Valves are mechanical devices that control the flow and pressure of liquids, gases, and slurries within a system. They are also known as regulators and are used in a wide variety of applications. Valves vary greatly in size, design, function, and operation. There are several methods that can be used to classify valves including the control mechanism and valve function.

Valve Function

Valves are a part of many daily- used machines and can perform a variety of functions. The three common valve functions include stopping and starting flow, throttling (control) flow, and acting as a non-return check for flow (check).

Stop/Start valves are used for systems that do not need the flow throttled. The valve opens to allow the flow and closes to stop flow.

Throttle or control valves control the speed and capacity of flow through the system.

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Non-return or check valves control the direction of flow. Flow in the desired direction opens the valve, while flow in the oppose direction forces the valve closed. These valves are important for preventing backflow to systems in applications such as wastewater management.

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Method of Control

The mechanism to control flow can vary based on the application of the valve. In general, there are two means of controlling flow through a valve.

Linear motion valves use a closure member that moves in a straight line to allow, stop or throttle the flow. The closure device could be a disc, slat or flexible material, like a diaphragm. The closure device can be used to:

• Move a disc or plug into or against an orifice
• Slide a slat, cylindrical, or spherical surface across an orifice
• Move a flexible material into the flow passage

Rotary motion valves rotate a disc or ellipse about an angular or circular shaft extending across the diameter of an orifice. Quarter turn valves will be in their fully open or fully closed state after a 90° turn of the stem.

Types

Below is a summary of the basic and most common types of valves, more information can be found by clicking the link in their title.

Valve Components

Valves can vary greatly in size and design but there are several basic components to valve functionality.

The body of the valve holds the parts together. The ends are designed to connect into the pipe or equipment in the system and generally are butt or socket welded, threaded or flanged. The body is the first pressure boundary to come into contact with the surrounding environment and system media. The environment is an important consideration when selecting the body material.

The bonnet is the cover for the opening in the body. This is the second most important boundary of a pressure valve and is made from the same material as the body. The bonnet can also support internal valve parts, such as the stem, disk, and actuator.

Trim is a term used for the replaceable internal parts such as the disk, seat, stem, and sleeves used to guide the stem. The trim is responsible for the basic motions and flow control features of the valve.

The disk and seat provide the capability for permitting and prohibiting fluid flow. The system is under full pressure when the disk is closed. The seat provides a surface for the disk to seal too in order to stop the flow. The valves may have one or more seats depending on the type. For example, a gate valve has two seats; one on the upstream side and the other on the downstream side. The design of the disk is generally where valves get their name.

The stem is responsible for the movement of the disk, plug or the ball for opening or closing the valve. It is usually forged and connected to the valve hand-wheel, actuator, or the lever by threading. The stem moves the disc in a linear or rotary movement to open or close the valve. There are five types of valve systems depending on the application. 

• Rising stem with outside screw and yoke-The exterior of the stem is threaded, while the portion of the stem in the valve is smooth. The stem threads are separate from the flow medium by the stem packing. This type of valve is common for larger valves.
• Rising stem with inside screw- The threaded part of the stem is inside the valve body, and is in contact with the flow medium. When rotated, the stem and the hand-wheel rise together to open the valve.
• Non-rising stem with inside screw- The valve disc travels along the stem, like a nut as the stem is rotated. Stem threads are exposed to the flow medium so this model is appropriate when space is limited to allow linear movement, and the flow medium does not cause erosion, corrosion or abrasion of the stem material.
• Sliding stem- The valve stem slides in and out of the valve to open or close the valve. This design is for hand-operated, rapid opening valves, and control valves that operate by hydraulic or pneumatic cylinders.
• Rotary stem- This is a commonly used model in ball, plug, and butterfly valves. A quarter-turn motion of the stem opens or closes the valve.

Stem packing is used to form a tight seal between the stem and the bonnet. The packing is fitted with one of several components: a gland follower, a gland, stuffing box, packing material, or a backseat. Packing is important in preventing damage to the stem and fluid or gas loss. It is commonly a fibrous material or compound (such as Teflon®) that forms a seal between the internal and the outside parts of a valve.

The yoke and yoke nut are used to connect the body with the actuating mechanism. The yoke must be strong enough to withstand the forces, movements, and torque developed by the actuator. The nut is used to control the movement of the stem.

