Chapter 10.12 - Control Valve Features: Packing Materials

Packing Materials

The most common packing materials available today are:

1. Graphite: This is a pure form of ordinary carbon. In yarn or foil form, this material can meet most industrial plant criteria.
   
   
2. TFE: TFE is highly resistant to chemicals and has low friction. The useful temperature limit is about 500° F. TFE is not abrasive to shafts and has a low extrusion tendency.
   
   
3. Glass Fiber: Generally mixed with other materials, glass fiber has high strength and chemical resistance. It resists wear and abrasion but can damage shafts. Dimensionally stable, it does not cold flow when under compression.
   
   
4. Metals: To meet the requirements of sealing and accommodating the walls of the packing box, most metals in this use are in the form of foil or ribbon. It is twisted or rolled, often in combination with other materials, to make a shape that will conform with the square cross section of braided or die-formed packing. Babbit, aluminum and copper are examples.

Other packing material variations are:

• Graphite yarn interbraided
• Graphite foil die-molded
• Asbestos braid was a common material, but is not generally supplied presently.
• PTFE V-ring

Packing must be largely impermeable to the fluid that it is sealing. When fibers, yarn or other forms constitute the packing, there are many passages between the neighboring fibers or yarns. Compression on the packing will eliminate most of these, but enough will remain in connected form to cause leaking. This requires special fluids in the packing to decrease its permeability.