Background information: Lubricant over-pressure testing in low runout conditions
Our literature teaches that there are three ways to address skew-induced wear:
- The use of our specially designed axially constrained seal (ACS).
- Axial spring force.
- Lubricant over-pressure.
The term “skew-induced wear” refers to rotary seal wear that occurs when abrasives in the process fluid impinge on a portion of the seal that is skewed due to circumferential seal growth related to initial compression and thermal expansion (Figure 1).
The term “lubricant over-pressure” refers to maintaining the pressure of the seal lubricant greater than the pressure of the process fluid that is being retained by a set of rotary seals. The resulting differential pressure acting on the seal prevents seal skewing, and therefore prevents skew-induced wear.
Our laboratory tests of 2.75” Kalsi-brand rotary seals with 0.020” radial extrusion gap clearance (Figure 2) indicate that 15 psi lubricant over-pressure is enough to prevent skew-induced wear at 162°F, and 22 psi over-pressure is enough to prevent skew induced wear at 375°F. The tests were performed with minimal shaft runout, and used abrasive oilfield drilling fluid as the process fluid.
A related series of tests, also performed with drilling fluid and minimal shaft runout, was performed with 0.010” radial extrusion gap clearance. This testing demonstrated that extrusion gap hydraulic effects that influence rotary seal abrasive wear increase as the extrusion gap clearance becomes smaller, and as the extrusion gap width (Figure 2) increases.
The extrusion gap width was varied in tests with 15 psi lubricant over-pressure, and we found that there was less abrasive seal wear with 0.020” extrusion gap width, compared to a 0.040” extrusion gap width. With 0.010” radial extrusion gap clearance, however, 15 psi lubricant over-pressure was no longer enough to eliminate abrasive seal wear.
When we increased the lubricant over-pressure to 100 psi with a 0.040” extrusion gap width and 0.010” radial extrusion gap clearance, seal abrasive wear was negligible. With 300 psi lubricant over-pressure, seal abrasive wear was still negligible, even when a larger extrusion gap width was used.
Current testing: Lubricant over-pressure testing in high runout conditions
Since not all rotary seal applications have minimal runout, we decided to perform parametric testing with a significant amount of shaft runout to determine the most appropriate range of lubricant over-pressure. The parameters that are being varied are lubricant over-pressure, extrusion gap width, and radial extrusion gap clearance. Although the testing is not complete, some significant insights have already been attained, and are being publicly disseminated for the first time in this blog.
Unless otherwise noted, the tests were conducted with 2.75” PN 507-5-11 Wide Footprint Kalsi Seals. Test constants were a 480 rpm shaft speed, 0.010” shaft runout, an abrasive drilling fluid outboard of the test seal, and an ISO 320 viscosity grade seal lubricant maintained at a bulk temperature of 200°F. The duration of most tests was in the range of 136 to 143 hours.
The most important test observations to date from the tests with 0.010” shaft runout are:
- With a 0.020” radial extrusion gap clearance and 0.020” extrusion gap width, there was little to no rotary seal wear in the range of 50 to 125 psi lubricant over-pressure.
- With a 0.020” radial extrusion gap clearance, and a 0.020” extrusion gap width, there was less abrasive wear to the seal, compared to a 0.040” extrusion gap width.
- With a 0.010” radial extrusion gap clearance and a 0.040” extrusion gap width, there was much less abrasive wear at 300 psi lubricant over-pressure, compared to 100 psi lubricant over-pressure (virtually no abrasive wear at 300 psi).
- At 100 psi lubricant over-pressure and 0.040” extrusion gap width, there was much less abrasive wear with a 0.020” radial extrusion gap clearance, compared to a 0.010” radial extrusion gap clearance (virtually no abrasive wear at 0.020” radial extrusion gap clearance).
We also tested a seal geometry variation that suggests it is possible to make rotary seals for lubricant over-pressure conditions that are more tolerant of extrusion gap clearance and width, even with less lubricant overpressure. Testing along these lines continues. We also plan to test axially spring loaded -10 HNBR seals and -11 ACS seals with 0.010” shaft runout. Results will be reported as they become available.