How to choose on-off valves and flow-control valves

When selecting a valve, your choices may seem overwhelming. Start by asking the question: What do I want the valve to do? The following article outlines considerations for two of the most popular functions of a valve, namely on-off and flow control. Matching valve type to function is the first and most important step in the valve selection process.

On-off valves

On-off control is the most basic valve function. Valves in this category stop and restart fluid flow. Primary on-off valves are ball, gate, diaphragm, and bellows valves.

Designed for on-off control, ball valves use quarter-turn actuation, which starts or stops flow by positioning a metallic ball in a straight-through flow path. The ball has a large hole through its centre: when the hole is aligned with the flow path, it enables flow; when it is turned 90 degrees, it stops flow. Ball valves offer quick shutoff and high flow capacity. The position of the handle provides a quick visual indication of whether the valve is open or closed and, for safety purposes, ball valves are easy to lock out and tag. They are most practical and economical at sizes between 1/4 inch and 2 inches (6 and 50 mm).

Typically used for process control applications, gate valves are commonly chosen for on-off control, particularly for lines above 2 inches (50 mm). They are also frequently used as the first valve off the process line for process instrumentation, often in a double block and bleed configuration. Gate valves are typically specified in applications such as large process or transmission lines. Multiple rotations of the handle raise and lower a sealing mechanism in and out of a straight flow path. Shutoff is gradual.

Packing surrounds the stem, 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 do not contain the soft packing material (such as gaskets and O-rings) normally found around the stem in other valves. The valve stem is the cylindrical part that connects the handle (or actuation mechanism) with the inner mechanism for shutoff, flow control and directional control. Usually the stem turns and/or moves up and down.

All stem seals or packing are subject to wear, which can lead to leakage. Valves with packing must be serviced or replaced at regular intervals.

Diaphragm valves are packless and provide rapid shutoff and precise actuation speeds. In some cases, they may also deliver consistent quantities of process fluid. Typically diaphragm valves are employed in high-purity applications. Among all valve types, they provide the highest cycle life, a product of highly engineered anatomy. Each valve contains a thin metal or plastic diaphragm, which flexes up and down, creating a leak-tight seal over the inlet. This robust valve is usually small, with the largest orifice - or internal pathway - typically less than 2 inches (50 mm).

Bellows valves are also packless, making them a good choice when the seal to atmosphere is critical and access for maintenance is limited. A welded seal divides the lower half of the valve, where the system media resides, from the upper parts of the valve, where actuation is initiated. The stem, which is entirely encased in a metal bellows, moves up and down (without rotating) to seal over the inlet.

In globe valves, fluid does not flow straight through on a level plane as it does in a ball valve. The flow path enters the valve under the seat and exits above the seat. Globe valves have lower flow rates than valves with a straight-through flow path of the same orifice size.

Flow-control valves

Flow-control valves enable the operator to increase or decrease flow by rotating the handle. The operator can adjust the valve to a desired flow rate, and the valve will hold that flow rate reliably. Primary flow-control valves are needle, fine metering, quarter-turn plug, and rising plug.

Needle valves provide excellent flow control and, depending on design, leak-tight shutoff. They consist of a long stem with a highly engineered stem-tip geometry that fits precisely into a seat over the inlet. The stem is finely threaded, enabling precise flow control. Stem packing provides the seal to atmosphere.

Some designs contain a metal-to-metal seat seal; consequently, needle valves are a good choice for high-temperature applications. Flow is limited because of the globe-style flow path. Needle valves are a good choice with lighter, less viscous fluids.

For the most precise flow control, consider fine metering valves, typically found in laboratory settings. Fine metering valves are a type of needle valve with a long, fine stem that lowers through a long, narrow channel. This makes for a pronounced globe pattern that is excellent for marking fine gradations of flow. Some fine metering valves are not designed to shut off.

Quarter-turn plug valves are economical utility valves. Quarter-turn actuation rotates a cylindrical plug in a straight-through flow path. The plug contains an orifice to permit flow. Plug valves are commonly used for low-pressure throttling applications and shutoff.

In a rising plug valve, as in a needle valve, a tapered plug lowers into an orifice to reduce flow. It differs from a needle valve in its flow path, which is straight-through rather than globe-patterned. Because of the straight path, the valve is not as effective at providing fine gradations of flow. The rising plug is also roddable, which is a good choice if the valve becomes clogged with system media.

Conclusion

Once you have matched valve type to function, you are well on your way in the valve selection process. Many details remain, though. You will need to give detailed attention to the following: installation issues; maintenance schedules and access; safety and code requirements; and system parameters such as pressure, temperature, flow rates, and the system media.

Ultimately, you will need to determine valve size and actuation, as well as materials of construction, which must be compatible with the chemical composition of the system media, pressures, and temperatures.

For more information about on-off valves and flow-control valves, go to www.swagelok.co.uk.