Compression packing is the primary method employed to seal the stem of many types of valves. It is made from soft, pliable material that is cut or formed into rings and compressed in an annular cavity to seal around a moving shaft. Compression packings are contained and compressed by a packing gland within a cavity called a stuffing box. It is a technology that has existed for a long time.
The perception is often that this is an outdated technology suited to the steam-powered equipment of past centuries rather than one capable of meeting the needs of modern industry. But nothing could be further from the reality of contemporary packing products. Current technology utilized to design and manufacture high performance ensures that packing will be keeping up, not only with current but also with future needs and requirements.
Many factors need to be considered for a properly functioning sealing system, starting with packing construction, the fiber utilized for sealing, the reinforcements to prevent extrusion, and the blocking agents and lubricants to prevent permeation and to reduce friction. Then, it is necessary to establish the load sufficient to compress the packing against the stem to achieve sealing while maintaining friction as low as possible.
The fibers used to braid the packing have evolved from natural fibers, either vegetable or mineral, toward high-performance synthetic fibers. Some of the materials that are currently used to make the fibers are carbon, graphite and Polytetrafluoroethylene (PTFE). The fibrous material is then braided into a square shape. Typical constructions are square braid (Figure 1) or lattice braid (Figure 2). Each have their own characteristics in terms of formability and malleability.
But a compression packing is much more than braided fibers. Braided mechanical packing is a “system” made of fibers, braid style, lubricants for break-in or blocking, and corrosion inhibitors. General requirements for packing are resilience, chemical resistance, strength and temperature resistance.
Modern formulations provide high performance, reliability and long life. Testing of individual installations can be used to determine the performance, but torque required, friction on the valve stem, and emission levels have been left to experts and experience.
Test standards have been developed to validate compression packing emission performance. These require sophisticated equipment for the packing tests per API 622, and for valve emission testing per ISO 15848 and API 624.
One of the areas that has been lagging is the predictability of performance. There is an increasing demand from valve end users for low-emission sealing systems, but it remains important that the valve continue 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 behavior in a valve.
Development of a Predictability Tool
This is the reason 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 required to reach certain characteristics in relation to friction and sealing performance. Part of the project is the development of a testing procedure that determines the packing ring characteristics needed for use in a calculation method for the torque required to achieve certain performance characteristics.
Mechanical tests will be carried out on individual rings, but the test rigs have been developed to be able to test full packing sets up to 6 rings. The application of a gland stress, applying stem movements, heating the stuffing box assembly to the test temperature and then repeatedly reducing the stress while applying stem movements and measuring the friction will generate data to establish a number of parameters, including the ring compression as a function of applied stress, the dynamic coefficient of friction and the axial force to radial force coefficient of transmission. Further testing will generate the ring modulus of elasticity, the relaxation coefficient of the packing ring, the deflection variation over time of the packing ring due to creep and the coefficient of thermal expansion of the packing ring.
The last part of the test determines the sealing performance of the packing. After application of stress on the packing and actuating the stem several times, the leakage is measured by helium mass spectrometry. Then the stress on the packing is reduced a number of times after which the leakage is measured at the lower stress level. This procedure is repeated several times to determine a reliable relational curve between packing stress and leakage.
The test setup consists of a hydraulic cylinder to actuate the stem, a stem movement transmission section with load or torque sensor, a hydraulic press to apply the gland load, and a test cell containing a stuffing box with up to 6 packing rings (Figure 3).
The test cell can be interchanged depending on whether leakage or mechanical behavior is to be measured. The stuffing box is dimensioned and toleranced as defined in API622. The sealing test cell is able to measure the leakage rate of 1’’ stem diameter packing for internal helium pressures up to 8 MPa.
The mechanical test cell allows the testing of a variable number of packing rings. It is equipped with strain gauge chains each made of 10 strain gauges to measure the stuffing box external diameter deformation. This enables determination of the value of the axial-to-radial contact pressure transfer coefficient. The test cell is also equipped with a load sensor to measure the transferred axial load to the bottom of the packing, and the three displacement transducers positioned at 120-degree angles to measure the packing deflection (Figure 4).
Preliminary test results show the leakage is indeed dependent on the stress level applied to the packing, and that extremely low leakage levels can be achieved. Figure 5 details the relationship between stress level and leakage rates.
The ISO 15848 Valve Testing Standard sets three classes of leakage performance. Class C was originally intended for graphite packing, class B for PTFE packing and Class A for bellows valves. It can be seen that the highest level of performance, Class A, can be achieved with the right amount of axial compression with 4 graphite rings. Certainly axial loads of 60 and 80 MPa are high, and the valve construction must be such that it can handle these loads without deformation, but these results show how modern packing formulation and manufacture can achieve the most stringent requirements.
Testing of packing in a fixture such as that specified by API 622 is not a guarantee the same emission level will be achieved in a given valve. Factors such as surface finishes, concentricity, tolerances on diameters, bolt stress and deformation under load all affect emission results. After tests have been done to qualify the packing, further tests are then needed to qualify the valve with the packing. But information is now available to predict stress levels required for a given performance level. Cooperation between the valve and packing manufacturers is essential to achieve the best results.
Acceptable emission levels are lower now than ever. The sealing products have higher performance than ever. Compression packing formulation, design and testing utilize some old technology but also state-of-the-art engineering tools. Parameters such as compressive loads and friction will no longer just be based on experience and occasional verification, and there is no doubt that compression packing emission sealing is a modern technology.