PTFE packing options

[joeswoes]

Does anyone have any strong feelings about the relative merits of chevron vs cup and cone PTFE packing ? Why do the two co-exist, are there applications one is better ?

Thanks in advance ;

Joe

 

[marklobo]

I only have experience with chevron PTFE packing. Since this design functions as a lip seal to both the valve stem and packing cavity wall, a fine surface texture is required on both parts. I'm familiar with the stem having a 8 AARH or better finish and the packing cavity a 16 AARH or better finish. Also, the edges of the rings must be consistent and free from flash in excess of .005 inch for small stems. .125" to .25" diameter. When everything is right, the packing works very well.

Just a guess: the "cup and cone" designs are more robust and allow for the PTFE to become embedded in the stem/packing cavity material.

Without more specifics, the answer to your question is "it depends".

 

[JimCasey]

TFE Chevron packing is directional. Cup and cone works in both directions. If you experience vacuum it would be necessary to invert TFE Chevron, or if the vaccum is not continuous,to use an upright set, lantern ring, and inverted packing set. Or just throw in a C&C set and don't worry about it. The chances of having a C&C set installed properly after the first rebuild are substantialy greater.

 

[bcd]

Hello,

Felt it important to dispell the myth in the preceeding reply. There is absolutely no reason to invert chevron packing in vacuum service. It is not directional sealing. The maximum possible differential pressure from vacuum is miniscule compared to the force required to compress the packing and effect a seal. Contrary to popular belief, chevron packing is not "pressure energized". Pressure is not going to generate sufficient force to cause the chevron to expand and seal on the edges. The benefits of chevron packing it excellent sealability (multiple sealing points), lower torque, and accomodates more generous tolerances between the sealing parts.

The two types of packing shapes has more to do with the materials they are made from and the available manufacturing processes.

 

[JimCasey]

Thanks, BCD. You have given me cause to think.

I was, in fact, just repeating the commmon wisdom which is, as you point out, not entirely correct but has enough truth that it has always worked.

Thinking about it, it makes plenty of sense that with a max of 15 psi (OK: 14.696) in the reverse direction that you can still seal appropriately regardless of the direction of the chevrons. I once had dealings with a valve that has spring-energized TFE Chevron packing and the knucklehead that designed it put the spring ON TOP instead of underneath. At about 150 psi the spring preload was overcome and the packing set displaced to the top of the stuffing box, and leaked. Spring UNDER would have been right, and would still have provided enough preload for vacuum.

If you DO get any packing leak on vacuum, it doesn't take much of a leak to make a difference.If you're trying to get 10 to (some negative exponent) torr, and there is any leak at all you won't maintain vacuum. Maybe it's the customary double packing set with lantern ring that protects from leakage. Certainly if the LR is supplied with a sealant (gland seal water in the power industry) it's more likely to seal and a leak of the sealant will not degrade system performance.


By your criteria, the performance of cup and cone packing is then equivalent. Both are virgin TFE or some loaded variation of TFE, Both are energized to seal by compression of the gland into the stuffing box( compression causes the id to be reduced and the OD to be expanded) , with supplemental process loading that assists sealing by providing additional compression. TFE chevron has reasonably delicate sealing edges that are a bit more subject to dirt and or poor-stem-finish abrasion, and TFE Chevron and C&C both must be assembled and installed as a packing SET in a particular sequence, not just random rings loaded into the stuffing box until it's full.