Seals play a crucial role in centrifugal pumps. They serve at both the "dry" and "wet" ends of the pumps, primarily to retain lubricant, exclude contaminants, separate fluids and confine pressure. Without effective seals, contaminants (solid or liquid) can find openings to infiltrate both the lubricant and bearings -- leading to potentially dire consequences impacting both the cleanliness and integrity of the lubricant and the life of the bearing. And, in cases where seal failure causes lubricant loss, dry-running operation can prompt premature and rapid deterioration of bearings and, in turn, the pump.
The correct specification of seals consistent with operating conditions ultimately can help maximize component service life and keep pumps up-and-running as intended.
Bearings in centrifugal pumps support hydraulic loads imposed on the impeller, the mass of the impeller and shaft, and loads due to couplings and drive systems. They also keep shaft axial and radial deflections within acceptable limits for the impeller and shaft seal.
Dynamic radial shaft seals, which come in a variety of designs and materials, commonly protect the power frames of API heavy-duty process pumps and ANSI light- and medium-duty class pumps; bearing isolators or labyrinth-type seals usually are located at the pump's wet end. Many API pumps are migrating gradually to bearing isolators at both the thrust and line ends.
In general, dynamic radial seals create a barrier between surfaces in relative motion (one usually is stationary while the other rotates). These seals -- more often than not made from nitrile rubber -- may feature a plain, wave or helix lip design. In many cases, a garter spring holds the primary sealing lip in position and also promotes oil retention. Standard seals usually incorporate a simple L-shaped shell with the sealing material bonded to it; others also may include an inner shell to help protect the lip from damage or distortion during installation.
Bearing isolators or labyrinth-type seals (Figure 1) provide highly effective exclusion capabilities. Instead of a contacting radial lip element, these seals rely on a labyrinth or maze-like internal structure to collect and eject contaminants before they can intrude. Designs typically integrate a stator pressed into the housing and a rotor fixed to and turning with the shaft. The two components are locked together for easier assembly and to prevent damage during handling. Standard versions usually will incorporate polytetrafluoroethylene (PTFE) for the structural body material and fluoroelastomer O-rings to promote high chemical and temperature resistance.
MAKING A CHOICE
Seal specification for centrifugal pumps begins with identifying the correct general design for the application, followed by proper sizing of the hardware (never mix inch and metric dimensions and tolerances). Then, for optimized seal performance, evaluate all relevant operating conditions to narrow the field.
Important operating parameters to consider include:
Surface speed. Radial shaft seals are designed to perform within designated surface speed limits. Generally, surface speed capability is inversely related to parameters such as seal torque, power consumption, under-lip temperature and the effect of dynamic run-out. All these speed-related influences can affect seal life.
The majority of standard small-bore (under 8 in. shaft diameter) radial seals are rated up to 3,600 ft/min, while larger diameter seals are rated to approximately 5,000 ft/min. PTFE bearing isolators usually can work at up to 5,000 ft/min, while metallic versions can handle 10,000 ft/min. An application calling for higher speeds requires specialized design considerations.