Can someone explain the suitability of RPTFE and PCTFE for ball valve seating?
Can we say that PEEk is the best seat material?

PTFE is pure teflon seating. It is good but has limited strength and thus is suitable for lower pressure valves. Immune to most environments.

RTFE is reinforced (typically glass) seating. It is better and will be suitable to valves with higher pressures but may still be limiting in Class 900 and 1500 valves which might not be able to have full flange rating. Again, immune to most environments.

PEEK is good to all ASME pressure classes and is highly resistant to most environments.  Great seat material but extremely expensive. It is also unlikely to pass a low pressure valve test (such as in API 598) as the material is so tough.

Nylon is a commonly specified seat material for valves up to Class 1500 as it allows higher ratings than teflon and is relatively inexpensive.  There is conflicting data available with regards to the suitability of Nylon in sour service. 

Just to add a little more the rneills excellent summary.  The choice of material can greatly affect the torque required to turn the valve.  PTFE and RPTFE have a low coefficient of friction.  PEEK, especially virgin PEEK has a relatively high coefficient of friction, so the torque can be excessive.

In addition, you have to look carefully at how the PEEK is manufactured.  It can affect the properties tremendously.  At about 6 inch and smaller diameter bar, it is typically injection molded.  Elongation is around 20%, so the seat will be somewhat flexible.  Once you get larger than 6 inch, it is typically compression molded from powder.  The elongation drops to 2%.  It doesn't flex very much until it breaks.

Just mentioning because you have to be very careful when working with PEEK.

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