Prevention of Fires and Explosions in Dryers - A User Guide

127 Pages·1990·5.513 MB·127\127

by  Abbott, John (Eds.)

PREVENTION OF FIRES AND
EXPLOSIONS IN DRYERS -
A USER GUIDE
J. A. Abbott (Technical Editor)
Second Edition
THE INSTITUTION OF CHEMICAL ENGINEERS
The information in this Guide is given in good
faith and belief in its accuracy, but does not
imply the acceptance of any legal liability or
responsibility whatsoever, by the Institution, the
Technical Editor or by individual members of the
Working Party for the consequences of its use or
misuse in any particular circumstances.
Published by
The Institution of Chemical Engineers
Davis Building
165-171 Railway Terrace
Rugby, Warwickshire, CV21 3HQ, England
Copyright 0 1977 and 1990
The Institution of Chemical Engineers
All rights reserved. No part of this publication
may be reproduced, stored in a retrieval system,
or transmitted, in any form or by any means,
electronic, mechanical, photocopying, recording
or otherwise, without the prior permission of the
copyright owner.
ISBN 0 85295 257 0
First Edition 1977
Second Edition 1990
Printed and bound by Antony Rowe Ltd, Eastbourne
Cover phutograph by kind permission of APV Pasilac Umited
..
I1
PREFACE
In 1977 the Institution of Chemical Engineers published the User guide to fire
and explosion hazards in the drying of particulate materials. This Guide has
now been updated to take account of recent developments in process safety.
This User Guide has been prepared by a working party of the
Engineering Practice Committee of the Institution of Chemical Engineers.
Its aims are to help those responsible for the design, installation, operation
and maintenance of dryers to:
0 recognise and assess the potential fire and explosion hazards;
0 specify systems, equipment and operating procedures which will elimi-
nate or reduce these hazards as far as is reasonably practicable;
0 provide adequate protection for personnel and equipment when required.
The dryer is defined to include the associated heating system, dust
recovery system, feeding system and product collection system, as well as
the drying enclosure. The scope of the guide is restricted to dryers handling
a particulate or granular product. It does not cover the drying of materials
commonly classed as explosives or as having explosive properties.
The information given in this guide should assist suppliers and users
of drying equipment to comply with current relevant legislation. It is not a
Code of Practice as defined in the 1974 Health and Safety at Work, etc Act.
The recommendations in this guide will provide a basis for good
practice in most situations, but it must be recognised that from time to time
there will be situations and materials where strict adherence to its recom-
mendations will be inappropriate.
The Working Party which carried out the revision is listed overleaf.
iii
CONTENTS
PART 1: GENERAL PRINCIPLES
1 INTRODUCTION 1
2 HAZARDS WHEN POWDERS ARE HEATED 4
2.1 INTRODUCI'ION 4
2.2 IGNITION OF DUST CLoUDS 6
2.3 IGNITION OF DUST LAYERS, DEPOSITS AND MATERIAL IN BULK 14
2.4 PRODUCT QUALITY CONSIDERATIONS 24
