Corrosion monitoring improvements

Case study examines prediction, monitoring and mitigation of corrosion-related effects at a West European refinery operating in a high temperature, naphthenic acid environment, and shows how metal loss was prevented when processing heavy acidic crudes.

Richard E Mathers
Nalco Ltd

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Article Summary

The corrosivity of naphthenic crude oils1, its mitigation with chemical inhibitors2 and the implementation and continual review of a detailed risk assessment3 focusing on predicting, monitoring and mitigating corrosion-related effects are well documented. For the processing of naphthenic acid feedstocks at high temperature operations, programs such as the proprietary Scorpion II have been applied to monitor corrosion and increase asset reliability. This program was applied at a West European refinery, along with the proprietary Field Signature Method (FSM) due to
the facility’s high temperature environments associated with naphthenic acid crude processing. 

Accurate and reliable corrosion monitoring is one of the keystones to processing naphthenic acid crudes. Through experience, continuous review, knowledge capture and risk-based analysis, the Western European refinery in this case has successfully, safely and reliably processed high acid crudes (HACs) for many years. In 1996, following a complete unit survey, risk assessment and joint research into the corrosivity of a specific HAC, a detailed corrosion monitoring plan and frequency was identified. The monitoring techniques and best practices identified, dependent on the monitoring location, consisted of a combination of:

• Stream analysis
• Corrosion coupons
• ER probes
• High temp UT
• HVGO fast loop
• Visual inspections

Additionally, a continuous review of the frequency and techniques was carried out as new crudes were introduced, processes optimised and experience gained.  

Calculating metal loss
In 2000/01 the refinery purchased ten FSM matrices for use in both high temperature naphthenic acid and low temperature sulphidic environments with initial locations selected to assess the techniques’ capability. The FSM technique was originally developed (1988) for the upstream business sector, for fixed internal corrosion monitoring of carbon steel pipe and has been used extensively on subsea and offshore installations. FSM was later developed as a portable system (FSM-IT) to fit the needs of the downstream sector (eg, refinery high temperature corrosion monitoring).

As a non-intrusive method for monitoring large areas and geometries, detecting changes in the actual pipe wall, providing information on internal corrosion mechanisms and operational in high temperature environments (up to 500°C), the technique was ideal for monitoring the localised attack and pitting corrosion associated with processing naphthenic acid crudes. The FSM-IT has been recognised as an important monitoring tool for this application, especially as there are few inspection techniques that can currently meet these requirements.

Corrosion monitoring systems, based on the FSM method, are a relatively new technology manufactured by CorrOcean and supported by iicorr. Essentially, the method enables a direct measurement of metal loss in a metal section (pipe or vessel) while in service. The technique is based on the application of an electrical current between two distant points across a pipe section (or elbow/T-piece) and making differential voltage measurements along the pipe between the current feed points. These potential drop measurements are used in the calculation of metal loss for each pin pair. The calculations of even metal loss are based on Ohm’s Law, as the objects’ resistance is proportional to its geometry. These measurements will be sensitive to changes in the thickness (and temperature) of the pipe/vessel.  Readings are taken with a portable instrument and downloaded to a PC with the proprietary FSMTrend software where the data analysis and presentation is done. Before calculation of metal loss, data is compensated for any temperature-induced noise and often filtered to reduce any random noise. Different algorithms are then available for quantification of metal loss.

The first high temperature FSM installations at the refinery were five high temperature matrices installed in July 2000. An additional three high temperature matrices have been supplied along with four low temperature (less than 120°C) matrices on the HF alkylation plant.

Location selection
The initial monitoring locations were identified through a detailed risk assessment using a combination of variables (3) and the experience of the on-site inspection group. It is well known that napthenic acid can create both general and localised corrosion under high turbulence and high velocity conditions, especially at process temperatures between 200°C and 420°C. Therefore, in preparation for running the high acid equity crude, the inspection group evaluated the anticipated velocities in all of the high temperature circuits on both the crude and the vacuum distillation units. The following three key areas were highlighted as being at highest risk: two vacuum transfer lines due to temperature, two-phase flow and supersonic velocities, the vacuum heater outlet bends, and the vacuum gas oil circuit, which is in a temperature region where reactive naphthenic acids generally concentrate or build up and result in the highest TAN values.

FSM matrices were therefore located at the heater outlets and on the bends of the two 36-inch transfer lines. The matrices are 20 pin stud-welded design and were cabled to allow data acquisition at ground level. The data is collected from these units on a frequency dependent on the crude slate being processed, at least twice weekly, then processed and incorporated into the CREDO database system of data management.

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