Advertisement
News
Advertisement

What’s Protecting Your Pumps?

Mon, 03/20/2006 - 7:47am
Many bearing types and arrangements have been developed to satisfy even the most difficult centrifugal pump conditions.
The service life of pumps and their bearings can be maximized if bearing options are evaluated based on their designs and operating environments. This exclusive report takes an analytical look at bearing solutions for today’s centrifugal pumps. By Daniel R. Snyder, P.E. Centrifugal pumps working in chemical plants help keep operations flowing smoothly. In turn, their bearings play vital roles in pump performance. Bearings in centrifugal pumps support hydraulic loads imposed on the impeller, the mass of the impeller and shaft, and induced loads due to couplings and drive systems. They also help keep the shaft end-play and axial deflections within acceptable limits for the impeller and seal assembly. In addition, they reduce friction, which can result, if uncontrolled, in loss of power, excessive heat generation, increased noise and wear or early seal failure. If and when pump bearings fail over time, they usually do so for root causes other than true metal fatigue. Causes can include impact damage sustained during assembly, contamination (both liquid and solid), excessive or reverse thrust loads, inadequate or ineffective lubrication, static vibration, insufficient loading, improper installation or misalignment. All these influences suggest that the service life of pumps and their bearings can be maximized when bearing options are evaluated in the context of their designs and operating environments.
Single-row deep groove ball bearings are used most often in the radial position in nearly every centrifugal pump.
Identifying Requirements In centrifugal pumps, which are classified as kinetic or dynamic pumps, energy is added continuously to the system. Compared with positive displacement pumps, which are used to move viscous fluids, centrifugal pumps exhibit relatively lower radial and axial loads, fewer moving parts, lower operating costs and minimal pressure pulsations.Typical bearing arrangements for these pumps include radial bearings to support radial loads and thrust bearings to support axial loads created by hydraulic forces in the pump. Certain parameters initially can help guide the selection process such as external loads and direction, shaft and housing fits, as well as shaft material, and speed. These are reviewed below. 1. External Loads and Direction: The proper size and type of a bearing will hinge on its capacity to carry radial and thrust or axial loads. Bearings with low contact angles are designed primarily for radial loads. Bearings with higher contact angles are most suitable for thrust loads. In real-world pump applications, though, loads may vary from heavy to minimal and reverse direction. Applying an axial preload to the pair can promote adequate minimum thrust load for each bearing. 2. Shaft and Housing Fits: If the load-carrying capability of a bearing is to be realized fully, the inner and outer rings must be supported firmly and evenly around their complete circumference and across the whole width of the bearing raceway. The bearing rings alone cannot support the applied loads. The purpose of the rings is to pass that load to the shaft and housing so that they can support the applied forces. Rings must be reliably secured to prevent them from turning on or in their seatings under load. Inadequately or incorrectly secured bearing rings can damage the bearings and associated components. 3. Speed of Rolling Bearings: There is a limit to how fast rolling bearings can be operated. Generally, the limit will be governed by the operating temperature that can be permitted with respect to the lubricant being used or the material of the bearing components. Bearing type and size, internal design, load, lubrication and cooling conditions, cage design, precision tolerance class and mounted internal clearance all will determine permissible speed.
Selecting bearings involves reviewing the pump application, operating conditions and related on-site parameters.
