I have too high water pressure (120 psi) on the main line. If we assume the water pressure on is constant, then is it possible to reduce it by reducing the flow using a ball valve?
The reason I'd like to go this route, is that installing a pressure regulator is a lot of work, and there's not much room where the main line enters the stucco of the house. Turning the pre-existing ball valve, would be a lot easier than installing a regulator.
To answer your question; A ball valve only controls flow not pressure. A few seconds after water is flowing a ball valve will limit the pressure based on the flow. A pressure regulator requires no more plumbing than a ball valve. A regulator will limit the maximum pressure but not flow until the set point is reached. If you can answer the question. Why do you want to limit pressure or flow. We may be able to point you in the direction you need.
The relationship between resistance, pressure, and flow is complicated. But in brief, a restriction like a half-open ball valve will only reduce the downstream pressure and volume when the water is actually moving. It does nothing to alter the "static pressure", which is the pressure in the lines when no water is being used. So a half-open valve may help in some situations (like if you shower head is too powerful) but it will not help with problems caused by high static pressure, like running toilet valves and other leaks.
As @EdBeal said, the right tool for this job is a pressure regulator.
you can go with using ball valve. ball valve is commomly used to regulate flow and it will not reduce pressure. however, installing ball valve on a branch very away from desired exit and not opening it fully will give desired output
