Talcyon uses acoustic pulse reflectometry for QA/QC of newly fabricated heat exchangers
04/10/2022
In this article, an acoustic pulse reflectometry (APR) system called APRIS is described. It has been designed specifically for detecting faults commonly found in industrial tube systems such as heat exchangers, condensers and boilers, including leaks, an increase in the internal diameter caused by wall thinning and constrictions due to blockages, deposits or scales. Vignesh Sivanandam, from Talcyon Pte Ltd, explains…
Acoustic pulse reflectometry (APR) has been applied extensively to tubular systems in research laboratories, for the purposes of measuring input impedance, bore reconstruction and fault detection. While industrial applications have been mentioned in the literature, they have not yet been widely implemented. Academic APR systems are extremely bulky, often employing a sophisticated set-up, which severely limits their industrial use. Furthermore, the leak detection methods that are described in the literature are based on indirect methods, by carrying out bore reconstruction and finding discrepancies between the expected and the reconstructed bore.
Heat exchangers play a vital role in cooling and heating by transferring heat between fluids at different temperatures. The solid wall in the heat exchanger separates the flowing fluids. Heat is transferred from the hot fluid to the wall by convection, via the wall by conduction and from the wall to the cold fluid by convection. Such heat exchangers are widely used in chemical plants, petrochemical plants, power generation, food processing plants, heating, ventilation and air conditioning (HVAC) and oil & gas processing plants.
Design and fabrication followed by testing are the key aspects of a heat exchanger prior to its installation on site. There are different types of heat exchanger and selection is based on the heat transfer rate, the heat transfer coefficient and the heat transfer surface area. The design is based on analysing the application and identifying the fluid properties, energy balance, geometry of heat exchangers, thermal calculation, noise, vibration, standards, environmental conditions and so on. Fabrication involves drilling, cutting, forming, rolling, welding, expansion and painting. When the final stage is reached, which is the inspection, the most commonly used method is hydrotesting in order to find any leaks.
Preventative to predictive maintenance requires more data and a rigorous inspection. Such data points can be obtained via multiple testing techniques, with APRIS offering a fast, reliable and intuitive option to meet these requirements.
APRIS works on the APR principle. An acoustic pulse injected into a semi-infinite straight-walled tube will propagate down the tube without generating any reflections. This pulse can be measured by mounting a small microphone, with its front surface flush with the internal tube wall, through a hole in this wall. The microphone will measure the pulse only once, as it passes over the microphone’s diaphragm.
The APRIS system employs a hardware set-up that is extremely portable but creates a large degree of overlap between forward and backward propagating waves in the system. A series of patented algorithmic innovations enables the system to perform the wave separation mathematically and then identify faults automatically, with a measurement time in the order of ten seconds per tube.
If the pulse encounters a discontinuity in the cross-section, a reflection is created. The amplitude and form of the reflection is determined by the characteristics of the discontinuity: a constriction will create a positive reflection, whereas a dilation (increase in cross-section) will create a negative reflection. Neither of these discontinuities will change the shape of the pulse in their vicinity, but the reflection measured by the microphone will be an attenuated and smeared replica of the impinging pulse, due to propagation losses.
A hole or leak in the tube wall, on the other hand, will create a reflection with a more complicated shape, affected by the size of the hole and the radiation of acoustic energy to the space outside the tube.
Though some of the acoustic energy present in the original pulse is reflected at the discontinuities, some of this energy continues to propagate down the tube. Any further discontinuities will once again create reflections. Therefore, diagnosing the internal condition of the tube is a case of correctly interpreting the reflections as they arrive back to the microphones. One aspect of the interpretation is straightforward: the time of arrival of a reflection can be used to calculate the precise location of the discontinuity, since such reflections propagate at the speed of sound. The second aspect of interpretation is more complicated, as it involves inferring the exact nature of the discontinuity from the detailed shape of the reflection.
In this example, the end-user wanted to know if there were any imperfections on the tubes due to expansion and the welding performed during fabrication.
APRIS was applied on a newly commissioned U-tube bundle. 291 U-tubes were inspected in less than one hour as it is not essential to inspect them from both the top and the bottom as the sound propagates from the top to the bottom of the U-tubes. This saved 50% of the time compared to deploying conventional tube testing techniques. Once data was collected for all of the 291 U-tubes, it was processed and a report was presented to the end-user in two hours.
The client was surprised to see a few of the tubes in yellow, which indicates a blockage, as the tubes were cleaned prior to testing. As per APRIS reporting terminology, a blockage does not mean that the tube is fully blocked/clogged. If there is any cross-section reduction in the tube, it will be reported as a blockage with its precise location or spread and percentage of cross-section reduction in order to determine the extent and severity.
Whatever obstructs the propagation of sound waves is reflected. Based on the signature, the defect type is determined. By carrying out the root cause analysis, the end-user found that the blockage was due to weld imperfections and the report indicates a 2% cross-section reduction. This helped the end-user to understand the sensitivity, probability of detection (POD) and accuracy in indicating the position and the size of the defect.
On completion of this heat exchanger inspection, the client made the decision to use APRIS for all of the company’s newly fabricated heat exchangers. Over a period of four months, 16 newly commissioned heat exchangers were tested to identify the abnormalities in the tube due to welding imperfection, tube expansion, rolling and other defects. This data will be used as a baseline when an inspection is carried out after a few years of service.
