Costly fallout from corrosion

Corrosion costs the global process industry an estimated $40 billion a year, industry figures show. The massive bill breaks down between: oil & gas production, $2.8bn; refining, $7.4bn; pulp & paper, $12bn; chemical process, $3.4bn; and utilities $14bn.

The recent problems at BP Exploration Alaska further highlight how corrosion can have a damaging impact on both business and environmental performance.

Last March, BP operators discovered a leak in the transit line at its Prudhoe Bay refinery that caused around 200,000 gallons of oil to be lost. It proved to be the largest spill in the history of Prudhoe Bay. The company later found that internal microbiological corrosion caused by sulphate-reducing bacteria had eaten a one-quarter-inch hole in the bottom of the transit pipeline. To date, BP has spent many millions responding to the incident.

Process operators, clearly, have huge incentive to invest in technologies that detect corrosion or prevent it from occurring in the first place, as well as systems to repair damaged pipeline with minimal downtime and disruption to production.

For its part, Rohm & Haas has turned to a monitoring technology from Honeywell to troubleshoot corrosion differences in two similar chemical plants at its Deer Park, Texas site. Though similar, one plant had low corrosion rates, while the other corroded at very high rates, causing rapid failure of stainless steel piping.

As traditional monitoring techniques, such as corrosion coupons, failed to detect the root cause of the problem, the company installed corrosion transmitters that provide real-time corrosion information and could be linked into the control system at the site.

The SmartCET system from Honeywell is designed to give operators access to current, actionable, process-variable information, including the time-trended general corrosion rate, as well as indicating localised, or pitting, corrosion.

The system employs electrochemical noise (ECN) technology to measure localised corrosion. The instruments measure electrical current noise between working and counter electrodes, explains Ayman Mehdawe of Honeywell.

With localised corrosion the current distribution deviates from a standard bell curve and the standard deviation of the signal will increase, said Mehdawe. The system, he added, can also determine a pitting factor, which is calculated as the ratio between the standard deviation of the ECN current signal to the corrosion current.

After installing the sensing probes and connecting them to the respective control systems, Rohm & Haas operators and engineers found that one unit’s corrosion rate was higher immediately after shutdown. On further inspection, they located and replaced a leaky valve that was allowing water into the system on shutdown and increasing the corrosive nature of the system.

The corrosion monitoring system also revealed that corrosion was higher in the process when a certain recirculation condition occurred. By making some process changes, the plant was able to avoid these conditions on the second unit.

The system communicates via the HART protocol and can connect to existing control systems. As an input to the process control system, corrosion data can be alarmed, assembled historically, trended and assigned to process groups, as it will seamlessly correlate with other process variables.

“Having a corrosion sensor that connects to the DCS is a real benefit,” said Andrew Wheeler from the Deer Part operations team. “We are able to view corrosion just like any other process reading and this allows us to make decisions that preserve our equipment.”

Carbon fibre composite technology offers a means of permanent on-line plant repair and rehabilitation. It is also being turned to increasingly by process industry companies, particularly in sectors such as petrochemical, chemical and pharmaceutical, where downtime and lost production represent high cost.

According to technical services company Furmanite, epoxy resin-impregnated carbon fibre composites offer the ability to restore full pressure integrity and structural strength to corroded, damaged or weakened plant without requiring shutdown. A typical repair has 10 times the strength of steel.

“Factors such as operating pressures and temperatures, size of repair, extent of any through-wall defect and the design life required determines the resin mix, number of layers, area of overlap, surface preparation, curing requirements and so on,” Furmanite head of sales Mike Tucker explains. “With these calculations incorporated into the repair design, the materials are applied accordingly using a layering process building up to the specified thickness.”

According to Tucker, a key benefit relates to the cost savings derived from being able to apply these repairs without downtime. He cites how a major pharmaceutical plant avoided a nine-day shutdown and significant craneage costs by using composites to solve a problem of severe corrosion over eight metres of a main 20-inch cooling water feed line, much of it in an area of awkward and restricted access. The resin repair restored full structural strength and pressure containment, with no disruption to plant operation.

When composite repairs are applied during a plant’s scheduled shutdown, the fact that no hot-work is required brings further benefits, including significant time and cost savings, by avoiding the need to purge the plant, added Tucker.

Composite repairs can be used for structural loads as well as pressure containment, as highlighted by a repair to a 24-inch crude oil line elbow at Shell’s Stanlow manufacturing complex. Fretting damage on the 45° elbow at the support, due to movement caused by thermal expansion, had caused a wear groove. A subsequent design study revealed that the elbow was overstressed and required repair and strengthening.

“The loads on the pipe due to thermal expansion were significant,” said Tucker. “We engineered a composites solution that optimised the fibre orientation to carry the necessary loads, while minimising the thickness and maximising the effectiveness of the repair. By ensuring that the repair’s hoop and axial capacities were equally utilised for optimum efficiency, we were also able to keep materials cost and installation time to a minimum. The resulting repair was designed for a 25-year lifetime.”

During a shutdown at the BP Grangemouth refinery, composite repairs were used to restore a number of refrigerated LPG lines to their original condition. This was achieved within a five-day window without having to clear hydrocarbons from the lines, which would have demanded a longer and more costly shutdown.

“The fact that these repairs eliminate hot-work is a major benefit on any hydrocarbon plant, and meant we didn’t have to gas-free the lines,” said a BP spokesman.