3 HAZARDS WITH FLAMMABLE VAPOURS 26
3.1 CHARA~sIlcOsF VAPOUR EXPLoSONS 26
3.2 HYBRID MIXTURES 28
4 EXPLOSION PRECAUTIONS 30
4.1 INTRODUCXON 30
4.2 EXPLosIoN PREVENTION 30
4.3 EXPLOSION PROTECTION 33
5 REDUCING THE IGNITION RISK 40
5.1 SELF-IGNITION 40
5.2 DIRECT HEATING SYSI'EMS 41
5.3 ELECTRWATIC DISCHARGES 41
5.4 ELECTRICAL EoUIPMENT AND bWI'RUMENTS 42
5.5 FRICTIONS PARKS AND FRICXONAL HEATING 43
5.6 WELDING, (IvTI1NG AND SIMILAR OPERATIONS 44
6 COMMON EQUIPMENT 45
6.1 HEATING SYSI'EMS 45
6.2 FEEDING SYSlEMS 46
6.3 DUSC OMYING AND RECOVERY 47
6.4 PRODUCr STORAGE VJSSZIS 53
7 COMMON OPERATING AND MAINTENANCE PRECAUTIONS 55
7.1 PLANT OPERATION 55
7.2 PLANT MAINTENANCE 61
V
PREVENTlON OF FIRES AND EXPLOSIONS IN DRYERS
PART 2: APPLICATION TO SOME COMMON TYPES OF DRYER
8 SPRAY DRYERS 63
8.1 b4TRODUCnON 63
8.2 HAZARDS 64
8.3 POTENTIAL IGNITION SOURCES 65
8.4 SAPETY FROM EXPLOSIONS 66
8.5 PROCESS SPECIFICATION 67
8.6 EQUlPMENT SPECIFICATION 69
8.7 OPERATION AND bhU"ANCE 69
9 PNEUMATIC CONVEYING DRYERS 71
9.1 INTRODUCI'ION 71
9.2 HAZARDS 72
9.3 POTENTIAL IGNITION SOURCES 73
9.4 SAPETY FROM -0NS 74
9.5 PROCESS SPECIFICATION 75
9.6 EQUIPMENT SPECIFICATION 76
9.7 OPERATION AND MAINTENANCE 77
10 FLUID BED DRYERS 78
10.1 h4l"TODUCI'ION 78
10.2 HAZARDS 79
10.3 POTENnAL IGNITION SOURCES 81
10.4 SAFETY FROM EXPLOSIONS 81
10.5 PROCESS SPECIFICATION 84
10.6 EQUIPMENT SPECIFICATION 85
10.7 OPERATION AND bhU"ANCE 86
11 ROTARY DRYERS 88
11.1 bJl"TODUCI'I0N 88
11.2 HAZARDS 89
11.3 POTENTIAL IGNITION SOURCES 90
11.4 SAPETY FROM EXPLOSIONS 90
11.5 PROCESS SPECIPICATION 91
11.6 EQUIPMENT SPECIFICATION 92
11.7 OPERATION AND MAINTENANCE 92
12 BAND DRYER5 93
12.1 b4TRODUCI'ION 93
12.2 HAZARDS 94
12.3 POTENnAL IGNITION SOURCES 95
vi
CONTENTS
12.4 SAFElT FROM EXPUXIONS 95
12.5 PROCESS SpECrmcAnON %
12.6 EQUIPMENT SPECIFICATION %
12.7 OPERAlTON AND h4AINTMANCE 97
13 BATCH ATMOSPHERIC TRAY OVENS 98
13.1 INTRODUCIION 98
13.2 HAZARDS 98
13.3 POIENTIAL IGNITION SOURCES 99
13.4 SAFETY FROM EXPLOSIONS 99
13.5 PROCESS SPECIFICATION 101
13.6 EQUIPMENT SPECIFICATION 101
13.7 OPERATION AND bfAINTENANCE 102
14 BATCH VACUUM DRYERS 103
14.1 blTRODUCllON 103
14.2 HAZARDS 104
14.3 POTENTIAL IGNITION SOURCES 104
14.4 SAFElT FROM EXPIAXIONS 105
14.5 PROCESS SPECIFICATION 105
14.6 EQUIPMENT SPECIFICATION 106
14.7 OPERATION AND MAl”ANCE 107
15 TROUGH DRYERS 108
15.1 INTRODUCX~ON 108
15.2 HAZARDS 108
15.3 POIENTIAL IGNITION SOURCES 109
15.4 SAFETY FROM EXPUXIONS 109
15.5 PROCESS SPECIPICATION 110
15.6 EQUIPMENT SPECIFICATION 110
15.7 OPERATION AND MAl”ANU3 111
REFERENCES 113
APPENDIX A INITIAL SCREENING PROCEDURE TO
ISOLATE DEFLAGRATING OR DETONATING EXPLOSIVES 117
APPENDIX B: TEST TO DETERMINE FIRE AND
COMBUSTION PROPERTIES OF POWDERS 121
vii
PART 1: GENERAL PRINCIPLES
1. INTRODUCTION
One of the most common unit operations in chemical engineering is heating
a solution, slurry, paste, or wet powder to drive off moisture and give a
dry particulate product. In recent years in Britain this operation has resulted
in an average of one explosion per year and over 30 fires per year serious
enough for a local authority fire brigade to be called.
