Valves Guide

What are Valves?

Valves are mechanical devices that controls the flow and pressure within a system or process. They are essential components of a piping system that conveys liquids, gases, vapors, slurries etc..

Different types of valves are available.. gate, globe, plug, ball, butterfly, check, diaphragm, pinch, pressure relief, control valves etc. Each of these types has a number of models, each with different features and functional capabilities. Some valves are self-operated while others manually or with an actuator or pneumatic or hydraulic is operated.

Functions from Valves are..

• Stopping and starting flow
• Reduce or increase a flow
• Controlling the direction of flow
• Regulating a flow or process pressure
• Relieve a pipe system of a certain pressure

There are many valve designs, types and models, with a wide range of industrial applications. All satisfy one or more of the functions identified above. Valves are expensive items, and it is important that a correct valve is specified for the function, and must be constructed of the correct material for the process liquid.

Regardless of type, all valves have the following basic parts.. the body, bonnet, trim (internal elements), actuator, and packing. The basic parts of a valve are illustrated in the image on the right.

Valve Body

The valve body, sometimes called the shell, is the primary boundary of a pressure valve. He serves as the main element of a valve assembly because it is the framework that holds all the parts together.

The body, the first pressure boundary of a valve, resists fluid pressure loads from connecting piping. It receives inlet and outlet piping through threaded, bolted, or welded joints.

The valve-body ends are designed to connect the valve to the piping or equipment nozzle by different types of end connections, such as butt or socket welded, threaded or flanged.

Valve bodies are cast or forged in a variety of forms and each component have a specific function and constructed in a material suitable for that function.

Valve Body

Valve Body

Valve Bonnet

Valve Bonnet

Valve Bonnet

The cover for the opening in the body is the bonnet, and it is the second most important boundary of a pressure valve. Like valve bodies, bonnets are in many designs and models available.

A bonnet acts as a cover on the valve body, is cast or forged of the same material as the body. It is commonly connected to the body by a threaded, bolted, or welded joint. During manufacture of the valve, the internal components, such as stem, disk etc., are put into the body and then the bonnet is attached to hold all parts together inside.

In all cases, the attachment of the bonnet to the body is considered a pressure boundary. This means that the weld joint or bolts that connect the bonnet to the body are pressure-retaining parts. Valve bonnets, although a necessity for most valves, represent a cause for concern. Bonnets can complicate the manufacture of valves, increase valve size, represent a significant cost portion of valve cost, and are a source for potential leakage.

Valve Trim

The removable and replaceable valve internal parts that come in contact with the flow medium are collectively termed as Valve trim. These parts include valve seat(s), disc, glands, spacers, guides, bushings, and internal springs. The valve body, bonnet, packing, et cetera that also come in contact with the flow medium are not considered valve trim.

A Valve's trim performance is determined by the disk and seat interface and the relation of the disk position to the seat. Because of the trim, basic motions and flow control are possible. In rotational motion trim designs, the disk slides closely past the seat to produce a change in flow opening. In linear motion trim designs, the disk lifts perpendicularly away from the seat so that an annular orifice appears.

Valve trim parts may be constructed of assorted materials because of the different properties needed to withstand different forces and conditions. Bushings and packing glands do not experience the same forces and conditions as do the valve disc and seat(s).

Flow-medium properties, chemical composition, pressure, temperature, flow rate, velocity and viscosity are some of the important considerations in selecting suitable trim materials. Trim materials may or may not be the same material as the valve body or bonnet.

Valve Disk and Seat(s)

Disc The disc is the part which allows, throttles, or stops flow, depending on its position. In the case of a plug or a ball valve, the disc is called plug or a ball. The disk is the third most important primary pressure boundary. With the valve closed, full system pressure is applied across the disk, and for this reason, the disk is a pressure related component. Disks are usually forged, and in some designs, hard surfaced to provide good wear properties. Most valves are named, the design of their disks.

