This past Saturday, we received a call from a homeowner that his water heater was leaking. When we arrived, we found that the leak was caused by a fitting and not the water heater itself. Now in order to replace the fitting, we would have to shut the water off. We turned the water off at the gate valve and completed the work. When we went to turn the water back on, the handle just spun and spun. The gate valve had sheared off and wouldn't allow any water to pass. The gate valve would need to be replaced...this time with a more formidable ball valve.  

A gate valve is a cheaper alternative to a ball valve, although by only a few dollars. Depending on the size, a gate valve costs around $5 and a ball valve is usually under $10.

Both types of valves have the same purpose, which is to control the flow of water. The design of each, however, is vastly different.
A gate valve has a round knob on top that you turn. This motion raises or lowers a gate inside of the housing to stop or start the flow of water. These valves are prone to failure as they age because they can corrode.

A ball valve is a man among boys. Instead of a knob, it uses a lever. This valve has positive stops machined in that allows the lever to move only 90 degrees. When the lever is parallel to the pipe, the valve is open and water will flow. If the lever is perpendicular to the pipe, the valve is closed.

If you were to cut a ball valve open, you would see that the lever is connected to a ball with a hole bored through the center of it. This is where the water passes through. Turning the water on and off is only a matter of throwing the lever 90 degrees.

Because of the round handle on a gate valve, it's hard to tell if the valve is open or closed, and when the valve is broken, you end up turning the knob one way and then the other without accomplishing anything. In some cases, the gate will corrode and separate from the lifting mechanism and drop. It will come to rest at the bottom of the valve and cut off the flow of water.
The long and short of it is if you are going to replace the valve, spend a few extra dollars and install a ball valve.