Introduction
The accuracy of fluid manipulation is the key to the reliability of operations in industrial fluid systems. The capability to initiate, terminate, and control the circulation of fluid is the most important, whether in petrochemical refineries, water treatment plants, or chemical plants. The valve is the centre of this operation. Nevertheless, the complexity of valve engineering is simplified when it is treated as a unit. A valve is a combination of separate parts that are designed to resist certain fluid pressures, temperature differences, and mechanical forces.
To engineers, procurement managers and maintenance technicians, it is important to know the particular components of a valve to be able to troubleshoot and source efficiently. This guide gives an in-depth examination of the valve anatomy, the functional differences between the designs of the valves, and the importance of manufacturing quality in long-term performance.
The Two Main Categories: Pressure Boundary vs. Valve Trim
In order to study the valve construction in a systematic manner, it is necessary to differentiate between the parts that hold the pressure and the parts that regulate the flow.
The Pressure Boundary: Body and Bonnet
The pressure boundary is made up of the stationary components that hold the fluid and pressure in the piping system. The main part in this category is the valve body.
The main structure of the assembly is valve bodies. They give the physical framework to join the valve to the piping system (through flanges, threads, or welds) and hold the internal components of the valve. The flow of the fluid is determined by the design of the body. In globe valves, the fluid is forced to change direction by the body, forming a tortuous flow that helps in throttling but raises pressure drops. In ball valves or gate valves, the body is designed to hold a straight-through passage, which means that it can have a high flow capacity.
The valve bonnet is attached to the body. The valve body opening is covered by the bonnet. It is attached to the body with bolts or a threaded connection. The bonnet has two main purposes: firstly, it closes the top of the valve body to avoid leakage, and secondly, it holds the valve stem and actuator assembly. Since the main pressure vessel is made of valve bodies and bonnets, their structural integrity is not negotiable. Any defects in these castings, e.g. porosity or inclusions, undermine the safety of the whole system.
The Valve Trim: Disk, Seat, and Stem
The trim is the name given to the working components of the valve that are in direct contact with the fluid. The basic logic of flow control is due to these trim components. The typical trim is the valve disc (or valve disk), valve seat and valve stem.
The moving obstruction in the flow stream is the valve disc. This component can be in the form of a ball, a gate, a plug or a butterfly depending on the type of valve. In the closed position, the disc rubs against the valve seat to stop the flow. The valve seat may be part of the body or a removable ring. The disc-seat interface should be machined to very high standards to provide a tight fit.The internal disc is attached to the external operator (handle or actuator) by the valve stem.
The valve stem joins the internal disc with the external operator (handle or actuator). The valve stem runs through the bonnet and it transfers the torque or linear force needed to move the disc. Since the stem is in motion under pressure, the surface finish and straightness of the stem is important in ensuring a seal at the packing gland.
Sealing and Actuation Components
Valves use stem packing to ensure that the fluid flow does not escape through the valve stem. This is usually a fibrous substance (PTFE or graphite) pressed around the stem in the bonnet. The compression does not allow leakage but the stem is free to rotate or slide.
The method of actuation is application-dependent. Manual valves have handwheels or levers. Automated systems use either pneumatic actuators or electric actuators. Pneumatic actuation is also used in applications where quick response or fail-safe operation is needed, and compressed air is used to actuate the valve stem. Electric actuators provide accurate positioning, but are generally slower than pneumatic.
Critical Components by Valve Type (Ball, Gate, Globe)
Although the above general anatomy is applicable to most of the valves, some architectures use special components to meet their performance objectives.
Ball Valves: The flow control element in a ball valve is a spherical disc that has a hole cut through the center of it. Ball valves are classified as quarter-turn valves since a 90-degree turn is required to change the valve to fully open or fully closed. This design has good shutoff properties. The most important components in this case are the ball and the seats, which are usually composed of soft materials such as PTFE to provide a bubble-tight seal.
Gate Valves: The gate valve employs a flat or wedge-shaped gate that slides linearly in the direction perpendicular to the flow stream. The design of gate valves is on/off. In the open position, the gate closes completely into the bonnet, and the flow path is not blocked. This causes little pressure loss. But a half-open gate will shake and wear away soon because of the speed of the fluid; therefore, they are not suitable for throttling.
Globe Valves: Globe valves have a different internal geometry. The movement of the valve disc is parallel to the fluid flow, either towards or away the seat. This design enables accurate control of flow. The direction of flow in the body is reversed and this results in a large drop in pressure but allows much better throttling control than with gate or ball valves.
