Environmental Limitations

Every pumping application results in either positive or negative pressure as the throat of the pump stuffing box. A possitive pressure will force the liquid pumped through the packing to the atmosphere side of the pump. Higher pressure will result in greater leakage from the pump. This results in excessive tighening of the gland, which causes accelerated wear of the shaft or shaft sleeve and packing. For pressure at the stuffing box greater than 76 lb/in² (5.1 bar), some mean of the throttling the pressure should be considered. A combination of hard and soft rings die formed to the exact stuffing box bore and sleeve dimension can be used. Harder rings at the inboard end of the box and at the lantern ring and gland break down the pressure and prevent the extrusion of the packing.

I the packing itself cannot be used to break down the pressure then a throttle bushing must be used. This is a typical arrangement for a vertical turbine where the stuffing box is the discharge pressure of the pump. Here the throttle bushing to used to bring the liquid to almost suction pressure, and most of the leakage through the bushing is bled back to the suction. When the suction pressure in the stuffing box. This prevents air leakage into the pump and excessive flow and wear at the bushing.

When a pump to fitted with a bypass line from the discharge, a valve can also be used to reduce the pressure at the stuffing box. This is another method for reducing pressure for the benefit of the installation.

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 seal 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 the 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.

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 a to enable a degree of freedom for the primary ring to follow the mating ring.

An additional consideration in the selection of the primary and mating ring materials in sliding contact in 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 seal. Limiting PV values for various face combinations.

Common Material of construction for mechanical seals

Components	Material of Construction
Secondary Seals: O-rings	Nitrils, Ethylene Propylene, Chloroprene, Fluoroelastomer
Bellows	Nitrile, Ethylene Propylene, Chloroprene, Pluoroelastomer
Wedge or U Cups	Fluorocarbon
Metal Bellows	Stainless steel, Nickel base Alloy
Primary Ring	Carbon, Metal-filled Carbon, Tungsten Carbide, Siliconized Carbon, Bronze
Hardware (retainer, disc, snap rings, set screws, springs)	Stainless Steel, Nickel-base Alloy
Mating Ring	Ceramic, Cast Iron, Tungsten Carbide, Silicone Carbide