The invention of PTFE plays a great role in the development of ball valves, without which I am afraid there would not have been a great development of ball valves. However, as time progresses, PTFE has many advantages while gradually revealing some disadvantages, such as the aforementioned "cold flow" and thermal expansion, as well as pressure-temperature limitations of the use of class. It is therefore necessary to discuss the development of sealing materials and their scope of application.
  Sealing materials can be divided into two categories in terms of a wide range, one is a polymer material PTFE and other polymers, can guarantee reliable sealing; the second category is metal, in general, can only guarantee a low leakage rate, metal sealing materials will be discussed in the metal sealing ball valves, this paper only introduces the polymer material polymer soft sealing materials.
  Certain polymer sealed ball valves, factory tests can be achieved without visible bubble leakage, that is, "zero" leakage. In many cases, this performance is maintained for the duration of use. The important factor affecting the sealing performance is the use of media, if the valve is used in very severe conditions, such as corrosive media, extreme high temperature, high pressure, etc. will be greatly affected by the sealing performance.
  Sealing performance of the life of the following factors: pressure, temperature, pressure fluctuations, media flow rate, valve operating speed and opening and closing frequency. In practice, these factors are cross-influence. Therefore, to obtain long-term service life and severe working conditions, the development of sealing materials.
  1, polytetrafluoroethylene (PTFE) pure PTFE is the most basic sealing material for ball valves, it has a certain temperature range, from low temperature to a maximum of 350 ℉ (177 ℃), but also has excellent chemical stability and self-lubricating properties, a wide range of applications.
  2, filled PTFE Filled or reinforced PTFE plastic, in fact, has all the characteristics of pure PTFE, and its temperature range increases, for the following 4in ball valve can be expanded to 400. F (205 ℃). Filled materials are generally glass fibre, carbon, graphite and so on.
  3, metal-reinforced PTFE Filled materials copper powder or other metal powders, the design of this sealing material is specifically for stronger pressure fluctuations in the occasion, mainly used in pulp mills digester discharge system.
  4, special nylon special nylon seal seat for PTFE can not adapt to higher pressure occasions, according to the type of valve and size of the size of up to 4500psi, but it is less resistant to chemical stability, widely used in high-pressure liquids, air and other gases (excluding oxygen) in the system.
  5, Danlin is the trade name of the polyacetal, with similar hardness, strength and rigidity of the metal. Wider temperature and humidity has good self-lubrication, low friction factor, elasticity, good corrosion resistance to most solvents, the disadvantage is that it is sensitive to strong acids, strong oxidants. Applicable temperature -40 ~ 116 ℃, tensile strength of 69MPa, shear strength of 65MPa, bending strength of 97MPa, the coefficient of linear expansion of (9.6 ~ 12.8) X10-5- ℃, forming method for injection moulding, the highest pressure up to 2400lbf / in2, covering 600 level pressure rating.
  6, PEEK chemical name is polyether ether ketone, is an aromatic polymer containing ketone groups. Has good high temperature performance, resistance to organic and inorganic chemicals and long time resistance to hot water decomposition. In the range of 200 ~ 260 ℃ can maintain its strength and toughness. It can be processed by injection moulding, extrusion and compression moulding. Relative density 1.51, tensile strength 156MPa, flexural strength 250MPa, coefficient of linear expansion 1.2×10-5°C-1. Can be effectively used in steam systems, for ½ ~ 2in ball valves, saturated steam at a pressure of 475lbf/in2 can be achieved reliable seal. In other occasions, can be used for temperature 550℉ (280℃) and has good chemical stability.
  7, polyethylene UHMW is a polymer polyethylene materials, PTFE can not be used for highly radioactive occasions, greater than 104rad [1rad (rad) = 10mGy (mGy)] need to use polyethylene as a material for the applicable radiation level of the dose of up to 2 × 107rad This material can also be used for PTFE prohibited occasions such as the tobacco industry.
  8, polyimide is a high-performance plastic, with high heat resistance, dimensional stability, flame retardant, radiation resistance, UV resistance and alkali resistance is poor. Its tensile strength of 95MPa, bending strength of 170MPa, coefficient of linear expansion of (3.5 ~ 4) × 10-5 ℃ -1; friction factor of 0.15 ~ 0.2. can be used for higher temperatures (to 25)