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Serious savings from service assessment

Connect, January/February 1999

Power plant
Power plant

Dramatic economies, secured as a direct result of working with TWI on a recent project, are being claimed by one of the world's biggest oil and gas companies.

In a fatigue life assessment project the conclusions delivered to the client allowed it to extend the inspection interval on part of its plant from one year to two, and possibly further in the future.

The investigation concerned steam heating/cooling coils within a large reaction vessel producing acetic acid. The client was concerned that circumferential butt welds attaching the coils to nozzles set through the vessel wall may be subject to cyclic stresses, and hence at risk of fatigue failure, as a result of turbulence of the product during the process cycle.

TWI proposed a strain measurement and recording exercise to establish typical stress cycles in the coils adjacent to the welds.

It was decided that strain gauges would not survive the hostile environment within the vessel. Instead, gauges were placed inside a steam coil.

Since the coil tube was of only 90mm internal diameter, and the closest point of access was a flanged joint in the coil external to the vessel approximately 600mm from the desired measurement location, this was a difficult undertaking.

TWI's strain gauging team devised a purpose-built fixture that allowed the gauges to be placed remotely. Pre-wired gauges with a layer of adhesive applied were mounted in the fixture and the whole assembly was then moved into position.

A pneumatically operated system then applied uniform pressure to each gauge to hold it in place as the adhesive cured. A total of twelve strain gauges and four thermocouples were successfully installed by this remote method.

Strain and temperature records during start-up and subsequent operation of the vessel established that stress cycles were of very low amplitude, posing no significant risk of fatigue failure.

'This helped to eliminate one twenty day planned plant shutdown every two years on two plants' declared the company in a post project appraisal. It also increased production contribution by £2M a year.

TWI understands that a maintenance saving of £300 000 a year associated with the shutdown was also made.

For more information on how TWI can help you with fatigue life assessment, please contact Peter Tubby in the Structural Integrity Department. E-mail:

For more information please email: