Pumps & Systems - September 2007

Figure 7. Close-up of output shaft showing burned oil around secondary seal.

arrangement, with an API Plan 52 that was vented to flare. The primary process seal was normally found to be in excellent condition. However, the secondary seal would always show signs of dry running and excessive heat (Figure 5).

This was evident as well from the buildup of burned oil on the output shaft (Figures 6 and 7).

Likewise, this buildup on the shaft certainly contributed to the unbalance of the rotor. There were several factors that contributed to the failed secondary seal:

• Poor circulation of the buffer fluid – This was primarily due to the design of the original API Plan 52, which incorporated a separate heat exchanger from the carbon steel seal oil reservoir (see Figure 8).

Additionally, the original 3-gal reservoir was constructed from 8-in pipe, which made it quite short in length. This, combined with the heat exchanger separate from the reservoir, reduced the thermo-siphon effects in the system. There was no measurable temperature differential across the buffer fluid exchanger (i.e., no flow and no thermo-siphon). This caused the temperature of the buffer fluid around the secondary seal to rise considerably and possibly flash, which contributed to the heat buildup at the secondary seal.

• Poor secondary seal rotating face design – The original secondary seal design incorporated a design where the sleeve and rotating face are one-piece steel, with the face of the seal coated with tungsten carbide, similar to a “top hat” in appearance (Figures 9 and 10).