Valve Primer

205 Pages·1997·11.703 MB·205\205

by  Stojkov, Brent T.

The
Ualue
Primer
S
Brent T. tojkou
Industrial Press Inc.
Library of Congress Cataloging-in-Publication Data
Stojkov, Brent T.
The valve primer / Brent T. Stojkov.
240 p.
Includes index.
ISBN 0-8311 -3077-6
1. Valves. I. Title.
IN PROCESS
621.8’4-dC21 96-3713 3
CIP
Copyright, 1997 0 by Industrial Press Inc., New York, N.Y.
10016. All rights reserved. This book, or parts thereof, may
not be reproduced in any form without permission of the
publishers.
Printed in the United States of America
5 6 7 8 9
Preface
Several years ago I was asked if I knew of a book about
valve basics that could be used in training new operators
at the oil refinery where I was working at the time. When I
replied that I did not, it was suggested that I write one. This
book is the result of that suggestion. It is in large part a
compilation of information scattered throughout different
valve manufacturers’ catalogs and design guides, but it
also includes knowledge that I have accumulated in over
25 years of experience in valve sales, selection, specifica-
tion, procurement, inspection, troubleshooting, and repair.
The Valve Primer is written for those individuals in the
power generation, oil, chemical, paper, and other process-
ing industries whose jobs as engineers, operators, and
maintenance technicians bring them into daily contact with
the hundreds to thousands of valves that are found in the
power stations, refineries, plants, and mills in which they
work. It is a primer in the true sense of the word: a small
introductory work. Its objective is not to produce valve ex-
perts, but individuals who have a basic knowledge.of valve
types and designs, the materials valves are made of,
where they should and should not be used, when and how
they are actuated, and other related topics. Just as impor-
tant is the reader’s introduction to valve terminology. With-
out a knowledge of valve terms, useful discussion with
supervisors, coworkers, valve salesmen, and valve ser-
vicemen is greatly hindered.
Many manufacturers of valves and valve actuators have
been helpful in allowing catalog photographs of their prod-
V
vi Preface
ucts to be used as illustrations in The Valve Primer. I ex-
press my thanks for their generosity. I have also had fa-
vorable experiences with products made by many
manufacturers that are not represented here; therefore,
omission of manufacturers’ names does not mean that
their products are unsatisfactory.
Brent T. Stojkov
Loveland. Ohio
Contents
PREFACE V
ACKNOWLEDGMENTS vii
REGISTERED TRADEMARKS viii
LIST OF FIGURES ix
1 INTRODUCTION 1
2 GATEVALVES 5
3 GLOBE VALVES 25
4 CHECK VALVES 37
5 BUTTERFLY VALVES 55
6 BALLVALVES 65
7 PLUGVALVES 79
8 DIAPHRAGM VALVES 93
9 VALVE MATERIALS 101
10 SIZES, CLASSES, AND RATINGS 121
11 FLUID FLOW THROUGH VALVES 131
12 OPERATORS AND ACTUATORS 141
13 CONTROL VALVES AND
PRESSURE RELIEF VALVES 163
14 SELECTION 173
15 MAINTENANCE AND REPAIR 181
16 MISCELLANEOUS TOPICS 189
APPENDIX 1 STANDARDS 201
APPENDIX2 GLOSSARY 205
1
Introduction
A valve is a mechanical device whose function is to con-
trol the flow of fluids in piping systems. The fluids con-
trolled in industrial settings can be the common liquids,
gases, and vapors; but they also can be liquids carrying
suspended solid particles (called slurries) and gases car-
rying suspended solid particles. In some instances, even a
dry powder can be considered and handled as a fluid. The
control applied to these fluids can take one or more of the
following forms:
1. Starting and stopping flow
2. Regulating flow volume (frequently called throttling)
3. Preventing reverse flow (called anti-backflow)
4. Changing flow direction
5. Limiting fluid pressure
A valve performs its control function by placing an ob-
struction (hereafter called the flow control element) in the
fluid path through the valve. The nature of the flow control
element determines the valve type and the form of control
for which the valve is suited. There are many types of
valves; however, those used in industrial and power pip-
ing applications are almost always one of the following
valve types: gate, globe, check, butterfly, ball, plug, or dia-
l
2 Introduction
phragm. Variations in the designs of each of these valve
types have been developed to satisfy many different ap-
plications. The flow control element, the form of control,
the different designs, and the applications for each of
these types are discussed in detail in the subsequent
