Volume-II of IV - GAIL

MECON LIMITED
Delhi
PROCESS & PIPING
DESIGN SECTION
TECHNICAL SPECIFICATION FOR
PERMANENT CATHODIC
PROTECTION SYSTEM
TECHNICAL SPECIFICATION NO. : MEC/TS/05/62/016B REV-1 Page 21 of 24
12.3 Wherever foreign pipelines which may or may not be protected by an independent CP
system run in parallel to the protected pipeline, either in the same trench or very near
to the protected pipeline and are not bonded to it, investigations shall be performed
for current discharge points on both the pipelines.
12.4 Mitigative measures shall be provided depending upon the type of interference. These
shall include installation of bond with variable resistor and diodes, installation of
galvanic anodes for auxiliary drainage of current, adjustment/ relocation (if possible)
of offending interference source, provision of electrical shield etc. depending on the
type of interference.
Bond with foreign pipelines / structures as a mitigation measure shall be provided
where the owners of the foreign pipelines / structures have no objection. Otherwise
alternative mitigatio measures shall be provided. Wherever bonding is provided for
mitigaiton, the bonding resistor shall be adjusted for optimum value for minimum / no
interference. Galvanic anodes installed as a mitigaiton measure shall be adequately
sized for the design life specified for permanent CP system.
12.5 Where high voltage (66 KV and above) transmission lines run in parallel or cross the
pipeline, the pipeline shall be grounded through polarisation cells at regular intervals
to ground any surge on the pipeline that would appear in case of transmission line
faults. Alternatively, grounding could also be done with galvanic anodes (Magnesium
or Zinc depending on pipeline voltage) at the discretion of the owner. These grounding
installation shall be provided wherever transmission lines run within 25 mtr. of the
pipeline.
A.C. voltage measurements shall also be made on the pipeline to find out continuous
induction of voltage beyond safe limits. In such a case, the pipeline shall be grounded
to ht enearest transmission galvanic (zinc or magnesium) anodes at the discretion of
the owner.
13.0 CLOSE INTERVAL POTENTIAL SURVEY
Where specified, contractor shall carry out a close internal potential survey over the
entire length of pipeline by computerised potential logging method and identify
suspected holiday spots on the pipeline coating, after the back filling has been
consolidated sufficiently and CP system has stabilized. Contractor shall identify the
necessary pipe coating required to be repaired. Detailed procedures for running this
survey shall be submitted for approval.
The close interval potential logging survey shall be carried out at approx. 0.75m
intervals & both “ON & “OFF” potentials shall be measured by operating the current
interrupters of CP TR/ ECU unit during survey
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