Valves perform various functions throughout the gas delivery system. They block flow by not allowing gas to pass beyond a certain point. Valves isolate pressure by separating two sections of the system that operate at different pressures. They direct flow by determining what path is open for gas to flow. Valves also manage pressure by partially blocking a specific path resulting in a downstream drop in pressure. Different types of valves are used for different functions and are differentiated by how they control flow.
Ball valve: A ball valve contains a ball with a hole in the center. If the hole is aligned with the pipe, gas will flow through the valve. But if the valve is turned one quarter of a turn, the ball will block flow through the pipe.
Ball valves are used throughout the gas delivery system. They are used for blocking or directing flow in the transmission system and at the customer meter to allow service to be turned on and off.
Plug valve: A plug valve is similar to a ball valve in that it has a hole in the center that can be aligned with the pipe to allow gas flow, or it can be turned one quarter of a turn to block flow. The difference from a ball valve is that a plug valve has a cylindrical or conical shape.
Like ball valves, plug valves are used throughout the gas delivery system for blocking and directing flow on the transmission system and as customer service valves. Due to maintenance and operation factors, many older plug valves in gas systems are being replaced with ball valves.
Gate valve: A gate valve contains a gate that is lowered into the pipe to stop flow or raised out of the pipe to allow flow to pass. Gate valves require several turns of the handle to fully open or close the valve.
Gate valves are used throughout the gas distribution system. They are used to control flow on distribution feeders as well as inlets to regulator stations and customer service pipes.
Spring valve: In a spring valve the opening is controlled by a spring. There are two commonly used types: pressure relief and excess flow.
- Pressure relief valve: A pressure relief valve uses the force of a spring to control the position of the relief valve and allow for venting gas into the atmosphere when pressure is too high. Under normal operation, the force of the spring is sufficient to press the valve seat all the way down, thus blocking the inlet nozzle and preventing flow of gas. But if the pressure of the system gets too high, the pressure will push on the spring, thus raising the valve seat and allowing gas to flow through the relief valve. This release of gas to the atmosphere will reduce the pressure of the system. Once the excess pressure has been alleviated, the force of the spring will again close the relief valve and return the system to normal operation.
Spring-operated pressure relief valves are often used at regulator stations to ensure that downstream pressures do not exceed maximum allowable operating pressures in the event of regulator failure.
- Excess flow valve: Instead of controlling pressure by venting gas to the atmosphere, an excess flow valve acts to shut off all gas flow. Under normal operation, gas flows around a ball or other type of plug. But when the gas velocity exceeds a specified flow rate, a mechanism releases the spring, forcing the ball into the narrow end of the chamber and shutting off the flow. Many excess flow valve mechanisms are capable of resetting automatically and returning to normal operation once the downstream issue has been resolved.
The excess flow valve ensures that if a major leak occurs in the service line, gas flow will be shut off to prevent dangerous flow of gas through leaking pipes.