Research data for five types of gland packings are reported. Relationships are shown with due regard for the number of loadings and the number of packing layers: relative deformation of the packing due to unit load, unit load on the packing due to its relative deformation, and compression and recovery modulus arising from relative deformation of the packing.Nowadays, gland packings made from heat-expanded graphite (HEG) are being increasingly used in global practice (in power, chemical, petrochemical, and oil and gas industry). This material has a low density and a relatively high elasticity, is resistant to many corrosive working media, and has a low friction coefficient and a high wear resistance.For calculating and designing gland seals with soft packing, it is essential to know the deformation characteristics of the HEG-based packings (coefficient of lateral pressure, shrinkage and recovery of the packings, friction coefficient, etc.).The coefficient of lateral pressure k is a ratio of unit loads on the packing in the radial q r and axial q z directions, i.e., k = q r /q z , and depends on many factors (material of the packing, its preliminary compaction, and axial unit load q z on the packing). It is determined only experimentally. In [1-3], the coefficient k was determined from the peripheral deformation of the outer surface of the chamber wall at a fixed axial load on the packing placed in the chamber. This deformation can be determined from the readings of the strain resistors attached to the wall [1] or of an elastic element in contact with the outer surface of the chamber wall [2,3]. The accuracy of k determination depends on the radial rigidity of the chamber wall (on the wall thickness): the greater the rigidity, more reliable the results. However, acceptable readings of the strain resistors or of the elastic element can be obtained only at a relatively low rigidity of the wall amenable to deformation. All k values obtained by this method are somewhat underestimated, and the degree of underestimation depends on the difference between the absolute rigidity (taken as absolute) and the actual radial rigidity of the wall of the gland chamber of the experimental unit.In all the referred works, the coefficient of lateral pressure was determined on the outer contour of the packing and the same value was taken for its inner contour.
An assessment has been made of the intensity of axial deformation of test fluoroplastic-4 (polytetrafluoroethylene) specimens upon their loading (compression) and unloading (recovery) in the 20-200°C temperature range. It has been proved that the nature of loading (compression or recovery) exerts significant influence on the intensity of deformation (instantaneous modulus) of the fluoroplastic element. For assessing the influence of the type of load, conditional calculating compression and recovery moduli have been proposed and calculating expressions for their assessment have been obtained. Also, calculating expressions for determining residual deformation of the fluoroplastic-4 specimen under static and cyclic loads have been derived.Currently, various polymeric materials and, in particular, fluoroplastic-4 (polytetrafluoroethylene), also called Teflon, are used extensively for making components of seals of chemical equipment.Let us consider the influence of the type of loading of fluoroplastic-4 gasket on the nature of its deformation.In the process of assembling detachable joint, the gasket is compressed under the force of tightening Q t of threaded fastening elements. With increase of pressure of the working medium being sealed, partial unloading (recovery) of the gasket and additional loading of the fastening elements occur under the action of axial force Q g .In accordance with the force diagram of the sealed detachable joint [1] shown in Fig. 1 (Q w is the axial force of the working pressure), the forces on the bolt Q b and gasket Q gk are determined by the expressionswhere [Q] b is the maximally permissible force on the fastening elements of the joint in terms of their strength, [Q] gk is the minimally permissible force on the gasket in terms of leak-tightness of the joint, α is the coefficient of joint rigidity determinable by the expression α = λ b /(λ b + λ gk ), λ b is the total axial pliability of the fastening elements and flanges of the detachable joint, λ gk is the axial pliability of the sealing gasket determinable by the expression λ gk = h/(F gk E gk ), h is the thickness of the gasket, F gk is the area of the gasket, and E gk is the modulus of elasticity of the gasket material.In calculation practice [1], tabulated values of modulus of elasticity of respective materials are used to determine the pliabilities λ b and λ gk , which adds significant errors in calculation of the key parameters of the sealed detachable joint, especially when fluoroplastic-4 gaskets are used. This is because the types of loading of the joint elements, including gaskets, in the operation process are different.In the case of joint elements (bolts, flanges, etc.) operating in the elastic stage, the nature of loading has no effect on the intensity of their deformation that depends on the modulus of elasticity of the material. But in the case of gasket loaded to the plastic state, the nature of loading exerts specific influence on its deformation. For instance, in accordance with the force dia-
No abstract
It is shown that the mode of setting gasket between flanges significantly affects its axial pliability, which is a key flanged-joint calculation parameter. gasket has been studied for four modes of setting gasket between flanges: with primary compression (without lateral restraint), with one-sided internal restraint, with one-sided external restraint, and with volumetric (three-dimensional) compression. It is shown that the mode of setting gasket markedly affects the leak-tightness of the joint and the strength of its individual components. The described calculation procedure can be used to obtain the required data also for flanged joints with gasket made of materials that differ from fluoroplastic-4 in physicomechanical parameters.Calculation of detachable flanged joints for leak-tightness and strength is based on an assessment of the rigidity (pliability) of their components: flanges, fastening bolts, gasket, etc. The pliability values for the joint components are included in the calculating equations for the determination of the key parameters of the joint: diameter and number of fastening bolts, geometric dimensions of the flanges, force of gasket preloading, etc.[1]. The pliability of the bolts and flanges depends on their geometric parameters and the modulus of elasticity of the material, which, for these components, are taken as constant in the elastic stage of operation in the case of both loading (compression) and unloading (recovery) of the component. The deformation characteristics of the gasket with due account of its material depends, as a rule, on the magnitude and nature of loading (loading or unloading) and mode (conditions) of gasket setting between the flanges.Investigations were made on the axial pliability of an annular gasket from fluoroplastic-4 (polytetrafluoroethylene), also known as Teflon, set between the flanges with axial compression without restraining radial deformation, with one-sided internal restraint, with one-sided external restraint, and with volumetric (three-dimensional) compression. The investigation results [2] are plotted in Fig. 1. As evident from these plots, radial deformation under axial compression occurs essentially along the external contour of the annular gasket.The gasket is loaded with a unit compressing axial load q, under the action of which the gasket thickness diminishes, the outer radius r 2 increases, and the inner radius r 1 diminishes. However, the changes in these parameters differ owing to curvature of the gasket contour. Upon axial compression of the gasket tensile and compressive annular stresses appear in certain sections (across the gasket breadth). The gasket radius on which the annular stresses change sign ("neutral radius") [3]:
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