№ 12 (December 2006)
Selecting the Right Valve for Your Application
Industry loses millions of dollars each year through the consequences of improper valve selection.
By John Wawrowski
Improper valve selection can promote valve failures, which can result in loss of system fluids, out of «spec» production, downtime expenses, unsafe workplace conditions, and environmental damage.
So, how can you confidently select a valve that will install easily, perform safely and reliably, and offer the lowest maintenance and overall cost in your system?
As you get ready to specify or replace your next valve, first analyze your system and consider these simple guidelines, designed to help you select valves that will meet your unique system requirements.
What type of fluid will the system carry?Before selecting a valve, consider the type of fluid the system will carry. Is the fluid viscous or thin? Gas or liquid? Corrosive or inert? Such variables can affect system components and operation.
For example, fluid viscosity affects system flow and valve requirements. Fluids that are more viscous reduce system flow and leakage. On the other hand, a high-pressure, light gas will move freely along its flow path, but can be more difficult to seal.
Some gases, such as hydrogen and methane, present significant ignition hazards, and even the smallest leak to the atmosphere can be catastrophic. If the system fluid is a toxic gas, such as arsine or phosphine, leakage to the atmosphere can be harmful to plant personnel. Corrosive gases or liquids such as hydrogen chloride, hydrogen sulfide, or even steam can damage components and actually remove material by chemical or physical attack.
What are the system operating conditions?System operating conditions, such as temperature and pressure, are also important factors in choosing a valve. For example, consider material selection in high- or low-temperature applications; component materials with varying expansion rates can allow fluid leaks. Plastic components can shrink and leak, or they can absorb water and other system media and become brittle at low temperatures. Elastomers, too, can harden and crack in cryogenic service, and they have high thermal coefficients of expansion.
In addition, differential pressure can affect sealing capability. For example, a system operating at 1,000 psig can leak 10 times the amount of the same system operating at 100 psig.
Will the valve be used in severe service?If you need a valve that will perform reliably in a severe service system, consider a valve that is especially designed for that service, and confirm that it meets current industry codes or standards. Below are a few examples of applications and the corresponding recognized industry codes.
• Valves used in fire safety applications - Fire Safety Specification API 607.
• Valves for sour gas service - NACE (National Association of Corrosion Engineers) Specification MR0175.
• Valves used in thermal fluid applications - ANSI/FCI 70-2 Specification for leak-tight shutoff and a fire hazard standard like API 607.
• Valves used in chlorine systems - Chlorine Institute Pamphlet #6, «Piping Systems for Dry Chlorine».
What specific valve design features will be required?After you have examined fluid characteristics and operating conditions, it is also important to understand valve design features that are critical to performance. While valve manufacturers cannot control your system\'s design parameters, such as the system fluid and operating conditions, they can control design features that affect the valve\'s performance.
One important feature is the way a valve seals to atmosphere. Valves can be packed or packless. Packed valves have either conventional or «live-loaded» packing. In conventionally packed valves, a PTFE packing cylinder fits closely around the valve stem (Fig. 1). When the packing nut is tightened, the PTFE is forced outward against the valve bonnet and inward against the stem to form a seal. Another design for packed valves is a «live-loaded» seal (Fig. 2). Live loading subjects the packing to consistent compression that ensures it remains leak-tight, even in systems with frequent pressure or temperature changes or high-cycle rates.
Well-designed, live-loaded packing exerts a minimum amount of pressure to achieve the seal - without increasing the amount of torque required for valve actuation. This way, live loading also reduces wear and tear on the stem packing in high-cycle applications. The two most common methods of live loading are an elastomeric O-ring seal and a spring-loaded plastic packing.
The simplest live-loaded seal uses an elastomeric O-ring. The resilience of the elastomer provides the live load. In the spring-loaded method, a seal may employ plastic packing, but because plastics are not as resilient as elastomers, a series of springs above a metal gland provide the live load. A packing nut compresses the springs to maintain a more consistent load on the packing.
Packless valves, such as diaphragm valves or bellows valves, provide static, metal-to-metal seals. Again, there are several factors within the valve manufacturer\'s control that can affect the integrity of the metal-to-metal seal. For example, there is a direct relationship between the quality of the surface finish and valve performance and seal integrity. That is, mating a stem tip and a seat with a smooth surface can result in a smaller gap between those two surfaces than would exist if the two surfaces were rough.
Another factor that affects metal-to-metal seal integrity is differential hardness of the materials. The stem tip must be made of a harder material than the seat so that the seat will deform slightly and create a leak-tight seal.
Sizing of ValvesValve size is often described by the nominal size of the end connection. But for most fluid systems, a more important measure can be the flow that a valve can allow. The principles of flow calculation dictate that certain aspects of the flow path be known, such as:
• size and shape of the orifice and flow path;
• internal diameter of the pipe or tube;.
• characteristics of the fluid, such as density and temperature;
• pressure drop from inlet to outlet.
It is easy to understand that a straight-through flow path, like that of a ball valve (Fig. 3), would allow for a greater flow than an equivalently sized needle valve (Fig. 4), which presents a much more tortuous flow route.
Rather than doing complex calculations to gain a clear understanding of flow, you can compare the flow coefficient (Cv). The Cv incorporates the combined effects of all the flow restrictions in a valve and gives a single common reference number. Other valve design features to consider include manual or automatic actuation and end connection methods. Experience shows that valves with integral end connections minimize potential leak points, and make installation and maintenance procedures less labor intensive.
What installation procedures will you need to follow?Once you have selected the right valve for your application, consider how it will be installed, and look for features that maximize performance and minimize maintenance issues. Improper installation will affect performance and reliability. Consider these suggestions:
• Install valves with panel mounts, bottom mounts, or special brackets. Remember that valve mounts should handle external loads, such as system expansion, and should absorb torque from valve actuation so that stress is not transferred to end connections, piping, or tubing.
• Install a valve so that it is supported by the valve mounting and not by the tubing or piping system.
• Install valves so they are easily seen, easily reached, and protected from accidental damage or accidental actuation.
• Install valves with flow in the direction of the flow arrow.
• Don't install valves in areas where they can become footrests or hangers.
In summary, when you choose the right valve, you help maintain a safe environment, eliminate costly downtime, and benefit from increased reliability and leak-tight operation and performance.