Valve Actuator

The valve actuator operates the stem and disk to open and close the valve. There are several types of actuators depending on the needs of the system such as the torque necessary to operate the valve, speed and the need for automatic actuation.

Manual/ hand operated actuators use a hand-wheel or crank to open or close the valve. They are not automatic but offer the user the ability to position the valve as needed. Manual actuators are used in remote systems that may not have access to power, however they are not practical for applications that involve large valves. The hand-wheel can be fixed to a stem or hammer which allows for the valve to be pounded open or closed if necessary. Gear heads can be added for additional mechanical advantage and open/close speed.

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Electric motor actuators permit manual, semi-automatic, and automatic operation of the valve. The motor is usually reversible and used for open and close functions. The high speed motor is connected through a gear train to reduce the motor speed and thereby increase the torque. The actuator is operated either by the position of the valve or by the torque of the motor. A limit switch can be included to automatically stop the motor at fully open and fully closed.

Solenoid operated valves use hydraulic fluid for automatic control of valve opening or closing. Manual valves can also be used for controlling the hydraulic fluid; thus providing semi-automatic operation. A solenoid is a designed electromagnet. When an electric current is applied, a magnetic field is generated around the wire. An iron "T" or plunger is put in the center of the coil to concentrate the magnetism. Since iron is a strong magnetic conductor and air is not, the "T" is drawn by the magnetic field into a position where the magnetism can travel 100% through the metal conductor. The moveable "T" acts as the actuator of the valve. Solenoid valves can be arranged such that power to the solenoid either opens or closes the valve. One application of solenoid valves is to supply the air to systems like pneumatic valve actuators. These valves are not practical for large systems because their size and power requirements would be excessive.

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Pneumatic operated valves can be automatic or semi-automatic. They function by translating an air signal into valve stem motion by air pressure acting on a diaphragm or piston connected to the stem. Pneumatic actuators are fast-acting for use in throttle valves and for open-close positioning.

Hydraulic actuators provide for semi-automatic or automatic positioning of the valve. They are used when a large force is required to open the valve, such as a main steam valve. With no fluid pressure, the spring force holds the valve in the closed position. Fluid enters the chamber, changing the pressure. When the force is greater than the spring force, the piston moves upward and valve opens. To close the valve, hydraulic fluid (such as water or oil) is fed to either side of the piston while the other side is drained or bled.

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Self-actuated valves use the system fluid to position the valve. These are commonly found in relief valves, safety valves, check valves, and steam traps. Because these actuators use the fluid in the system, no external power is required.

Speed of Power Actuators

Actuators can vary in operating speed. The speed should be selected based on the speed and power requirements of the system and availability of energy to the actuator.

• Fast acting actuators are best used when a system must be quickly isolated or opened. Fast action is provided by hydraulic, pneumatic, and solenoid actuators. The speed of actuation is set by installing the correct orifice in the lines and the valve is closed by spring pressure, which is opposed by hydraulic or pneumatic pressure to keep the valve open. Electrical motors can also provide fast actuation when the speed is set through the motor speed and gear ratio.

• Slow acting actuators are best used when cold water is injected into a hot system or slower opening is needed.

Actuator Size

Due to the wide variety and variations in valves, the actuator must be sized to the specific valve in the system. If the actuator is undersized, it will be unable to overcome the forces against it. This will cause slow and erratic stroking. If the actuator is not stiff enough to hold the close position, the closure element will slam into the seat, causing a pressure surge. If the actuator is oversized, it will cost more, weigh more, and be more sluggish in terms of speed and response. Larger actuators may also provide a higher thrust that will damage internal valve parts. Actuators tend to be oversized because of safety factors but smaller sizes function just as well when the built-in safety factors are considered.

Material of Valve Construction

Valves are made of a wide variety of materials including metallic and nonmetallic options. When selecting a material, the operating environment (i.e. ambient heat), lifespan (i.e. maintenance), and media (i.e. gas or corrosive liquid) should be considered. The most common material is carbon steel because it does very well in high heat, is easily available and inexpensive but it is not suited for corrosive materials. Stainless steel is strong and exhibits resistance to both corrosion and high temperatures, but costs more than carbon steel. Special alloys are used for severe applications such as high pressure or extremely corrosive materials.

