A discussion of current compression packing technology will help set the stage for an example of how this can be accomplished. Compression packing is still widely used to seal rotating equipment such as pumps. The most common type of compression packing used in rotating equipment is braided yarn made from a variety of materials such as graphite, aramid, and PTFE (Polytetraflouroethylene). Many braided packings will also incorporate blocking agents and other coatings to minimize leakage through them and provide cooling and lubrication during startup. The braided packing is cut into rings, inserted into a stuffing box and compressed using a bolted gland. Under axial compression, the packing expands radially, creating a seal between the stationary body of the equipment and the dynamic surface, usually a rotating shaft.
Figure 1. Standard packing system
There are a myriad of factors that can affect the performance of packing. These include:
Equipment condition and design—stuffing box finish, shaft finish, shaft run-out and concentricity with the stuffing box, number of packing rings, clearances between the shaft, gland and stuffing box.
Application parameters—temperature, pressure, surface speed, and the properties of the media being sealed (i.e., corrosive or abrasive).
Packing properties—thermal conductivity, coefficient of friction, material temperature limits and chemical compatibility, material strength, construction, density and permeability.
With all mechanical packing, some leakage of the sealed fluid is to be expected, but it must be controllable. Friction between the shaft and packing during operation generates heat. The controlled fluid leakage provides some measure of cooling and lubrication which helps manage the heat generation. That is the tradeoff—for a given application, lower leakage rates can translate to higher operating temperatures. Higher leakage rates can yield lower operating temperatures. Lower leakage rates could mean less lost product but shorter packing life and vice versa.
Some established, general guidelines on expected packing leakage can be found in the Third Edition of the FSA/ESA Compression Packing Technical Manual, but end-user experience and/or requirements for a given application and packing type can vary widely. One operator may look to achieve no visible leakage from an application whereas another may be satisfied with 20 milliliters per minute.
Compression packing is considered a cost effective sealing solution, particularly in applications with larger shaft diameters for which other sealing options are more costly. The standard packing designs and materials in use today can handle some pretty extreme conditions, i.e. pressures to 500 psi (34 bar), shaft speeds to more than 4,000 feet per minute (more than 20 meters per second) and temperatures to more than 600 degrees F (more than 315 degrees C).
What about the severe service applications with operating parameters beyond these conditions? The example application described in the next section will show that using techniques gained through field experience and laboratory testing can extend the effective sealing capabilities of compression packing to severe service conditions beyond the published limits for the standard packing.
High Pressure Slurry Pumps
In mining operations, there are applications in which ore slurry has to be moved from the mining site to the processing facilities. This is done with pumps in series that develop enough pressure to move this viscous abrasive fluid long distances. The particulates are abrasive and can wear packing fibers, pump shafts and sleeves and increase frictional heat. The elevated pressure creates high compression on the packing that can increase packing friction and wear, cause extrusion of the packing through clearances between the gland and shaft and make controlling leakage difficult. Leakage control can be complicated at these pressures because small adjustments of the gland bolts can cause drastic changes in leak rate and, subsequently, the frictional heat and operating temperature of the packing set.
Often in these harsh applications, the packing is flushed through a lantern ring (See Figure 1.). The introduction of the flush is meant to cool and lubricate the packing and keep particulate from getting between the I.D. of the packing set and the O.D. of the shaft. The flush pressure has to be higher than the stuffing box pressure to ensure a positive flow.
Figure 2. Modified Packing System