Engineers unfamiliar with cathodic protection (CP)requirements often overlook the need for CP on buried sumptanks during the design and construction phase of newfacilities. The owner may only discover the need for CP aftermechanical completion and commissioning of the site arecomplete. This scenario recently occurred in Canada, wherea buried sump tank was installed and commissioned withouta 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 constructedwith double-walled steel and a fusion-bonded epoxy (FBE)coating. Numerous flanged connections and the requirementfor electrical grounding of the sump motor make electricalisolation of sumps challenging. In addition, isolation isgenerally not possible where buried piles support the sump.
In these cases, the standard approach for cathodic protectionis a flood-style cathodic protection system that providescurrent to all facility structures. These flood style systemsdemand current capacities far above what is practical withsacrificial anodes in soil. The current requirement for thesubject facilities was estimated to be 20 amperes, with thesump requiring less than 100 milliamperes.
A client in Alberta, Canada retained Corrpro to providedesign and installation services for the protection of anin-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 thecoated sump while avoiding excessive current loss to nearbyfacilities, Corrpro designed and installed a distributedmagnesium anode system. The system wasinstalled using hydrovac excavation with anodes arranged ina polar array of five semi-deep vertical holes spaced roughly5 feet (1.5 m) from the tank shell with multiple anodes perhole.
A cathodic protection coupon with integrated zinc referenceelectrode was installed equidistant to the anodes on thefacilities side. The CP coupon represents a coating defecton the sump and helps to determine whether anode-sumpspacing is adequate to overcome the current loss to theelectrically continuous facilities.
The cathodic protection system was commissioned andallowed to polarize. Potential data obtained by Corrpro afterthe installation indicates that adequate CP was provided tothe sump despite the current loss to the facilities.