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The refinery is introducing a new crude blend with higher acidity and sulfur content. CDU and VDU units are expected to be affected due to CS/5Cr metallurgy. Learners must determine where to measure corrosion and which inspection methods to use to assess potential damage.
Select a topic to explore its main ideas, practical considerations, and subtopics.
Explain key features and consideration for two popular intrusive monitoring methods: Coupons and Electrical resistance (ER) corrosion probes.
Key point: Select the best fit for purpose intrusive monitoring method
Weight-loss corrosion coupons are removable metal specimens exposed to the process stream for a defined period. Their measured mass loss gives an average corrosion rate for the coupon material and exposure interval.
CR (mm/y) = 87.6 x mass loss (mg) / [density (g/cm3) x exposed area (cm2) x exposure time (h)]Select monitoring points from the corrosion-circuit review and the damage mechanism being assessed. Choose a representative, fully wetted location for the target stream; consider locations downstream of crude blending or mixing once the stream is representative, and locations associated with relevant temperature, flow, or phase changes. Add points where the expected damage distribution differs, rather than assuming one coupon represents an entire circuit. Avoid stagnant or poorly mixed locations unless they are the specific environment being investigated. Final locations, holders, insertion depth, and orientation must be approved for the actual process conditions and site procedures.
Electrical resistance (ER) probes estimate metal loss by measuring the change in electrical resistance of a probe element exposed to the process. They provide a time-based signal that can help identify changes in corrosive conditions between coupon retrievals.
Install the probe where the process is representative of the circuit and where the probe element sees the intended phase, temperature, and flow. Confirm the probe, access fitting, cable, transmitter, and data historian are suitable for the service and that the measurement location is protected from avoidable fouling or dead legs.
Screen the crude unit for locations where chemistry, temperature, velocity, metallurgy, and phase behavior combine to increase damage potential.
Key point: Prioritize representative measurement locations and high-consequence components, then validate the screening with inspection evidence.
Start with the crude and vacuum unit circuit review. Mark hot transfer lines, furnace outlets, atmospheric and vacuum fractionator sections, pump-arounds, side draws, and downstream locations where temperature, velocity, mixing, or phase behavior changes. Then rank components using expected damage severity, inspection history, metallurgy, and consequence of failure.
Place monitoring points where the stream is representative and where the damage mechanism is expected to be active. Include more than one point when conditions change materially along a circuit; do not extend a local result across the entire circuit without supporting evidence.
A new crude blend raises TAN and sulfur while the CDU and VDU continue to operate near the upper end of their temperature envelope. An ER probe shows a gradual increase, but the latest coupon result is only an average rate. Decide what evidence to review before changing the inspection plan.
The ER trend is an early warning, not a complete diagnosis. Review blend and operating history, confirm instrument health and point representativeness, compare coupon and thickness data, and check whether deposits or localized attack could make the local result more severe than the average rate suggests.
Recommended action: Validate the monitoring point and trend, then target follow-up thickness examination or coupon retrieval at the susceptible circuit locations before revising the inspection interval.
Use this checklist when a crude blend or operating change could alter corrosivity or when monitoring signals and inspection results do not agree.
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