Maximise reliability to optimise pump performance
07 October 2016
Phil Burge, Country Communications Manager at SKF outlines key factors that impinge directly on the reliability and smooth running of pumps.
Pumps are in use in almost every single industry, and are often production-critical pieces of equipment. In recent years, industrial pump development has centred on reducing total cost of ownership (TCO) and compliance with tough environmental legislation. A new challenge for pump manufacturers is to increase the meantime between failures of their products, while at the same time reducing their energy consumption, which accounts for nearly half of a pump’s TCO.
Pump reliability should start with the manufacturer. Apart from the materials of construction - which will be determined by the operating environment - and the overall hydraulic design, there are two key components that play a considerable role as far as reliability is concerned: the bearings and the bearing seals. Once a pump is in duty, it is the operator’s responsibility to maintain the reliability of his installation and here, two factors come to the fore: correct lubrication choice and procedures, and condition monitoring. So, there are four things to consider when embarking on pump performance optimisation and they involve both manufacturer and end-user: bearings and seals, lubrication and monitoring.
Taking each one in turn:
Choosing the right bearing
Pump bearings will often be subjected to high axial loads or receive marginal lubrication; they are also likely to suffer both high operating temperatures and mechanical vibration. Bearings are also designed to minimise friction, which, if uncontrolled, can result in power loss, excessive heat generation, increased noise or wear and early bearing failure. So, first and foremost, evaluate bearings (types, designs and arrangements) in the context of their anticipated operating environment.
Industrial pumps are some of the most common applications for angular contact ball bearings (ACBB), and these are generally used in small and medium sized pumps due to their high speed capability and low friction. They are also used when high axial load capabilities are needed for greater pump operational reliability.
Sealing the system
The pump bearing and its seal form an integrated system that performs four crucial tasks: lubricant or liquid retention, contaminant exclusion, fluid separation and confinement of pressure within the pump housing. Seal selection will be influenced by the application and any environmental concerns; for example, seals exposed to relatively constant pressure differentials are best served by pressure seals (in which the seal cavity is pressurised). Dynamic radial seals are generally the best choice for centrifugal pumps as they create the necessary barrier between surfaces in relative motion.
Ensuring proper lubrication
Improper lubrication accounts for around 36 percent of all bearing failures. Common methods for pump bearing lubrication include: grease filled, oil bath, oil ring, oil mist and air-oil. Oil mist generates the least amount of friction (allowing rotational speed to be based on the bearing design instead of lubrication limitations) and creates a positive pressure within the bearing housing, keeping contaminants at bay. Regardless of lubrication method, it is important to specify the lubricant according to the demands on vertical shafts and resistance to solids, pressure, temperatures, loads and chemical attack.
Monitoring pump health
Regular measurement and analysis of key physical parameters such as vibration and temperature can detect the onset of pump system problems, helping operators to avoid costly unscheduled downtime. Many problems will manifest as vibration, which is widely considered the best operating parameter to judge pump-train condition. Vibration can highlight problems such as imbalance, misalignment, bearing oil-film instabilities, rolling bearing degradation, mechanical looseness, structural resonance and even an inadequate foundation. Vibration measurements are quick and generally non-intrusive because the pump equipment remains undisturbed.
While these tools are extremely useful, it should be remembered that operators also play a pivotal role in the proactive maintenance strategy - by performing basic maintenance activities and serving as the ‘eyes and ears’ to detect equipment faults before problems escalate.
SKF helps manufacturers and end-users achieve optimum pump performance with a range of products and services. Where bearings are concerned, the company can supply ACBBs in basic and SKF Explorer performance classes, the latter providing up to three times longer life than regular bearings. Double row angular contact ball bearings, which correspond in design to two single row ACBBs, but which take up less axial space, are also available. These bearings have a high load carrying capacity, especially in the axial direction.
On the seals front, SKF’s ISO 6194 and DIN 3760 compliant standard seals and custom engineered sealing systems come in a broad range of elastomers, thermoplastics and thermoplastic elastomers. For worn pump shafts, SKF Speedi-Sleeve offers a useful solution as it can be installed without having to disassemble the shaft or specify a new replacement seal size.
SKF has developed multiple tests to assess the performance of lubricants under bearing operating conditions; these tests are widely respected and are used by lubricant manufacturers worldwide. The company can also supply lubricant delivery units such as SKF SYSTEM 24; these timed, gas or electro-mechanically driven grease cartridges are fitted to the bearing housing lubrication point to dispense lubricant in precise quantities at selected intervals.
Condition monitoring is an important maintenance tool and SKF provides a full portfolio of measuring and reporting solutions, for assessing vibration, temperature and other parameters.
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