Selecting Bearings Many bearing types and arrangements have been developed to satisfy even the most difficult centrifugal pump conditions such as those found in vertical and balance pumps. An overview of types and functions is below. 1. Single-Row Deep Groove Ball Bearings: Most often utilized in the radial position in nearly every centrifugal pump, these types are made in series and sizes to handle moderate to heavy radial loads, thrust load in either direction or combined radial and thrust loads. They have extremely low friction and noise characteristics. However, they are seldom used as a primary thrust bearing in centrifugal pumps. Deep groove ball bearings are available typically with two-piece pressed steel cages or may be available with pressed or machined brass cages on larger sizes. Hybrid designs also are available, combining steel rings with silicon nitride rolling elements. Hybrids can help promote longer service life in applications where there may be ineffective lubrication, excessive contamination or stray electrical currents. 2. Single-Row Angular Contact Ball Bearings: Single-row 40-degree angular contact ball bearings are the most popular API pump thrust bearings in service today. They generally are used in moderate-speed centrifugal pumps as thrust bearings where high thrust loads can be expected. Variations designed with robust machined bronze cages can run particularly well in applications where thrust loads vary greatly during operation and periods of ball skidding are likely. These bearings will resist destructive vibration forces when cavitation occurs. Other variations featuring 29-degree and 15-degree contact angles allow for higher speeds, lower thrust load requirements and greater radial loads. The bearings are mounted normally in pairs to accommodate reversing thrust loads.
Bearings often can make or break performance and productivity.
3. Double-Row Angular Contact Ball Bearings: These arrangements are used extensively as the primary thrust bearing in ANSI standard centrifugal pumps and some older API style pumps. The most effective types feature ABEC-1 precision tolerances, a 30-degree contact angle per row, ABMA C3 internal radial clearance, one-piece heat-treated pressed steel cages, and multiple sealing options. Since their contact angles diverge outwardly, the bearings exhibit greater rigidity and increased resistance to misalignment. Variations include styles with steeper 40-degree contact angles that deliver increased thrust capacity, machined brass cages that offer robust performance in heavy-duty and poor lubrication conditions, reduced axial internal clearances that promote load sharing between the two rows of balls, as well as a reduced possibility of skidding in the inactive ball set, and ABEC-3 (P6) tolerances that contribute to better control of the bearing’s mounted condition and smoother bearing operation.
Pump and bearing service life can be maximized when bearing options are evaluated in context of plant operations.
4. Specialized Angular Contact Ball Bearing Sets: High-performance types in this category consist of matched sets of 40-degree and 15-degree angular contact ball bearings with a computer optimized internal design. Depending on the arrangement, they can be utilized for centrifugal pumps with heavy thrust loads that are not reversing or reverse only periodically, for pumps involving very heavy primary thrust loads or for pump applications operating with light or no thrust loads at high speeds. (In pumps with minimal thrust, paired 15-degree angular contact bearing versions should be utilized.) The primary benefit of these sets is that their 15-degree bearing is designed with considerably less internal clearance than the 40-degree bearing, making it less susceptible to centrifugal and gyroscopic forces that produce ball sliding and shuttling, while still carrying a high radial load. 5. Angular Contact Split Ring Bearing Sets: Split inner rings designed into these types enable the assembly of an optimum ball complement while accommodating thrust in either direction. The split inner ring bearing is usually matched with a single-row angular contact ball bearing with the same angle (29-degree or 40-degree). This bearing set is commonly used in vertical pumps to handle the primary thrust load. Because two bearings acting in tandem share the thrust load, this solution offers an extremely high thrust-carrying capacity. Reversing thrust load can be accommodated on the back side of the split inner ring bearing. These two-bearing sets behave like “triplex” sets with the added advantage of saving space and costs. Whenever evaluating bearings for a particular pump application, the first rule of thumb is to understand that each type of bearing is designed for specific operating conditions and a particular bearing solution for one application may be inappropriate for another. Partnering at the outset with an experienced manufacturer can help chart the most appropriate decision-making course. About the Author: Daniel R. Snyder, P.E., is director of applications engineering for SKF Industrial Div., SKF USA Inc., 1510 Gehman Rd., Kulpsville, PA 19443. SKF is a global supplier of products and services in the rolling bearings, seals and related businesses. Questions about this article can be addressed to Snyder at 215-513-4680 or daniel.r.snyder@skf.com. Additional information is available at www.skfusa.com.
Advertisement

Share this Story

X
You may login with either your assigned username or your e-mail address.
The password field is case sensitive.
Loading