Seat(s) The seat or seal rings provide the seating surface for the disk. A valve may have one or more seats. In the case of a globe or a swing-check valve, there is usually one seat, which forms a seal with the disc to stop the flow. In the case of a gate valve, there are two seats; one on the upstream side and the other on the downstream side. A gate valve disc has two seating surfaces that come in contact with the valve seats to form a seal for stopping the flow. To improve the wear-resistance of the seal rings, the surface is often hard-faced by welding and then machining the contact surface of the seal ring. A fine surface finish of the seating area is necessary for good sealing when the valve is closed. Seal rings are not usually considered pressure boundary parts because the body has sufficient wall thickness to withstand design pressure without relying upon the thickness of the seal rings.

Valve Stem

The valve stem provides the necessary movement to the disc, plug or the ball for opening or closing the valve, and is responsible for the proper positioning of the disk. It is connected to the valve handwheel, actuator, or the lever at one end and on the other side to the valve disc. In gate or globe valves, linear motion of the disc is needed to open or close the valve, while in plug, ball and Butterfly valves, the disc is rotated to open or close the valve.

Stems are usually forged, and connected to the disk by threaded or other techniques. To prevent leakage, in the area of the seal, a fine surface finish of the stem is necessary.

There are five types of valve stems..

• 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 isolated from the flow medium by the stem packing. Two different styles of these designs are available; one with the handwheel attached to the stem, so they can rise together, and the other with a threaded sleeve that causes the stem to rise through the handwheel. This type of valve is indicated by "O. S. and Y." is a common design for NPS 2 and larger valves.
• Rising Stem with Inside Screw
  The threaded part of the stem is inside the valve body, and the stem packing along the smooth section that is exposed to the atmosphere outside. In this case, the stem threads are in contact with the flow medium. When rotated, the stem and the handwheel to rise together to open the valve.
• Non Rising Stem with Inside Screw
  The threaded part of the stem is inside the valve and does not rise. The valve disc travels along the stem, like a nut if the stem is rotated. Stem threads are exposed to the flow medium, and as such, are subjected to the impact. That is why this model is used 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
  This valve stem does not rotate or turn. It slides in and out the valve to open or close the valve. This design is used in hand-operated lever rapid opening valves. It is also used in control valves are operated 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 open or close the valve.

Below you will find some links to detailed (large) images of Rising and NON Rising Stem valves.

Valve Stem Packing

For a reliable seal between the stem and the bonnet, a gasket is needed. This is called a Packing, and it is fitted with e.g. the following components..

• Gland follower, a sleeve which compresses the packing, by a gland into the so called stuffing box.
• Gland, a kind of bushing, which compressed de packing into the stuffing box.
• Stuffing box, a chamber in which the packing is compressed.
• Packing, available in several materials, like Teflon®, elastomeric material, fibrous material etc..
• A backseat is a seating arrangement inside the bonnet. It provides a seal between the stem and bonnet and prevents system pressure from building against the valve pakking, when the valve is fully open. Back seats are often applied in gate and globe valves.

An important aspect of the life time of a valve is the sealing assembly. Almost all valves, like standard Ball, Globe, Gate, Plug and Butterfly valves have their sealing assembly based upon shear force, friction and tearing.

Therefore valve packaging must be properly happen, to prevent damage to the stem and fluid or gas loss. When a packing is too loose, the valve will leak. If the packing is too tight, it will affect the movement and possible damage to the stem.

Typical sealing assembly

1 Gland Follover

2 Gland

3 Stuffing Box with Packing

4 Back Seat

Valve Yoke and Yoke Nut

Yoke

A Yoke connects the valve body or bonnet with the actuating mechanism. The top of the Yoke holding a Yoke nut, stem nut, or Yoke bushing and the valve stem passes through it. A Yoke usually has openings to allow access to the stuffing box, actuator links, etc.. Structurally, a Yoke must be strong enough to withstand forces, moments, and torque developed by the actuator.

Yoke Nut

A Yoke nut is an internally threaded nut and is placed in the top of a Yoke by which the stem passes. In a Gate valve e.g., the Yoke nut is turned and the stem travels up or down. In the case of Globe valves, the nut is fixed and the stem is rotated through it.

