X-radiographic monitoring of circular welded joints in a tube – tube sheet system

Abstract

Structures for tube -tube sheet welded joints (TTSWJ) and schemes for x-radiographic monitoring are considered. Monitoring problems connected with further development of heat exchanger structures, and their solution, are defined. Description of a developed and manufactured test example of an automated device for TTSWJ x-radiographic monitoring with programmed control is provided. The device makes it possible to monitor for one exposure joints with a diameter from 8 to 20 mm. The manual operating part includes placing an unexposed film holder into the device and its removal after exposure. Fulfillment of exposure conditions (voltage, current, etc.) and subsequent passage around a TTSWJ is accomplished automatically by a prescribed regime. The test example is tested on TTSWJ specimens and models made of titanium and stainless steel; results are provided.An integral part of contemporary heat exchangers, operating with hazardous or corrosive media (atomic power, chemical industry, etc.), are tube -tube sheet welded joints (TTSWJ) whose construction has been defined in [1] (Fig. 1).Stainless steel and titanium alloys are normally used as the weldable material. A requirement for carrying out 100% nondestructive monitoring of these welded joints is explained by the metallurgical and structural features of welded joints, for which the main monitoring method is radiographic (RM). TTSWJ RM reveals pores, tungsten inclusions and other volumetric discontinuities.The generally accepted schemes for x-raying TTSWJ have been determined in [2]. Monitoring by a scheme (Fig. 2) provides relatively good productivity (obtaining a photograph of a whole welded joint by one exposure), although it requires use of small sharply focused radiation sources, with radionuclide iridium-192 with a typical size of the active part of 1.0 × 0.5 mm used as such sources for some decades.Research and development aimed at replacing iridium-192 (the effective radiation energy of iridium-192 is ~400 keV, and it has a relatively short half life of ~74 days) by other radionuclides has not ceased, for example by ytterbium-169, but they have not been used extensively in industry.The sensitivity of RM for steel TTSWJ using iridium-192 and films of class C3 (EN 548-1) is normally W11 (0.32 mm) for a wire standard (IQI).Direct confirmation of the complexity of performing RM by the scheme (see Fig. 2) is a limitation [3, para. 9.11.15]: "welding of tubes in a tube plate with a normal internal diameter, tubes up to 15 mm, is not amenable to radiographic monitoring if there are no special indications in the construction documentation."