Sealing technology manual for the process industry - Stuffing box packings

STUFFING BOX PACKINGS From centrifugal and piston pumps to applications on spin- dles and valves – stuffing box packings are a type of seal broadly used throughout the food and beverage industry where sealing is achieved by compressing the packing. The functional principle of a stuffing box packing consists in the axial compression of endless woven cords pressed into rings. This process generates a radial compression on the sealing location, which must be greater than the inter- nal pressure to ensure reliable sealing. Using different types of stranding and packing materials to match the respective chemical and thermal requirements, stuffing box packings can be tailored to a variety of application scenarios. In addi- tion to the classic woven packings, Freudenberg also offers the special Valtec variant, made of a non-woven material impregnated with PTFE which is wound and compressed into rings. Pressures play an important role in the sealing function. When there is no pressure on the packing, a good compres- sion set is necessary to maintain a good seal using resilience. Stuffing box packings are self-amplifying under pressure: their maximum pressure load is influenced by the gap width. To improve their gliding ability and therefore the cross-section impermeability, stuffing box packings are also impregnated with lubricants. Function • Sealing in pressure-free environments by means of restor- ing force • Self-amplifying sealing function under pressure (the higher the pressure, the better the sealing) Dimensions and types • Double, triple and quadruple diagonally braided, with quadratic cross-section from 3 mm to 25 mm, available by the meter • Tube-braided packings, also available as double-layer bands • Braided packings as pre-cut components or pressed in ring form • Graphite versions as pre-pressed rings and bushings • PTFE-impregnated packings (Valtec) as rings or bushings • Combination of different packing rings to a set, e.g. with anti-extrusion versions on the outside and low-friction rings on the inside • Use of “lantern rings” for lubrication, shut-off or cooling • Versions with internal or external springs for plunger pumps • Grafiflex as cover seals in internal pressure-supported systems DESIGN NOTES Packing cross-sections • For centrifugal pumps, plunger pumps and mixers: s = 1.4 x ½ d (d = shaft diameter in mm) • For valves: s = 1.2 x ½ d (d = spindle diameter in mm) Surfaces • Surface roughness for shafts, spindles, and valves: Ra <0.25 µm • Surface roughness for housings: Ra <0.25 µm Spindle hard- ness Min. 40 HRc, for heavier loads 60 HRc Amount of guide play in the spindle Less than 1 /1,000 of the spindle diameter Gap widths • Max. permissible gap between spindle and gland packings/housing bore = 2 /100 of the packing width • For larger widths use packings with high-strength aramid yarns Radial shaft deflection Max. 0.001 x the shaft diameter Double Triple Quadruple Concentric Types of stranding Product portfolio 80

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