Valve Actuator

Hand-operated valves are usually equipped with a handwheel attached to the valve's stem or Yoke nut which is rotated clockwise or counter clockwise to close or open a valve. Globe and gate valves are opened and closed in this way.

Hand-operated, quarter turn valves, such as Ball, Plug or Butterfly, has a lever for actuate the valve.

There are applications where it is not possible or desirable, to actuate the valve manually by handwheel or lever. These applications include..

• Large valves that must be operated against high hydrostatic pressure
• Valves they must be operated from a remote location
• When the time for opening, closing, throttle or manually controlling the valve is longer, than required by system-design criteria

These valves are usually equipped with an actuator.
An actuator in the broadest definition is a device that produces linear and rotary motion of a source of power under the action of a source of control.

Basic actuators are used to fully open or fully close a valve. Actuators for controlling or regulating valves are given a positioning signal to move to any intermediate position. There a many different types of actuators, but the following are some of the commonly used valve actuators..

• Gear Actuators
• Electric Motor Actuators
• Pneumatic Actuators
• Hydraulic Actuators
• Solenoid Actuators

For more information about Actuators see main Menu 'Valves'

Classification of Valves

The following are some of the commonly used valve classifications, based on mechanical motion..

• Linear Motion Valves. The valves in which the closure member, as in gate, globe, diaphragm, pinch, and lift Check Valves, moves in a straight line to allow, stop, or throttle the flow.
• Rotary Motion Valves. When the valve-closure member travels along an angular or circular path, as in butterfly, ball, plug, eccentric- and Swing Check Valves, the valves are called rotary motion valves.
• Quarter Turn Valves. Some rotary motion valves require approximately a quarter turn, 0 through 90°, motion of the stem to go to fully open from a fully closed position or vice versa.

Classification of Valves based on Motion

Valve Types Linear Motion Rotary Motion Quarter Turn Gate YES NO NO Globe YES NO NO Plug NO YES YES Ball NO YES YES Butterfly NO YES YES Swing Check NO YES NO Diaphragm YES NO NO Pinch YES NO NO Safety YES NO NO Relief YES NO NO

Class Ratings

Pressure-temperature ratings of valves are designated by class numbers. ASME B16.34, Valves-Flanged, Threaded, and Welding End is one of the most widely used valve standards. It defines three types of classes.. standard, special, and limited. ASME B16.34 covers Class 150, 300, 400, 600, 900, 1500, 2500, and 4500 valves.

Different types of valves are used in process plants, so there are many standards for valves. Below are the most common..

ASME B16.10 covers Face-to-Face and End-to-End Dimensions of Ferrous Valves

API STANDARDS

API 6D is used to design a valve that is used in the transportation pipeline.

API 600 is used for Bolted Bonnet Steel Gate Valves for Petroleum and Natural Gas Industries

API STD 602 is used to design Compact Steel Gate Valves – Flanged, Threaded, Welding, and Extended-Body Ends.

API STD 594 covers the design, material, face-to-face dimensions, pressure-temperature ratings, examination, inspection, and test requirements for Check Valves with Flanged, Lug, Wafer, and Butt-welding type ends.

API STD 599 covers design for Metal Plug Valves with Flanged, Threaded, and Welding Ends, in sizes NPS 1 through NPS 24.

API STD 609 is for Butterfly Valves with Double Flanged, Lug- and Wafer-Type.

API STD 526 covers Flanged Steel Pressure Relief Valves.

API RP 520 Part 1 overs Sizing and Selection, and Part 2 covers Installation of Pressure-Relieving Devices in Refineries.

API STD 598 covers inspection, supplementary examination, and pressure test requirements for both resilient-seated and metal-to-metal seated gate, globe, plug, ball, check, and butterfly valves at the valve manufacturer’s plant.

Most of the standards above, are used for dimensional inspection of piping components. Some are also used for component design.

Summary

On this page are defined a number of basic information from valves.

