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Chapter 1: Valve Functions and Basic Parts

Introduction

A valve is a mechanical device that controls the flow of fluid and pressure within a system or process. Valves can perform a variety of functions essential for fluid management, including:

• Stopping and starting fluid flow
• Varying (throttling) the amount of fluid flow
• Controlling the direction of fluid flow
• Regulating downstream system or process pressure
• Relieving component or piping over pressure

With numerous designs and types available, valves can adapt to meet the specific needs of various industrial applications.

Despite their differences, all valves generally share the following key components: the body, bonnet, trim (internal elements), actuator, and packing. These components are essential in defining the functionality and performance of a valve.

Valve Body

The body of a valve, often referred to as the shell, acts as the primary pressure boundary. It supports the overall structure of the valve assembly and is responsible for withstanding the pressure loads from connected piping.

Valves can take on various shapes, such as cast or forged designs, to accommodate specific functional and manufacturing needs. Factors to consider when creating valve shapes include pressure resistance, ease of assembly, and costs. These influences often lead to diverse shapes, from block designs to more complex configurations involving a removable bonnet.

Many valve designs utilize the venturi effect, whereby the fluid passage narrows, reducing size and cost while maintaining functionality.

Valve Bonnet

This component covers the valve opening and can vary in design. It is essential as it serves not only as a cover but also supports internal parts such as the stem, disk, and actuator.

The bonnet is a crucial pressure boundary and often made from the same material as the body. Its attachment to the body through threaded, bolted, or welded joints must maintain pressure retention.

Valve Trim

The trim refers to the internal components of a valve, typically including a disk, seat, stem, and guiding sleeves. The interaction between the disk and seat dictates the valve's performance and flow control capabilities.

Disk and Seat

The disk provides the means for controlling fluid flow, serving as a pressure boundary when closed. Fine surface finishes ensure an effective seal when the valve is closed. Most valves derive their names partially from their disk designs.

Seal rings may be used as seating surfaces for the disks, where their design can range from simple machined body surfaces to threaded or welded rings for improved sealing effectiveness.

Stem

The stem connects the actuator to the disk, facilitating the proper positioning of the disk. Stems may vary between rising and non-rising designs, affecting valve operation.

Valve Actuator

The actuator is responsible for the operation of the stem and disk assembly, which can be a manual, motor-driven, pneumatic, or hydraulic mechanism.

Valve Packing

Most valves feature packing to prevent leakage at the stem-bonnet junction. Properly compressed packing is crucial to ensure effective sealing without hampering stem movement.

Introduction to the Types of Valves

Various valve types have emerged to serve a myriad of applications and environments, such as globe, gate, ball, plug, butterfly, diaphragm, check, pinch, and safety valves. Each valve design is tailored to meet specific operational needs and environmental conditions.

In summary, there are four basic methods for flow control through valves:

• Moving a disk or plug to restrict or permit flow (e.g., globe or needle valves).
• Sliding a disk across an orifice (e.g., gate and plug valves).
• Rotating a disk about a shaft (e.g., butterfly or ball valves).
• Using flexible materials to control flow (e.g., diaphragm and pinch valves).

Summary

Key takeaways from the chapter include:

• Four basic flow control methods exist in valve design.
• Proper packing compressing can control valve stem leakage.
• Seven basic components are commonly found across most valve types.

Chapter 2: Types of Valves

Gate Valves

Gate valves are linear motion valves that start or stop fluid flow but do not regulate it. The design of the disk allows it to effectively remove resistance to flow when fully opened.

These valves provide a tight seal when closed but are not suitable for leverage applications due to vibrations during partially open states. Gate valves are adaptable to various fluids, offering solid performance across many conditions.

• Gate valves can be classified based on their disk type, including solid, flexible, split, or parallel disks, suitable for various applications.

Globe Valves

A globe valve is a versatile linear motion valve that can stop, start, and regulate flow. Its design allows for exceptional throttling capability, combined with low leakage rates when closed.

However, its structure may lead to higher pressure losses compared to gate valves. When utilized for quick-acting stop functions, the globe valve's performance can be advantageous.

Ball Valves

Using a ball-shaped disk, a ball valve is capable of quick on-off operation via a 90° handle turn. While generally cost-effective and low maintenance, their throttling characteristics may be limited under certain conditions.

Plug Valves

Plug valves, resembling a tapered plug, provide on-off control and can offer easy adjustments for multiport operations depending on specific service applications.

Diaphragm Valves

These valves utilize a flexible diaphragm to control fluid flow, making them ideal for handling corrosive or fibrous materials while ensuring no contamination of the flowing media.

Butterfly Valves

Known for their lightweight design and compact structure, butterfly valves are excellent for large flows and low-pressure applications. Their quick-acting nature further enhances system efficiency.

Check Valves

Check valves provide automatic flow direction control, utilizing the pressure of the flowing media to open and close, preventing backflow under reverse conditions.

Summary

• Gate valves excel in non-throttle applications.
• Globe valves are preferred for their throttling abilities and low leakage.
• Ball valves are compact and quick-acting, with limited throttling options.
• Diaphragm and pinch valves isolate mechanisms from the fluid.
• Butterfly valves save on space and costs in larger installations.
• Check valves prevent undesired flow reversals.

Chapter 3: Valve Actuators

Introduction

Valve actuators are essential for establishing the operational capability of valves, based on factors like required torque, control type needed (manual or automatic), and varying operational contexts.

Summary

• Manual actuators, such as handwheels, allow direct control, while electric motors and pneumatic systems enable automation.
• Hydraulic actuators scale with required fluid pressure support.
• Self-actuated valves operate without external energy inputs, relying solely on system fluid characteristics.