Module 4: Crude Corrosivity - Monitoring | Hugo Docsy Training
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Module 4: Crude Corrosivity - Monitoring

Master intrusive and non-intrusive thickness tracking to detect sulfidation and naphthenic acid attack before containment breach.

The 2:00 AM Operational Alert

Plant crude slate shifts to a heavy opportunity blend (high TAN, high S). Skin temperatures in the atmospheric tower bottom circuit hit 380°C. An Electrical Resistance (ER) probe signals sudden wall growth while manual UT shows metal loss. Which instrument do you trust?

1. Choose Your Monitoring Technology

Select a technology tab to inspect operational pros, cons, and exposure formulas.

Intrusive • Gravimetric

Weight-Loss Corrosion Coupons

Standard reference coupons exposed directly to high-temperature crude stream inside a retractable holder.

Exposition Time (h) = 50 to 2000 / Expected Rate (mpy)
✅ Advantages
  • Direct physical measurement of mass loss
  • Allows microstructural examination of sulfide scale
  • Inexpensive initial setup
⚠️ Operational Constraints
  • Slow response (weeks or months of exposure)
  • Requires line pressure depressurization or insertion valve
  • Up to 50% error on short exposure intervals
Interactive Diagnostic Case Study

The "Metal Gain" Mystery

Live DCS Anomaly

At day 15 of running heavy sour crude, your ER probe data spikes upward, indicating remaining metal thickness is increasing. Examine the live plot simulator below and use progressive diagnostic hints to uncover the physical root cause.

Tag: ER-PROBE-401A (Atmospheric Tower Bottoms)

Engineering Diagnostic Clues:

Hint 1: Check Operating Temperature & Crude Chemistry
Process temperature is steady at 375°C. Total Sulfur is 2.1 wt%, and TAN is 1.8 mg KOH/g. At these conditions, high-temperature sulfidation forms massive Iron Sulfide ($FeS$) corrosion scale layer on steel surfaces.
Hint 2: Electrical Resistance Measurement Physics
ER probes calculate thickness reduction based on increased electrical resistance. If a deposit layer forms on top of the sensing element that is electrically conductive, what happens to total measured electrical resistance?
Reveal Engineering Root Cause Solution

Root Cause: Conductive Iron Sulfide ($FeS$) Scale Bridging

In high-temperature crude lines, sulfidation creates an outer layer of semi-conductive iron sulfide ($FeS$). As this scale builds up across the probe element, it provides a parallel electrical pathway.

This lowers the net measured resistance, causing the transmitter algorithm to falsely calculate an increase in metal thickness ("Metal Gain").

Action Required: Cross-verify wall thickness using non-intrusive high-temp Ultrasonic Testing (UT) before adjusting inhibitor dosage.

2. Field Verification Checklist: UT Sensor Mounting

Ensure proper procedures are executed before signing off an on-line UT installation.

0 of 5 Completed

3. Knowledge Check & Mastery Assessment

Score: 0 / 2

Q1: Why are weight-loss corrosion coupons rarely relied upon as the sole monitoring technique for high-temp crude corrosivity?

Q2: What primary non-intrusive technology combination provides both point thickness trend data and pipe wall profiling?

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Unlock Advanced Refinery Failure Case Studies

Gain instant access to downloadable ASME/API rate calculators, automated ER-trend smoothing scripts, and Module 5: Amine Unit Cracking & High-Temp Sulfidation Models.