chapters. First, some additional comments about valves in
general are appropriate.
In the process of performing its control function, a valve
must satisfy two conditions. First, the fluid cannot be al-
lowed to leak into the environment. Second, there can be
no internal leakage. That is, when the valve is closed there
can be no flow, either along the normal fluid path or be-
tween valve parts, where flow is never intended. These
two conditions are not always met absolutely. Valve testing
standards, such as the American Petroleum Institute Stan-
dard API 598, permit a very small amount of leakage at the
seating surfaces for some types and sizes of valves. In ad-
ditlon, external leakage past stem packing and shaft seals
is not uncommon in valves that have been in service for
some time.
All valve types are manufactured with ends that mate
with the common piping connection methods: threaded
(also called screwed), flanged, butt-weld, socket-weld, sol-
der, and grooved. The appropriate connection method to
be used on the valves in a specific pipeline is decided by
the piping designer. The designer’s decision is based on
factors such as line size, fluid pressure, materials of con-
struction, and ease of assembly. However, some stan-
dardization exists. Small valves are usually manufactured
with threaded or socket-weld ends; whereas large valves
are usually manufactured with flanged or butt-weld pre-
pared ends. The threaded and flanged connections permit
easy assembly and disassembly and must be used where
the valve material cannot be welded. However, fluid leak-
Introduction 3
age at these connections is possible. Socket-weld and
butt-weld joints are leak-tight and are preferable for high-
pressure pipelines; however, they are more difficult to
make and are much more permanent than are threaded
and flanged joints. Note that the commonly accepted con-
vention is that piping 2 inch NPS (nominal pipe size) and
smaller is referred to as smallor small bore, and piping 2%
inch NPS and larger is called large or large bore.
Some valves are made with no “ends.” There are two dif-
ferent styles, wafer valves and lug valves, both of which
are designed to be used with flanges. The wafer-style
valve is used between mating flanges. Its circular body fits
just inside the circular bolting pattern of the flanges. When
tightened, the extended-length flange stud bolts cause the
flanges to seal the ends of the valve and hold the valve in
position. In this design, the valve does not contribute to
holding the pipeline together. Wafer-style valves are
shown in Figures 4-4 and 5-2. The lug-style valve is also
circular but has projections (called lugs) with threaded
holes spaced around its perimeter. The locations of the
holes match those of the mating flanges, and the threads
match the flange stud bolts used in assembly. The lug-style
valve can be situated between flanges by using short stud
bolts with each flange, with the valve body acting to hold
the pipeline together, or the lug valve can be bolted to a
single flange at the end 0f.a pipeline. A lug-style valve is
shown in Figure 5-1.
2
Gate Valves
The flow control element of a gate valve (called a gate,
wedge, or slide) enters the fluid path from the side and tra-
verses it until the fluid path is completely closed off, stopping
the flow. When the valve is open, the gate is entirely out of
the fluid path. Thus flow is in a straight line, with very little re-
sistance from the valve. Because the gate valve is symmet-
rical, either end can be the inlet, and thus flow can be from
either direction through the valve. The form of control for
which gate valves are suited is starting and stopping flow.
Gate valves, as are other valve types used for this kind of
control, are frequently referred to stop valves or block valves.
GATE VALVE DESIGN
Figure 2-1 shows a typical gate valve. A manufacturer
might describe this valve as a "flanged end, bolted-bonnet,
outside-screw-and-yoke, flex-wedge, gate valve." Each
term describes the particular design features of the valve
with respect to the end connections, body-bonnet joint,
stem design, and gate design, in that order. Considering
the choices for each of these design features in turn, the
many variations of gate valve designs that are available
will become evident. End connections have been dis-
cussed in Chapter 1 and, therefore, will not be repeated
here.
5
6 Gate Valves
Backseat Bushing
Stem Shoulder
Figure 2- 1. Gate valve