To effectively apply PTFE in a axial split case pump , it is important to understand the properties of this material. Some of the unique properties of PTFE make it an excellent material for braided packing:

1. Excellent chemical resistance. A major reason for using PTFE in packing is that it is not affected by a variety of corrosive fluids, including strong acids, bases, and solvents. Perhaps most importantly, PTFE can withstand strong oxidizing agents such as nitric acid, chlorine dioxide, and highly concentrated sulfuric acid (oleum).

2. Low coefficient of friction when in contact with most surfaces. PTFE is known to have non-wetting, smooth, and low coefficient of friction properties. This helps reduce power consumption and heat generation at the packing-shaft interface.

While PTFE has its advantages, some of its properties are not ideal in many pump packing applications. Problems encountered with PTFE packing are generally due to its poor thermal and mechanical properties:

1. Cold deformation or creep under pressure. Creep increases with increasing temperature. When pressure is applied to 100% PTFE packing for a period of time, the packing may become a dense solid and require frequent adjustments to maintain a seal. It also has a tendency to squeeze out the top and bottom gaps of the stuffing box of a axial split case pump.

2. Low thermal conductivity. When frictional heat is generated in contact with a high-speed rotating shaft, pure PTFE has a tendency to absorb heat and is unable to dissipate it to the surrounding environment. To prevent PTFE packing from burning or scorching, a high leakage rate is required at the packing-shaft surface.

3. High thermal expansion coefficient. As temperature increases, PTFE expands much faster than the surrounding metal. This expansion increases the pressure of the packing on the axial split case pump shaft and bore.

PTFE Fiber Packing

Many manufacturers produce packing that use PTFE as the base fiber. These products can be supplied as dry fibers, fibers coated with PTFE dispersions, or fibers coated with various lubricants. It is good practice to use these products only when there is no other PTFE alternative, including applications with corrosive chemicals such as strong oxidizers, or for food or pharmaceutical processes.

For PTFE fiber packing, it is especially important to adhere to the manufacturer's limits on temperature, speed, and pressure. These packing are very sensitive to adjustment when used in rotating equipment. Typically, lower gland pressures and higher leakage rates are required than with other packing.

Expanded Polytetrafluoroethylene (ePTFE) Packing

ePTFE yarns are similar in appearance to wound PTFE tape. The most common form is ePTFE impregnated with graphite to improve its thermal conductivity and speed rating. ePTFE braids are less sensitive to heat buildup than PTFE fiber packing. ePTFE packing may experience cold deformation and extrusion at higher pressures.

PTFE Coated packing

When the excellent chemical resistance of pure PTFE is not required, PTFE can be coated on many fiber materials to improve packing performance and take advantage of the benefits of PTFE. These fibers can also help reduce or eliminate some of the weaknesses of pure PTFE braids.

Synthetic and glass fiber blended yarns can be coated with PTFE to produce economical, versatile packing that have higher resilience, greater extrusion resistance, and less tuning sensitivity than PTFE fiber braids. They can also be coated with a dispersed blend of PTFE and graphite to further improve the braid's speed capabilities and heat dissipation characteristics.

Aramid fiber packing with PTFE coatings can be used where extreme wear resistance is required. Novoid fiber packing with PTFE coating can be used in mildly corrosive services and has better resilience and extrusion resistance than PTFE fiber braids.

PTFE-coated carbon and graphite fiber braids are among the most versatile packing. They have excellent chemical resistance (except for strong oxidizing agents), high-speed performance, high-temperature performance, and very good resilience. They do not tend to soften or extrude at high temperatures and also exhibit good abrasion resistance.

By understanding the advantages and limitations of various forms of braided PTFE packing, you can select the product that will most effectively meet your axial split case pump or valve process sealing requirements.