As you may have seen in the main Menu "Valves", you can find also information about several and often applied valves in petro and chemical industry.
It can give you an impression, and good understanding of the differences between the various types of valves, and how these differences affect the valve function. It will help to a proper application of each type of valve during the design and the proper use of each type of valve during operation.

Related Post(s)

API 600 Valve Trim No
The removable and replaceable valve internal parts that come in contact with the flow medium are collectively termed as Valve trim...

Article

Introduction to Valve Part Fundamentals

Industrial valves are made up of many different components that allow them to regulate flow. The main parts of a valve designs can be divided into the body, trim, actuators, and ancillary accessories. This table provides a brief overview of the primary valve components and their functions:

Valve PartDescriptionValve BodyThe main pressure boundary of a valve that contains the flow. Usually made of cast or forged metal.BonnetThe cover that allows access to the valve internals for assembly and maintenance. Bolted, threaded, or welded to the body.TrimThe internal moving components that modulate the flow, such as the disc, ball, plug, or gate.SeatThe stationary surface against which the movable trim seals off flow. Precision machined based on trim type.StemThe component that connects the actuator to the trim, allowing motion to control flow.PackingCompressible rings that seal around the valve stem to prevent leakage. Requires periodic replacement.GasketUsed to seal non-moving parts, like between the bonnet and body. Provides high integrity seal.ActuatorProvides the force to open and close the valve’s internal parts. Can be manual, pneumatic, hydraulic or electric.PositionerControls the actuator so the valve moves to the precise flow control position demanded.Limit SwitchesFeedback devices to indicate open and closed position for monitoring.Gear OperatorsGears that allow manual handwheels or actuators to produce high valve torques with lower input effort.

Valve Bodies – The Pressure Containment Shell

The valve body, also called the shell, housing, or casing, is the primary pressure boundary of a valve. It serves as the framework that holds together all the other valve parts in proper alignment. Valve bodies are designed to withstand pipeline pressure, temperature, and mechanical stresses. The inlet and outlet of the valve body connect to the piping system. There are various body styles and configurations, with the most common being globular, straight-through, angle, and Y-pattern. The body shape depends on the valve’s intended flow control function. Gate, globe, check, ball, plug, and butterfly valves all have distinctive body designs tailored to their unique flow control application. Valve bodies are cast or fabricated from materials like carbon steel, stainless steel, cast iron, alloy steel, and forged steel. The material is chosen based on the process fluid composition, pressure, and temperature. Many valve bodies have flanged ends to enable connection to piping. Others may have threaded, socket weld, or butt weld ends. No matter the style, the valve body must be strong enough to withstand the system pressure when the valve is in the closed position. It must also be rigid and resistant to warping or cracking that could cause leaks.

Valve Body Materials for Different Services

There are various options when selecting valve body materials based on the service conditions. Carbon steel is suitable for water, oil, and gas service. Stainless steel handles more corrosive fluids like acids or wet chlorine gas. For extremely high temperatures, alloy steel and cast iron are better choices. Cryogenic valves for frigid liquids like LNG use stainless steel or forged carbon steel bodies. The body material also affects maintenance requirements. For example, carbon steel is prone to rusting or corrosion over time and may need frequent repairs. Stainless steel and alloy steel have higher corrosion resistance, extending the service life. In highly abrasive applications, a hardened valve body is required to resist wearing. No single material is ideal for all applications. Consider fluid composition, pressure, temperature ranges, and desired valve life when choosing a body material. Partnering with an experienced valve supplier is key to getting the right metal for reliable service.

Valve Bonnet Styles for Access and Assembly

The valve bonnet is the cover on the upstream side that completes the pressure shell of a valve body. It also provides the means for assembling the internal valve parts and accessing them for maintenance. There are three main bonnet configurations: screw, bolted, and welded. A screw bonnet has threads that engage with the body and provides a compact means of assembly. It is easy to open and close for routine inspection and repairs. Bolted bonnets have a separate flanged head that connects to the body using long bolts. This allows very large valves to be assembled in sections. Bolted bonnets are common on gate, globe, and check valves over NPS 2. Welded bonnets have the cover permanently welded to the body. No threads or bolts are used, creating a tight seal. However, this does not allow accessing internal parts without cutting. Welded bonnets are preferred for high pressure and temperature systems where bolts or threads could leak. They also cost less than bolted styles. When selecting a bonnet type, consider the maintenance needs, potential leakage risks, valve size, and expense. The bonnet must withstand system pressure and temperature fluctuations. Leak-proof, easy disassembly and assembly is ideal for most applications.

