Solving Seal Challenges in Vertical-Up Applications
The basics of sealing in vertical-up applications such as cooling towers usually seems simple at first glance. You are sealing in oil at the top of a gearbox. Not only does this area see little lubrication -- a splash on occasion at most -- but gr...
September 1, 2004 | By Paul Yager
The basics of sealing in vertical-up applications such as cooling towers usually seems simple at first glance. You are sealing in oil at the top of a gearbox. Not only does this area see little lubrication — a splash on occasion at most — but gravity is also working in the seal’s favour by forcing the oil to flow down and away from the seal.
As you are probably already aware, the failure of a gearbox is not likely to be due to lubricant loss at this location. The primary sealing objective in vertical-up applications is excluding contamination. But all the things that help keep lubricant from escaping — the small amount of lubricant and gravity — make it impossible for typical seal arrangements to provide protection for any length of time.
To study the problem, a unique test unit was designed and built by JM Clipper that duplicated a cooling tower environment. Seals were installed in a vertical-up position, were poorly lubricated, and were subjected to a heavy and continuous seawater condensate. The shaft was supplied by a well know manufacturer of gearboxes designed specifically for cooling towers. The condensate was allowed to pool on the gearbox face to a depth of 1/4 in. and the unit was allowed to dry completely to simulate start-ups after weeks of non-operation.
The problem with typical seal arrangements was easy to see. Some of the seal arrangements that were tested are shown in Figure 1. While these designs have proven successful in many harsh applications, they are not the best choice for cooling tower gearboxes.
If a single seal is used, single or double lip, there simply is not enough lubricant available to provide an adequate oil film for the seal lips to ride on. Without the proper oil film present under the lip, excessive friction is generated. The friction generates heat which in turn hardens the contacting lip — and this leads to excessive wear.
Figure 2 shows how quick a lip can wear. The lip wear shown here occurred in just 300 hours at 250 fpm on a 4-in. dia shaft under the test conditions described previously.
Using multiple lip seals did not improve performance as well as expected. With little lubrication, the presence of additional lips contacting the shaft built up even more heat and accelerated failure of the inboard seal by 50 hours. Developing a method to lubricate between the seals was viewed as impractical and expensive.
Another arrangement that used a slinger or V-type seal was also tested. While this arrangement did show some benefit by keeping large droplets from running down the shaft, it did not stop the condensation from pooling on top of the seal. The high humidity levels also caused severe rusting and pitting of the sealing surface of the shaft.
Figure 3 shows how destructive the cooling tower environment is to a typical shaft. Such a surface will make mincemeat out of any contact lip seal. The scale and sludge sitting above the seal will find its way into the gearbox within weeks of operation.
Figure 4 shows the residual rust deposits trapped under the seal lips after 200 hours of operation. Premium lip materials were also tested but due to the rapid deterioration of the shaft sealing surface, none performed past 400 hours.
The solution for this application was to install a JM Clipper’s ProTech Bearing Isolator. The material of this unique seal design is PTFE so it will not rust or corrode. ProTech is a two-piece design that is unitized without the use of internal O-rings or other internal seals.
ProTech designs do not use internal seals because these components wear over time and as they wear, the seal’s ability to exclude water declines rapidly. The ProTech stator has an O-ring press fit into the gearbox housing. This is the stationary part of the seal. The rotor has a static O-ring press fit on the shaft and rotates with the shaft.
Since the rotor spins with the shaft, there is zero wear on the shaft and the surface condition of the shaft does not have to be maintained in order to achieve long-term performance. A shaft that is going to rust is not a problem.
Figure 5 shows a cross sectional view of a ProTech Bearing Isolator in a vertical-up application. Both the housing and the shaft are permanently sealed against water or any other type of contamination.
Testing of the ProTech seal started by installing the seal on the same shaft that was grooved by the previous contact seal testing. Outside of using emery cloth to remove the rust buildup, no additional dressing was used. This was done to simulate replacing a seal in the field.
The bearing isolator was cycled on for 200 hours and off for eight hours, and the direction of the shaft rotation was reversed every 500 hours. The pooled seawater was maintained at a depth of 1/4 in. for the duration of the test.
Figures 6 and 7 show the progressive nature of the contaminate buildup over time. After 1,600 hours, not a drop of water had leaked past the seal. The test unit was then allowed to dry for 30 days, with zero shaft rotation, in order to simulate a shutdown mode. At restart, the seal worked as new with no signs of binding or sticking. Those with cooling tower experience are probably familiar with lip seals tearing away under such conditions.
Green dye was added to the saltwater solution so that any leakage would be easy to see. Figure 8 shows lip seal leakage running down the shaft after 200 hours in the original test. Figure 9 shows a clean shaft after the ProTech seal had run for 1,600 hours.
Once the upper shaft had been upgraded to a ProTech Bearing Isolator, the life of the lower seal needed to be extended in order to minimize downtime. A failure here will create a total loss of gearbox lubricant. A solution is to use a hybrid isolator design also manufactured by JM Clipper, the ProTech 360. As shown in Figure 10, this seal consists of an outboard labyrinth to exclude dirt and water and two inboard PTFE lips to retain lubrication.
The combination of the hybrid low-wear PTFE lip design, internal sealing surface and a labyrinth to exclude contaminates provides three times the life of typical lip seals.
Paul Yager is with JM Clipper. For more information, telephone 1-800-233-3900 or visit www.jmclipper.com.