Picking a valve for an instrument system means sorting through what may seem an overwhelming number of choices. Just to name a few, there are ball, diaphragm and bellows valves, as well as check, excess flow, fine metering, gate, multi-port, needle, plug, rising plug, relief and safety valves. And each of these comes in many sizes, configurations, materials of construction and actuation modes.
To make the best choice, it's always good practice to first ask: What do I want the valve to do?
Most valves fulfill one of five primary functions -- on/off, flow control, directional flow, over-pressure protection and excess flow protection. Matching valve type to function is the first and most important selection step. It's not unusual to see misapplied valves, such as a ball valve used for throttling flow. In some cases, the mismatch can be catastrophic, say, if a ball valve were in a high-pressure oxygen system. With a source of ignition, the sudden burst of oxygen -- enabled by the fast opening of the valve -- could lead to a fire.
So, here, we'll review the basic types of valves, how they work, what functions they fulfill, and what to think about when choosing one over another.
On/off control -- stopping and restarting system fluid flow -- is the most basic valve function. Primary on/off options are ball, gate, diaphragm and bellows valves.
Perhaps the most common of all valve types, ball valves (Figure 1) are designed for on/off control. Quarter-turn actuation starts or stops flow by rotating a metallic ball with a large hole through its center. Straight-through flow occurs when the hole is lined up with the flow path. When the hole is turned 90° from the flow path, flow stops. If you're seeking an on/off valve with quick shutoff and high flow capacity, a ball valve is a good choice. The position of its handle provides a quick indication of whether the valve is open or closed; ball valves are easy to lock out and tag for safety purposes. They are most practical and economical at sizes between ¼ in. and 2 in.
Typically used for process control rather than instrumentation applications, gate valves commonly are chosen for on/off control -- particularly for lines above 2 in. They also frequently serve as the first valve off the process line for instrumentation, often in a double-block-and-bleed configuration. Among the oldest types of on/off valves, they usually are specified in general industrial applications, such as large process or transmission lines. Some can exceed 100 in. Multiple rotations of the handle lower or raise a sealing mechanism in or out of a straight flow path. Shutoff is gradual.
Packing surrounds the stem, the cylindrical part connecting the handle (or actuation) with the inner mechanism, preventing system media from escaping to atmosphere where the stem meets the valve body. Valves that seal to atmosphere with metal-to-metal seals are referred to as "packless" because they don't contain soft packing material, e.g., gaskets and O-rings, normally found around the stem in other valves.
All stem seals or packing are subject to wear, which can lead to leakage. Valves with packing require servicing or replacement at regular intervals, although some types of packing, such as the two-piece chevron design, create more effective seals and last longer than others.