For your second question, it is my understanding that triple offset valves are used in applications where low-leakage is a must and gaskets are not practical. High temperature gases would be a good candidate for example.
Best companies for triple offsets are Xomox, Velan, Dezurik, Tricentric and Adams.
As an addition to above, European experience, to highlight the uses for centric contra double eccentric contra triple eccentric:
For pure (particle free) water distribution and purification, water pipelines and hydroelectrical power plants you would normally not use triple offset because of cost and availability.
Double offset (double eccentric) would normally be higher in cost than centric butterfly valves for theese applications, but double eccentric would give an advantage in cost/lifetime and dependability. The benefit would increase with diameter and pressure classes. Double eccentric are used at least up PN25 (pressure class 25 bars) before you go for other special valve constructions, but normally not (?) triple eccentric valves.
For water the double eccentric valves would be leakage thight to the normal required leakage tests and rates.
This would lead you back to the typical applications for triple eccentric QMax1 has given.
Zwick and Vanessa also do triple offset butterfly valves. I use triple offset for critical service where fire safe reliable shutoff is required (example, ESD valve on a TurboExpander)
Triple-offset design usually also means a metal-to-metal, torque seating mechanism with the relevant advantages (more mechanical and chemical resistance, wider temperature range, etc.) and mainly just one disadvantage, i.e. a higher cost...
Triple-offset, metal seated valves may be used in many processes of oil & gas, chemical, power generation, pulp and paper and other industries (where "piggability" is not a must, of course), including replacement of existing ball or gate valves, for both control and on/off sevices (with the proper actuation)... but in the last years, with the growth of LNG business, they also found more and more success in cryogenic applications.
