Bearing down on pump seal failures

Phil Burge, marketing manager at SKF, looks at how bearing and seal technology can extend pump life:

Luton, UK – Attempting to apply predictive maintenance practices to the ubiquitous centrifugal pump can be an elusive exercise to say the least. The problem with these pumps — the workhorses of the oil and gas, chemical processing, mining and many other manufacturing industries — is that they rarely fail due to commonplace wear and tear, which can be predicted through MTBF (mean time between failure) and similar analyses. Rather, they are more likely to succumb to unexpected and premature breakdown of some vital component.

Bearings in pump power frame applications are a case in point. Pump manufacturers may point to an expected life for a radial ball bearing that can run into years based purely on the unlikely possibility of metal fatigue, but in reality a bearing can fail prematurely without proper specification and selection of the sealing systems for bearings and lubricants.

Various industry studies have attributed premature bearing failures primarily to poor lubrication. Such failures — estimated at somewhere between a third and a half of all bearing failures — can occur at any time: within hours of installation, after a year or more of service, or even just before metal fatigue sets in.

While excessive wear on pump bearings can be caused by poor installation and shaft misalignment, most bearings that fail prematurely do so for one of two reasons — either the lubricant breaks down due to over-heating, or it becomes contaminated with water or solids. If a bearing seal in a pump fails, contaminants have an immediate opening to infiltrate both the bearing and the lubricant. And moving in the opposite direction, lubricant can be lost, leading to poor lubrication and eventual dry-running — major contributors to pump over-heating.

Although pump bearings and their seals should ideally be selected to suit the operating conditions, all bearings have to be able to perform specific functions. Clearly, they have to support the loads from couplings and drive systems as well as the hydraulic loads imposed on the pump impeller and shaft.

But they also have to maintain axial and radial deflections within acceptable limits for the impeller and shaft seal. And, of course, they have to minimize friction on the pump shaft. If left unchecked, friction results in power loss, heat generation, increased noise and wear, and ultimately bearing failure.

Fortunately, bearings are available for every application with leading manufacturers such as SKF, offering products suitable for the most benign in, say, low-pressure water pumping, to the highly aggressive conditions that can be encountered in the oil and gas industry.

In the latter heavy-duty applications, for example, centrifugal pumps conforming to API (American Petroleum Institute) standards will typically have an angular contact ball bearing set fixed in the bearing housing in the thrust position at the coupling end. Handling the thrust load and a portion of the radial load, this bearing secures the impeller in the proper axial position. The bulk of the radial load, meanwhile, is taken up by a single-row, deep groove ball bearing that floats in the housing near the impeller.

In other industries, light and medium duty pumps may be specified to ANSI (American National Standards Institute) standards. These are similar to API configurations except for the replacement of the angular contact bearing set with a double-row bearing, a single bearing with two rows of balls in the thrust position. With outwardly diverging contact angles, these double-row ball bearings offer greater rigidity and increased resistance to misalignment.

Whatever types of bearing are selected for the relevant duty, however, their efficient lubrication is vital in maintaining pump performance. Process pump bearings can be lubricated with grease, mineral or synthetic oils, or oil mists using manual or automatic dosing systems such as those produced by SKF. All of the materials will fulfil the primary purpose of providing a separating film between the roller elements and raceway contact surfaces, lubricating the sliding surfaces within the bearings, and providing corrosion protection and cooling.

Viscosity is probably the single most important property of a lubricant. Choosing the correct viscosity for the speed, operating temperature and load on the pump should ensure the development of a full film of oil between rotating parts. Get the viscosity wrong and the oil can degrade under load to a point where it becomes too thick to flow between the moving surfaces.

Viscosity aside, however, cleanliness of lubricant is paramount in avoiding premature bearing failure. Some industry estimates suggest that as little as 0.002% of water in the bearing oil can reduce bearing life by nearly a half, while a 6% water content will reduce the working life by over 80%.

