Valves are one of the most important pieces of the drinking water distribution system. There are many different types that are used to start, stop or throttle flows.
Gate valves are used for isolation of a section of town, an individual line or tank. They should be operated in the fully open or closed position and should not be used for flow control (throttling). If they are fully open or closed, they produce very little head loss and turbulence. Gate valves should always have thrust blocking or mechanical restraints installed at dead end lines or changes in direction.
Another type of valve used in the distribution system is the butterfly valve. It is used primarily for isolation, but may also be used for low-pressure throttling. A butterfly valve creates more head loss than gate valves, but is cheaper, lighter and shorter and wears better than gate valves and must be installed in the closed position. It requires minimum maintenance, but has to be removed from the pipeline for repair or cleaning.
Another valve used in the distribution system is the globe valve. The globe valve is the most versatile of the valves. It may be used to control pressure, flow, altitude and throttling. The globe valve has the highest head loss of all valves in the open position. It also requires minimum maintenance, but must be removed from the pipeline for repair or cleaning.
A plug valve is normally used for isolation where it is frequently operated. It is not recommended for throttling. The design of the valve minimizes the closing surge, and it has a longer life than a gate valve. A plug valve is generally heavier and more expensive than a gate valve, produces lower head loss and is normally dependable and easy to operate.
A version of the plug valve is the ball valve, which is used as a corporation stop on service connections. It has the same characteristics as a plug valve, except a much lower turning torque is required, and there is no metal-to-metal contact, which extends the life of the valve.
The air release valve allows trapped air to be released, which can act as a partially closed valve. A vacuum release valve relieves negative pressure due to vacuum conditions, which could collapse pipes. Both air and vacuum relief valves should be installed at high points in the distribution system.
A pressure-reducing valve is one that automatically maintains a constant outlet pressure, regardless of changes in flow rate or upstream pressure. Usually, it is a globe-type valve with a spring-loaded diaphragm. These valves will hold a partially open position in response to the settings of the spring-loaded diaphragm. The use of two pressure-reducing valves in sequence can be used to handle a wide range of flow rates.
As with all valves, maintaining a valve inspection/exercising program will help keep the valves in good working order. Each valve should have a valve card containing, at a minimum, the number of turns to open or close the valve, direction to open the valve, valve location, type of valve, size and date of installation, repair, inspection or maintenance. Technology is beginning to catch up with the valve card. In most cases, the valve card has been replaced or is being replaced with computer record keeping, which should be easier and more efficient to use.