Selection Tip: Will the valve be mostly open or mostly closed? Some materials show different characteristics in stagnant verses continuous flow conditions.

Media

Media is a term used to describe the material that will be going through the pump system. The media plays an important role when selecting the material the valve body and disc will be made of as well as the type and speed of the actuator. There are a wide variety of materials that could be in the valve system; these include:

Gas- Valves for gas systems seal tightly to minimize specified leakage rate at rated operating temperatures and pressures.

• Air -- used to describe all non-pressurized air
• Compressed air -- used to describe pressurized and potentially explosive air.
• High purity, natural gas, sour, specialty or corrosive
• Liquefied petroleum
• Steam

Liquid: Valves for liquid systems require tight seals to prevent leakage.

• Water (hot or cold, clean or dirty, fresh or salt)
• Gasoline (diesel fuel)
• Hydraulic fluid
• Highly viscous or gummy fluids

Solid: Valves for solid materials must be durable and have few parts to prevent clogging.

• Slurry- A slurry is a solution with suspended particles. For this media type, the valve must be able to operate effectively in aggressive conditions.
• Powder 

Specifications

Flow

A valve can be used to stop and start as well as throttle or regulate the flow or movement of a media through a system. The given and desired properties of the flow can be used when selecting a valve.

Variables for flow calculations

Pressure Drop

The pressure drop is the pressure change between the inlet and outlet of the system. The formula is as follows:

ΔP = G (q/F_pC_v)²

If the pressure drop is too high, a larger valve or a valve with a higher C_v can be used to lower the pressure.

Flow Coefficient

The valve flow coefficient is the number of U.S. gallons per minute of 60°F water that will flow through a valve at a specified opening with a pressure drop of 1 psi across the valve. The coefficient is used to determine the size that will best allow the valve to pass the desired flow rate, while providing stable control of the process fluid. It can be used to compare flow capacities of valves of different sizes, types, and manufactures. The flow coefficient is different for gases, liquids, and steam and is also dependent on the pressure drop across the valve. The C_v calculated will apply to either the opening or closing depending on the function.

For air and gases: Compressible media - The density of the gas changes with a change in pressure and therefore the flow rate changes. Low pressure is defined as P₂ > P₁/2 and high pressure is defined as P₁ > P₂/2

Low pressure: C_v= q/ (16.05) √(P₁² - P₂²) / G * T

High pressure: C_v= q/ (13.61) √(1) / G * T

For liquids: Incompressible media- The flow rate only depends on the difference between the inlet and outlet pressures so the rate stays the same as long as the change in pressure remains the same.

C_v= q/F_p √(G/ΔP)

For steam: Compressible media- The density of the steam changes with a change in pressure. There is a critical (choked¹) and non-critical pressure drop. Critical is defined as the pressure dropping by 58% or more from the inlet to the outlet.

Critical pressure drop: C_v = m/1.61 p_i

Non- critical pressure drop: m/(2.1 ( p_i + p_o))

If C_v calculated is too small the valve will be undersized and the process system may be starved for fluid. This also causes a higher pressure drop across the valve causing cavitation or flashing. If C_v is too high the valve will be too big leading to a waste of money and the machine being too difficult to maneuver. A larger C_v can also be a problem for throttling because the flow cannot be effectively controlled at the openings. The location of the closure element leads to the possibility of creating a high pressure drop and faster velocities causing cavitation, flushing or erosion.

Flow Characteristic

The flow characteristic describes the relationship between the flow coefficient and the valve stroke. It is inherent to the design of the selected valve. For example, as the valve is opened, the flow characteristic allows a certain amount of flow through the valve at a particular percentage of the stroke. This is especially important for throttle control because it controls the flow in a predictable manner.

Rangeability

Rangeability is a very important factor when selecting a valve type. It is defined as the maximum to minimum flow rate that can be controlled by a given valve type. The characteristic is affected by three factors: the geometry of the valve, the seat leakage, and the actuator's accuracy or stiffness at near closure of the valve. Geometry is inherent due to the design of the seat and closure and excessive seat leakage can cause instability in the valve as it lifts off the seat.

