AMPP NACE SP0193 Overview

Why SP0193 Matters

Aboveground storage tank bottoms are difficult to inspect from the soil side after installation. SP0193-related concepts help guide CP design, monitoring, and evaluation for tank bottoms where external corrosion can occur.

Tank-bottom CP interpretation differs from pipeline CP interpretation. Fixed reference cells, tank pad conditions, anode layout, current distribution, and accessibility limitations must be considered.

The standard should not be cited from memory. Exact requirements and limitations must be checked in the official document.

Core Concepts Connected to SP0193

Tank-bottom protection

The protected surface is the underside of the tank bottom where it contacts soil, sand, concrete, or another electrolyte beneath the tank.

Fixed reference cells

Permanent reference electrodes are often installed beneath tank bottoms to provide repeatable monitoring locations. Their reliability must be evaluated because permanent cells can fail, drift, dry out, or become questionable.

100 mV polarization

Tank-bottom evaluations may use polarization-based methods where allowed by the applicable standard and project requirements. The test method and timing must be valid.

Current distribution

Current distribution beneath a tank is affected by anode layout, tank diameter, pad material, moisture, bottom contact, shielding, and foundation conditions.

Documentation

Tank CP reports should identify readings, reference cells, current sources, test conditions, interrupted sources, and limitations affecting the interpretation.

Field Application

SP0193 concepts are used during tank-bottom CP design, annual surveys, rectifier inspections, fixed reference cell testing, depolarization testing, and troubleshooting.

Tank status matters. Out-of-service tanks, repairs, intentionally de-energized CP systems, disconnected RMUs, and unavailable reference cells must be documented because they affect interpretation.

Common Mistakes

1. Applying pipeline assumptions to tank bottoms.
   Why it is wrong: Tank-bottom CP has different geometry, access limitations, and current distribution behavior.
2. Trusting fixed reference cells without evaluating reliability.
   Why it is wrong: Permanent reference cells can fail or drift over time.
3. Assuming rectifier output proves tank-bottom protection.
   Why it is wrong: Output does not prove current reached all areas beneath the tank.
4. Ignoring tank pad conditions.
   Why it is wrong: Moisture, sand, concrete, liners, and shielding affect CP current flow.
5. Quoting the standard from memory.
   Why it is wrong: Official language, exceptions, and applicability must be verified from the current standard.

Field Example

A tank has three fixed reference cells. Two show stable readings consistent with prior years, while one shifts sharply electropositive and differs from the remaining cells.

The abnormal cell may indicate a localized CP problem, but it may also indicate reference cell failure. The correct response is to evaluate the CP system, historical data, test setup, and reference cell reliability before making a conclusion.

Practice Questions

1. Why should SP0193 be consulted directly before citing requirements?
2. Why are tank-bottom CP measurements more difficult than many pipeline measurements?
3. Why can fixed reference cells become unreliable?
4. Why does rectifier output not prove complete tank-bottom protection?
5. Why must tank status be documented during a CP survey?

Related Pages

• CP for Aboveground Storage Tanks
• Reference Electrodes
• 100 mV Polarization Criterion
• Tank Bottom Surface Area
• 100 mV Polarization Criterion