Check Valves: Check valves are automatic and they are used to avoid backflow. They do not require a stem or handle. Instead, they depend on fluid pressure to work. As an example, in a swing check valve, the forward flow causes the disc to open, and the reverse flow causes the disc to push against the seat. Other designs use internal springs to help in closing the valve quickly to avoid the water hammer.
Butterfly Valves: Butterfly valves operate on a rotating shaft with a disc attached to it. The disc closes the bore of the pipe when it is closed. They are small and economical, particularly in large pipe sizes. The sealing is dependent on the quality of the resilient liner or seat.
Diaphragm Valves: Diaphragm valves are designed to separate the operating mechanism and the fluid by means of a flexible membrane (diaphragm). This is essential in pharmaceutical or corrosive applications where corrosion resistance is of concern and contact between the fluid and metal components should be kept to a minimum.
Pressure Relief Valve: A pressure relief valve has a calibrated spring which holds a disc against a seat. The valve is opened when the pressure in the internal system is greater than the spring force to release the excess pressure and prevent over-pressurization of the system.
| Valve Type | Flow Control Element | Key Components | Operation Description | Advantages | Disadvantages |
| Ball Valve | Spherical disc with a hole | Ball, Seats (usually PTFE) | A 90-degree turn changes the valve from fully open to fully closed. The ball is the flow control element, providing a bubble-tight seal. | Good shutoff properties, quick operation, suitable for on/off control | Not suitable for throttling; can wear out with frequent use |
| Gate Valve | Flat or wedge-shaped gate | Gate, Seat | The gate moves linearly perpendicular to the flow, fully opening or closing to block the flow. When open, the gate fully retracts into the bonnet. | Low pressure loss when fully open, ideal for on/off control | Not suitable for throttling, can wear out when partially open |
| Globe Valve | Valve disc | Disc, Seat | The disc moves parallel to the fluid flow, either towards or away from the seat, providing precise flow control. The design results in a pressure drop. | Excellent throttling control, precise flow adjustment | Causes large pressure drop, higher flow resistance |
| Check Valve | Disc (automatic operation) | Disc, Seat, Springs (optional) | The valve opens with forward flow and closes with reverse flow, preventing backflow. Some designs include springs to close the valve quickly. | Automatic operation, prevents backflow | Cannot control flow, may be affected by water hammer |
| Butterfly Valve | Rotating disc | Disc, Shaft, Liner/Seat | The valve operates on a rotating shaft, with the disc closing off the pipe bore when in the closed position. Suitable for large pipe sizes. | Economical, compact design, easy operation in large pipes | Sealing quality depends on the liner, lower sealing efficiency |
| Diaphragm Valve | Flexible diaphragm | Diaphragm, Seat | The diaphragm separates the fluid from the operating mechanism, offering a flexible membrane for fluid control, especially in corrosive or sanitary systems. | Ideal for applications requiring isolation from the fluid | Limited to lower pressures, more complex maintenance |
| Pressure Relief Valve | Calibrated spring and disc | Spring, Disc, Seat | The spring holds the disc in place, and the valve opens when the system pressure exceeds the spring force to release excess pressure. | Prevents over-pressurization, protects system safety | Requires accurate calibration, can be prone to wear over time |
Material Selection: Balancing Durability and Cost
Material choice is critical to the life of valve components.
Stainless steel is the standard for valve bodies and trim in environments requiring both strength and corrosion resistance. Grades such as 316 stainless steel are resistant to chlorides and acidic media that are present in chemical plants.
In the case of trim materials, engineers tend to choose harder materials than the base body material to offer wear resistance. The trim parts, which are the seat and disc, are subjected to high velocity and friction. The contact surfaces are usually welded with hard-facing materials such as Stellite to avoid galling and erosion.
Carbon steel bodies are adequate in less aggressive uses, but do not have the natural corrosion resistance of stainless steel. In very specialized applications, exotic alloys such as Monel, Hastelloy or Inconel are employed to resist high temperatures or highly reactive fluids.
Common Valve Part Failures and Troubleshooting
Identifying the root cause of valve failure often requires analyzing specific internal parts.
- Stem Leakage: This is the most prevalent failure mode. It normally shows that the stem packing has worn or lost compression. It may also be caused by a scratched or bent valve stem which tears the packing material.
