Valve Packing Characterisation & Design
There is an increasing demand from valve end users for low emission sealing systems, but it remains important that the valve continues to move smoothly and efficiently. Since the frictional load of the stem packing has a contrary effect on these two requirements, valve and valve packing manufacturers are challenged with respect to the prediction and improvement of packing behaviour in a valve
This is the reason that the European Sealing Association (ESA), in collaboration with the Fluid Sealing Association (FSA) and the Fluid Equipment Committee of French research house CETIM (a working group composed of French valve and sealing product manufacturers who initiated this program) are developing a tool to predict the initial packing tightening force that is required to reach certain characteristics in relation to friction and sealing performance.
A European Standard is already in place for flanges that allows end users and engineering firms to engineer a bolted flange connection in such a way that the minimum leak rate is being achieved at the minimal cost. EN1591-1 has been developed to predict flange behaviour through mechanical calculations. The parameters on which this method feeds are being generated by EN13555 gasket parameters tests. However, unlike this situation for flange gaskets there is no current standardized method that characterizes valve stem packing parameters, such as mechanical behaviour, friction, sealing performance versus packing load, in a similar way.
Several emission standards have been developed in the last 2 decades that test either the valve or valve packing for their fugitive emissions tightness. VDI2440, API622, ISO15848-1 and API624 are all standards that measure valve or valve packing leakage under a certain set of given conditions such as temperature, pressure, mechanical cycles, etc. These standards however, do not model packing behaviour. On the other hand calculation methods have been developed to model the behaviour of mechanical packing, but these have never been linked to actual leakage and mechanical behaviour of the packing in a real stuffing box.
This is why the initiative has been taken to develop this new calculation tool for valve packing. The development project for this new tool consists of a calculation part and a parameter testing part.
The calculation method that predicts the behaviour of packing in the stuffing box is simplified in a number of ways to assure the practicality of the method. It consists of the calculation of the initial bolt force required to fulfil the tightness criteria at installation and at operation, a check on the mechanical integrity of the bolts and gland and the check of packing friction. Thermal expansion and contraction are considered as well as creep behaviour of the packing rings in the theoretical calculation.
The second part of the project has been the development of a testing procedure to determine the packing ring characteristics needed for the calculation method. Tests have been carried out on individual rings. Parameters that are being generated are static and dynamic coefficients of friction (µs and µf), the axial to radial transmission coefficient (K), Young’s modulus (ER), relaxation coefficient /variation of ring thickness (Rx, Dep) and packing thermal expansion (aR).
Sealing behaviour at room temperature has also been tested by applying the initial packing stress, operating the stem, measuring the leakage, lowering the gland stress and repeating this procedure for each lower stress level.
Applying these tests to any specific packing will now enable a valve designer or any other packing user to predict the leakage and friction behaviour of a packing in a valve, therefore enabling him to more effectively design the valve, carry out packing installation in a calculated and planned way and in the end save costs by improving the efficiency of the whole valve system.