DESIGN OF VENT GAS COLLECTION AND DESTRUCTION SYSTEMS

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Process Safety Guide:
GBHE-PSG-019
DESIGN OF VENT GAS
COLLECTION AND
DESTRUCTION SYSTEMS
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DESIGN OF VENT GAS COLLECTION AND
DESTRUCTION SYSTEMS
CONTENTS
1 INTRODUCTION
1.1 Purpose
1.2 Scope of this Guide
1.3 Use of the Guide
2 ENVIRONMENTAL ISSUES
2.1 Principal Concerns
2.2 Mechanisms for Ozone Formation
2.3 Photochemical Ozone Creation Potential
2.4 Health and Environmental Effects
2.5 Air Quality Standards for Ground Level Concentrations of Ozone, Targets
for Reduction of VOC Discharges and Statutory Discharge Limits
3 VENTS REDUCTION PHILOSOPHY
3.1 Reduction at Source
3.2 End-of-pipe Treatment
4 METHODOLOGY FOR COLLECTION & ASSESSMENT OF PROCESS
FLOW DATA
4.1 General
4.2 Identification of Vent Sources
4.3 Characterization of Vents
4.4 Quantification of Process Vent Flows
4.5 Component Flammability Data Collection
4.6 Identification of Operating Scenarios
4.7 Quantification of Flammability Characteristics for Combined Vents
4.8 Identification, Quantification and Assessment of Possibility of Air Ingress
Routes
4.9 Tabulation of Data
4.10 Hazard Study and Risk Assessment

4.11 Note on Aqueous / Organic Wastes
4.12 Complexity of Systems
4.13 Summary
5 SAFE DESIGN OF VENT COLLECTION HEADER SYSTEMS
5.1 General
5.2 Process Design of Vent Headers
5.3 Liquid in Vent Headers
5.4 Materials of Construction
5.5 Static Electricity Hazard
5.6 Diversion Systems
5.7 Snuffing Systems
6 SAFE DESIGN OF THERMAL OXIDISERS
6.1 Introduction
6.2 Design Basis
6.3 Types of High Temperature Thermal Oxidizer
6.4 Refractories
6.5 Flue Gas Treatment
6.6 Control and Safety Systems
6.7 Project Program
6.8 Commissioning
6.9 Operational and Maintenance Management
APPENDICES
A GLOSSARY
B FLAMMABILITY
C EXAMPLE PROFORMA
D REFERENCES
DOCUMENTS REFERRED TO IN THIS PROCESS GUIDE

TABLE
1 PHOTOCHEMICAL OZONE CREATION POTENTIAL REFERENCED
TO ETHYLENE AS UNITY
FIGURES
1 SCHEMATIC OF TYPICAL VENT COLLECTION AND THERMAL
OXIDIZER SYSTEM
2 TYPICAL KNOCK-OUT POT WITH LUTED DRAIN
3 SCHEMATIC OF DIVERSION SYSTEM
4 CONVENTIONAL VERTICAL THERMAL OXIDIZER
5 CONVENTIONAL OXIDIZER WITH INTEGRAL WATER SPARGER
6 THERMAL OXIDIZER WITH STAGED AIR INJECTION
7 DOWN-FIRED UNIT WITH WATER BATH QUENCH
8 FLAMELESS THERMAL OXIDATION UNIT
9 THERMAL OXIDIZER WITH REGENERATIVE HEAT RECOVERY
10 TYPICAL PROJECT PROGRAM
11 TYPICAL FLAMMABILITY DIAGRAM
12 EFFECT OF DILUTION WITH AIR
13 EFFECT OF DILUTION WITH AIR ON 100 Rm³ OF FLAMMABLE GAS

1 INTRODUCTION
1.1 PURPOSE
The purpose of this guide is to provide guidance on the safe design of
vent gas collection and destruction systems including, in particular,
thermal oxidizers and their associated equipment for destroying volatile
organic compounds (VOCs). It is based on experience gained from
operating units and capital projects and on the application of sound
engineering practice and good safety principles.
The standards which are applied to any particular project or plant will differ
based on the geographic location and local legal requirements as well as
site and business preferences. Any relevant company, local, national or
international codes or standards should therefore be applied to the design
of the system.
Most operating problems that are experienced with thermal oxidizers
derive from process deviations upstream of the unit. Therefore, in any
project or installation it is essential to consider the vent collection headers
and the destruction unit as a complete system and not as an assembly of
separate entities.
1.2 Scope of this Guide
This guide does not replace, or provide a substitute for, national or
international standards but should be considered in conjunction with them.
When consulting this document it should be remembered that it is
intended as a guide and not a set of hard and fast rules. Good engineering
judgment should be applied to the design at all times in order to produce a
safe and efficient collection and destruction system.
This guide is applicable to the safe design of:
o Vent collection headers whether connected to destruction units,
flare stacks or vent stacks;
o Ancillary equipment including knock-out pots, fans, pumps etc.;
o Thermal oxidizer units;
o Process and vent gas burner control systems.