- Internal Leakage (Pass-by): When a valve does not offer a tight seal when closed, the problem is typically with the valve seat or valve disc. The debris that gets stuck in the line may cut the seating surface, leaving a path of leakage. In globe valves, the trim can be destroyed by wire drawing (erosion due to high-velocity fluid leaking through a small opening).
- Seizing: When a valve is not able to be turned to the open position, the stem might be corroded, or the internal parts might be locked by thermal expansion. Thermal binding may cause the wedge to become stuck in the seats in gate valves.
Manufacturing Quality: Why Casting Process Matters
Even though choosing materials and designing the valve are important steps in the process, the way we manufacturer’s valve parts, in particular, the body and bonnet casting, will ultimately determine the safety and reliability of the product. Faulty castings are the leading reason of serious failures of the body.
Silica Sol Investment Casting for Precision Valve Parts
Despite the many techniques to cast metal, for high-pressure, high-precision valve parts, silica sol investment casting (or lost wax casting) is better than sand casting.
While sand casting is cheaper, it also tends to have a rougher surface finish, and a greater chance of internal porosity is present (this means small pockets of air are trapped in the metal). Under high fluid pressures, these areas will tend to become points of stress concentration, leading to the potential for cracking, or leaking, through the body wall.
Silica sol investment casting uses a ceramic shell created from high-quality silica sol. This results in fabrication of parts with unparalleled dimensional accuracy and surface finish quality. This is beneficial for valve bodies since they will have internal flow paths that are much smoother, leading to a decrease in turbulence and associated pressure drops. Also, the uniform, fine-grained-structure of the casting will enhance mechanical strength and pressure containment for the valve bodies. For this casting method, chemical composition of the stainless steel or alloy is tightly controlled to meet quality and regulatory specifications.
BesserCast: Your Partner for Custom Valve Body Manufacturing
Valve manufacturers and industrial suppliers know that the reliability of their products depend on the quality of their castings. At BesserCast given the silica sol investment casting method, we produce high-precision valve body shells.
A valve body shell might seem like just any ordinary shell, but we know that it is actually a pressure vessel that must withstand certain pressure and temperature conditions. We focus on completing valve body shells that are used in ball valves, gate valves, and complex check valves. Our process not only ensures that the components are within the tolerances but also have a high surface quality, which means that secondary machining will not be required. This results in a lower cost of production, and a better functioning and longer lasting end product.
Sand casting is the lower cost option than silica sol investment casting, but this is a situation where lower cost will also correlate with higher risk; we advise against it. Our valve body shell products come in many high-performance materials: stainless steel, carbon steel, and various alloys, so you know that your valve will perform under high pressure and high temperature conditions.
To know more about us, please visit our website: https://www.bessercast.com/.
Tips for Sourcing Replacement Parts vs. OEM Manufacturing
How you obtain valve components for maintenance as opposed to manufacturing is different.
As for maintenance, replacement components can mostly be purchased as repair kits that contain soft parts, including gaskets, stem packing, and O-rings. When it comes to hard parts, however, such as the valve stem or valve disc that need to be replaced, it is essential to confirm whether the trim materials compliantly match the other valve parts. The immediate corrosion failure from instilling a 304 stainless steel stem in an application that needs a 316 stainless steel would be a problem.
When it comes to Original Equipment Manufacturers (OEMs) that design and assemble valves, the main concern becomes the supply chain for the raw castings. The reliable and consistent supply of valve bodies and bonnets comes from foundries that have advanced casting technology. Such foundries, which incorporate full process control from wax pattern injection to heat treatment, can offer the quality control necessary for compliance and recognition to other country’s requirements (like ASME or ISO).
Conclusion
Each of the valve components has its own unique role while also working together towards the same goal. The ability of valve bodies to withstand pressure, to the safe and accurate sealing of the trim components, and beyond, every piece has their own important role.
The same mechanics are true regardless of the use of pneumatic actuators for automation or manual handwheels. The valve stalk has to transfer motion, the valve seat has to close and seal against the valve disc, and the packing has to retain the liquid or gas.
But the manufacturing does have to be to a certain threshold of quality for this to work. Knowing the difference between rough sand casting and precision investment casting is the basis for sound purchasing and engineering decisions. The investment casting of silica sol for the valve and control industry is a quality manufacturing investment and the industry has a responsibility to ensure flow control for safe, effective and dependable use for many years.
