Many processing units, including pumps, compressors and blowers, depend on rotating shafts to operate safely and productively. These shafts must be equipped with reliable seals to prevent releases of potentially hazardous materials.
The choices of seal technology for the given application are many. Mechanical seals are one option. “In my experience, double-seal cartridge types are most common,” says Dan Towse, senior consultant at T.A. Cook’s consulting office in Raleigh, North Carolina.
“We deal primarily with process machinery, with the notable exception of pumps, so mechanical packing is the most common seal we find in service,” says Starkey Steuernagle, general manager at Meco Seal, a subsidiary of Woodex Bearing.
“Specific seal types and models vary significantly from industry to industry, because the pump type, operating conditions and fluid characteristics might require the design features that are unique to a specific seal type,” says Michael Huebner principal engineer at Flowserve. Pusher style mechanical seals are the most common type, says Huebner, but the choice depends on the specific industry. “Most industries have moved toward cartridge style seals because they simplify installations and generally improve reliability,” he says.
Like nearly every other device, sooner or later, regardless of technology chosen, the shaft seal will exhibit signs of impending failure. The concept of mean time between failures (MTBF) might be elusive. “Any discussion about MTBF must begin with a caveat that not all users define MTBF the same way,” warns Huebner. He clarifies this by saying that, while the concept is simple, the actual definitions and recording methods used in industry vary widely. For example, if a seal fails because of a bearing failure, does that count as a seal failure? How does a user account for a seal that was changed out before failure? Different methods might be used even within the same company, which makes it difficult to compare MTBF data from different sources accurately.
Figure 1. This air-purged, double-faced sanitary mechanical is designed for service in an ATEX environment on a pharmaceutical product mixer. The carbon brushes prevent triboelectric discharge. (Meco-Woodex)
Much depends on what went into the seal selection and its installation and ongoing care. “If the shaft run out, motor/pump alignment, coupling condition, cooling, and PM and PdM of pump and motor are within design specifications,” says Towse, “I’d expect 18-month to 2-year MTBF in a centrifugal pump application.” Other factors include minimizing shock loading by means of soft starts and barrier fluid tank operation.
Attention to such details should extend the average MTBF. “I’ve witnessed dramatic improvement in MTBF after the implementation of best practices methodology, timely PM/PdM practices and an increase in operator awareness of asset reliability best practice,” reports Towse.
But such improvements aren’t automatic. “MTBF has a direct correlation to an end user’s focus on reliability,” says Huebner. “End users who continuously identify causes of seal failures and address underlying problems see improvements in MTBF. Customers who don’t try to improve reliability systematically generally don’t see any improvements.”
Monitor and predict
Failures come in many flavors. “Seals often fail as a result of other equipment damage,” says Steuernagle. “Constant high shaft runout causes packing to wear and fail prematurely; the presence of abrasive process material between shaft and packing often damages shafts. Bearing failures frequently result in failure of packing and mechanical shaft seals.”
Towse cuts to the chase. The primary causes of failure are “poor PM and PdM practices, process upsets and operator error,” he says.
“There have been numerous studies on seal failure that show the wide range of failure modes seen in industry,” added Huebner. “The predominant seal failures are caused by poor equipment condition and operation of the seal outside of the seal’s rated operating window. These manifest themselves in failure modes such as dry running, fretting, broken faces, hang-up and heavy face wear. Most seal failures can be addressed by ensuring pumps are in good condition, the seal OEM understands the application and the user operates the equipment correctly.”
Maintenance teams have been focused on predictive technologies for many years now. It’s a source of competitive advantage. The most useful predictive technologies for seals are those that are actually performed, says Paul Wehrle, chief engineer at Meco. “In purged, double-face seals, monitoring the quality of purge retention often can predict the need for seal maintenance,” he says.
“Monitoring purge/barrier fluid pressures and retention is useful, but performance monitoring should always include regular visual inspections of the seals,” adds Steuernagle. “This informs, not only regarding the condition of the seal, but also of other aspects of machine operation that might affect sealing.”