Mohammed Diany

An Experimental-Numerical Procedure for Stuffing Box Packing Characterization and Leak Tests

The sealing of valve stems is ensured by the traditional systems of packed stuffing boxes. The performance of this type of sealing system, which is also used in rotating equipment, is dependent on the radial contact pressures generated by the packing axial compression. The mechanical behavior of a packing seal is characterized by the transmission ratio of the radial stress over the axial stress known as the lateral pressure coefficient, which is one of the required parameters used to select packing seals. However, the modeling of the packed stuffing box requires the knowledge of other packing seal mechanical characteristics such as compression modulus and Poissons ratio. In this paper, the mechanical characteristics of packing seals are obtained using a hybrid experimental-numerical procedure. The experimental study is carried out on an instrumented stuffing box packing test bench. The tests results of the experiments are coupled to the ones obtained by a finite element simulation of the test rig to determine the packing seal mechanical characteristics. Two packing types are used: one based on Teflon and the other one based on flexible graphite. In addition, leak rates are measured.for different axial compressive stresses and gas pressures in order to estimate the tightness performance of such seals.

Short term relaxation modeling of valve stem packings

The long term tightness performance of stuffing-box packings, used in valves, is conditioned by the capacity of its sealing material to maintain a contact pressure to a predetermined minimal threshold value. Due to the creep, this contact pressure decreases with time depending on the creep properties and the stiffness of the housing. Assessing relaxation is a key parameter in determining the tightness performance of a valve stem packing over time. An analytical model based on the packing viscoelastic behavior is developed to assess the contact pressures between the packing material and the stem and the housing and their variation with time. In parallel, an axisymmetric 2D finite element model was build to validate and support the analytical model. The valve stem packing relaxation performance is an important design parameter to consider when selecting compression packings.

Determination of gasket effective width based on leakage

The concept of the effective gasket width is used in the ASME code for flange designs to account for flange rotation. Its origin has never been revealed and it is suspected that only limited experimental testing was conducted for verification. In addition, with the new design rules based on the new gasket constants obtained from ROTT tests conducted on rigid platens, there is a need to revise the effective gasket width formulas. This paper presents some work development conducted under the auspices of the PVRC and related to the investigation of the effect of the non-uniform gasket contact stress due to flange rotation on the leakage behavior of sheet gasket materials. A experimental investigation coupled with some numerical work were conducted to determine the effective gasket width and its variation with gasket type, flange flexibility and load.Copyright

Creep Constitutive Law of Packing Materials Based on Relaxation Tests

The tightness of valves, compressors and pumps is ensured by superposed braided rings installed in a system of stuffing-box. The nature of the packings material and structure, which is like a rectangular braided cord, influences the proper stuffing-box assembly behavior. During installation, a minimum compressive load is required to ensure a minimum level of tightness. A fairly large percentage of this axial compression load is transferred to the radial direction to generate the contact pressures at the packing-stem and packing-housing interfaces necessary for sealing. The packing is considered in several studies as a viscoelastic material with its creep-relaxation behavior assumed as one-dimensional rheological model. In the present work, relaxation tests in a test-bunch containing all the components of the packed stuffing-box, are carried out to define a creep constitutive law for packing braids of different materials. Based on three-dimensional compression tests the developed method is applied to three different packing materials.Copyright

An Experimental-Numerical Procedure for Stuffing Box Packing Characterization

The sealing of valve stems is ensured by the traditional systems of packed stuffing boxes. The performance of this type of sealing system used also in rotating equipment is dependent on the radial contact pressures generated by the packing axial compression. The mechanical behavior of a packing seal is characterized by the transmission ratio of the axial stress over the radial stress known as the lateral pressure coefficient which is one of the required characteristic used to select packing seals. However, the modeling of the packed stuffing box requires the knowledge of some mechanical characteristics other than the lateral pressure coefficient. In this paper, the mechanical characteristics of packing seals are obtained using a hybrid experimental-numerical procedure. The experimental study is carried out on an instrumented stuffing box packing test bench. The tests results of the experiments are coupled to the ones obtained by a finite element simulation of the test rig to determine the packing seal mechanical characteristics. Two packing types are used; one based on Teflon and the other one based on flexible graphite.Copyright

Short Term Relaxation of Stuffing Box Packings

The long term tightness performance of stuffing box packings, used in valves, is conditioned by the capacity of its material to maintain a contact pressure to a predetermined minimal threshold value. Due to creep, this contact pressure decrease with time depending on the creep properties and the stiffness of the housing. Assessing relaxation is a key parameter in determining the tightness performance of the stuffing box packing over time. Using Ansys software, an axisymmetric 2D finite element model is developed to assess the contact pressures between the packing material and the stem and the housing and its variation with time. The assessment of the packing relaxation is a major obstacle to the good leakage performance of the Stuffing Box Packing.Copyright

On the validity and limits of the gasket effective width concept

In the current ASME code procedure for the design of the gasketed bolted joints, the concept of the effective width is introduced to take into account the effect of the non-uniform gasket contact stress distribution. The code fixes a threshold value of the gasket width above which an adjustment of the gasket contact width is introduced. The validity of this threshold has never been verified. Under the normal operating conditions of flanges used with flat gaskets, the definition of this threshold is independent of the bolt load, the average gasket stress, the internal pressure and the flexibility of the joint. In this paper a study on the new limits of the concept of the effective gasket width is conducted. A new approach for the calculation of the effective gasket width, based on the results of an experimental investigation and undertaken in parallel to a numerical study, is presented. This approach takes into account the non-uniform distribution of the gasket stress, the flexibility of the flanges, the mechanical behavior and the sealing performance of the assembly.Copyright