An improved method for testing stainless titanium tubing

摘要

Condenser and heat exchanger tubing typically requires a final air test prior to boxing and shipment. One process, defined as an air-underwater test, involves pressurizing individual tubes with air (150 PSI), submerging in clean water for a minimum of 5 seconds and examining the tube for, want of a better term, bubbles. The other process employs what is called a pressure differential or vacuum decay test. This test is dependent on a pressure or vacuum established and stabilized between tubes in pairs. Sensitive pressure transmitters are used to detect extremely small changes in the internal pressure (or vacuum) between the two tubes. A dichotomy currently exists in terms of these two final testing options for condenser and heat exchanger tubing. Certain ASTM Supplements ("S" Requirements) listed under A-249, A-268, A-269, A-688, A-789, A-803 and general specification A-450 require an air-underwater test (1). Other Standards including A-498, B-338, B-111, B-359, B-395, B-543 & B-467 permit either the air-underwater or alternatively, a pressure differential (P-D) test (2). This procedural paradox within the ASTM has led to confusion, spawning recent initiatives to develop and upgrade testing procedures and language consistency throughout the standards. The Pressure Differential Test (P-D) is an improved method of testing titanium and stainless tubing. It is highly sensitive to very small leaks and arguably, less operator dependent than the traditional air-under-water method as specified under ASTM specifications for stainless tubes. The tests are independent of the operator (visual acuity, physical condition, etc.) and calibration and test results can easily be recorded. In addition, the test is fully automatic and water is not present during the process eliminating any drying requirements and attendant corrosion potential activities. During testing, all parameters are fixed and constant for each production run of the same size tubing. The testing device is calibrated at regular intervals with results documented. Standard test times are adequate to detect small leaks with a 2:1 margin of safety even in the largest volume tube manufactured. Based on the positive experience of the P-D test method for titanium tubing (B-338), Cu.Ni. (B-111, B-359, B-395, B-543 & B-467), Ferritic (A-498) [plus pending ASTM finned tube ballot measure] and supporting test data presented within this paper, the P-D test method is proposed as an alternative, arguably superior methodology, to air-underwater testing for titanium and stainless steel tubing.