It also covers:
o Flammability and explosion hazards in vent headers;
o Environmental aspects of vents treatment and destruction systems;
o Heat recovery systems;
o Flue gas scrubbing;
o Specification and purchase of destruction units.
This guide does not deal with:
o Detailed mechanical or engineering design of the thermal oxidation
unit itself, except where applicable to safety issues;
o Choice of materials of construction for oxidizer refractory linings;
o Choice of specific type of oxidation unit, except for general
considerations around environmental and safety performance.
Guidance on different types of VOC abatement technology can be found
in Process Safety Guide: GBHE-PSG-017
PRACTICAL GUIDE ON THE SELECTION OF PROCESS
TECHNOLOGY FOR THE TREATMENT OF AQUEOUS
ORGANIC EFFLUENT STREAMS
.
Guidance on the detailed design and operation of flare stacks can
be found in Process Safety Guide: GBHE-PSG-008
PRESSURE RELIEF

1.3 Use of the Guide
This guide is split into six main Sections:
1 Introduction.
2 Environmental Issues.
3 Vents Reduction Philosophy.
4 Methodology for Collection & Assessment of Process Flow Data.
5 Safe Design of Vent Collection Header Systems.
6 Safe Design of Thermal Oxidizers.
Section 2: discusses environmental issues, mechanisms for ozone
depletion and air quality standards.
Section 3: provides guidance on reduction at source in compliance with
the principles of inherent SHE.
Section 4: outlines a methodology for collecting and assessing the data
required to design a vent header system. This is based on
previous experience on a number of previous projects in
GBHE.
Section 5: contains guidance on the design of vent header systems.
This is equally applicable to all header systems whether
venting to atmosphere, flare stack or thermal oxidation unit.
Section 6: deals with the design of thermal oxidizers. These are the
most common form of destruction system used for VOCs.
Specific guidance on the design of flare stacks can be found
in GBHE-PSG-008 PRESSURE RELIEF

2 ENVIRONMENTAL ISSUES
2.1 Principal Concerns
Some VOCs are toxic and some are implicated in damage to the
stratospheric ozone layer. However, the principal concerns with most
VOCs are:
(a) Their involvement, together with oxides of nitrogen and in the
presence of sunlight, in the production of photochemical oxidants in
the lower atmosphere (see Section 2.2).
(b) Odors which may be offensive at concentrations well below the
Occupational Exposure Limit (OEL).
VOCs can be classified according to their Photochemical Ozone Creation
Potential (POCP) referenced to a standard of unity for ethylene (see
Section 2.3). Ozone is the photochemical oxidant that has been studied
most widely but there are others including peroxyacetyl nitrate (PAN) and
hydrogen peroxide. Ozone can pose a health risk and cause
environmental damage (see Section 2.4).
Some VOCs also present an odor nuisance, even at very low
concentrations. For example, ethyl acrylate has an odor threshold of about
0.02 ppb. This can create major difficulties for design and operation as the
emission to atmosphere of only a few mg/sec can cause odor problems. It
is therefore vital that odorous materials are contained within process
equipment. Where this cannot be achieved, then destruction or capture
techniques should be very efficient and stacks discharging directly to
atmosphere should usually be very tall.
2.2 Mechanisms for Ozone Formation
The atmospheric chemistry of ozone formation is very complex and
involves a multitude of interacting chemical reactions [Refs. 2 & 3]. The
principal reactions are shown below which illustrate the involvement of
VOCs in a simplified form.

