Analysis of the Plate Heat Exchanger Failure
Abstract
The present article describes the effect of the pressure on the operability of the plate exchanger of the SWEEP IC 5 x 20 type as well as the results of the numerical analysis of the stresses at three different loads. The exchanger was tested at the testing pressure defined by the manufacturer (2 MPa). Maximum stresses were observed at the places of soldered joints on individual exchanger plates and the stress was highly above the carrying capacity of the solder joint. With an increasing distance from the place where the boards were connected, stress exhibited a sharp increase. The testing was also focused on the stresses on the surface area (shroud) of the exchanger at the permissible operating pressure of 1.6 MPa defined by the manufacturer. An important observation was the stress identified at the measured real operating pressure of 0.7 MPa ̶destruction of the exchanger used by the operator.
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Introduction
The supply of heat via heat distribution systems may be accompanied with failures of plate heat exchangers which may occur in the following two cases:
1) At the formation of the total hydraulic impact in the system when the pressure in the conveyed fluid exceeds the permissible value of the stress in the material of the soldered joint;
2) If there is a hidden defect in the exchanger which is detected during the exchanger operation.
The article presents the case of analysing the plate heat exchanger destruction which occurred during the unsteady flow of water in the distribution system, characterised with time changes in the flow rate and pressure of water. Such unstable condition may be induced by various causes, most frequently by a failure of the pump or a sudden opening or closing of the regulating element in the pipeline during the steady flow of the fluid from the heat exchanger to a consumer. The shorter the time of closing the valve, the higher the proportion of kinetic energy of water transforms into the deformation work, and the more intensive the exerted hydraulic impact force in the system [1], [2], [3]. The intensity of the hydraulic impact may be one of the causes of the failures of the inner and outer walls of the exchanger, as shown in Fig. 1 and Fig. 2.
Conclusion
Provided that the soldered joints exhibit full and functional carrying capacity, the exchanger may be pressurised up to the value of p = 2 MPa. At such pressure, the walls of the front plate are exposed to the stress of only 10 MPa which is sufficiently below the permissible value P = 115 MPa. All the primary forces are then transferred through the soldered joints. Therefore, the exchanger components that are loaded the most are the soldered joints. The exchanger works reliably until these joints are destructed. The soldered joints become destroyed, and hence the exchanger losses its function, in the following two cases:
1) A sudden increase in the pressure in the exchanger above the carrying capacity of the soldered joint;
2) Due to the effects of the cyclic fatigue life of the soldered joint which gradually loses its carrying capacity due to the cyclic loading, and after the material reaches its kinetic carrying capacity, the joint becomes destructed and the exchanger walls gradually experience plastic deformation. This is a well-known correlation between the “stress (force) and the number of cycles” equal to the fatigue life.
Reliable operation of SWEEP exchangers, IC 5 x 20 types, depends on the carrying capacity of the soldered joints. As operators cannot affect this qualitative property of the exchanger, the problem of exchanger deformation, with the given solder material and during the operations at pressures above 0.47 MPa, must be addressed by the manufacturer.