The judgement way of heat exchanger heat pipe over-expansion and control methods

The judgement way of heat exchanger heat pipe over-expansion and control methods

^{1. How to judge the heat exchanger tube over-expansion?}

^{(1) Observe the tube plate deformation:}

^{After the tube plate surface is cleaned up, use high-precision flatness measuring tools, such as flatness ruler, per centimeter, etc., to measure the flatness of the tube plate surface. If you find that the tube plate has obvious local bumps, depressions or warping deformation, especially in the heat exchanger tube near the expansion area, it may be caused by over-expansion.}

^{(2) Check the appearance of the heat exchanger tube:}

^{Carefully observe the surface condition of the heat exchanger tube expansion area, if there is a tube wall thinning, wrinkles, or even visible cracks and other conditions, it is likely that over-expansion has occurred. These defects are usually due to the expansion of the expansion force exerted when too large, more than the capacity of the pipe caused.}

^{(3) Check the sealing condition of the expansion port:}

^{You can use halogen leak detector, helium mass spectrometer leak detector and other professional equipment for testing. If there is a medium leakage detected at the expansion port, in addition to the possible expansion of the joint is not tight, over-expansion of the expansion port caused by material damage, sealing structure damage is also one of the common reasons.}

^{(4) Measure the outer diameter of the heat exchanger tube:}

^{Before and after the expansion, measure the size of the expansion part of the heat exchanger tube, if the measured value exceeds the upper limit of the permissible expansion range, it can be assumed that there is an over-expansion problem.}

^{(5) Measure the gap between the hole of the tube plate and the heat exchanger tube:}

^{Before expansion, measure the inner diameter of the hole of the tube plate and the outer diameter of the heat exchanger tube, and calculate the gap between the two. Expansion after the measurement again, if the gap is too small or even a negative gap (i.e., heat exchanger tube O.D. is greater than the inner diameter of the tube plate hole), and beyond the scope of the design allowable, which is an important sign of over-expansion.}

^{(6) Hydraulic test:}

^{In accordance with relevant standards and design requirements, the heat exchanger for hydraulic test. During the test, closely observe whether there is leakage at the expansion port. If there is leakage at the expansion port, in addition to the quality of the expansion joint, the expansion port damage caused by over-expansion is also a possible cause.}

^{(7) Microstructural testing:}

^{Take samples from the expansion area and conduct metallographic section analysis. Observe the metallographic organisation through metallographic microscope to see whether there are signs of excessive plastic deformation such as grain deformation, slip, twinning, and whether there are microcracks and other damage. If there are these phenomena, it means that the heat exchanger tube has been subjected to excessive stress during the expansion process, and over-expansion may have occurred.}

^{(8) Hardness test:}

^{The use of hardness testing equipment, such as Rockwell hardness tester, Brinell hardness tester, etc., the expansion part of the heat exchanger tube and tube plate material hardness test. Over-expansion will cause the material to undergo work hardening, resulting in an increase in hardness. If the test results show that the hardness of the expansion part is significantly higher than the original hardness of the material, and beyond the normal expansion caused by the hardness change range, can also be used as one of the basis for judgement of over-expansion.}

^{2. How to control the size of the expansion force to avoid over-expansion when expanding the joint?}

^{(1) Reasonable choice of expansion process}

^{-Mechanical expansion:}

^{For different pipe diameters and materials, it is necessary to reasonably select the diameter of the expansion rod, taper and expansion length and other parameters. Pipe diameter or material hardness is higher, need to appropriately increase the diameter and taper of the expansion rod, but according to experience and test data to determine the reasonable range. At the same time, control the stroke of the expansion joint, through the precise mechanical limit device or stroke control system, to ensure that the expansion rod in the appropriate stroke to complete the expansion joint, to avoid over-expansion.}

^{-Hydraulic expansion:}

^{Precisely set and control the pressure and holding time of the hydraulic expansion joint. According to the material properties of the pipe and tube plate, pipe diameter, wall thickness and other factors, through calculations and tests to determine the appropriate expansion pressure. In the process of expansion, the use of high-precision pressure sensors and pressure control system, real-time monitoring and adjustment of pressure to ensure that the pressure is stable within the set range. Pressure holding time should also be reasonably determined according to the material and pipe diameter and other factors, should not be too long, so as not to cause over-expansion.}

^{(2) Selection of suitable equipment and tools}

^{-Selection of high-precision expansion equipment:}

^{Select the expansion equipment with high-precision pressure control, stroke control and speed control functions. The pressure control system of the equipment should be accurate to a small pressure unit, the accuracy of the stroke control system should reach the millimetre or even smaller level, the speed control system should be able to stably control the rotational speed of the expansion rod to ensure the stability and controllability of the expansion process.}

^{-Regular calibration and maintenance equipment:}

^{Regular calibration and maintenance of the expansion equipment to ensure that the various performance indicators of the equipment meet the requirements. Calibrate the equipment pressure sensor, stroke measuring device and other key components, check the equipment hydraulic system, transmission system and other normal, timely replacement of worn or damaged parts to ensure the accuracy and reliability of the equipment.}

^{-Use appropriate expansion tools:}

^{According to the material of the heat exchanger tube, tube diameter and wall thickness, etc. to choose the appropriate expansion tool, such as expansion rod, expansion bead, etc.. Expansion tool size, shape and surface quality will affect the transmission and distribution of expansion force, should choose high dimensional accuracy, smooth surface, hardness of the appropriate expansion tool, and regular inspection and replacement to ensure the quality of expansion.}

^{(3) Strictly control the quality of materials}

^{-Check the quality of pipe and tube sheet:}

^{Before expansion, the materials of the pipe and tube plate are subject to strict quality inspection, including the chemical composition, mechanical properties, hardness and other indicators of the material. Ensure that the performance of the material meets the design requirements, to avoid uneven or non-compliance with the requirements of the material properties and lead to difficulties in controlling the expansion force.}

^{-Match the hardness of the pipe and tube plate:}

^{The hardness difference between the pipe and the tube plate has an important effect on the quality of expansion. The hardness of the pipe should be slightly lower than the hardness of the tube plate, so that the pipe can better plastic deformation during expansion, while the tube plate to maintain good rigidity, which is conducive to controlling the expansion force and ensure the strength of the expansion. According to the material properties of the pipe and tube plate, reasonable choice of hardness matching material combinations.}

^{(4) Strengthen operator training}

^{-Improve the operation skills:}

^{Professional training for operators, so that they are familiar with the expansion process, equipment operation methods and quality control requirements. Operators should master the expansion parameters of the setting and adjustment methods, according to the actual situation to determine whether the expansion process is normal, and take appropriate measures in a timely manner.}

^{-Emphasis on quality awareness:}

^{Strengthen the operator's quality awareness education, so that they understand the importance of controlling the size of the expansion force to ensure the quality of expansion. Operators in the expansion process should be strictly in accordance with the operating procedures, carefully record the expansion parameters and abnormalities in the process, to ensure the stability and reliability of the expansion quality.}