Edited by Kenneth J. Korane
Selecting valves for instrumentation or fluid-control applications may, at times, seem overwhelming. There are lots of choices, including ball, bellows, check, diaphragm, excess flow, fine metering, gate, multiport, needle, plug, relief, rising plug, and safety valves. Further, each comes in many sizes, configurations, and materials, and can be actuated in various ways.
To narrow the options, it is always good practice to first ask: What will the valve do?
Most valves perform one of five primary functions — on-off, flow control, directional control, overpressure protection, and excess-flow protection. Ensuring a valve performs the intended function is the first and most-important step in the selection process. It is not unusual to see valves misapplied in the field, such as using a ball valve for throttling flow. In some cases, the mismatch can be catastrophic, say, if a ball valve were used in a high-pressure oxygen system. Near a source of ignition, the fast opening valve would allow a sudden burst of oxygen that could start a fire.
Here’s a review of the basic types of valves, how they work, functions they perform, and trade-offs when choosing one over another.
On-off control is the most basic valve function. Valves in this category stop and start fluid flow. Primary types are ball, gate, diaphragm, and bellows valves.
Ball valves are, perhaps, the most common design for on-off control. As the name implies, a metal ball with a large hole through its center lies in the flow path. Aligning the hole with the flow path lets fluid pass through; turning the ball 90° stops flow. A ball valve, with its quick shutoff and high-flow capacity, is a good choice for an onoff valve. The handle position provides a quick indication of whether the valve is open or closed and, for safety, ball valves are easy to lock out and tag. They are most practical and economical in sizes from 0.25 to 2 in. (6 to 50 mm).
Gate valves are among the oldest types of on-off valves. Turning the handle raises and lowers a metal plate or other sealing mechanism in and out of a straight flow path. Shutoff is gradual.
Gate valves are typically used for process-control applications, particularly in lines larger than 2 in. However,they are also frequently used as the first valve off an instrumentation process line, often in a double block-andbleed configuration. And they’re used in general industrial applications such as in large transmission or process lines. Some exceed 100 in. (2,540mm) in size.
The valve stem, a cylindrical shaft, connects the handle (or actuator) with the inner mechanisms in gate valves, as well as other shutoff, flow control, and directional-control valves. Usually, the stem turns and/or moves up and down. Packing — soft material such as gaskets and O-rings — surrounds the valve stem where it meets the valve body. This prevents gases or fluids from escaping. Valves that seal to atmosphere with metal-tometal seals are referred to as “packless” because they do not contain soft packing material.
Stem seals and packing are subject to wear that can lead to leaks. Thus, packing must be regularly serviced or replaced. Some types, such as the two-piece chevron design, create more-effective seals and last longer than others.
In contrast to packed valves, diaphragm valves are packless and provide rapid shutoff. The valve contains a thin metal or plastic diaphragm which flexes up and down, creating a leaktight seal over the inlet. Depending on the type of actuation, opening and closing speeds can be precisely controlled to deliver consistent quantities of fluid.
Diaphragm valves tend to provide the highest cycle life among all valve types. This robust valve is usually small, with the largest orifice or internal pathway generally less than 2 in. (50 mm). High-purity biopharmaceutical and semiconductor applications typically use diaphragm valves.
Like the diaphragm valve, bellows valves are also packless. A welded seal divides the lower half of the valve that contains system media from the upper parts that house the actuator. The stem, entirely encased in a metal bellows, moves up and down (without rotating) to seal over the inlet.
Bellows valves are a good choice in applications with critical sealing demands and limited access for maintenance. Frequently, they are specified for containment areas in nuclear-power plants.
Bellows and diaphragm valves have a globelike flow path. Fluid does not flow straight through globe valves on a level plane, as it does in ball valves. Flow enters the valve below the seat and exits above the seat. Therefore, globe valves have lower flow rates than valves with a straightthrough flow path of the same orifice size.
Flow-control valves let operators adjust flow by rotating a handle, and the valve reliably maintains that flow rate. Some flow-control valves also provide shutoff. Common flow-control valves include needle, fine metering, quarter-turn plug, and rising plug.
Needle valves provide precise flow control and, depending on design, leaktight shutoff. They have a long, finely threaded stem with a highly engineered tip geometry (for example, vee or needle-shaped) that fits precisely into a seat over the inlet. Stem packing provides the seal.
Some designs contain a metal-to-metal seat seal, making needle valves a good choice for high-temperature applications. Globe-style flow paths limit flow, so needle valves are a better choice with lighter, less-viscous fluids.
For the most-precise flow control consider fine-metering valves, typically found in laboratories. Fine-metering valves are needle valves with a long, fine stem that lowers through a long, narrow channel. This construction makes for a pronounced globe pattern, ideal for controlling flow in fine gradations. Some fine-metering valves cannot be shut off.
