The primary factors affecting seal selection are temperature, wear resistance, abrasion, sealed pressure, face materials, vibration, and expected life. Usually, bellow seals are needed for high-temperature applications.
Temperature affects all seal materials, but its most important effects are on the secondary seals. The general limitation on temperature for standard synthetics is about 225°F although some are available for uses up to 600°F. PTFE can be used over a temperature range of -400 to 550°F, although most manufacturers rate PTFE seals on the basis of 500°F maximum temperature.
Asbestos elements have been used up to 650°F. Above 650°F, metal bellows, U-cups, or piston rings can be used, but these seals are considered specials.
Lubrication can reduce heat generation at the seal interface, but care must be taken to prevent coking. Direct cooling with a cooling chamber and heat exchanger can help control thermal problems. In this method, an integral pumping ring on the rotating seal element circulates coolant through an inner chamber in the stuffing box.
Wear resistance depends largely on temperature and chemical factors, and on abrasives. To minimize wear, the sealed fluid should be a good lubricant for the materials of the seal head and seat. Furthermore, all seal materials should be virtually impervious to corrosion by the sealed fluid.
Face materials subject to dry running because of malfunctioning equipment can fail prematurely. Double seals with isolated liquid circulation avoid this hazard. For systems with external circulation, pressure drops can be detected with a pressure-sensitive switch.
Abrasion is the bane of face seals. Faces should be cleaned before initial start-up to prevent premature failures. With liquids that form abrasives on contact with air, a buffer zone or quench gland should be provided between the atmosphere and the seal faces. With liquids that form abrasives at certain temperatures, heating or cooling is necessary to dissolve abrasives near the seal faces. With liquids that are inherently abrasive, a neutral clean liquid can sometimes be injected into the seal chamber. If the sealed liquid cannot be contaminated, a double-seal design can be used, or a centrifugal separator should be inserted ahead of the seal.
Face materials must be compatible with each other and the sealed fluid. Because of their good mechanical and thermal properties, graphites are generally used as one of the primary sealing elements. The opposing element can be made of ceramics, iron, bronze, stainless steel, tool steel, and various other metals plated with dense chrome. The ceramics are some of the hardest face materials available and have excellent wear and corrosion properties. However, they cannot stand tensile stress and are subject to cracking by thermal shock.
Vibration can shorten the life of a seal, particularly if imposed vibration during operation has a frequency near the natural frequency of the seal. The basic precaution is to ensure the seal's natural frequency is higher than the highest imposed frequency. This precaution is particularly necessary with metal bellows.
Life expectancy depends on both shelf life and operational life. The shelf life of metal bellows is practically unlimited, whereas organic secondary seals may have shorter shelf life, particularly at elevated temperatures. Within their temperature limits, elastomeric bellows have better operational life than metal bellows. However, metals withstand higher temperatures.