The industry loses crores of rupees each year through the consequences of improper valve selection. Improper valve selection can promote valve failures, which can result in loss of system fluids, out of spec production, downtime expenses

As you get ready to specify or replace your next valve, first analyse your system and consider these simple guidelines, designed to help you select valves that will meet your unique system requirements. The industry loses crores of rupees each year through the consequences of improper valve selection. 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 one confidently select a valve that will install easily, perform safely and reliably, and offer the lowest maintenance and overall cost in the system? As you get ready to specify or replace your next valve, first analyse 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 1000 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 recognised industry codes.
• Valves used in fire safety applicationsFire Safety Specification API 607
• Valves for sour gas serviceNACE (National Association of Corrosion Engineers) Specification MR0175
• Valves used in thermal fluid applicationsANSI/FCI 70-2 Specification for leak-tight shutoff and a fire hazard standard like API 607
• Valves used in chlorine systemsChlorine Institute Pamphlet #6, Piping Systems for Dry Chlorine

What Specific Valve Design Features will be required?
After the fluid characteristics and operating conditions are examined, it is also important to understand valve design features that are critical to performance. While valve manufacturers cannot control the system’s design parameters, such as the system fluid and operating conditions, they can control design features that affect the valve’s performance.






Fig 1: (W-PH-0242) In conventionally packed valves, a PTFE packing cylinder fits closely around the valve stem. When the packing nut is tightened, the PTFE is forced outward against the valve bonnet and inward against the stem to form a seal.

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 sealwithout 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 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.

Valves in chemical process industry

It was the need to start and stop fluid flows, that lead to the invention of valves. Today they are used to stop or control flow in chemical process industries, power plants, cross-country pipelines, oil and gas installations, pharmaceutical industries and sewage system.

All the valves are designed to control flow. However some throttle the flow, while others perform on-off duties. The valve can be operated manually, remotely or automatically. They come in a variety of materials ranging from steels to exotic alloys, non-ferrous metals, plastics, glass and ceramic. Selecting the right type of valve for duty as well as cost effective design, is an involved process.

This article reviews the different types of valves commonly used in industries and the factors to be considered while selecting the most appropriate and cost effective valve. We have given specific attention to the materials used for their construction and various design standards concerning valves and piping systems.

Types of valves

Manual valves may be grouped according to the way the closure member moves onto the seat. Accordingly valves have been divided into following four groups.

Closing down valves: A stopper like closure member is moved to and from the seat in the direction of seat axis

Slide valves: A gate like closure member is moved across the flow passage

Rotary valves: A plug or disc or ball like closure member is rotated valve within the low
passage, around an axis normal to the flow stream

Flex-body valves: The closure member flexes the valve body

Non-return valves: These valves automatically open with forward flow and closes with reverse flow

Each valve group represents a number of distinct types of valves, which use the same method of flow regulation, but differ in the shape of the closure member.

Valves in use

The majority of valves used in industry are one of the four, that is gate, globe, quarter turn (plug, ball, butterfly) and check valves. However, other kinds of valves are used for specific applications.

All the valves consist of the same basic components. The body contains the fluids, which moves along the flow path of the valve. The movement of the fluid is manipulated by a device, such as disc, plate, and ball, which is inserted into the flow path. A stem moves the flow-changing device with either linear or rotary motion. All the joints of the valve are sealed with suitable seals to prevent any sort of external leakage.

Gate valve

A gate valve consists of a body and an internal wedged shaped plate. When the valve is closed, the plate is wedged between the seats, totally blocking the flow. In the open position the plate is completely removed from the path of the fluid flow.

Gate valves are used exclusively for on-off operation where opening up or shutting off the flow is relatively infrequent. Gate valves have two sealing surfaces, one of each side of the gate. In a closed position, the seals traps liquid in a cavity bounded by the body and two sealing surfaces. If liquid temperature rises, the fluid will expand and its pressure will increase. If not vented, the liquid will leak internally or externally. To avoid such leakage, manufacturers make a positive vent, a small drilled hole in the upstream seat. Such vents make valve a one way valve. The one way flow direction of the valve must be marked on its exterior to prevent installation in the wrong direction.

A variation of the gate valve, the knife gate, employs a gate with a flat leading edge rather than a tapered one. With no wedging action to provide a tight seal, flexible elastomeric or plastic sealing surfaces must be used. One more variety is parallel slide valve. In parallel slide gate valve, the shape of the closure is not wedge shaped but parallel. These valves offer low resistance to flow. These are generally used in high-pressure application. Gate valves start and stop the flow of gases and liquids and are used infrequently. They are useful in case of fluids with solids in suspension, slurries, fibres, powders, granules, vacuum, and cryogenic fluids.

Globe valve

Globe valve achieves the same level of tightness as a gate valve and can be used for shut-off conditions. However globe valves are mainly used for throttling the flow. The flow changing component or plug of a globe valve has a circular cross-section but its overall shape ranges from conical to cylindrical or a variety of other forms. It engages a circular seat, which can be of the same or a different alloy. Each plug shape produces unique flow characteristics which generates volumetric change in the flow relative to the amount the valve is open.

Source – Engineering Review
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