Full port cost more, and lots of applications don't need them. Installations like domestic water service (especially commercial & residential), many compressed air lines, isolation valves for steam traps, etc.

TBP,

Thanks for providing some applications where reduced port ball valves are commonly used.  I can understand how the reduced port style might be less expensive in smaller sizes, especially if the manufacturers are using the same casting for both types of balls and simply machining a larger bore in those that will be sold as full port. However, my catalogues list reduced port valves up to 42". I would think that in larger sizes... say 12" and up, manufacturers would save money by using two different castings (one for boring full ports, one for boring reduced ports), thereby saving on initial material and subsequent machining costs?
But manufacturers' costs aside, I still don't understand when a reduced port ball valve would be a better choice than a full port valve.  That is, under what circumstances would one want to achieve pressure/energy loss and restriction of flow with a ball valve when a simple (and less expensive) venturi device would affect the same results? What am I missing here?

A well defined problem is half solved.

Reduced port ball valve is the normal choice because of its less cost as compared to full port ball valves.When less pressure drop is desired (but not very critical) then I have seen reduced port ball valves. I have seen the full port valves normally in inlet of PSVs( here the minimum the loss the better).Ball valves can be used for control purpose as the flow and pressure drop can be set in the range from full to completely closed. Is this possible with
venturi.And ball valves are not only for control thay are used as block valves also (very often in gas service).

When only considering pressure drop and the ability to pig the line, there is no benefit or justification to use reduced bore ball valves.  The diameter of the hole through the ball determines the required OD in order to have full contact around the seat when shut.  Usually, the diameter of the waterway is around 60% of the ball OD.  So, the ball OD for full and reduced port valves cannot be the same size for a given valve size.  Said another way, the ball OD for a 3 inch full port is much larger than a 3 inch reduced port valve.  Because of this, manufactures cannot not use the same castings for full and reduced port.  A larger body is needed to enclosed a full port ball, so there is more material, more machining time, heavier valve, more cost.  In addition, the valve operating torque is a function of the ball diameter cubed.  So a larger, actuator is need for the full port valve.  The weight of a full port valve with actuator can be up to 600% more than a reduced bore valve.  The cost goes along with the weight increase.

Now considering cost sensitive general services such in HVAC chilled water, city water, instrument air systems, etc. where many decades ago, used globe valves (high pressure drop, but tight shut-off) or suffered with leaking gate valves (low pressure drop, but leaked), the cost and performance benefits of using reduced bore ball valves was huge.

Because of the cost difference, significantly more reduced port valves are sold than full port (by a factor of around 3:1).  This fact says a majority of valve applications can live with the increased pressure drop.

I hope this helps end an 8 year dilemna.

bcd,

Wow!  What an excellent explanation.  You've not only clarified a long standing question I've had, in doing so you've also enabled me to be better at my job.

Although 80% of the ball valves I spec' and purchase must be piggable and minimize energy loss, the cost savings of installing reduced port valves, where those qualities are not decisive factors, will be considerable.

I guess the root of my misperception was assuming that the two styles shared common balls, differing only in their bores.  Your thorough explanation makes perfect sense.

I guess I could've called a manufacturer, but it's like the old story where a guy is frantically and laboriously sawing down a tree, and it's taking all day. Another guy walks up, watches awhile and tells him he'd have a much easier time if he's sharpen the saw.  The first guy replies, "I'd like to sharpen my saw, but I'm too busy sawing."

Anyway, thanks again bcd, and everyone else who took the time to help me sharpen my saw.

A well defined problem is half solved.