Rangeability is easily calculated based on the geometry and actuator of the valve. If the valve is not accurate at 5 percent of stroke, then the rangeability is 20:1 (100 percent divided by 5%). As the rangeability increases, a wider range of flow rates can be controlled by the valve. It is not imperative that the valve has the highest rangeability because most systems do not have that wide of a flow rate range. V-notched ball valves have the highest rangeability at 200:1, while globe valves have a high rangeability of 100:1. Higher rangeability usually indicates the sensitivity is lower when the closing element is near closed and increases as the valve opens.

Flow Rate

Flow rate is affected by the flow characteristic and the pressure drop across the valve.

Valve Sizing

The science behind valve sizing is determining the flow through the diameter of the valve. Valves may contain two different sized openings designed to take a pressure drop. This is why valve sizing is almost always done for throttling valves. Although, sizing for open/close valves should also be considered.

• Open/Close valves are expected to pass 100% of the flow without a significant drop in pressure. They do not throttle the flow so the openings are generally the same size. If the valve is too small, the flow will be restricted, defeating the point of the on/off valve. A large valve will cost more because increasers will need to be installed.
• Throttle valves are expected to produce a certain amount of flow at certain positions of opening to create a pressure drop. Throttle valves work best when the valve uses the full range of stroke while producing desired flow characteristics and maximum flow output.

Oversizing a valve happens more frequently than undersizing because the manufacturer adds safety factors to the specifications they receive from the user, which are generally the maximum specifications of the system. Having an oversized valve is more managable and safer than undersized valves

The sizing for valves is based upon type. More information is available on the valve type pages in the table above.

The use of increasers or reducers to create nonstandard piping configurations can be corrected in the C_v equation. In order to determine the piping geometry factor, F_p, the inside diameter of the pipe is required. "d" is the inside diameter and "D" is the outside diameter.

Piping- geometry factor for valves with reducers and increasers on both ends. Table Credit: Valtek International

Installation Cost 

• Open/close- Some of these valves are considered to be a onetime purchase; use until they break and then discard them

• Throttle- Throttle valves have a number of cost factors, not including the initial cost and true installed cost.

The true installed cost over time includes the purchase price, installation and start up, training, maintenance, and cost of spare parts. The reliability of the valves can also affect the cost because the more reliable valves will not need as much maintenance.

Features

Valve position indicator: Indicators on the valve allow the operators to identify the position of the valve. The indicator can be a light, position switch, rising stem, or some other visually discernible characteristic. This feature will indicate if the valve is on or off and in the case of throttling valves, how open the valve is.

Standards

ANSI B16.34 -- covers process valves with pipe flange, butt weld, socket weld, and pipe thread ends. It defines pressure-temperature rating, materials, design requirements, and wall thicknesses for process valves and does not apply to instrument valves.

MSS Standards -- applies to steel and alloy valves of 1- inch nominal pipe size and smaller and pressure rating of 10,000 psi and lower at 100°F.

API Standards -- API 598 defines the inspection and test requirements for valves purchased under API valve standards. It may be applied to other valves and is widely used in the petroleum and chemical industry.

NACE Standards -- for valves dealing with corrosive materials.

ASME standards and code cases -- ASME has developed two codes for pressure vessels and piping systems. The role of the Boiler and Pressure Vessel Code and the Code for Pressure Piping has been to foster safety in the design and construction of pressurized systems and promote standardization in the design, materials, equipment, and construction.

Resources

Wermac- Introduction to valves- Only the basics

VMA- Common Actuator Types

Sixteen Considerations for Valve Selection You Can't Afford to Ignore

4 Steps to Valve Selection (pdf)

The Engineering ToolBox- Flow Coefficient

ROSS DECCO Solenoid Valves

Engineers Edge- Hydraulic Pneumatic Knowlege

Skousen, Philip L. Valve Handbook. New York: McGraw-Hill, 1998. Print.

Image credits:

Pexuniverse | Beswick Engineering | Grainger

¹Choked pressure drop is when a further increase in pressure drop does not affect the valve's flow rate. For fluid systems, choked flow is when the volume increases at a faster rate than if the flow increased due to the pressure differential. When this happens the valve cannot pass any additional flow, even if the downstream pressure is lowered.