Distributed Sacrificial Anode System for In-Service Sump Tanks

Date:January 2018
Capability:   Cathodic Protection
CP for sump tank

Engineers unfamiliar with cathodic protection (CP) requirements often overlook the need for CP on buried sump tanks during the design and construction phase of new facilities. The owner may only discover the need for CP after mechanical completion and commissioning of the site are complete. This scenario recently occurred in Canada, where a buried sump tank was installed and commissioned without a cathodic protection system.

Below-grade sump tanks are typically 3 to 6 feet (1 to 2 m) in diameter by 6 feet (2 m) in height. They are constructed with double-walled steel and a fusion-bonded epoxy (FBE) coating. Numerous flanged connections and the requirement for electrical grounding of the sump motor make electrical isolation of sumps challenging. In addition, isolation is generally not possible where buried piles support the sump.

In these cases, the standard approach for cathodic protection is a flood-style cathodic protection system that provides current to all facility structures. These flood style systems demand current capacities far above what is practical with sacrificial anodes in soil. The current requirement for the subject facilities was estimated to be 20 amperes, with the sump requiring less than 100 milliamperes.

A client in Alberta, Canada retained Corrpro to provide design and installation services for the protection of an in-service sump tank. Design constraints included:

  • Provide adequate CP to the sump
  • No impressed current CP system
  • No mechanical excavation
  • No electrical isolation of sump
  • No ground disturbance within 3 feet (1 m) of sump

In order to provide adequate cathodic protection to the coated sump while avoiding excessive current loss to nearby facilities, Corrpro designed and installed a distributed magnesium anode system. The system was installed using hydrovac excavation with anodes arranged in a polar array of five semi-deep vertical holes spaced roughly 5 feet (1.5 m) from the tank shell with multiple anodes per hole.

A cathodic protection coupon with integrated zinc reference electrode was installed equidistant to the anodes on the facilities side. The CP coupon represents a coating defect on the sump and helps to determine whether anode-sump spacing is adequate to overcome the current loss to the electrically continuous facilities.

The cathodic protection system was commissioned and allowed to polarize. Potential data obtained by Corrpro after the installation indicates that adequate CP was provided to the sump despite the current loss to the facilities.

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