Overview of General Purpose Valves - a PDH Online Course for Engineers

Overview of General Purpose Valves

Course Outline

In almost any nuclear, chemical, or mechanical system, valves are used to regulate the flow of fluids in piping systems. There are a wide variety of valves developed for specific applications such as throttle flow, shut-off flow, reduce pressure, relieve pressure and prevent reverse flow. A basic understanding of the purpose, construction, function and principle of operation is essential for right selection of valve for your service.

This 3-hr course material is based entirely on US Department of Energy training materials (DOE-HDBK-1018/2-93, Mechanical Science, Vol. 2 of 2, Module 4).

This course includes a multiple-choice quiz at the end, which is designed to enhance the understanding of the course materials.

Learning Objective

At the conclusion of this course, the student will:

Describe the purpose of valves and the basic types of flow control elements;
Describe the key terminology including the body, bonnet, stem, disc, seat, packing, actuator etc;
Describe the physical construction and operation of different valve types;
Learn relative merits of different types of throttling and shutoff valves, including gate, globe, ball, needle, butterfly, check, plug and pinch valves;
Understand various types of check valves, including swing, tilting disk, lift, piston and butterfly check valves;
Describe the application of diaphragm and pinch valves;
Understand Do’s and Don’ts of valve selection for particular application;
Differentiate between relief valves and safety valves; and
Describe the construction and principle of operation for different types of actuators, including manual, electric motor, pneumatic, hydraulic and solenoid actuators.

Intended Audience

This course is aimed at students, mechanical and process engineers, HVAC and facility designers, contractors, estimators, energy auditors, plant layout professionals and general audience.

Course Introduction

Valves are integral components in piping systems and they are the primary method of controlling the flow, pressure and direction of the fluid. Valves may be required to operate continuously e.g. control valves, or they may be operated intermittently e.g. isolation valves, or they may be installed to operate rarely if ever e.g. safety valves. A valve can be an extremely simple, low cost item or it may be and extremely complicated, expensive item. In piping design the valves probably require more engineering effort than any other piping component. In this course, you are required to study the following U.S. DOE Fundamental Handbook - DOE-HDBK-1018/2-93, Mechanical Science Volume 2, Module 4, 1993 edition.

Course Content

This course is based entirely on US Department of Energy training materials (DOE-HDBK-1018/2-93, Mechanical Science, Vol. 2 of 2, Module 4 - Valve Functions and Basic Parts).

The link to the document is Valve Functions and Basic Parts.

Course Summary

The piping industry, over the years, had developed a wide range of valve designs and material to handle virtually all of the fluids being handled. Important considerations for selection are summarized below:

Isolation of flow
The isolation of the downstream system from the upstream system by use of and isolation /stop valve is a critically important function. The prime requirements of this valve are tight shut off when closed and minimum restrictions to flow when open. Valves used for this function include gate valves, globe valves, ball valves, plug valves, butterfly valves, diaphragm valves and pinch valves

Regulation of flow
Many applications require the flow of the fluid be regulated (throttled) at some fixed or variable level between fully zero and maximum flow limits. This is achieved by introducing resistance to flow, or by bypassing flow or by changing the direction of the flow. An important feature for control valves is that the output variable (flow) is related to the input variable (valve position). An ideal operating characteristic of a hand operated flow control valve is that the flow is directly proportional to the position of the handwheel.

Valve types for this function include globe, needle, ball, and butterfly. Globe and needle valves are best suited for this duty but ball valves are also easily adapted to give reliable flow control.

Back flow prevention
In some circumstances it is important to prevent reversed fluid flow. The type of valve for this duty is a non-return-valve (NRV) or check valve. The important criteria when selecting these valves are tight shut off against reverse flow, low resistance to flow for forward flow, fast response. The valve can be operated to close by gravity, fluid flow, or spring.

Two main valve types are available for this function lift check valves or swing check valves.

Pressure Regulation
In many applications there is a need to reduce the supply pressure to a set fixed value. It is also necessary to maintain this reduced pressure over a range of fluid flow conditions. The pressure regulator valve is engineered for this application. The pressure regulator valve operates using the downstream fluid pressure as feedback. This is mostly taken from within the valve (self acting). For more accurate control a feedback connection can be taken from the downstream piping. The valve is basically a spring loaded globe valve.

The pressure regulation at low near zero flows is difficult and it is often necessary to include internal or external relief valve functions to ensure no high pressures in the downstream system.

Pressure Relief Valves- Safety valves
A very important valve for safety is the pressure relief valve. This valve is used in applications where excessive pressure in the system can cause damage or failure or can introduce a safety risk. Relief valves are mainly spring loaded but they can also be gravity operated and other more specialized designs are available. Relief valves when used for safety applications are engineered in line with safety regulations and require regular inspections to confirm the settings and the operation. An important part of the relief valve installation is the routing of the relieved fluid. This pipe route must be to a safe location and must be engineered such that it is always fully open.

In general valves are one of the simplest and the most important components in piping systems. The valve failure can result in loss of fluid pressure or excessive pressures, which often leads to undesirable consequences. Uncontrolled excessive pressures can result in disastrous accidents particularly when potentially hazardous and explosive fluids are being controlled. Before attempting to select a valve, the engineer should first gain a basic understanding of each type of valve and how it operates.

Quiz

Once you finish studying the above course content, you need to take a quiz to obtain the PDH credits.

DISCLAIMER: The materials contained in the online course are not intended as a representation or warranty on the part of PDH Center or any other person/organization named herein. The materials are for general information only. They are not a substitute for competent professional advice. Application of this information to a specific project should be reviewed by a registered architect and/or professional engineer/surveyor. Anyone making use of the information set forth herein does so at their own risk and assumes any and all resulting liability arising therefrom.