Tips: Stuffing Boxes and Quarter-Turn Valves

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Recently I had the pleasure of listening to a presentation on stuffing box design. The speaker mentioned that an ideal stuffing box contains 5 rings of packing and that deeper stuffing boxes are not preferred as only the two packing rings nearest the outside of the valve are fully compressed by the force from the packing follower. This is because of friction between these packing rings and the stuffing box walls that prevents compression forces reaching the inward packing rings.

I fully agree with the speaker’s comments.

As supplied there were 17 rings of PTFE chevron-type packing, including solid inermost and outermost rings. The chevron shape, it was thought, would provide a pressure-energized seal and each subsequent ring would serve as backup to inward rings shoould they allow leakage.

What was found, however, was that the packing required continual re-tightening of the follower nuts. This occurred due to the lack of compressive forces reaching the more inward rings, plus the tendency of PTFE to cold flow under load. Tightening the nuts did cause the outer rings to develop a sealing relationship to the body and shaft but they soon relaxed, cold flowing into the air gaps that exist between the chevron rings. As for the chvron shape allowing pressure to cause their sealing the hydrogen, the material is simply too stiff to react to pressure and the stacking of the rings made each ring even more resistant to deformation under pressure.

Solution

The solution was to replace the 17 rings of packing with five rings but not with stacked chevron-shaped rings.

Note that, had the soft PTFE rings been used alone, they would have quickly flowed through the small clearances between the shaft and follower and shaft and body. The solid inner and outer rings were there to prevent flow of the soft rings, not to perform a hydrogen seal of their own.

The result of the packing modification was excellent, and I have employed the same concept on numerous high-performance valves, always with excellent results.

Obviously the service temperatures and fluids must be compatible with PTFE as used above, but the same design can be used with graphite packing with solid inward and outward rings preventing flow of softer, more flexible contained rings.

Ed Holtgraver is CEO and president of QTRCO. Reach him at ed@qtrco.com.