Study on Radial Clearance Sealing Performance of Graphite Ring

Firstly, the suitable forming method of graphite ring was screened according to compression deformation test, considering the characteristics of radial clearance seal. Then, the compression resilience, radial contact stress and axial sealing performance of graphite packing rings with different density were researched by testing. Research shows: 25% compressibility is the limit of radial clearance compensation ability of graphite packing ring; And the radial contact stress of graphite packing ring on the pipe wall is linearly positively correlated with the axial load, density has little effect on it. Based on the porous media model of gaskets, axial leakage models of three kinds of graphite rings with different density were obtained by experimental fitting and the influence of external working conditions on leakage mode was analyzed, which provide a theoretical basis for the design of graphite ring seal based on leakage rate.

Prediction of Liquid and Gas Leak Rates in Packed Stuffing Boxes

The prediction of gas and liquid leak rate through packed stuffing boxes subjected to gas flow is a subject of very few studies in the literature. For better prediction of leakage, the change of porosity with length due to the non-uniform axial stress must be accounted for. There are few theoretical models on the prediction of leak rates in packing rings with capillary models. However, a model that incorporates the change of the capillary area with stress gives a better prediction. In this paper, the first slip flow model is used to predict gas and liquid flow considering the straight capillaries and capillaries having an area dependent on the axial stress in the packing rings. An approach that uses an analytical-computational methodology based on the number and the size of pores obtained experimentally is adopted to predict gas and liquid leak rates in uniform and non-uniform compressed yarned packings. The Navier-Stokes equations associated with slip boundary condition at the wall are used to predict leakage. Experimental tests with helium, argon, nitrogen and air for gazes and water and kerosene for liquids will be used to validate the models. The porosity parameters characterization will be conducted experimentally with helium at a reference gas at different gland stresses and pressures.

Predicting Leak Rate Through Valve Stem Packing in Nuclear Applications

Leaking valves have forced shutdown in many nuclear power plants. The myth of zero leakage or adequate sealing must give way to more realistic maximum leak rate criterion in design of nuclear bolted flange joints and valve packed stuffing boxes. It is well established that the predicting leakage in these pressure vessel components is a major engineering challenge to designers. This is particularly true in nuclear valves due to different working conditions and material variations. The prediction of the leak rate through packing rings is not a straightforward task to achieve. This work presents a study on the ability of microchannel flow models to predict leak rates through packing rings made of flexible graphite. A methodology based on experimental characterization of packing material porosity parameters is developed to predict leak rates at different compression stress levels. Three different models are compared to predict leakage; the diffusive and second-order flow models are derived from Naiver–Stokes equations and incorporate the boundary conditions of an intermediate flow regime to cover the wide range of leak rate levels and the lattice model is based on porous media of packing rings as packing bed (Dp). The flow porosity parameters (N, R) of the microchannels assumed to simulate the leak paths present in the packing are obtained experimentally. The predicted leak rates from different gases (He, N2, and Ar) are compared to those measured experimentally in which the set of packing rings is mainly subjected to different gland stresses and pressures.

Experimental Investigation of Interfacial and Permeation Leak Rates in Sheet Gaskets and Valve Stem Packing

The quantities of leak rate through sealing systems are being regulated because of the global concern on the hazardous pollutants being released into the atmosphere and their consequences on the environment and health. The maximum tolerated leak is becoming a design criterion, and the leak rate for an application under specific conditions is required to be estimated with reasonable accuracy. In this respect, experimental and theoretical studies are being conducted to characterize the gas flow through gaskets and packing rings. The amount of the total leak that is present in a gasketed joint or a valve stem packing is the sum of the permeation leak through the sealing material and the interfacial leak at the mating surfaces between the sealing element and mechanical clamp assembly. The existing models used to predict leakage do not separate these two types of leaks. This paper deals with a study based on experimental testing that quantifies the amount of these two types of leaks in bolted gasketed joints and packed stuffing boxes. It shows the contribution of interfacial leak for low and high contact surface stresses and the influence of the surface finish as a result of a 32 and 250 micro-inch RAAH phonographic finish in the case of a bolted flange joint. The results indicate that most of the leak is interfacial reaching 99% at the low stress while the interfacial leak is in the same order of magnitude of the permeation leak at high stress reaching 10−6 and 10−8 mg/s in both packing and gaskets, respectively.

Modeling Dry Wear of Piston Rod Sealing Elements of Reciprocating Compressors Considering Gas Pressure Drop Across the Dynamic Sealing Surface

The wear of dynamic sealing elements, i.e., elements that seal against a moving counter-surface, is highly complex. In dry-running reciprocating compressors, these sealing elements, commonly referred to as packing rings, have to seal the compressed gas against the environment along the reciprocating rod. Since the packing rings' seal effect arises from the differential pressure to be sealed, it is of paramount importance to take into account the gas pressure drop across the dynamic sealing surface. This paper presents a numerical model that allows us to calculate how the wear of such a packing ring evolves with time. An analytical solution is used to verify the numerical model.

Gauging the Force Effects of Valve Stem Packing on Valve Stem Actuation

Stem friction in an operating valve is a function of the dynamic interaction of a number of variables — packing material of construction, number of packing rings, compressive load, lubrication, stem surface finish, temperature, cycling, etc. Forces due to friction can be reduced by modifying these factors. Attaining low actuation force and good sealing requires a balanced approach. Packing manufacturers have their own procedures for determining the frictional properties of different packing materials. This paper will show one such procedure and how varying materials and packing set configurations affect actuation force. The focus will be on linear reciprocating valve stems. The equation F = π × d × H × GS × μ × Y can be used to calculate the force of the packing on the valve stem: Where F - Force needed to overcome packing friction; d - Stem diameter; H - Packing set height; GS - Compressive stress on the packing; μ - Packing coefficient of friction; Y - Ratio of radial to axial load transference, commonly equal to 0.50. Knowing the force, F, by test allows the calculation of the packing set’s frictional characteristics. . This knowledge can guide valve designers and builders to properly size actuating units for consistent and reliable valve performance. Paper published with permission.

Experimental and Numerical Investigations on Exfoliated Graphite Seals

In order to improve the sealing performances of porous braided packing rings, an experimental study is currently performed on a dedicated device. This device has been specially designed to measure the stresses during cyclic loading tests with compressed exfoliated graphite valve packing. Indeed, different tests which reproduce the opening and closing cycles of the valves have been performed to enhance the understanding of the response of the packing mechanical behavior. To this respect, an Hyperelasto-Hysteretic model is now being developed to reproduce the mechanical behavior of the compressed exfoliated graphite. The material parameters of this model are then identified to describe the stress-strain response of the studied packed stuffing-box under these cyclic loading tests.