by Phil Mahoney
December 17, 2011

Case Study

So, how can compression packing be applied in this service? Let's look at an example of a slurry pump in an ore transport system. The operating pressure of this particular pump is 650 psi (44 bar) with potential surges to over 700 psi (48 bar). It is flushed with water at a pressure of 670 psi (46 bar). The flush was recycled plant water and contains some particulate. Shaft diameter was 6.25 inches (159 millimeters) rotating at about 600 rpm (980 feet per minute or 5 meters per second); packing cross section was ¾ inch ( 19 millimeters). The stuffing box was packed in a 3/LR/3 configuration (three rings below the lantern ring, three rings above).

Standard Packing

Using a para-aramid reinforced graphite coated PTFE fiber packing, the service life of the application was typically weeks—maybe a month or two. The intent of using this design was to reduce the extrusion of the packing rings by using a material with very strong corner reinforcement. Failure mode was charring of the packing ring I.D. (overheating due to friction), heavy sleeve wear (abrasion) and sometimes a complete loss of a seal (heavy leakage).

Inspection of the packing sets removed from the stuffing box revealed that the high pressure was compressing the packing significantly, which moved the lantern ring out of position relative to the flush port. The flush would then start leaking past the packing O.D. in roughly the same location as the flush injection port, thus reducing the cooling and lubrication of the dynamic sealing surface.

Laboratory Tests Point to a Solution

Laboratory testing confirmed leakage control with the standard packing rings and configuration was extremely difficult. Leakage dropped to zero at startup and the packing set quickly overheated. After a number of tests, the packing system shown in Figure 2 yielded consistently positive results—tolerable leakage and temperature control from startup. This system incorporated the same para-aramid reinforced graphite coated PTFE packing with dimensional changes and system component enhancements.

The solution used a close-tolerance bushing with an integral lantern ring in place of the old lantern ring and two packing rings instead six. These served a number of purposes, Specifically, they:

Helped create a pressure drop to reduce the effective pressure that the packing set is sealing

Reduced extrusion clearances for the packing

Ensured that as the packing is properly compressed, the flush port is always kept clear and the flush is distributed to the full circumference of the packing set I.D.

To avoid possible clogging of the restriction bushing with particulate, steps should be taken to remove or minimize the particulate content in the fluid entering the packing set. This can be done either by filtering or screening the flush fluid prior to use or with other solids separation technologies. 

Packing Configuration

The packing rings were cut differently than is standard for butt cut rings (90 degrees to the packing axis). They were cut slightly shorter than nominal by 1/8” (3 millimeters). This left a slight gap in each ring when inserted into the stuffing box, ensuring there was an effective leak path for the flush fluid to get through the packing set. The use of two rings seems counterintuitive but in laboratory testing with the extra ring gap, the leakage was controllable at startup and stayed consistent during longer term operation. This configuration has since been considered for standardization based on these results.

The standard packing selected and used in the application had a maximum recommended pressure rating well below the application pressures. The packing material has low tensile strength but good resistance to thermal fluctuations and gland adjustments. The modifications to the packing dimensional configuration and stuffing box resolved the performance problems by reducing the severity of the conditions under which the packing set operated.

Conclusion

Similar types of modifications have been successfully used in applications that operate at very high speeds. Altering the conditions at the seal can make the difference between a maintenance headache and a consistently good performing sealing solution for severe service applications.

Contact your sealing system manufacturer if you have a severe service application. They will have the experience with compression packing in a broad range of applications and industries that can likely provide an effective solution for sealing your severe application.

Next Month: What are the most important considerations before ordering expansion joints & upon receipt?

We invite your questions on sealing issues and will provide best efforts answers based on FSA publications. Please direct your questions to sealingsensequestions@fluidsealing.com.

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