Process Vessels Subject to Explosion Risk - Design Guidelines for the Pressure Rating of Weak Process Vessels Subject to Explosion Risk

82 Pages·2000·5.015 MB·English

by  Pilkington, Stan (Eds.)

Process vessels
subject to
explosion risk
Design guidelines for
the pressure rating of
weak process vessels
subject to explosion risk
Edited by Stan Pilkington
lChemE
INSTITUTION OF
CHEMICAL
ENGINEERS
The information in this book 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,
British Materials Handling Board, Health and
Safety Executive or by the editor, for the
consequences of its use or misuse in any
particular circumstances.
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 publisher.
Published by
Institution of Chemical Engineers,
Davis Building,
165-189 Railway Terrace,
Rugby, Warwickshire CV213HQ, UK
IChemE is a Registered Charity
0 2000 British Materials Handling Board
ISBN 0 85295 428 X
Printed in the United Kingdom by Bell & Bain Limited, Glasgow
11
The process industry has long been in need of a structured method of manufac-
turing equipment that may be subject to dust explosions, that would enable suit-
ably protected plant to withstand a transient explosion. The use of pressure
vessel design codes produces vessels that are much stronger than required, and
more expensive. During a Search Meeting into the problems of dust explosions
in 1984, the British Materials Handling Board (BMHB) was asked to initiate
work on suitable guidelines. While these guidelines were agreed by 1991, they
were purely theoretical and it was agreed that the equations needed validation.
With substantial input from the Health and Safety Executive (HSE), this valida-
tion was undertaken by means of a test rig and by finite element analysis, which
took much time and effort. HSE has incorporated these guidelines in its
computer-based expert system.
Despite not covering all situations, these guidelines should enable engineers
to calculate the strength of weak vessels and thus enable explosion venting and
suppression systems to have a more consistent foundation, removing many of
the difficulties associated with our lack of knowledge of vessel strength.
The guidelines represent the best information currently available, and it is
recognized that the equations are not complete. Readers are invited to indicate
to HSE or BMHB what features they would like to see included in a future
edition.
Acknowledgement is made to the extensive work undertaken by the prin-
cipal editor, Stan Pilkington. Thanks are also due to the small Working Group
that oversaw the finalization of the document:
Norbert Gibson Burgoyne Consultants
Charlie Beardsell Eutech
Graham Norton Health and Safety Laboratory
Alan Tyldesley Health and Safety Executive
Peter Middleton
Director, BMHB
December 1999
...
111
...
Foreword 111
Industrial sponsors V
1. 1
2. Method of analysis 4
3. Validation of theontical formulae 11
3.1 Validation testing 11
3.2 Finite element analysis 12
3.3 Results of validation tests 13
3.4 Allowable strain level 14
4. Appliion of formulae to obtain a vessel pressure rrting 16
4.1 The need for formulae 16
4.2 Pressure rating of 0.5 bar 17
4.3 Pressures other than 0.5 bar 17
4.4 Material properties 18
4.5 Common materials of construction 19
4.5.1 Steel 19
4.5.2 Aluminium 20
5. Units and nomenclature 22
5.1 Units 22
5.2 Nomenclature 23
5.3 Conversion factors 23
vii
6. Formulae for weak v dfeat ures 24
6.1 Vessels 24
6.1.1 Cylinder 25
6.1.2 Cone (including a truncated conical section) 26
6.1.3 Hemispherical end or dome 27
6.1.4 Application 27
6.1.5 Worked examples for vessels 28
6.2 Plates 30
6.2.1 Circular flat plate 30
6.2.2 Square and rectangular flat plates 32
6.2.3 Polygonal flat plates 33
6.2.4 Application 33
6.2.5 Worked examples for plates 34
6.3 Duct-vessel intersections 38
6.3.1 Square duct into square plate 39
6.3.2 Circular duct into circular plate 41
6.3.3 Square duct into circular plate 42
6.3.4 Circular duct into square plate 44
6.3.5 Application 45
6.3.6 Worked examples for intersections 45
6.4 Bolted joints 50
6.4.1 Longitudinalf langed joint in a rectangular vessel or duct 51
6.4.2 Longitudinalf langed joint in a cylindrical vessel or duct 53
6.4.3 Bolted circular plate 54
6.4.4 Flanged hoop joint in a cylindrical vessel or duct 56
6.4.5 Bolted square plate 57
6.4.6 Bolted lapped joint 58
6.4.7 Application 59
6.4.8 Worked examples for bolted joints 60
6.5 Welded joints 68
6.5.1 Butt weld 68
6.5.2 Lapped joints 69
6.5.3 Corner or fillet welds 69
6.5.4 Modified factors for thin material 71
6.5.5 Application 72
6.6 Other features 72
Rcferences 75
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Vlll
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Various industrial processes carry the risk of an explosion within the process
plant, if the process is not properly controlled and sources of ignition are
present. This is common in the powder handling industries, where many dusts
can cause explosions, but the risk is also found in equipment used to evaporate
solvents from coated surfaces and in some large-scale gas-fired combustion
plant. A common precaution in such equipment is to provide an area of weak-
ness, such as an explosion vent, to prevent destruction of the plant in the event
of an internal explosion. An alternative precaution is the provision of an explo-
sion suppression system, to snuff out incipient explosions.