Trim Materials for Erosive and Corrosive Fluids

Valve trim refers to the internal moving parts that modulate flow such as balls, plugs, discs, and gates. The trim comes in contact with the process fluid, so its material must handle the chemical, temperature, and abrasive characteristics. Hardened trim materials include tungsten carbide, Stellite alloys, titanium, Inconel high-nickel alloys, and 440C stainless steel. These withstand highly erosive or corrosive substances. Softer trim materials like bronze, aluminum, Monel, and 304 stainless suit less destructive fluids. The trim material really depends on the fluid composition. For example, a high nickel alloy is better for hydrofluoric acid compared to regular stainless steel. Cryogenic valves need trim that handles freezing temperate without becoming brittle. Abrasive slurries require durable trim that resists wearing. Partnering with an experienced supplier is key to getting the right trim materials for your specific process conditions. This ensures long service life and minimal erosion damage.

Valve Packing vs. Gasket Sealing Methods

Valves use sealing systems to prevent fluid leakage between the stationary body and moving parts. Packing and gaskets are the two main sealing methods. Valve packing consists of rings made of soft, deformable material like graphite, PTFE, or flexible graphite. The rings fit around the valve stem and compress when the bonnet or gland follower tightens. The soft packing deforms to create a tight seal. Packing can leak over time and must be periodically tightened or replaced. It allows some controlled leakage for lubrication. Gaskets provide a more permanent seal between two mating surfaces. Common types are spiral-wound metal, ring joint, kammprofile, and flat paper or plastic. Gaskets require more precision machining for leak-proof performance. Packing handles frequent disassembly better since gaskets can be damaged during maintenance. For fugitive emissions control, metal gaskets are preferred over packing. However, packing enables easy stem movement and regular adjustment. Consider maintenance needs, allowable leakage, and emission regulations when choosing packing or gasket seals.

Flexible vs Solid Wedge Gate Designs

Gate valves use linear motion gates to start and stop flow. The gate and valve disk can be flexible or solid. Flexible wedge gates have a solid top edge but flexible sides made of metal bellows or laminated sheets. This allows the gate to match the bore when seating, creating a tight seal even on worn valve seats. However, bellows can burst or laminations separate after frequent flexing. Solid wedge gates are a one-piece solid gate that cannot flex. These provide a sturdier gate but require precision machining for effective sealing without leakage. Solid wedges are better for high pressure or frequent operation. Flexible gates suit low pressure modulating control where tight shutoff is needed. Gates must be resistant to cutting, scoring, and deformation from fluids. Flexible gates suit liquids and clean gases. Solid gates work for steam, gases with solids, and contaminated fluids where a bellows could rupture. Consider shutoff requirements, pressure, media properties, and desired service life when choosing between flexible and solid gate designs.

Rising and Non-Rising Valve Stems

The valve stem translates motion from the actuator to the flow controlling element inside the valve. It may have a rising or non-rising design. Non-rising stems remain vertical as the valve operates. The stem is threaded into the gate, plug, or ball and turns it without lifting. Rising stems lift up and down with valve motion while remaining attached to the flow control element. Rising stems indicate valve position and can automate control via positioners. Non-rising stems require separate shaft position indicators. Rising stems are common on gate and globe valves. Non-rising stems suit ball and plug valves where turning motion is required. Non-rising stems work well for buried valves or corrosive fluids where rising stems could get damaged or cause binding. The stem must align with the actuator and match its torque output. Consider maintenance needs, automation requirements, and environments when selecting between rising and non-rising stem designs.

