FACTORS IN RELIABILITY
Lubrication and sealing systems should play an important role in pump selection, design, manufacturing and purchase because they significantly affect pump operation and reliability. Often, repairing a pump doesn't solve the underlying problem and the failure recurs. In these cases, a truly effective solution requires correct root-cause analysis — this demands sufficient knowledge of the lubrication and sealing systems.
Lubrication. Oil bath lubrication may suffice for a small pump but can't be used if frictional heat generation is relatively high. It isn't suitable when the shaft size or speed of the pump exceed certain levels. As a very rough guide, don't consider oil bath lubrication for shaft diameters exceeding 1.5 in. (40 mm) or speeds over 3,000 rpm. If the oil level reaches the center of the bearing, a considerable amount of heat could be generated. So, set the oil level below the center of the bearings. When the linear speed in the bearings increases beyond a certain point, oil bath lubrication and oil rings become unsuitable and unreliable. The oil rings in particular often cause problems. For pump designs above a certain size or beyond a certain speed, an additional device to lift and spray the oil into the bearings is worthwhile. Today, cartridge-mounted bearings with flinger devices (to lift and spray oil into the bearings) are popular for pumps. Oil-mist (oil-fog) lubrication systems are gaining favor for small-size and some medium-size rotating machines using rolling element bearings.
Sealing system. Very few new pumps still use packing. Pumps instead rely on mechanical seals. In these, the seal faces are separated by a small gap into which a clean, cool flushing fluid is injected. This fluid not only prevents contact between the faces but also cools them. Use of a flushing fluid requires a number of connections, including ones for fluid inlet and outlet (both usually at the top of the pump), a vent (to remove vapor or gas from the seal cavity) and a leakage monitoring port. When ambient air must be prevented from penetrating the space between the seal faces or greater cooling, safety or reliability is required, pumps can be equipped with a dual seal; it has a secondary seal in addition to the primary one. In this design, a secondary fluid, known as the "barrier" or "buffer" fluid, is injected into the gap between the primary mechanical seal and the secondary one.
Pumps, e.g., for general applications and clean liquids, often use a "Plan 11" seal system in which a liquid side stream is recirculated. This side stream should have a pressure around 1.5–2 bar higher than that directly behind the pump impeller where the mechanical seal is located. Usually an orifice also is located in the flushing liquid route. Clogging of seal piping or the seal orifice reportedly is a common issue.
A "Plan 23" seal system includes a heat exchanger. It offers better thermal management of the flushing liquid and, thus, enhanced seal reliability and performance. This plan is popular for pumps in hot services or handling liquids working near their vaporization point. It can improve the vapor pressure margins in the seal chamber. Today, the "Plan 23" is very popular for hot water services, especially boiler feed-water pumps. The performance of the heat exchanger, which usually is cooled by cooling water, requires careful monitoring in all services.
Operators have avoided the "Plan 23" for a number of reasons, including its greater complexity. However, modern "Plan 23" cartridge-type mechanical seals easily can be used in a wide range of pump applications.
Dual seals using a secondary fluid need a special seal support system. "Plan 32" often has been used for these applications but now "Plan 53A" is more popular; in many pumps, the "Plan 32" has been converted to the "Plan 53A." Dual seals and "Plan 53A" are employed for harmful or hazardous services. This arrangement also is chosen for liquids that contain suspended solids or that have poor lubricity. Dual seals commonly are specified for services that can't tolerate even a small leakage to the atmosphere.
AMIN ALMASI is a rotating equipment consultant based in Brisbane, Australia. E-mail him at email@example.com