Shaft Seals Are Under Constant Scrutiny
They are shaft seals - seemingly simple items, which are the result of detailed chemical analysis, engineering, and precise manufacturing. For the benefit of contractors and technicians who fuss and sometimes fume over shaft seal failures, they are under constant scrutiny for improvement.
At the 9th Annual Compressor Focus Conference, sponsored and hosted by John Crane Inc. just a few miles from the company's Cranston, R.I., plant, 33 invited attendees came from throughout the industry and from within the company for two days of technical talks and tours. They were welcomed by Bob Wasson, president of John Crane Americas, who traced portions of the company's history back to its founding in 1917 at the Crane Packing Co. to its current configuration as a subsidiary of Smiths Group.
Then it was on to technical talks.
O-RingsJohn Lengenfelder, marketing manager for Parker Hannifin, detailed O-rings for refrigeration service. O-rings are one critical component in shaft seals. He said advantages of O-rings include the ability to seal for a large range of pressures and temperatures, the fact that there is a broad selection of materials, that they compensate for metal tolerances, that wear is normally gradual, and that they are reuseable and economical.
He noted the factors that apply to all O-rings include temperature, media, pressure, lubrication, squeeze, stretch, and compression force. "You should consider all of these factors," he said.
He maintained that companies like Parker "have solutions for hundreds of applications."
Lengenfelder reported on re-search Parker was doing on O-ring design for carbon dioxide systems.
He concluded that "lubricating oil compositions vary widely, specific elastomer formulations must be evaluated, explosive decompression issue considered, and a design approach must be included in the overall solution."
Mechanical Shaft SealsFactors affecting mechanical shaft seal performance comprised the topic of a presentation by Crispin Elt, a senior design engineer for John Crane in the United Kingdom. He said key factors include film thickness, viscosity, and pressure differential.
He encouraged attention be paid to the benefits of hydraulic balance because that factor reduces hydraulic closing force, heat generation, wear rate, and the power required to drive the shaft seal.
Another factor regarding film thickness is shaft seal face waviness. Elt cited causes of waviness including physical damage such as knocks, dents, and nicks to seal rings and housings; self-induced waviness because of unequal contact and wear; drive distortion because of uneven distribution of driving forces; non-uniform spring loading; and thermally induced waviness.
"What can we do to influence performance?" Elt asked.
He said seal design considerations directly influence performance. These include seal types involving face geometry, balance ratio, spring design and load, and the drive system for seal head and mating ring. Consideration also has to be given to face material properties and surface roughness.
"There is a seal designers' dilemma," he said. "Design is a compromise between low leakage/high wear and high leakage/low wear. The challenge is to create the optimum design for all operating conditions and that is not easy to do with variable operating conditions."
Refrigeration Shaft SealsDuring a session at the John Crane plant that followed a tour of the facility, the attendees were given a presentation on projects the company was undertaking.
Hai-Ping Ma, product development engineer, noted in his presentation the company's commitment to real-world testing, including the simulation of actual seal operating conditions. This is accomplished both in a live refrigeration compressor system and in dedicated test rigs. Ma noted that the "objectives of testing are to optimize the compressor operating conditions for seal performance and to optimize the seal design for worst operating conditions."
Short-term goals for current programs include finding ways to lower seal face temperature through flush and cooling system optimization, seal design, and cartridge design, as well as reducing leakage through seal face materials optimization, seal surface profile, and secondary seal optimization.
Back To BasicsDr. Kenneth Lilje, Director of R&D for CPI Engineering Services, noted that lubricants are used to cool, to clean, to seal, and to lubricate.
"The major factors that impact the effectiveness of a lubricant are viscosity, stability, and lubricity," he said. "Viscosity usually is the first property to consider in selecting a lubricant. Temperature, pressure, and dilution can significantly alter or lower the viscosity of a lubricant."
During his talk, Lilje reported on experimental methods in evaluating the stability of lubricants. Among key points, he noted that high temperatures can degrade lubricants to form sludge, varnish, and deposits. Oxygen and metal surfaces can further accelerate the speed of lubricant degradation, he indicated.
Lilje also noted that synthetic lubricants, such as esters, are more stable than mineral oils, meaning longer life and fewer oil changes.
CompressorsLarry Bradley, mechanical product engineer for FES Systems, took a look at the basic operation of screw compressors, which he noted were developed in the 1930s, became popular in Europe for refrigeration in the 50s and 60s, and caught on in the United States in the 1970s.
He said the lubrication system within such a compressor:
He also looked at pumps including full-flow (full-time) oil pumps; full-flow cycling oil pumps; part-time oil pumps; full-time, part-flow oil pumps; and internal oil pumps.
Attention was paid to thermosiphon oil cooling, which utilizes liquid refrigerant from the condenser to remove heat from the oil. In the concept there is a "shell-and-tube heat exchanger with oil on the shell side and refrigerant on the tube side."
The approach "acts as a gravity-fed flooded evaporator with an evaporating temperature equal to the condensing pressure. Heat is rejected at the condenser. Oil temperature is controlled with a thermostatic mixing valve."
Water-cooled oil cooling, Bradley said, "utilizes water or glycol to remove heat from the oil and a shell-and-tube heat exchanger with oil on the shell side and coolant on the tube side. The coolant is circulated with a pump. Heat is rejected to a cooling tower. Oil temperature is controlled with a thermostatic mixing valve."
Liquid injection cooling "cools the oil and discharge gas by directly injecting liquid refrigerant into a closed thread within the compression cycle."
Real TimeTwo York International representatives - Product Service Manager John Kemmerly and Senior Product Engineer Paul Snell - closed out the two days of talks with a report on real-time remote monitoring of shaft seal performance.
A successful shaft seal design begins, they said, with conceptual development that looks at mechanical constraints, the lubrication circuit, target leakage rates, and service life, as well as serviceability.
Then the research team builds and tests a prototype to "confirm initial seal design and gain confidence in basic performance."
Then the product is installed and operated in the "real world." There is a gathering of data to determine long-term performance and to prepare a design for full production.
Next, limited production is started. "We ask, â€˜Is the seal performing as expected? Why or why not?'"
Then comes the installation of real-time remote monitoring equipment to "monitor and record as many parameters as possible ... even those â€˜unlikely' to affect the seal."
This approach has proved successful in optimizing shaft seal design.
For more information, visit www.johncrane.com.
Publication date: 04/05/2004