Instrument Technology. Telemetering and Automatic Control

205 Pages·1957·3.811 MB·English

by  E. B. Jones (Auth.)

Dedicated to
KAREN AND KIM
Instrument Technology
Volume 3
TELEMETERING AND
AUTOMATIC CONTROL
E. B. JONES
B.Sc, F.Inst.P., F.Inst.M.C.
Butterworth Scientific
London-Boston-Sydney-Wellington-Durban-Toronto
All rights reserved. No part of this publication may be reproduced or
transmitted in any form or by any means, including photocopying
and recording, without the written permission of the copyright
holder, application for which should be addressed to the Publishers.
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publication is stored in a retrieval system of any nature.
This book is sold subject to the Standard Conditions of Sale of Net
Books and may not be re-sold in the UK below the net price given by
the Publishers in their current price list.
First published 1957
Reprinted 1961, 1968, 1970, 1971,
1973,1975, 1977, 1978, 1981, 1982
ISBN 0 408 23051 7
© Butterworth 8c Co (Publishers) Ltd, 1957
Printed in England by J. W. Arrowsmith Ltd, Bristol
PREFACE
ONE of the greatest advances in instrumentation in recent years has been
the development of instruments which control a process completely.
Whereas previously instruments had to be supplemented by human inter-
pretation and calculation, plants may now be built which carry out difficult
or complicated processes with little or no human intervention. This de-
velopment of instrumentation has led to a considerable increase in the
responsibilities of the staffs whose task it is to install and maintain the
instruments, for failure of an instrument may now have much more serious
consequences than it did when the instrument was merely an aid to the
human operator. It is, therefore, more necessary than ever that such staffs
should bring understanding as well as manual skill to their work. This
book, as with the previous two volumes, is written with the object of helping
the reader to understand the 'why' as well as the 'how' of his work, and
with the hope that it may in some small measure help to reduce the des-
perate shortage of men skilled in the craft and science of instrumentation.
This volume, together with volumes one and two, is an attempt to cover
most of the ground required for the Intermediate and Final Examinations
in Instrument Maintenance of the City and Guilds College, London.
The usefulness of the book is not, however, limited to examination candi-
dates for it is hoped that anyone in any way connected with the problem of
controlling industrial processes may find in it much that is of value.
The mathematics have been kept as simple as possible to avoid embar-
rassing readers whose attainments in mathematical subjects are limited, and
graphical illustrations have been used where these increase the clarity of an
explanation.
The selection of material has been a difficult problem, but the aim has
been to give as complete a picture as possible while emphasizing the more
important and the more common types of instruments.
The author cannot adequately express his appreciation of the very con-
siderable help received from the makers and users of industrial instruments.
He would thank firms and organizations who provided information and
photographs for the illustrations which are acknowledged in the text.
The terminology used in the section on process control is based on the
recommendations of the Committee set up by the British Standards Institu-
tion to revise the 1949 Standard. The proposals of this Committee were
published in 1954 under the following title and reference: Draft British
Standard Glossary (Revision of B.S. 1523 Section Two: Process Control)
CT (INE) 8280.
He finds it a great pleasure to acknowledge his indebtedness to Mr.
P. R. Higginson of Messrs. John Summers & Sons, Limited, Hawarden
Bridge Steel Works, who has kindly read the proofs and has pointed out
errors, obscurities and omissions, and has made many valuable suggestions
at all stages of the preparation of the book.
In conclusion, the writer would like to express his gratitude to Mrs. F.
PREFACE
Williams who kindly undertook the typing, to Mr. P. M. Booth, M.A.
for his considerable help in proof reading and for his valuable comments
and suggestions, and to the publishers and printers for their ready co-
operation in the preparation of the book.
It is too much to hope that the book is completely free from all errors and
obscurities, and the author would welcome corrections or suggestions for
improvement.
E. B. JONES
Holy well,
December, 1956
INTRODUCTION
As man's mastery of his environment increases he enlists more and more
of the earth's materials and sources of energy to his service. Not only does
he use and modify the materials which nature provides but he sets out to
create materials which may never have existed in the natural state. Having
harnessed fire, the wind and water to his service he is now taming what
might at present be regarded as the fundamental energy of the universe,
the source of the sun's energy, the energy stored up in the nucleus of the
atom. The processing of some of these new materials and the necessity for
keeping a tight rein on the new sources of energy render the use of some
closer form of control absolutely essential, and the choice between manual
and automatic control, or a combination of both, is made from" considera-
tions of safety and economy. From a careful study of the economics of
efficient plant operation, progressive industrial organizations have already
discovered that automatic control of processes does pay dividends, and the
increasing use of such control is resulting in improved quality of product,
increased productivity and decreased costs.
