Is this real? 1

[Tima57]

We are being told that we need "weep holes" on teflon lined piping in order to vent corrosive gasses away from the steel portion of the pipe because we are insulating. These weep holes are not cheap. Has anyone seen this before?

 

[pmureiko]

Process material will permeate through the teflon liner. This material normally vents out a weep hole in the pipe. If you insulate the pipe, you need to make special provisions to keep the vents open.

 

[moltenmetal]

Teflon liners are definitely permeable- more so for some compounds than others, but you're using Teflon for a reason so it's likely that whatever permeates will be corrosive to steel or ductile iron. The permeated material needs somewhere to vent, or it will accumulate in the annulus between the liner and the pipe, ultimately possibly collapsing the liner. The only exception to this is pipe which is blasted and lined with centrifugally-cast ETFE- in this case, the liner is bonded to the metal leaving no interstitial space for the vapours to permeate into.

There are two main styles of TFE-lined pipe: "swaged" (Dow-style) pipe and "loose" or "flared" liner pipe. The swaged pipe has internal rifling and permeation is vented via a special washer between the liner flange face and the metal flange face. The "loose" lined pipe has a liner which is not tight in the pipe. The pipe needs weep holes at intervals. If you're insulating, you need half-couplings welded over these weep holes, and "sacrificial" threaded nipples to carry the vented material outside of your insulation cladding- sacrificial because they often corrode away when the permeated stuff hits the moist atmosphere. If you don't use this method, the permeate can be trapped by the cladding and accumulate, eventually eating your pipe from the outside inward...

Crane Resistoflex offers very good info on this stuff in their catalog.

 

[kenvlach]

I had a liner permeation problem although in a different application. Had an anodizing tank of mild steel, 125 mil flexible PVC-lined, heat-bonded to the tank. Lead cathodes ran down one wall, across the bottom and up the other side. After ~16 years, a large ‘boil’ ~2 ft diameter and ~5 inches high in center formed under the liner in the center bottom. Very apparent; it lifted up the cathode and perforated plastic shielding (which prevents anodizing parts from contacting the cathode). Initially thought it was corrosion due to a perforation in the liner, but after emptying the tank, found a bubble of hydrogen (hydrogen is created at the cathodes during anodizing). Apparently the liner had de-bonded in the one area, allowing the hydrogen to accumulate underneath.

So, a gaseous bubble could collapse a piping liner, as moltenmetal warned. I think this would generally only be a problem if the pipe was used first in high pressure service and then low. Or, if an acid permeated the liner, its reaction with the steel would release hydrogen.