Such water ingress can come about in several ways — from inadequate storage facilities for the lubricant, to direct penetration of the bearing assemblies by water from hose-down procedures in the event of pump leakage. The former clearly calls for better system design, while efficient bearing seals can prevent the latter.

As with bearings themselves, seal selection begins with choosing the correct general design for the application. Working closely with your supplier should be an important part of this process, for example, SKF has a dedicated team of experts that provide technical and applications advice and practical support to both OEMs and end users of pumps.

For ANSI-type medium duty applications, dynamic radial shaft seals are the most common choice. They typically feature a steel or elastomer shell and the lip seal itself. The latter is bonded to the shell and enables the requisite interference fit of the seal in the housing to be maintained.

In pump power frames the lip seal, usually made of elastomer, should always point towards the material being retained — that is, inwards towards the bearing and its lubricant. Most sealing lips are of a nitrile rubber composition, although other materials have been introduced for use with synthetic or chemically aggressive lubricants.

Also known as bearing isolators, labyrinth seals are increasingly being specified for heavier duty applications, typified by ANSI enhanced and API rated process pumps, and are making inroads into other service classes. As the name suggests, these seals incorporate a dynamic, non-contacting internal pathway between the static and dynamic parts of the seal. Installed correctly, they will exclude contamination, retain lubrication and offer a long service life.

Having opted for a particular seal design, the user then needs to take stock of all relevant operating conditions that can further influence selection. Among the key operating parameters are surface speed, temperature, pressure and the surface finish preparation of shaft and seal.

? Each type of radial shaft seal has surface speed limits, but in general seal performance will be reduced by higher shaft speed, seal torque, power consumption, under-lip temperature and dynamic run-out. All these speed-related parameters can contribute to shortened seal life. Most standard small bore radial seals (under 200mm shaft diameter) are rated up to 18.3m/s, while larger diameter seals can run to around 25.4m/s (5000ft/min), which is also within the capability of plastic labyrinth seals or bearing isolators. Metallic labyrinth seals can be capable of speeds up to 50.8m/s (10,000ft/min).

The negative effects of higher shaft speeds can be combated by several design variations: for example, reducing the radial load of the seal lip; changing to a sealing material capable of handling higher temperatures; changing lubricant or its viscosity; optimising the shaft sealing surface; or simply opting for a labyrinth bearing isolator.

? As with surface speed limits, each type of seal material has an optimum temperature range within which it should operate. Outside of that optimum range thermal stress can harden the elastomeric compound, causing it to crack. In fact, this type of heat ageing can be a more common cause of failure for nitrile rubber seals than wear. A seal’s thermal limit can, however, be extended by upgrading the seal material to a thermopolymer or PTFE.

Another aspect of operating at too high a temperature is the impact it can have on the lubricant itself. Some estimates suggest that good quality, uncontaminated oil or grease lubricants should have a useful life of many years operating at 30ºC, but that operating life can be halved for each 10ºC rise in temperature. As heat is added to the lubricant it will start to lose its viscosity and eventually start to form varnish and coke, depositing solids into the bearing.

? Some types of pump will use radial shaft seals as the main pressure retention seal for pressurised seal cavities. Seal manufacturers, such as SKF, can supply particular lip profiles for this duty to resist deformation under pressure and moderate surface speeds. At higher speeds, the allowable pressure differential across the seal will decrease so the two parameters, pressure and speed, have to be balanced against each other (the PV factor). Some radial designs, mainly using PTFE as lip material, can operate with a PV in excess of 250,000 depending on the service life required.

? Self-evidently, the smoother the shaft surface the better should be the sealing contact with a lip seal. For optimum radial lip seal performance and service life, a surface finish of 0.20-0.43?m Ra (arithmetical mean roughness) with a machine lead of less than 0 +/- 0.05 degrees is recommended.

Following the above selection guidelines should help in warding off the unpredictability of premature seal or bearing failure, but it would certainly pay to also follow a rigorous seal inspection and replacement programme — preferably in partnership with an experienced bearing and seal manufacturer.