Materials Of Construction

All component parts of a seal are selected based on their corrosion resistance to the liquid being sealed. The National Association of Corrosion Engineers (NACE) Corrosion Handbook provides corrosion rates for many materials of construction for mechanical seals used with a variety of liquids and gases. When the corrosion rate is greater than two mils (0.05 mm) per year, double seals that keep the hardware items of the seal in a neutral liquid should be selected to reduce corrosion. In this design, only the inside diameter of the mating ring, the primary ring, and the secondary seal are exposed to the corrosive liquid and should be constructed of corrosion-resistant materials, such as ceramic, carbon, and Teflon. Common materials of construction are given in Table 3. Table 4 lists the properties of common seal face materials.

The operating temperature is a primary consideration in the design of the secondary and static seals in the assembly. These parts must retain their flexibility throughout the life of the seal, as flexibility is necessary to retain the liquid at the secondary seal as well as to enable a degree of freedom for the primary ring to follow the mating ring. The usable temperature limits for common secondary and static seal materials are given in Table 5.

An additional consideration in the selection of the primary and mating ring materials in sliding contact is their PV limitation. This value is an indication of how well the material combination will resist adhesive wear, which is the dominant wear in mechanical seals. Limiting PV values for various face combinations are given in Table 6. Each limiting value has been developed for a wear rate that provides an equivalent seal life of two years. A PV value for an individual application can be compared with the limiting PV

TABLE 3 Common materials of construction for mechanical seals

Components

Materials of Construction

Secondary Seals: O-rings

Bellows

Wedge or U Cups Metal Bellows Primary Ring

Hardware (retainer, disc, snap rings, set screws, springs) Mating Ring

Nitrile, Ethylene Propylene, Chloroprene, Fluoroelastomer, Perfluoroelastomer

Nitrile, Ethylene Propylene, Chloroprene, Fluoroelastomer Fluorocarbon

Stainless steel, Nickel-base Alloy Carbon, Metal-filled Carbon, Tungsten Carbide, Silicon Carbide, Siliconized Carbon, Bronze Stainless Steel, Nickel-base Alloy

Ceramic, Cast Iron, Tungsten Carbide, Silicon Carbide value for the materials used to determine satisfactory service. These values apply to aqueous solutions at 120°F (49°C). For lubricating liquids such as oil, values of 60 percent or higher can be used. Higher or lower values of PV may apply, depending on the seal face design.

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