Influence of Tightening Torque of Emergency Cooling Heat Exchanger Flange Studs on Strength

Abstract

Previous efforts on assessing static strength of the flange joint components of the emergency cooling heat exchanger 08.8111.335 SB (ECHE) performed using the finite element method (FEM) revealed that there is an excess of permissible stress values for flange joint components of heat exchanger Dn2130 and Dn2080. These static strength calculations considered the design values for tightening of flange studs.
 Failure to meet the strength conditions flange joint components is due to the fact that the effect of «protrusion» under pressure of the bottom and shell attached to the flanges, as well as depending on the tightening force of the studs, the system of equilibrium applied force changes. This, in turn, leads to deformation of flange joint components.
 The paper presents an approach to reducing stress in flange joint components of ECHE first used at ZNPP-4. As a result of additional iterative calculations, the boundary conditions under which the strength conditions in all flange joint components of the heat exchanger are met are determined. In addition, the compliance with tightness condition for flange connections is considered.
 Thus, the analysis of the calculations established that when the tightening value of flange connection Dn2080 studs equal to 6800 kgf (corresponding to the torque on the key of 65 kg ∙m), the strength conditions for all groups of reduced stresses in all elements of the flange unite. In addition, at this value of stud tightening, the tightness of the flange connection Dn2080 is maintained.
 Analysis of similar iterative calculations of flange connection Dn2130 shows that for this flange connection it is recommended to reduce the tightening of the flange connection studs to the value of 14600 kgf, which is equal to the torque on the key of 145 kg ∙ m.