Valve trim is the collective name for all the removable and replaceable internal parts of a valve. These components are responsible for controlling the flow of liquid and gas through the valve, and are exposed to the process elements, making them vulnerable to wear over time. The three basic items of a high-pressure control valve trim are the stem, ball, and seat. Other valve trim components can include back seats, glands, spacers, guides, bushings, retaining pins, and internal springs.
Selection of the right valve trim is essential for ensuring that the valve works properly within the system. Different types of valves require different valve trim elements, as shown in Table.
| Type of Valves | Valve Trims |
|---|---|
| Angle Valves | Stem, Seat, Disc, Disc nut, back-seat bushing |
| Gate Valves | Stem, Seat, Wedge, back-seat bushing |
| Globe Valves | Stem, Disc, Seat, Disc nut, back-seat bushing |
| Disc Valves | Disc, Disc nut, back-seat bushing |
| Lift-type check valves | Seat ring, Disc, Disc guide |
| Swing-type check valves | Seat ring, Disc, Disc nut, Disc nut pin, Disc holder, Disc holder pin, side plug, carrier pin |
Valve Trim Design: Importance and Components Explained
Valve trim plays a crucial role in determining the flow rate and isolation characteristics of a valve. The shape of the valve trim is responsible for the flow characteristics of the valve. The right trim material is necessary to withstand specific forces and conditions. Here is a detailed explanation of the components of valve trim.
Disc: The disc is a pressure-retaining part of the valve that allows, throttles, or stops the flow depending on its position. It can be moved away from the valve seat by the valve stem, except in the case of check and safety relief valves.
Seat: A valve seat is a non-pressure retaining part of the valve that forms a seal with the disc to stop the flow. The number of seats depends on the type of valve. Valve seats can be integral, renewable, or replaceable rings.
Back Seat: The back seat is a non-pressure retaining part of the valve made up of the shoulder of the stem. When the stem is in a fully open position, it forms a seal, preventing leakage from the valve shell to the packaging chamber and environment.
Stem: The stem connects the actuator, valve handwheel, and the disc, plug, or ball. In a gate or globe valve, linear motion of the disc is needed to open or close the valve. Whereas in the plug, ball, and butterfly valve, the valve disc is rotated to open or close the valve.
Bonnet Bolt: Bolting holds the bonnet and body, creating a tight pressurized seal between them. Bolting should be selected as per the application code and standards.
Gland Eyebolt: It connects the gland flange and the bonnet, tightening the bolts.
Yoke: The yoke is the mechanism that connects the valve body or bonnet. The yoke and bonnet are designed as a one-piece construction in many valves.
Yoke Bushings: Yoke bushings are the part where the internally threaded nut is held at the top of the yoke. They minimize friction between the hardened stem and the yoke bushing. Anti-freeze yoke sleeve bearings are provided on valves that require much effort to open or close.
Valve trim design is essential in determining the flow rate and isolation characteristics of a valve. The right trim material is necessary to withstand specific forces and conditions. Understanding the components of valve trim is crucial for proper valve selection and operation.
Gland Flange: Definition and Function in Valve Packing A gland flange is a component used to support the gland bush in a valve. Its primary function is to keep the gland packing under tension. This tension helps to prevent leakage of the process fluid from the valve. Learn more about the function and application of gland flanges in valve packing.
Stem Packing: Definition, Function, and Material Selection Stem packing is a component used to prevent the leakage of process fluid from a valve. It is contained in a stuffing box and is compressed by tightening packing nuts or gland bolts. Stem packing is made of materials like graphite or PTFE, depending on the requirements of the application. Learn more about stem packing and how to select the appropriate material based on factors like temperature and chemical composition.
Types of Control Valve Trims and Their Applications Valve trim shapes determine the flow characteristics of a control valve. There are three primary types of control valve trims: snap trim, nominal trim, and equal percentage trim. Each trim has a specific application, such as isolation, throttling, or pressure and flow control. Learn more about control valve trims and their applications.
Corrosion and Erosion Resistance in Valve Trims: Material Selection Valve trim materials are selected based on service conditions, such as temperature, pressure, chemical composition, and flow velocity. Corrosion resistance tables can be used to select trim materials that are sustainable in corrosive environments. Erosion is caused by high-velocity liquids with abrasive particles that are harder than the trim material. Learn more about the selection of valve trim materials for resistance to corrosion and erosion.
Valve Trim Materials: Metallic and Non-Metallic Options Valve trim materials can be metallic or non-metallic. Metallic materials like stainless steel, steel alloys, titanium, cobalt-chromium alloys, and nickel-boron alloys are used for general service applications. Non-metallic materials like PTFE and Teflon are selected based on operating temperature. Learn more about metallic and non-metallic valve trim materials.
Valve Trim Chart: Standardization of Trim Materials API has assigned a unique number to each set of valve trim materials for standardization. The valve trim chart consists of trim numbers or combination numbers grouped based on materials like disks, stems, back seats, and sleeves. The chart is defined in API 600 and API 602 and provides a list of trim materials that can be used in valves. Learn more about the valve trim chart and its application.
Refer to the figure below to find a typical Valve trim chart as provided by API 600 and API 602. The chart is defined by Trim numbers.