Most natural produds of animal or vegetable origin are combustible.
So are most synthetic organic powders, and also some oxidisable inorganic
compounds such as iron pyrites. The risk is increased substantially if a flam-
mable solvent is being evaporated from the material. Maloperation of an oil-
or gas-fired heater is potentially another source of flammable vapour or gas.
If combustible, dusty materials are handled there is a statutory
requirement (in the UK) to take practicable precautions. These requirements
are embodied in the Factories Act 1961. Sub-sections 31(1), (2) and (4)
provide as follows:
‘(1) Where, in connection with any grinding, sieving, or other process giving
rise to dust, there may escape dust of such a character and to such an extent
as to be liable to explode on ignition, all practicable steps shall be taken
to prevent such an explosion by enclosure of the plant used in the process,
and by removal or prevention of accumulation of any dust that may escape
in spite of the enclosure, and by exclusion or effective enclosure of possible
sources of ignition.’
‘(2) Where there is present in any plant used in any such process as aforesaid
dust of such a character and to such an extent as to be liable to explode
on ignition, then, unless the plant is so constructed as to withstand the
pressure likely to be produced by any such explosion, all practicable steps
shall be taken to restrict the spread and effects of such an explosion by the
provision, in connection with the plant, of chokes, baffles and vents, or
other equally effective appliances.’
‘(4) No plant, tank or vessel which contains or has contained any explosive
or inflammable substance shall be subjected:
PREVENllON OF FIRES AND EXPLOSIONS IN DRYERS
(a) to any welding, brazing or soldering operation;
(b) to any cutting operation which involves the application of heat; or
(c) to any operation involving the application of heat for the purpose of
taking apart or removing the plant, tank or vessel or any part of it;
until all practicable steps have been taken to remove the substance
and any fumes arising from it, or to render them non-explosive or non-in-
flammable; and if any plant, tank or vessel has been subjected to any such
operation, no explosive or inflammable substance shall be allowed to enter
the plant, tank or vessel until the metal has cooled sufficiently to prevent
any risk of igniting the substance.’
Furthermore there are obligations under the Health and Safety at
Work Act 1974, Part 1. Sections 2, 3 and 6 are particularly relevant.
It is the responsibility of the Health and Safety Executive to enforce
these legal requirements.
Ignition sources which may be present in dryers and which could
ignite flammable vapours and dusts, both as dispersions and in layers, in-
clude electric sparks, electrical heating, frictional sparks, frictional heating
and particles ignited in the burners of heating systems.
If dust clouds, layers or bulk accumulations of combustible powders
are exposed to elevated temperatures in air, they may self-heat and ignite
spontaneously; some may undergo exothermic decomposition and this may
sometimes be followed by ignition of the decomposition products. It is
possible to determine specific temperatures at which these processes will
occur.
The basis of safety from explosions may be either
(i) explosion prevention, or
(E) acceptance of the possibility of an explosion and provision of a method
of protecting personnel and equipment from its consequences.
Explosions may be prevented by drying in an inert atmosphere, avoid-
ing the formation of a dust cloud (provided no flammable vapour is present)
or rigorously excluding all passible ignition sources. The last is difficult to
guarantee and should only be used as the basis of safety when no other method
is reasonably practicable. Protection from explosions can be achieved by pro-
viding vents to relieve an explosion to a safe area, suppressing an explosion
by rapid release of inerting chemicals or making plant strong enough to contain
an explosion without damage. Whatever the basis of safety, all reasonably
practicable steps to avoid ignition should be taken.
2
GENERAL PRINCIPLES
Methods of dust explosion prevention and protection are described
in three IChemE guides1t2J.
The great diversity of industrial dryers and materials makes it im-
possible to give detailed recommendations for every type in a guide of
modest size. Moreover, this is a field where each situation must be judged
individually. What this guide presents, therefore, is an account of the general
principles of fire and explosion hazard assessment and control in the context
of drying particulate materials, followed by examples of their application
to some common types of dryer and ancillary equipment.
In any case of doubt concerning explosion hazard and precautions
in a given process the Health and Safety Executive lnspectors should be
consulted in addition to obtaining other expert advice.
3