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Achieve optimum centrifugal pump performance

By David R. Mikalonis, SKF USA Inc.

ChemicalProcessing.com

Keywords: centrifugal pump and fluid handling

Proper bearings, lubrication and seals as well as proactive maintenance are crucial.

Centrifugal process pumps often must perform in hostile and stressful operating conditions. As a result, the likelihood of their premature failure increases over time — raising risks of incurring considerable costs and lost productivity from downtime. With pressures mounting to achieve better pump reliability, reduced maintenance and lower energy consumption, the quest to identify and implement viable solutions for relief ranks as a top priority.

Evaluating pump maintenance and repair practices — paying particular attention to bearings, lubrication and seals — can serve as a crucial tool for attaining higher levels of pump reliability and longer service life. The proper bearings, lubricants and sealing systems can boost economies and efficiencies and, when supported by proactive condition-monitoring approaches, can help extend the service life of pumps and minimize the need for untimely pump replacement or rebuilds.
 

Selecting the bearings

Bearings in centrifugal pumps support hydraulic loads imposed on the impeller, the mass of the impeller and shaft, as well as loads due to couplings and drive systems.

They also keep the shaft axial and radial deflections within acceptable limits for the impeller and shaft seal.

A variety of conditions tend to continually put bearings in many centrifugal pumps to the test. The bearings often will face high axial loads, marginal lubrication, and high operating temperatures and vibration, all while the bearings attempt to minimize friction — which, if uncontrolled, can result in power loss, excessive heat generation, increased noise or wear, and early bearing failure.

All these influences can dramatically impact the service life and reliability of bearings and, in turn, pumps. So, first and foremost, evaluate bearings (types, designs and arrangements) in the context of their anticipated operating environment.

Suitable bearings are available to satisfy even the most difficult conditions faced by centrifugal pumps. As an example, thrust-type bearings (to support axial loads created by hydraulic forces in the pump) include various versions of angular contact ball bearings that suit many applications:

Single-row 40° angular contact ball bearings. These are the most popular API pump thrust bearings in service today and generally are used in moderate-speed centrifugal pumps where high thrust loads can be expected.

Variations designed with robust machined brass cages suit applications where thrust loads vary greatly during operation and periods of ball skidding are likely. These bearings also will resist destructive vibration forces should cavitation occur. They normally are mounted in back-to-back paired arrangements to accommodate reversing thrust loads and to provide adequate shaft support to promote long mechanical-seal life.

Pay particular attention to internal clearance. When mounted and at operating temperature, the bearings should have enough residual internal clearance to operate cool but not so much as to promote skidding of the inactive bearing.

Double-row angular contact ball bearings. These (Figure 1) are used extensively as the primary thrust bearing in ANSI standard centrifugal pumps and some older API-style pumps. The most effective types feature a Conrad-design, ABEC-3 precision tolerances, a 30° contact angle per row, one-piece heat-treated pressed steel cages, and multiple sealing options. Because their contact angles diverge outwardly, the bearings exhibit greater rigidity and increased resistance to misalignment. As with paired single-row angular contact ball bearings, operating conditions dictate the need for normal or greater-than-normal (C3) internal radial clearance.

Figure 1. ANSI standard pumps typically rely on such bearings to handle thrust loads.

Design variations are becoming more popular. Examples include steeper (40°) contact angles to deliver increased thrust capacity, machined brass cages to offer robust performance under heavy-duty and poor-lubrication conditions, reduced axial internal clearances to promote load sharing between the two rows of balls and a reduced possibility of skidding in the inactive ball set, and ABEC-3 (P6) tolerances to contribute better control of the bearing’s mounted condition and smoother bearing operation.

Specialized angular contact ball bearing sets. For pumps with minimal thrust, paired 15° angular contact bearings generally suffice. However, for high-thrust-load conditions, use high-performance matched sets of 40° and 15° angular contact ball bearings to provide improved robustness by reducing the susceptibility of ball skidding in the inactive bearing. These designs are intended for centrifugal pumps that don’t reverse or only periodically reverse.

The primary benefit of these sets is that the 15° bearing is designed with considerably less internal clearance than the 40° bearing, making it less susceptible to centrifugal and gyroscopic forces producing ball sliding and shuttling, while providing additional radial stiffness to maintain integrity of the shaft and seals.

Split-inner-ring angular contact ball bearing sets. These, by matching a single-row 40° angular contact ball bearing with a split-inner-ring ball bearing or four-point contact ball bearing, are designed to accommodate thrust loads in either direction. This arrangement commonly is used in vertical pumps to handle the primary thrust load but also can be utilized in horizontal arrangements, provided the loading is such that the split-inner-ring bearing doesn’t support radial load on its own.