Summary
Cathodic protection of hot tank bottom tanks requires specific design parameters to ensure an adequate current density and distribution for a sufficient level of polarization to give the right protection against corrosion.
This paper presents the design, installation details and commissioning results for two hot sulphur tank cathodic protection using impressed current titanium / Mixed Metal Oxide ribbon anode grid system.
1 Introduction
The system of protection which was designed and installed for the external bottom steel surfaces of the hot sulphur tanks is of the impressed current type and comprises a transformer rectifier to provide d.c. current to a grid of conductor bars and mixed metal oxide ribbon anodes installed in the tank foundation between an HDPE containment layer and the tank bottom. Ribbon anodes are of titanium strip coated with a MMO catalyst. Conductor bars are titanium of larger cross-section than the anodes and are used to reduce the voltage drop along the ribbon anodes and to ensure a uniform spread of protection current over the whole tank bottom area.
2 Sulphur Tank Details
The two sulphur tanks are of the vertical, floating roof type. The tanks are 21 m in diameter and 9,5 m in height for a total capacity of 2780 m3 and are based on concrete ring foundations. Tank operating temperature is 135 °C to maintain the sulphur in liquid phase.
3 Cathodic Protection System Design
The design of the tank bottom cathodic protection system is designed to ensure a tank bottom negative (cathodic) “Off potential” of at least -0.85 V measured between the structure steel surface and a saturated Copper/Copper sulphate (Cu/CuSO4) reference electrode in contact with the electrolyte surrounding the structure. Negative potential is to be limited to -1.2 V “Off” (Cu/CuSO4) to prevent over voltage protection. The current density used for tank bottom cathodic protection system calculation and sizing is 35mA/m2 based on similar cases feedback. No coating was applied to the tank bottom external surface. The cathodic protection system design life considered for external tank bottoms is 30 years as minimum. The sand resistivity used for cathodic protection calculation is 500 Om.
