What types of ball valves are there?
Every project we do, we leave up to the mechanical contractor to supply the valves. All we tell him is to use ball valves (service is oil, water, emulsions). Unfortunately this has lead to poor quality valves that seize up quite often requiring more maintenance than the operators are typically willing to do.
Any ideas on how to specify the ball valve type to find something middle range (I wouldn't like to spend too much buying the "Cadillac" valves).
Are there different types of ball valves?

Many ball valves are selected for specific applications as engineered items.

Start with engineering standards such as API 6D and ASME B16.34 as the minimum criteria.  This will eliminate the valves at Home Depot.

There are full port and reduced port.  Normally we buy reduced except for low pressure compressor suctions, pigable valves, etc.  The ends are flanged or welded.  Flanges are a minimum for line-class block valves.  Welded are applicable for acid gas such as the high percentage levels of hydrogen sulfide.  Most valves that I buy are NACE MR0175 compliant for sour service - even if the gas is not sour.

Most applications require bi-directional bubble-tight shutoff compliant with API STD 598 and fire safe per API STD 607 or SPEC 6FA, 6FB and 6FC.

Ball valves are available as trunion and floating end.  Someone else can explain this better than me.

Offshore we like Zylan bolts and special coatings to keep the valve around for a few years before they rust away.

It is good if the valve is maintainable.  That may require lubricating connections etc.  Some valves have welded bodies that must be removed and sent to the shop instead of serviced in the field.  I forgot to mention top entry valves that may permit service with the body in the line.

Elastomers have to be compatible with the fluid.  Some absorb CO2 then explosively decompress.  Some are eaten by amine, etc.

Usually floating ball valves are in the lower-pressure classes with resilient  seats but with extrusion-resistant polymers the valves are offered up to around 6000 psi.  As long as the valve relies on polymeric seats, the temp limit of the valve will be defined by the polymer.  Embellishing a bit on what zdas04 stated above: floating ball seats are usually flexible and preloaded (crushed) against the surface of the ball. This way they shut off at low pressure differentials and although the ball is designed to be able to move into the downstream seat, the actual travel is pretty small. Most of the time, both seats seal. This can trap high pressure liquid inside the ball and cavity, and for cryo liquids, Chlorine, etc. it is necessary to drill a hole in the upstream side of the ball to release the pressure generated if the liquids vaporize. A good supplier will have developed mind-boggling arrays of seat technologies for various materials, pressures, temperatures, and other applications criteria.   

Valves without elastomers can work at elevated temperatures; Valvtechnologies and Mogas use hard alloy balls and a hardfaced seating surface in the body, and the ball is lapped to the seat.  Generally the ball is loaded against the seat by a spring mechanism/belleville washer and these valves are unidirectional, but just the ticket if you want to isolate superheated steam.  

A 6" Ball weighs around 200 pounds.  That's a lot to support on two polymer seats.  Trunnion valves have an advantage for higher pressure valves as well as larger size valves.  The seats, as mentioned earlier, are spring loaded against the ball(springs, belleville washers, even o-rings are used) AND the upstream area of the seat is larger than the downstream area of the seat so that increasing process pressure loads the seat against the ball. The loading mechanism pretty much dertermines how much crud the valve can tolerate: O-rings are used for paper stock, but coil springs can be used on clean steam.   Trunnion valves can have seats with polymeric inserts, or the seats can be metallic for extended pressure/temp range.

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