Mehdi Kazeminia

Analysis of Stresses and Strains in Packed Stuffing-Boxes

Packed stuffing-boxes are mechanical sealing systems that are extensively used in pressurized valves and pumps. Yet there is no standard design procedure that could be used to verify their mechanical integrity and leak tightness. It is only recently that standard test procedures to qualify the packing material have been suggested for adoption in both North America and Europe. While the packing contact stress with the side walls is predictable using existing models there is no analytical methodology to verify the stresses and strains in the stuffing-box housing.This paper presents an analytical model that analyzes the stresses and strains of all the stuffing box components including the packing rings. The developed model will be validated both numerically using FEM and experimentally on an instrumented packed stuffing box rig that is specially designed to test the mechanical and leakage performance of different packing materials.Copyright

Characterization and Modeling of Time-Dependent Behaviour of Braided Packing Rings

The sealing performance of packed stuffing boxes used in valves and compressors depends on the ability of the structure to maintain a minimum threshold contact pressure through a sufficient period of time. Packing rings exhibit combined creep and relaxation behavior due to internal disordered porous structure and nonlinear material behavior in addition to the interaction with other structural components. A comprehensive understanding of the time-dependent behavior of packing rings is essential for increasing the sealing performance. In this paper, the time-dependent linear viscoelastic behavior of packing material is constitutively simulated. The experimental investigation is carried out in a special test bench which was designed and developed to study the characteristics of the time-dependent behavior of packing rings. The results show that the proposed model can successfully be exploited to determine the time-dependent behavior of packing rings for application in the design of packed stuffing boxes.Copyright

Prediction of leak rates in porous braided packing rings

The prediction of leakage is one of the most challenging tasks when designing bolted flanged connections and valves. Failure of these pressure vessel components can cause shutdowns but also accidents, loss of revenue and environmental damages. With the strict regulations on fugitive emissions and environmental protection laws new tightness-based standards and design methods are being adopted to improve the sealing performance of bolted joints and valves. However, there is a practical interest in using a reliable correlation that could predict leak rates of one fluid on the basis of tests carried out with another on compressed packings. The paper presents an innovative approach to accurately predict and correlate leak rates in porous braided packing rings. The approach is based onDarcy-Klinkenberg to which a modified effective diffusion term is added to the equation. Experimental measured gas flow rates were performed on a setof graphite based compression packing rings with a large range of leak rates under isothermal steady conditions. Leakage from three different gases namely Helium, Nitrogen and Argon were used to validate the developed correlation. In the presence of the statistical properties of porous packings, the leak rates for different gases can be predicted with reasonable accuracy.Copyright

Effect of reverse yielding on the residual contact stresses of tube-to-tubesheet joints subjected to hydraulic expansion

The analytical prediction of the contact stress in tube-to-tubesheet joints subjected to hydraulic expansion is conducted without any consideration to reverse yielding that can occur inside the tube. Most existing models consider the tube and tubesheet to unload elastically when the expansion pressure is released. These models are therefore less conservative as the overestimate the contact pressure.An analytical model that considers strain-hardening material behavior of the tube and tubesheet and accounts for reverse yielding has been developed. The model is based on Henckey deformation theory and the Von Mises yield criteria. The paper shows that reverse yielding that is present in tubes during hydraulic expansion unloading makes the joint less rigid and causes a decrease in the contact pressure depending on the gap clearance and the materials used. A good correlation between the analytical and FEM results is obtained on different treated cases which gives confidence on the developed model.Copyright