Low Leakage Packing – In North America, the Environmental Protection Agency (EPA) has recently enacted strict guidelines for the maximum allowable leakage of specific pollutants to the atmosphere. For many fluids and applications, the maximum allowable leakage from gasketed joints and stem packing is 500 ppm (parts per million). This low leakage requirement has led to the development of several new packing designs.
Figures 28 and 29 illustrate PTFE and graphite based, low leakage packing arrangements that are suitable for use in ANSI Class 125 through ANSI Class 600 valve bodies. Note the following general design features that have been developed to improve stem sealing:
• Low leakage packing arrangements include fewer packing rings than standard packings. The inclusion of fewer packing rings results in a higher stress on each ring (more force in terms of psi). The higher stress results in tighter sealing.
• A smaller packing surface results in less stem and packing wear, and improved packing life.
• The packing is live-loaded through the use of externally located Belleville springs. Live loading places a constant load on the packing. Live-loaded packings require less maintenance than jam style packings.
• Massive guiding prevents lateral stem movement that can result in packing damage.