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Valve Parts 101: The Basic Components of Industrial Valves

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.


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.

What are heart valves?

The heart consists of four chambers, two atria (upper chambers) and two ventricles (lower chambers). There is a valve through which blood passes before leaving each chamber of the heart. The valves prevent the backward flow of blood. These valves are actual flaps that are located on each end of the two ventricles (lower chambers of the heart). They act as one-way inlets of blood on one side of a ventricle and one-way outlets of blood on the other side of a ventricle. Normal valves have three flaps, except the mitral valve, which has two flaps. The four heart valves include the following:

  • tricuspid valve: located between the right atrium and the right ventricle
  • pulmonary valve: located between the right ventricle and the pulmonary artery
  • mitral valve: located between the left atrium and the left ventricle
  • aortic valve: located between the left ventricle and the aorta

How do the heart valves function?

As the heart muscle contracts and relaxes, the valves open and shut, letting blood flow into the ventricles and atria at alternate times. The following is a step-by-step illustration of how the valves function normally in the left ventricle:

After the left ventricle contracts, the aortic valve closes and the mitral valve opens, to allow blood to flow from the left atrium into the left ventricle.

As the left atrium contracts, more blood flows into the left ventricle.

When the left ventricle contracts, the mitral valve closes and the aortic valve opens, so blood flows into the aorta.

What is heart valve disease?

Heart valves can have one of two malfunctions:

  1. regurgitation (or leakage of the valve): The valve(s) does not close completely, causing the blood to flow backward through the valve. This results in leakage of blood back back into the atria from the ventricles (in the case of the mitral and tricuspid valves) or leakage of blood back into the ventricles (in the case of the aortic and pulmonary valves).
  2. stenosis (or narrowing of the valve): The valve(s) opening becomes narrowed or valves become damaged or scarred (stiff), inhibiting the flow of blood out of the ventricles or atria. The heart is forced to pump blood with increased force in order to move blood through the narrowed or stiff (stenotic) valve(s).

Heart valves can have both malfunctions at the same time (regurgitation and stenosis). Also, more than one heart valve can be affected at the same time. When heart valves fail to open and close properly, the implications for the heart can be serious, possibly hampering the heart's ability to pump blood adequately through the body. Heart valve problems are one cause of heart failure.


Valve Parts 101: The Basic Components of Industrial Valves

Heart Valves, Anatomy and Function





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