We perform dye penetrant testing on all bellows longitudinal seam welds before and after forming the bellows. The test is performed to insure there are no weld defects such as surface cracking or weld discontinuities. The weld surface is thoroughly cleaned and a red penetrant dye is applied to the surface of the weld and allowed to soak for a period of time. The dye is then removed and a white powder is sprayed on the weld surface that performs a blotting action. An indication of the dye with a white background characterizes the weld surface discontinuities of the bellows longitudinal seam weld. This testing before and after the bellows forming process ensures a sound quality bellows seam weld.
Procedure for Dye Penetrant Testing
Examples of Dye Penetrant Testing
This week’s featured article is on expansion joint burst testing. The objective of a burst test is to determine the ultimate pressure resistance that a bellows can absorb prior to failure. Burst tests help determine the margin of safety and safety factor to be used in a specialized bellows design. During a burst test, hydrostatic pressure is slowly increased until failure occurs, which usually happens in one to two hours. Burst testing is conducted at ambient temperatures to determine a bellows pressure rating. We follow all the necessary safety precautions for personnel involved, such as placing the bellows in a high strength steel cage and using a safety shut-off switch. Based on the burst test results, a safety factor is applied, thus establishing the ultimate pressure rating.
The video shows an expansion joint bellows being cycle tested through a specified design movement of 1-1/4″ to verify the calculated life span of 30,000 cycles. The number of completed cycles is recorded throughout the duration of the test. U.S. Bellows routinely performs this test to confirm designs and ensure a high quality product. We do extensive expansion joint product testing such as x-ray, burst, hydrostatic, ultrasonic, radiography, fatigue, PMI, pneumatic, magnet particle and helium leak testing depending on customer specifications. Each bellows and expansion joint unit goes through in-process quality control using a system of travelers, and a final in-house dimensional inspection is performed prior to shipping.
This photo shows a 42 inch bellows made by PT&P being tested by Leak Detection Specialist Inc. After sealing the bellows a full vacuum is pulled on the inside, then helium is sprayed over the surface of all welds. A pump pulls any gas which leaks to the interior of the bellows to a special mass spectrometer designed to detect the helium. If helium is detected, the point of the surface where it was sprayed is marked for repair. When no leaks are found, the flow rate of helium to the mass spectrometer is measured against a standard for aluminum, in this case 0.000001 cc/sec. Helium molecules are so small, they will penetrate aluminum at a slow rate when the pressure difference of a full vacuum is applied.
Leak Detection Test
A mass spectrometer functions by accelerating a narrow beam of ions of a gas sample through a magnetic field. When they reach the magnetic field, the amount they are deflected depends on their charge-to-mass ratio. In this application the detector is set to record helium ions. In order to measure the flow rate of the helium when no leaks are detected, the entire system is sealed in a plastic “bag”. LDS can measure helium flow rates as small as 0.00000001 cc/sec.