Process control consists of maintaining within desired limits, or altering
in a predetermined manner, the energy and material balance of matter
undergoing treatment in the process. Although many variables upon which
the efficiency of the process depends, such as the rate at which energy,
usually in the form of heat, is supplied or removed, the operating pressure,
the feed rate of raw material, the reaction time, the rate of removal of
product and other similar variables may be controlled, the process may still
be subject to disturbances. These disturbances are created by inevitable
variations in the composition of feed stock, changes in ambient temperature,
changes in the efficiency of portions of the plant such as heat exchangers,
and so on. In practice, therefore, it is usual to try to maintain at constant
values all process variables which can be controlled except one or two
variables the values of which are adjusted to maintain the quality of the
product at a desired value. As it is rarely possible to measure the quality
of a product directly, the condition most directly related to the quality
and quantity of product is chosen and maintained at the optimum value.
Processes may be simple or complex, but a complex process may be
regarded as a number of simple operations or 'unit processes'. As a very
simple example, consider the making of bread. The ingredients must be
mixed in the right quantities and in the proper order; a unit process.
The bread must rise for a known time at a given temperature; a unit
process. Finally, the bread must be baked for the correct time at the correct
temperature; a unit process. For the purpose of studying the process and
applying process control it is helpful to treat the relatively complex process
of making bread as a series of independent unit processes. The experience
gained in the operation of a given type of unit process may prove helpful
in designing a control system for a similar unit process on another material.
For example, information gained in the baking of bread may prove helpful
in designing a process for baking bricks, although the time and temperatures
are different in the two processes.
INTRODUCTION
Although a wide variety of mechanisms and devices are used to accomplish
automatic process control, most of them fit into a readily defined pattern.
The basic principle involved in controlling the release of energy in an
atomic reactor is merely a development of the principle involved in the
use of the simple governor to control the speed of a steam engine. Both
are based on the 'closed loop' principle which is discussed in section (1.7).
In order that the desired quality and quantity of product may be attained
at the lowest cost it is essential that as complete an indication as possible
of all the process variables should be readily available. This is usually
achieved by bringing together on one panel, which may be of the con-
ventional or graphic form, all the indicators and recorders which show the
state of the process variables. As many process fluids may be highly corro-
sive, inflammable or otherwise dangerous, it is undesirable that they be
brought into the control room. In such cases, it is usual to convert the
measured value of the process variable into an electrical or pneumatic
signal which may be readily transmitted to the control room.
The use of transmitters is not, however, limited to measurements on
dangerous or corrosive fluids, for in order to reduce controller lags to a
minimum it is sometimes necessary to install a controller near to the point
of measurement on the plant and to transmit to the control room a signal
which indicates the measured value of the variable.
This book is, therefore, devoted to the instruments which are used to
transmit to a control room an indication of the value of a measured variable
and to the instruments and mechanisms which are used to control process
variables. As in previous volumes basic principles are discussed before
the actual instruments and the instruments then classified according to the
physical principle upon which they are based.
1
TELEMETERING
Telemetering is the reproduction, at a convenient location, of measurements
made at a remote point. The use of telemetering systems makes it possible
to group several instruments in a centralized control room to enable the
operator to have a complete picture of the conditions on a plant, without
the use of long lengths of thermometer capillary or connecting piping from
pressure gauges, liquid level and flow meters. Measurements made in
hazardous and remote positions may be transmitted to the control room
where the operator can control the process in safety. When making measure-
ments on highly corrosive fluids it is necessary to keep the pressure lines
as short as possible, and it is often advisable to convert the measured variable
into a proportional air pressure or electrical quantity and transmit this to
a measuring instrument.
Telemetering systems are also useful in indicating levels in remote
reservoirs. In such instances telemetering systems are designed to use the
G.P.O. land lines for transmission over considerable distances.
In general, a telemetering system consists of:
(1) A measuring instrument which may measure flow, liquid level,
pressure, temperature or any other variable.
(2) A conversion element which converts the measured variable into a
proportional air pressure or electrical quantity.
(3) The pressure lines or connecting wires which carry the transmitted
variable from transmitter to receiver.
(4) A receiver which indicates the size of the transmitted variable and
may also record or control the measured variable.
1. THE PNEUMATIC SYSTEM
Basically, a pneumatic system requires the following:
(1) A constant supply of air, usually at a pressure lying between 15 and
20 p.s.i.g.
(2) A transmitter which reduces or 'throttles' the pressure of the air
supply in such a way that the pressure of the air transmitted is strictly
proportional to the value of the measured variable.