Quarter-turn plug valves are economical utility valves. Turning the handle 90° rotates a cylindrical plug in a straight-through flow path. An orifice in the plug permits flow. Plug valves are commonly used for shutoff and low-pressure throttling.
Another type of plug valve is the rising-plug valve. Like a needle valve, it lowers a tapered plug into an orifice to reduce flow. However, its flow path is straight-through rather than globe patterned. This means risingplug valves are not as effective as needle valves for fine gradations of flow. But the rising plug is “roddable.” That is, thanks to its straight-through bore design, the valve can be cleaned out while in line — making it a good choice if the valve may get clogged.
As the name implies, these valves direct fluid flow. Check valves ensure flow in only one direction. In most designs, upstream fluid pushes a springloaded poppet open, permitting flow. Should downstream or back pressure increase beyond preset limits, however, the force pushes the poppet back into the seat, stopping reverse flow. Check valves are available with fixed or adjustable cracking pressures.
Some ball and diaphragm valves are designed with more than two ports. In most multiport valves, fluid enters through a single inlet but may exit through one of several outlets, depending on actuator position. Multiport valves may or may not have a shutoff position.
Valves in this category prevent the buildup of system pressure beyond safe levels. Relief valves come in several versions. One type is a proportional-relief valve. It contains a vent to atmosphere that opens when system pressure exceeds operator-set levels. A springloaded poppet enables the measured release of fluid. The vent closes when pressure falls below preset levels.
Safety-relief valves are designed to open quickly and release large amounts of system media. Because of their critical safety function, regulatory codes require safety-relief valves in certain applications. Safety-relief and proportional-relief valves are not interchangeable with check valves, because the three have different functions.
Rupture discs are used mainly on sample cylinders to protect against overpressurization, for example, when temperatures rise during transport. Similar to relief valves, rupture discs vent to atmosphere. A metal diaphragm bursts when pressure reaches a point preset by the manufacturer. Once activated, the rupture disc must be replaced. Transportation codes require that compressedgas cylinders be equipped with pressure-relief devices. Rupture discs are an economical choice for this application.
Excess-flow valves stop uncontrolled release of system media if a downstream line ruptures. Under normal conditions, a spring holds a poppet open. When flow exceeds a specified limit, the poppet moves to a tripped position, stopping almost all fluid flow. The valve returns to its open position when the problem is corrected. These valves are available with fixed tripping values.
Matching valve to function is step one when selecting a valve. From there, other details need attention. See the accompanying sidebar, “Tips and traps,” for some additional advice on specifying valves.
Tips and traps
Beyond specifying the type of valve for an application, a number of other areas deserve attention:
Know the application. When choosing a valve, it is essential to know the chemical composition of the system media and the full range of pressures and temperatures the valve will see over its life. Make sure the valve (including the seals) can handle these conditions. Consult product data and test reports in re ning the choice. Manufacturers’ application engineers can help guide the process. Don’t rely on hunches or approximations.
Check for material compatibility. It is possible to have the right valve but the wrong materials. Valves often come in standard materials with standard O-rings and seals, but there are alternatives. Always check the catalog to identify temperature and pressure ranges, as well as compatibility with different system media (chemicals). When in doubt, consult the manufacturer.
Know the maintenance schedule. Different valves have different maintenance schedules. System parameters, including how often the valve cycles, affect this schedule. The valve’s maintenance schedule needs to be manageable for your maintenance team. This seemingly obvious point is often overlooked. Are you willing to service a valve once every 20 days when it is 100 ft in the air?
Understand pressure drops. Most every valve and component produces a drop in pressure. Be aware of the cumulative pressure drops that various components create along a flow path. Otherwise, there may be too little pressure at certain points in the line, and this can adversely affect processes or equipment. Every valve is rated with a flow coefficient (Cv) which describes the relationship between pressure drop across an orifice, valve, or other assembly and the corresponding flow rate. The higher the Cv, the lower the pressure drop. Ball and needle valves of the same size produce significantly different pressure drops. Ball valves generate little resistance to flow, whereas needle valves create sizable pressure drops.
Meet safety and code requirements. Regulatory codes demand specific types of valves in certain applications. Be cognizant of such requirements, or seek the advice of engineering experts.
Consider cost of ownership. The true cost of a valve is not just its purchase price. The true cost also includes the cost of owning, maintaining, and replacing that valve over time. To calculate the cost of ownership, determine how long a valve will operate in a particular system between maintenance checks. Estimate maintenance costs not only in replacement parts, but also in labor and downtime. Note that some valves are much easier to service than others. Some can be serviced in place; others must be removed from the process line. Finally, consider the chances of unscheduled maintenance and downtime for a given valve.