Purpose and Use of Valve Backseats

The backseat is a wearing surface on the valve stem that contacts the bonnet when the valve is fully open. It serves several purposes. First, it provides an additional seal between the stem and bonnet. This isolates the bonnet from system pressure when doing maintenance. It also gives the valve a bidirectional shutoff ability – seal both upstream and downstream. Backseats also enable packing adjustment and replacement while the valve is pressurized. Finally, backseats can act as a stopping point when fully open, preventing damage to seating surfaces. Backseats are common on gate, globe, and check valves. They should have enough surface area to prevent excessive wear. Stainless steel, brass, or carbon graphite materials work well. Consider whether backseats would facilitate safer maintenance when selecting valves. But they aren’t recommended for infrequently operated emergency shutoff valves.

Types of Valve Actuators: Linear, Quarter-Turn, Multi-Turn

Actuators provide the force to open, close, and position the valve. Common types are linear actuators, quarter-turn actuators, and multi-turn actuators. Linear actuators apply thrust along the stem’s axis to drive gates, globes, or diaphragms up and down. These are often pneumatic cylinders or hydraulic pistons. Quarter-turn actuators rotate 90 degrees to open/close ball, plug, and butterfly valves. Manual lever arms, electric motors, or pneumatic cylinders are common quarter-turn actuators. Multi-turn actuators use gearing to allow for multiple 360 degree rotations. These automate precise positioning of globe and gate control valves. Another benefit of gearing is the high output torque from a small electric motor or manual handwheel. When selecting valve actuators, consider torque requirements, speed, automation needs, space constraints, and hazardous area rating. The actuator output must match the torque demands of the valve, especially for 100% shutoff.

Valve Components Selection for Special Use Conditions

Valves contain many components, and material choices depend on the application. For steam systems, metal seats, bonnets, and steam-rated packing suit the high temperature and avoid oxidation. In cryogenic applications, the body, trim, and seals require materials that stay ductile at freezing temperatures like stainless steel. Highly corrosive fluids need stainless steel or alloy bodies and trim along with corrosion inhibiting sealant on gaskets. For throttling control valves, components supporting smooth stem movement and flow characterization are essential. This includes characterized trims, modified trim geometry, low-friction packings, and high resolution actuators. The principles are the same, but components must be tailored to safely handle the operating conditions. Partnering with an experienced supplier ensures that all valve parts are suited to the service.

How to Select Parts of a Valve Correctly

Properly selecting valve components requires careful consideration of the service conditions, performance requirements, and desired valve type. Here are some important considerations when choosing valve parts:

For welding end valves, ensure the valve-body ends and weld joint design suit the piping system and materials. Flow mediums that require tight shutoff may need metal-seated ball valves with properly matched stem threads, outside screw and yokes that prevent side loading. High pressure applications need sturdy valve bonnets, thick stem packing in the stuffing box, and sturdy yoke bushings.

Relief valves require precise trim parts and trim designs to provide accurate pressure control. Ball valves used for throttling duties require characterized ball and seats to control flow. The rotational motion of ball and plug valves depends on quality bearings and seals internal elements.

Control valves rely on valves bonnets, gaskets, stem packings and other parts to prevent leakage and enable smooth actuation. The top of the yoke and downstream side covers of a swing-check valve see the most wear, so durable materials are vital.

Choosing control valves also means matching the actuator style and output to provide the right type of motion – rotary for 90 degree ball valves or linear for globe designs. This ensures proper valve positioning and tight shutoff when closed.

No matter the valve type, considering spacing, installation, and maintenance requirements will guide the selection of compact wafer, lugged, or flanged designs. Partnering with experienced suppliers and following PMI standards ensures the parts selected provide long service life.

Conclusion

Understanding how the various internal parts of valves work together is critical for engineers, maintenance staff, and plant operators. The body, bonnet, trim, stem, seals, and actuators all play a role in controlling fluid flow safely and reliably. Component selections must be based on service conditions and performance goals. With the right knowledge of component functions and material differences, industrial valves can be kept in prime operating condition for their essential role in managing fluids.