Much experimental work over the last 20 years has refined the ways of spec-
ifying the area of a vent panel needed to control the explosion pressure down to
some manageable value. Similarly, the design of suppression systems is based
on much development work. Equipment users and designers realized some
years ago that often the major remaining uncertainty was no longer how to
design the explosion vent or suppression system, but instead the pressure that
any given item of equipment could withstand. For the majority of equipment
used in powder and gas processing vessels a high pressure strength is not
needed, but for both venting and suppression it is necessary that the equipment
can withstand some overpressure, commonly in the range 0.1 to 0.5 bar.
The standard pressure vessel codes are not intended to cover this situation
and often the shape is such that it could not be designed using such codes. If the
equipment cannot be designed to a set figure, the alternative is to test it. Some
manufacturers, especially in European countries, have adopted this approach.
However, some plant cannot withstand the static forces from filling with water
so hydraulic testing is impossible, and it may well not be leak-tight enough to
permit pneumatic testing. Testing by deliberately causing an explosion in the
plant is possible, but expensive, particularly if the unit is destroyed by the test.
To overcome these problems, design equations are needed. The first
approach is set out in the German Engineering Institute standard VDI 2263.
This extends the traditional pressure vessel codes to allow the full strength of
PROCESS VESSELS SUBJECT TO EXPLOSION RISK
the construction material to be used within its elastic limits. Vessels designed to
this code are described as pressure shock resistant. It is, however, largely
restricted to shapes that are covered by the pressure vessel codes.
The first steps in formulating a design guide for weak process vessels were
taken during a collaborative three-year research project into dust explosions in
vented vessels. The project was organized and administered by the British
Materials Handling Board (BMHB).
Funding was provided by the Department of Trade and Industry (DTI),
Health and Safety Executive (HSE) and by industry (see page v). The HSE's
Health and Safety Laboratory, the Fire Research Station, PERA and Salford
University all provided technical input to the project.
Initially a theoretical analysis was made of selected constructional features
typical of those found in weak process vessels. From this analysis a series of
formulae was produced for calculating the pressure capability of the various
vessel features, such as plates, cylinders and so on.
A programme of testing was then started with the aim of validating these
theoretical formulae. From the results of the validation tests it was recognized
that the validity of the formulae was critically dependent on having established
the correct mode of failure and also that considerable further work would be
involved in completing the validation programme for certain of the selected
features. At this stage HSE assumed responsibility for the remaining work in
order to derive appropriate equations which would adequately describe the
behaviour observed in the validation tests.
The results from this weak vessels project, together with a comprehensive
dust explosion protection programme, are brought together by HSE in the form
of a computer-based expert system known as 'DUST-EXPERT' I.
Conventional engineering equations are available covering a wide range of
cases, having been developed over many years. One typical example listing
such equations is Roark's Formulas for Stress and strain2 which contains more
than 5000 formulae for the mechanical loading of many differing types of
structure.
These guidelines contain some 30 equations for shapes and features such as
cylinders, plates, duct intersections and joints. While this may seem a small
number, it represents a novel approach to the pressure rating of weak process
vessels subject to explosion risk. Hopefully, the number of available equations
can be extended in the future by further research and testing. Suggestions for
further work are invited.
The equations in these guidelines draw on the principle that most metals
used in the construction of process plant can deform plastically to a significant
degree without tearing. Vessels with pressure ratings determined from the
INTRODUCTION
equations described may well suffer permanent distortion or be damaged
beyond repair in the event of an explosion within the plant but, for the compara-
tively rare event of an explosion, this may be an economically sound basis for
design.
