DCVG DC Voltage Gradient Equipment

John Mulvany & Dr.John Leeds Experimenting During The Initial  Development And Testing Of The DC Voltage Gradient Technique In The  Australian Desert In The Early 1980’s 

The DCVG technique is one of the two most important inspection tools used in the ECDA concept  to inspect buried pipelines. The other is CIPS. Both DCVG and CIPS utilise the pipelines CP  system, one of the two corrosion mitigation techniques applied to the outside of pipelines. The  NACE SP 0502–2008 which is a virtual reissue if the original 02 document perpetuates gross  errors in the way DCVG is classed the same as ACVG. It is NOT the same. See Page 3 for a list  of Coating Fault properties and Information Sheet 7 for limitations of AC and EM techniques. 

CIPS and DCVG are the only two inspection methods that assess the effectiveness of a pipeline  Cathodic Protection system.

Second To None. Predominant (29 YEARS) Expertise  In DCVG Technology – “We Lead Others Copy” 

DCVG was first was discovered by John Mulvany an Australian ex Telecom Corrosion Engineer,  who worked extensively with Dr.Leeds of DCVG Ltd. in developing the technique and its  application. This gives DCVG Ltd. an unparalleled 29 years experience in DCVG Technology.  This expertise backed by the excavation and inspection of several thousand DCVG fault  indications, plus the interpretation of DCVG electrical data to what is found during excavation is  passed onto users of our DCVG Equipment. Technology transfer is via detailed Training Courses  and ongoing e mail Technical Support. Finding coating faults is not difficult but the correct  interpretation of survey data to identify what coating faults to cost effectively repair requires  experience and the type of expertise that is available from DCVG Ltd. 

Principle of the DC Voltage Gradient Technique 

In Cathodic Protection when current flows through the resistive soil to the bare steel exposed at  faults in the protective coating, a voltage gradient is generated in the soil. The larger the fault the  greater the current flow and hence voltage gradient and this is utilised in the technique to prioritise  faults for repair. The voltage gradient is observed by measuring the out of balance between two  copper sulphate electrodes utilising a specially designed milli-voltmeter. When the two electrodes  are placed 1.5 meters apart on the soil in the voltage gradient from a coating fault, one electrode  adopts a more positive potential than the other which allows the direction of current flow to be  established and fault to be located. To simplify the fault location interpretation, the applied CP is  separated from all other DC influences such as Tellurics, DC Traction etc, by pulsing the local  CP ON and OFF in an unsymmetrical fashion. The pulsing DC can be from the pipeline CP system  itself, or from an independent source such as a portable DC generator or batteries utilising a  temporary groundbed and impressed on top of a pipelines existing CP system. 

In surveying a pipeline, the operator walks the pipeline route testing for a pulsing voltage gradient at  regular intervals. As a fault is approached, the surveyor will observe the milli-voltmeter needle begin  to respond to the pulse, pointing in the direction of current flow  that is towards the fault.

When the fault is passed the needle direction completely reverses  and slowly decreases as the surveyor moves away from the fault.  By retracing to the fault, a position of the electrodes can be found  where the needle shows no deflection in either direction (a null).  The fault is then sited midway between the two electrodes. This  procedure is then repeated at right angles to the first set of  observations, and where the two midway positions cross is the  epicentre of the voltage gradient. This is usually directly above  the coating fault.  

In order to determine various characteristics about a fault, such as severity, shape, corrosion  behaviour, etc, various electrical measurements around the epicentre and from epicentre to earth  are made for interpretation.  

The beauty of the DCVG technique is that it utilises the existing pipeline CP system at its normal  setting wherever possible and hence results reflect the intimate interaction between the protective  coating degradation and the effectiveness of CP at individual coating faults. This is very powerful  information in the fight against corrosion. No AC or Electromagnetic Technique offers such a direct  study capability. 

The DCVG Technique can be applied in City Streets, Process Plant and Refineries, across Rivers  and Estuaries, Swamps, Parallel Pipeline systems, to Gas, Oil, Chemical and Water Pipelines.  Also DCVG can be used to evaluate the protective coating on Telephone and Power Cables.

The detailed information provided by DCVG Technology when analysed with other information  such as Pipe to Soil Potential, Soil Resistivity and Composition, pH, Temperature, Operational  History, Inline Inspection Tool Data, etc through a program such as DCVG Ltd’s ECDA program  enables pipeline rehabilitation requirements for the Steel, Coating and CP to be prioritised for the  most cost effective repair.