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黄台电厂7号机组是东方汽轮机厂生产的 N300-165/535/535型高温高压30万 KW 机组。自1987年底投运以来,该机组第11级叶轮在1988年5月由于叶片断裂使第60片叶片处的轮缘产生了1条深8mm、长25mm 的裂纹,在对其进行焊补修复后,又对该处进行了着色探伤,未发现缺陷。在1989年3月的大修中提出要在不拆叶片的情况下,对轮缘焊补区进行超声波探伤,过去对2.5万 KW、5万 KW 等小机组轮缘的超声波探伤,因其轮缘厚度较薄,可以用遮挡法进行判断裂纹是否存在。但随着装机容量的增大,叶轮轮缘厚度也相应增厚,该机轮缘厚度为22mm,小脚为4mm×9mm(如图1所示),为使主声束切过 C 点,折射角β为23°,由公式 sinα/sinβ=C _有/C _钢可知,
Unit No. 7 of Huangtai Power Plant is N300-165 / 535/535 type high temperature and pressure 300,000 KW unit produced by Dongfang Steam Turbine Works. Since commissioning in late 1987, the 11th-level impeller of this unit produced a deep 8mm, 25mm long crack in the rim of the 60th blade due to blade rupture in May 1988, , But also the Department of color inspection, found no defects. During the overhaul in March 1989, it was proposed to carry out ultrasonic flaw detection on the rim welding area without dismantling the blade. In the past, the ultrasonic flaw detection on the flanges of small units such as 25,000 KW and 50,000 KW, Thin thickness, you can use the occlusion method to determine whether the existence of cracks. However, with the increase of installed capacity, the thickness of the rim of the impeller is also correspondingly thicker. The thickness of the rim of the rim is 22 mm and the size of the foot is 4 mm × 9 mm. As shown in FIG. 1, Angle β is 23 °, the formula sinα / sinβ = C _ Yes / C _ steel we can see,