(3) A suitable connecting pipe for transmitting the changes in air pressure.
(4) A receiver which translates the air pressure into terms of the original
variable.
THE SIMPLE FLAPPER AND NOZZLE SYSTEM
Suppose a nozzle A is tapped into the air supply as shown in Figure 1(a)
and the area of cross-section of the hole in the nozzle is small in comparison
1
2 TELEMETERING
with the area of cross-section of the supply line. A flapper B is pivoted as
C and actuated by the measuring element. When the flapper is well away
from the nozzle, air will flow freely through it, but the volume of air which
escapes is such a small fraction of the total volume of air supplied, that the
pressure transmitted is very little different from that produced when the
flapper is completely restricting flow of air from the nozzle. If, however, a
restriction having an orifice D, whose area of cross-section is smaller than
that of the nozzle A, is fitted in the supply line, as shown in Figure 1(b), the
air can escape from the open nozzle more rapidly than it can enter through
the orifice D. The position of the flapper will now have a considerable effect
on the transmitted pressure. If air at 20 p.s.i.g. is supplied, the orifice having
CM Linkage from
measuring element
B- or recorder pen
(a)
filtered air^ r- Transmitted air
suppfy usually U- to receiver
at f7j>.s.i.g.
Linkage from
measuring element
(b) B- or recorder pen
-£ZZZZ\
Filtered air ( ^_ -K Transmitted air
supply usuallyh—izzz^. -j to receiver
at 17 p.s.i.g.
Fig.l
(a) Simple flapper and nozzle system
(b) Simple flapper and nozzle system with restriction
a diameter of 0-010 in. and the nozzle a diameter of 0-025 in., the trans-
mitted pressure will be reduced to about 1 p.s.i.g. when the nozzle is fully
open, and were it not for leakage around the flapper, it would build up to
a pressure approaching the supply pressure when the flapper closes the
nozzle.
The pressure of the air stream escaping from the nozzle will produce a
force on the flapper which will depend upon the flapper position, the area
of cross-section of the nozzle and the pressure of the escaping air. As the
flapper is usually moved by the measuring element, which for accurate
measurement should be restrained as little as possible, the force required to
move the flapper should be as small as possible. In order to achieve this, the
area of the nozzle opening is made as small as the danger of blockage owing
to imperfectly cleaned air supplies permits. Nozzle diameters, therefore,
THE PNEUMATIC SYSTEM 3
usually measure between 0-020 in. and 0-040 in., and those of the corre-
sponding orifice about a third to a half that of the nozzle. Where an orifice
having a diameter of less than 0-010 in. would be required, a capillary tube
of about 0-010 in. internal diameter is used, the length being chosen to
produce the required restriction.
The amount of flapper movement required to change the output pressure
from a maximum to a minimum (throttling range) is usually very small in
this simple system, so that a slight amount of backlash or lost motion in the
lever system actuating the flapper, or vibration of the instrument, will
greatly affect the accuracy of the system. It is also very difficult to obtain a
linear relationship between flapper movement and output air pressure since
the output pressure will be seriously affected by variations in supply pressure.
"Feed-back* bellows
The above-mentioned difficulties may be overcome by including a 'feed-
back' bellows in the output side of the air line (see Figure 2). The 'spring
Linkage from
y actuating mechanism
>
*-r
Spring
~tt\ w 'Feed-back
_^ bellows
4- I Transmitted air
Filtered,
airssuupppp( lr V7i to receiver
Fig. 2
Flapper and nozzle system with feed-back bellows
rate' of the bellows system depends almost entirely upon the stiffness of
the spring fitted inside the bellows. Accurate calibration and a linear relation-
ship between measured variable and output pressure are thus obtained
because a definite compression of the bellows is required to counteract a
given movement of the measuring element.
The fixed pivot C is now replaced by a moveable pivot Z* With an increase
in the transmitted air pressure, the bellows contract, moving the bellows
rod F upwards. (A decrease in transmitted air pressure moves the bellows
rod downwards.) For a fixed value of the measured variable, the linkage G
will cause the pivot Z and the flapper to assume a definite position for each
value of the output pressure.
Assuming that a change in the measured variable causes the flapper to
move towards the nozzle, this will make the bellows contract, thus moving
the bellows connecting rod upwards and tending to move the flapper away
from the nozzle. The flapper will therefore come to rest in a position in
which the increased air pressure is enough to compress the bellows suffi-
ciently to counteract the effect of the change in the measured variable. As
the spring rate of the bellows is fixed, and the force on the bellows depends
upon the output pressure and the area of the bellows, there will be a definite