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CHARACTERISATION OF SCALE HARDNESS

Pearson, N. and Harvey, S.

An in-situ scale hardness measurement is required that is repeatable and simulates the descale process. This would provide objective assessment of scale hardness, and allow correlation of scale hardness with process operating conditions, and provide improved estimates of descale turn-around times when a vessel is taken off-line.

The standard methods of measuring rock strength, Unconfined Compressive Strength (UCS) and Brazilian Tensile Strength (BTS), do not necessarily reflect the issues faced by descale crews in regard to hardness of scale. Unlike scale in process vessels where descale is undertaken by impact on a confined bulk, UCS and BTS are slow crush measurements on a small unconfined sample. Due to the structure of scale, UCS and BTS measurements often have poor repeatability and vary with orientation of the scale. Two alternative methods for measuring scale hardness are the “Schmidt Hammer” that measures the rebound of a spring loaded mass, and “Penetration Resistance” tests that measure the depth of penetration of a “pin” fired into the surface. Both of these methods more closely simulate descale by impact compared to UCS and BTS, can be performed in-situ, return results quickly, and can indicate local variations in scale hardness.

Schmidt Hammer measurements have proven to be the quickest, easiest, and most reliable hardness tests for insitu measurement of scale hardness. Calibration of the Schmidt Hammer against standard tests such as UCS or BTS is possible. While these are technically not measuring the same properties, it does allow for crude comparison of scale to other materials such as concrete or granite, and thus selection of descale equipment with appropriate power and robustness.