EFFECTIVENESS OF CATHODIC PROTECTION FOR TANK SOIL SIDE AND TANK FOUNDATION BACKFILLING MATERIAL ELECTRICAL CONDUCTIVITY

EFFECTIVENESS OF CATHODIC PROTECTION FOR TANK SOIL SIDE AND

TANK FOUNDATION BACKFILLING MATERIAL ELECTRICAL CONDUCTIVITY

ABSTRACT

• Above ground storage tank bottom plate Soil-side corrosion is one of the main causes for tank deterioration and environmental impact leak, this costs a lot for corrosion mitigation, monitoring , tank outage for inspection ,repair and bottom plate replacement.
• Severe corrosion has been found on several tanks’ bottoms plate soil side at petrochemical Companies. Corrosion led to the costly outage and repair of tanks bottom plates ;however, tanks were provided with impressed current (ICCP )cathodic protection systems .
• Tanks designed and constructed on liquid asphalt/sand mix backfilling and provided with impressed current cathodic protection system with cast iron canister distributed anodes around the tank concrete wall ring. Tank bottom plate soil side found with sever corrosion due to the entrapment of the contaminated rain water.
• However Cathodic protection system operated at the design protection current and the measured tank to soil potential meets the protection criteria, cathodic protection current found picked up by the under-ground fire water piping network which connected to tank shell and the measured potential was for the under-ground piping.
• Effectiveness of ICCP cathodic protection system for above ground storage tank soil side corrosion mitigation depends on the design of tank foundation and electrical conductivity of the backfilling material under the tank .
• The measured Pipelines and cables current (amount and directions) revealed that cathodic protection current picked up by underground piping network and raised the measured negative potential,
• Tank bottom plate soil side did not receive sufficient CP current (isolated by the asphalt layer) and the measured potential reflect the potential of the underground piping which picked up the CP current.?

Introduction

This Document evaluates the effectiveness of tank bottom plate soil side cathodic protection systems for Tanks Designed and constructed on liquid asphalt/sand mix backfilling material.

Soil-side corrosion of the above ground storage tank bottom plates is a major concern and maintenance issue. Above ground storage tanks are typically supported by concrete ring walls.

It had been a common practice for most petrochemical Companies to construct the above ground storage tank on liquid asphalt/sand mix or use impregnated asphalt boards under the tank bottom plate which restricted by current API 650 and API 651 standards.

Due to tank bottom plate to foundation sealing aging and damages, contaminated rain water ingress under the tank and trapped for long time as it is encapsulated in the gap between bottom plate metal and asphalt (water evaporate and condensate on the steel surface for long time as asphalt will not absorb water)

The tank bottoms are typically bare steel and supported by a liquid asphalt/sand mix. The asphalt /sand mix is an electrical insulator.

Cathodic protection Pre design drain current was misleading as the provided DC current picked up by underground fire water piping network and the measured potential raise was for the pipelines not for the isolated tank bottom plate.

CP Annual survey records the potential of underground third party facilities (fire water piping) . As Tanks, pipelines and earthing cables found electrically shorted with other facilities.

Tanks MFL scan reports reviled sever spot tank bottom plate soil side corrosion and spot metal loss and many patch repairs installed as per API 563 requirement. The calculated short term corrosion rate found very high approached 0.45 mm/year.

pipeline and earthing cables current measurement carried out by using swan meter and DC clamp meters?to clarify the stray current to other assets , continuity and isolation of the associated pipelines and cables from other facilities.

Tank bottom plate’s soil side found electrically isolated from the cathodic protection anodes by the installed 100 mm thickness asphalt/ sand mix and did not receive sufficient cathodic protection.

Back ground

Tanks and Cathodic Protection systems data

The above listed tanks designed and constructed about 25 years ago.

Tanks designed and constructed on asphalt/sand mix backfilling as this was the corrosion mitigation method during the old days which is currently restricted by NACE and API 651 standards.

Cathodic protection systems with distributed canister anodes designed and installed for these tanks, however these systems are not effective as the tank bottom plates?are electrically isolated by the asphalt /sand mix backfilling.

Cathodic Protection System schematic.

Cathodic Protection Circuit Soft Ware Modelling and Simulation

API 651 Cathodic Protection of Aboveground Petroleum Storage Tanks

5.1.4 Limitations of External Cathodic Protection

Cathodic protection is an effective means of corrosion control only if it is possible to pass electrical current between the anode and cathode (tank bottom).

Many factors can either reduce or eliminate the flow of electrical current and, therefore, may limit the effectiveness of cathodic protection in some cases or preclude its use in others. Such factors include:

a) tank pads such as concrete;

b) a non-conductive external liner between the tank bottom and anodes (unless anodes are installed between liner and tank bottom);

c) high resistivity soil or coarse/large rock aggregate pads;

d) old aboveground storage tank bottoms left in place when a new bottom is installed.

These and other related factors are discussed in more detail in 5.3 and 5.4. It should be recognized that external

5.3.5 Oiled Sand Pad

Historically, in some cases oil has been added to the sand for various reasons, including compaction and corrosion control. However, if cathodic protection is applied, the higher resistivity of oiled sand may prevent it from being effective. For new tank construction, use of oiled sand is discouraged.

Field measurements by Swain meter and DC clamp meter.

Analysis and conclusion

From tank foundation drawings , tank bottom plate constructed on 100 mm thickness liquid asphalt/sand mix.

The measured on& instantant-off tank to soil potential showing that thank meet the protection criteria of -850 mV w.r.t CSE

Cathodic protection system design violated both NACE and API 651 strand requirement as Tanks constructed on asphalt/sand mix backfilling, which electrically isolate the tank bottom plate from the CP system anodes current.

Transformer rectifier CP current found picked up by underground piping (third party?facility) and tank bottom plate soil side did not receive any cathodic protection.

Cathodic Protection current returning through fire water pipe line metal may cause pipelines internal corrosion at high resistance flange joints if internal coating was deteriorated?(fire water/service water is a conductive media)?

Recommendations

New design of above ground storage tank shall not use oily sand or asphalt under tank bottom pate .RBI strategy shall consider these tanks bottom plate soil side without any cathodic protection.

New cathodic protection system based on NACE and API 651 standard requirement along with HDPE release preventive barrier and grid anodes shall be provided for each tank during bottom plate replacement when it is required (based on asset life assessment and RBI study).

Another corrosion mitigation method as VPCI (vapor phase corrosion inhibitor along with corrosion monitoring probes) can be used to extend tank bottom plate remaining service life.

REFERENCES?

API 650 Welded Tanks for Oil Storage

API 653 Aboveground Storage Tank Inspector

API 651 Cathodic Protection of Aboveground Petroleum Storage Tanks

SP0193-2016-SG (formerly RP0193) External Cathodic Protection of On-Grade Carbon Steel Storage Tank Bottoms.