Written by: Steve McKenzie at Piping Design Central

For accurate control of simple systems it is necessary for the control valve to have a fairly high head loss across it compared to the entire system pressure loss. The ratio of valve loss to total system (including valve) loss is called authority, and for good tight control it should be around 0.5 (1:2) or better. A system with all the head loss across the valve will have an authority of 1. An authority of 0.5 means 50% of the head is lost across the valve, and the remaining 50% is lost across the rest of the system. So for control we want the valve to have a high head loss which means its trim will be small, and the rest of the system to have a low head loss which means big pipes. So thats the reason why you see smaller that line-size control valves, apart from cost of course.

The big problems start when there is very little available head loss for the valve, and we cant have a high authority. This can mean we need a line size control valve but will need to introduce special valve trims, additional feedback control systems and/or characterized control algorithms in order to achieve acceptable controllability . About 50% of the control valves I see are oversized. You can tell an oversized valve because it runs almost closed during normal operation. The prices paid for oversize are poor control, high wear and unnecessarily high first cost.

It's a big question and cant be fully answered in a single posting. Fortunately you will find plenty of good information on the internet, and also in vendors brochures and manuals.