The humble ball valve is available with many different options. One of the most important distinctions is in how the ball is made. Manufacturers either join two hollow hemispheres or machine a hole through a solid ball. Machining takes more work, but produces a valve with better flow characteristics. Here’s why.
Quarter-turn Valve Basics
A ball valve is used for opening up or shutting off fluid flow through a pipe. Visualize a ball trapped in a pipe and at the top of the ball a shaft connects to a handle. Through the center of the ball there’s a hole, so when the hole and pipe are aligned fluid can flow. Turn the handle 90 degrees and the flow is shut off, hence the reason ball valves are classed as quarter-turn valves.
If the hole through the ball is smaller than the bore of the pipe it restricts the flow. This design is known as a reduced port valve. Making the hole the same size as the pipe eliminates any flow restriction, but means the valve itself must be bigger. This is a full port valve.
When designing a hot water supply, an irrigation network or any other kind of fluid system, it’s important to calculate the flow rates. Usually there is some minimum, and possibly maximum rate to achieve at the delivery point, specified in terms of cubic meters or cubic feet per second.
Fluid dynamic calculations rapidly slip into supercomputer realm, so flow coefficients or Cv values are used to simplify design work. The Cv indicates the gallons or liters per minute that will flow through a restriction with a pressure drop of 1 PSI or 1 bar. (It’s important to know which units you are using). Any good ball valve has an associated Cv that the designer can use to calculate flow.
In a solid ball valve the hole is the same diameter all the way through. That helps the fluid flow smoothly at a constant velocity. A hollow ball is, as the name implies, hollow inside. That creates a bigger space for the fluid to fill, which changes its velocity and creates turbulence.
Turbulence is almost always undesirable. It creates noise and it’s inefficient. The Cv drops and it takes more energy to pump fluid through the system.
In a few applications valve weight is an issue. In such cases the hollow ball may have an appreciable advantage, especially in larger valves.
Solid Ball, Better Flow
When evaluating ball valves, check the Cv values and be sure to compare identical port formats. (Full port to reduced port is not a good comparison.) In almost every case, the solid ball will give a higher Cv than the hollow ball.