Typical weak plant, especially that used in some of the dust-handling indus-
tries, is built with little documentation and to uncertain quality standards. There
is usually no specific allowance in the design for corrosion, or erosion of the
metal. There is no legal requirement for assessment of the initial integrity of the
unit or for periodic inspection and, where this is done, it may not be
documented.
If more scientifically based vent designs or suppression systems are to be
adopted, the vessels protected need to be better designed and some arrange-
ments put in place to ensure that they remain fit for service during their lifetime.
Those who design vessels using these guidelines are recommended to record
the basis of their designs, to take steps to ensure that vessels are constructed
properly and to suggest maintenance routines that will ensure they remain safe
for operation.
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Method of analysis
The aim is to provide simple formulae with which to calculate the pressure
capability of each constructional feature of a weak process vessel. Each section
or feature is examined separately and given a feature pressure rating Pf . The
pressure rating for the whole vessel or structure is then the pressure rating of the
weakest feature. This pressure is termed Pred, being the 'reduced explosion
pressure'. This is the term commonly used to describe the predicted maximum
explosion pressure in a vented process vessel after the explosion relief vents
have opened.
A menu of parts and constructional features was selected for analysis and
expressions developed for calculating the feature pressure rating.
The initial approach was to develop formulae based on conventional engi-
neering design methods with materials stress levels confined to the elastic
range, as in conventional engineering practice. This led to over-design and
under-rating of some features since no account was taken of possible perma-
nent deformation in the plastic range.
A dual approach to the derivation of the theoretical expressions was then
adopted. For shapes or features which are inherently stable, and which will
retain their original shape, the conventional engineering design method using
well-established formulae is satisfactory. This includes 'preferred' shapes such
as a sphere, which retain their original shape up to failure.
At the limit the vessel material is allowed to reach yield stress or proof stress
which, for mild steel, is equivalent to an elastic strain of about 0.001 or 0.1%.
Using this approach the vessel is described as being pressure shock resistant.
However, for vessel shapes which are not inherently stable and which could
be expected to undergo significant deformation and to change shape, a form of
plastic analysis was used.
To be able to calculate the strength of such a vessel it is necessary to know
the final shape, hence the mode of deformation must be known.
The analysis of each feature was based on plastic strain at yield stress.
Plastic deformation due to an internal vessel explosion can, provided an
METHOD OF ANALYSIS
allowable maximum strain is not exceeded, usually produce shapes which are
intrinsically stronger than the original shape. In other words a shape will, if
possible, tend to change to an inherently stable or preferred one. For example, a
square duct tends to assume a circular cross-section if it is free to do so.
The assumptions made in the initial theoretical analysis were:
The material yield stress (Sy ) is constant after the material has started to
yield.
The constructional feature being analysed can deform in the plastic range up
to a limiting value of strain (E) to be determined by validation testing.
For the theoretical derivation of formulae, the maximum explosion pressure
(Pf or Pd) is assumed to have the same effect as a uniform static internal
pressure.
The curved surfaces of cylindrical or spherical vessels, or similar features, are
assumed to have the same shape before and after an internal explosion. These
are said to be stable or preferred shapes. With no change in the shape of the
feature, the material from which it is constructed will not suffer plastic
deformation.
For features which undergo a significant change of shape as a result of an
internal explosion, it is assumed that the formula properly describes the mode
of deformation in the critical areas which determine its strength.
Features which may change their shape are free to suffer plastic deformation
without restraint. If this is not possible then high local strains may result,
causing premature failure.
The development of expressions for certain features proved difficult, partic-
ularly when attempting to reproduce the results of the validation tests described
in Chapter 3. An alternative approach was therefore adopted.
In order to describe fully the behaviour of shapes suffering a high degree of
plastic strain, it was necessary to use a non-linear analysis technique. A
non-linear finite element computer-based analysis was used to model these
systems.
The results of the finite element analysis were justified by comparison with
data from the validation tests. A series of empirical expressions was then devel-
oped relating vessel pressure and local material strain. Being empirical, the
equations are strictly valid over a limited range of input conditions such as
vessel dimensions and material properties.
The procedures derived for rating vessel features assume that the vessels are
in good condition. No allowance has been made for the effects of corrosion,
erosion or other conditions which could adversely affect the strength of the
vessel in service or the properties of the materials of construction.