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The industry suffers significant financial losses each year through the consequences of improper valve selection, as it can promote valve failures, which can result in loss of system fluids, out of spec production, downtime expenses, unsafe workplace conditions, and environmentaldamage.
Analyzing the system and considering the following guidelines can help users to select a valve that will install easily, perform safely and reliably, offer the lowest maintenance and overall cost, and will meet the unique system requirements.
What type of fluid will the system carry?
Before selecting a valve the type of fluid the system will carry needs to be considered. Is the fluid viscousor thin? Gas or liquid? Corrosive or inert? Such variables can affectsystem components and operation.
Fluid viscosity affects system flow and valve requirements. Fluids that are more viscous reduce systemflow 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, material selection needs to be considered 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 can harden and crack in cryogenic service, and they have highthermal coefficients of expansion. In addition, differential pressurecan affect sealing capability.
If a valve that will perform reliably in a severe service system is needed, a valve that is especially designed for that service and that meets current industry codes or standards should be considered.
What specific valve design features will be required?
After fluid characteristics and operating conditions have been examined, it is important to understand valve design features that are critical to performance. While valve manufacturerscannot control a system’s design parameters, such as the system fluidand operating conditions, they can control design features that affect the valve’s performance.
An 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 valvestem (fig 1). When the packing nut is tightened, the PTFE is forced outward against the valve bonnet and inward against the stem to form aseal. 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 ortemperature 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 packingin high-cycle applications. The two most common methods of live loading are an elastomeric O-ring seal and a spring-loaded plastic packing.
Packless valves, such diaphragm valves or bellows valves, provide static, metal-to-metal seals.There are several factors within thevalve 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. Mating a stem tip and a seat with a smooth surface can result ina 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 deformslightly and create a leak-tight seal.
Sizing of Valves
Valve 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 thatcertain aspects of the flow path be known, such as the size and shapeof the orifice and flow path, the internal diameter of the pipe or tube, the characteristics of the fluid, such as density and temperatureas well as pressure drop from inlet to outlet.
To gain a clearunderstanding of flow, the user can compare the flow coefficient (Cv).The Cv incorporates the combined effects of all the flow restrictionsin 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 labour intensive.
What installationprocedures need to be followed?
Once the right valve for the application has been selected, the user should consider how it will be installed, and look for features that maximize performance and minimize maintenance issues. Improper installation will affect performance and reliability. Valves should be installed with panel mounts, bottom mounts,or special brackets. The valve mounts should handle external loads, such as system expansion, and should absorb torque from valve actuationso that stress is not transferred to end connections, piping, or tubing. Installed valves should be easily seen, easily reached, and protected from accidental damage or accidental actuation. They should not beinstalled in areas where they can become footrests or hangers.
In summary, choosing the right valve, helps to maintain a safe environment, eliminates costly downtime, and benefits from increased reliability, leak-tight operation and performance.
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