Gamma Radiography of Weldments - Comparison of Radiation Sources

摘要

Weld inspection on oil and gas pipelines as well as in chemical and petrochemical production plants has always been one of the most important areas of gamma radiography. The small equipment size and weight in comparison to corresponding X-ray tubes are advantages of gamma radiation devices. They do not need an electric supply and are very useful for the mobil inspection. The most common radiation source is Ir-192. Due to its high radiation energy its application is limited for thick wall inspections. Since about three years the new gammagraphic equipment on the basis of Se-75 is commercially available. It is of about the same size as Ir-192 containers but significantly less in weight. The Gamma constant is almost the half the Ir-192 one and the half live time amounts to 120.4 days. From the gamma-spectrum of Se-75 its application range is expected to be between Ir-192 and Yb-169. Radiographic parameters were measured like specific contrast and image quality in comparison with Ir-192 and X-ray to characterise the suitable wall thickness range. The parameters were measured with different film types and lead screens. Test measurements of welds are compared to demonstrate the potential of the new equipment. The evaluation of special flaws is based on digitized images and image enhancement. Spectroscopic measurements in comparison to simulations are used to evaluate hardening effects. As result of these measurements the new international standard proposal of ISO 5579 and the European proposals of EN 1435 (weld inspection) and EN 12681 (casting inspection) have considered Se-75 for radiography.