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Strain gauging in extremis

Connect, no.82, January 1997

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Ambient temperatures over 30 degrees C and the possible presence of hydrofluoric acid made for a demanding working environment when TWI was asked to assist in the structural assessment of acid evacuation system pipework in a middle-eastern oil refinery.

The pipework normally transfers, very rapidly, large quantities of hydrofluoric acid from the process vessels to a storage area. Concern had been expressed over the level of dynamic stresses generated by the transfer and TWI's Site Services team was asked to measure the pipework system performance during plant commissioning using alkylate for a test transfer.

After discussions with the designers, 18 locations on various outlets, supports, T-pieces and elbows were selected for strain measurement. At each location, a pair of gauges was used to measure hoop and axial stress. In addition, five pressure transducers were installed at selected drain plug locations in the pipework.

To reduce the time needed on-site for installation, the 36 120W, 6mm gauges were pre-bonded with cyanoacrylate adhesive to stainless steel shim, and triple-twist wire tails were pre-soldered. This had the added advantage that it eliminated the problem of trying to perform the delicate tasks of bonding and wiring the gauges whilst wearing heavy duty rubber gloves and over clothes needed for protection in areas where hydrofluoric acid might be present.

On-site, in temperatures exceeding 30 degrees C, TWI's two-man team prepared the 18 locations, resistance spot welded the shims and connected the instrumentation to a computer controlled logging system. Over 3.5 million readings were taken before, during and after the fluid transfer.

Such wide ranging readings gave a very accurate picture of the stress levels. It was found that the maximum changes in stress were less than 40% of the allowable level. This, therefore, did not represent any significant threat to the structural integrity of the acid evacuation system and the plant engineers were satisfied that remedial action was not required.

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