bolts shall be tightened up so that an even stress is maintained on the flange faces.
  • Isolating flanges inside bunded and other designated hazardous areas, with the exception of flanges used in H2S service, shall be profiled with moulding putty and wrapped with self-adhesive reinforced bitumen/PVC laminate tape. The tape shall be suitable for the expected service temperature and shall be resistant to degradation by ultra violet radiation.
  • Isolating flanges shall be painted with non-conductive paint or shall be tape wrapped using a standard contrasting colour for easy identification.

Testing after installation

  • After installation, an isolating flange shall be tested similarly to an isolating joint.
  • If an isolating flange is found to be defective, the bolt to flange resistance shall be measured in dry condition using a 500 V insulation tester. If the resistance between any bolt and the flange is less than 100 kW the bolt shall be marked and the insulation repaired.
  • If all bolt to flange resistances are in excess of 100 kW and the flange has not been accepted due to the test results, the gasket shall be replaced.


Bolts showing low resistance to flange shall be removed and the insulation sleeves and washers replaced. Should the gasket require replacement the flange shall be dismantled and reassembled using new insulating materials.

When existing isolating flanges are repaired, bolts shall only be removed from, and replaced in, a line in service if arrangements cannot be made to shut down the line, and in addition the following conditions are met:

  • the flange has eight bolts or more;
  • prior approval shall be obtained from the responsible operations department using the required procedure;
  • bolts shall only be removed and replaced one at a time;
  • bolts shall be correctly tightened after replacement;
  • the line shall not be subject to pulsating pressure;
  • the line shall not be blocked off and subject to a static pressure exceeding the maximum operating pressure.

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