Nitrogen dioxide absorbs natural radiation and breaks down into nitric
oxide and oxygen radicals:
The oxygen radicals combine with oxygen to form ozone:
However, ozone oxidizes nitric oxide to nitrogen dioxide:
Hence there is a natural balance of ozone concentrations at ground level
involving oxides of nitrogen. However, peroxy radicals (RO2) produced by
the attack of hydroxyl radicals (OH) on VOCs act as a sink for nitric oxide
and thereby disturb the above equilibrium towards higher concentrations
of ozone:
It is believed that hydroxyl radicals are formed in the atmosphere by
photochemical dissociation of ozone and subsequent reaction with water.
It should be noted that the above reactions require the simultaneous
presence of precursors in the appropriate meteorological conditions.
Furthermore, not only are some of these reactions slow, but ozone, once
formed, can persist for several days and so may be transported long
distances. Therefore, elevated ozone concentrations often appear over
widespread areas up to several hundred kilometers from the sources of
the precursors.
2.3 Photochemical Ozone Creation Potential
As stated above, VOCs and other substances can be classified according
to their POCP referenced to a standard of unity for ethylene [Ref. 5] as
shown in Table 1.

2.4 Health and Environmental Effects
A high concentration of ozone can affect human soft tissues such as the
eyes and nose. It may also affect respiratory functions including changes
to the airways and an increase in the sensitivity to some inhaled allergens
such as pollen. Although there is no evidence that it can cause asthma, it
has been claimed that it might trigger allergic reactions and it is widely
reported to be involved in the significant rise in reported cases of asthma.
It is recognized that ozone at commonly found concentrations can damage
a wide variety of crops and other vegetation including grapevine, beans,
beet, spinach, clover, peanut, cotton and turnip. It has been reported that
soybean yield is reduced by up to 15% by concentrations of ozone at
about 50 ppb.
Ozone and other photochemical oxidants cause material damage to
rubber, plastics, painted surfaces, dyed fabrics and synthetic elastomers
which is estimated to cost billions of US dollars annually.

It is well known that smog in warm still air, such as regularly experienced
in the Los Angeles area, can be caused to some degree by photochemical
oxidants.
It is worthy of note that ozone is the only atmospheric pollutant that is
commonly present in concentrations that can be significant fractions of the
occupational exposure limit (OEL). Further information on the health and
environmental effects of ozone can be found in Refs. 4 and 5.
2.5 Air Quality Standards for Ground Level Concentrations of Ozone,
Targets for Reduction of VOC Discharges and Statutory Discharge
Limits
The World Health Organization guideline for ground level ozone
concentrations on an 8-hour average basis is 50-60 ppb. The National
Ambient Air Quality Standard for ozone in the USA is 120 ppb hourly
average, not to be exceeded on more than one day per year. The UK
Expert Panel on Air Quality Standards has proposed an Air Quality
Standard of 50 ppb as a running 8-hour average [Ref. 4]. The 8-hour time
weighted average (TWA) occupational exposure limit (OEL) for ozone is
100 ppb; the 3-minute TWA limit is 300 ppb.
A 1991 Protocol to the 1979 United Nations Economic Commission for
Europe (UNECE) Convention on Long Range Transboundary Air
Pollution, calls for voluntary reductions in VOC emissions across Europe
and North America by at least 30% by 1999 relative to 1988 levels.
There is increasing pressure from both legislative authorities and public
opinion to completely eliminate all vents containing VOCs.
In general, discharge limits for VOCs are set at national level and are
usually in the form of emission concentration limits. Some of these are
defined by statute as in TA Luft [Ref. 6] in Germany whereas others
appear as strict guidance limits as in IPR Guidance Notes [Ref. 7] in
the UK. Although the principles of POCP are becoming generally
accepted, it is likely to be some time before they are adopted formally by
the statutory control authorities.

3 VENTS REDUCTION PHILOSOPHY
3.1 Reduction at Source
It is most important that, wherever possible, vents should be eliminated at
source according to the principles of inherent SHE. This is not only
environmentally responsible, but also good business practice as vented
material is wasted material and, furthermore, end-of-pipe treatment is
invariably expensive. If vents cannot be eliminated at source, they should
be reduced as far as possible or mitigated. Large volumes of
vented material will require proportionately larger and more expensive
collection and treatment systems and have higher operating and
maintenance costs. Vents minimization can therefore have a large positive
benefit on the overall project cost.
Technical options for control at source include the:
o Increased vessel design pressure may eliminate the need for
pressure relief systems at minimal extra cost for the stronger
vessel. Consideration should also be given to the possibility of
uprating the design pressure of existing vessels, tanks and pipe
work. Stock tanks should be fitted with PV valves instead of open
vents;
o Instrumented, high integrity protective systems may be fitted
utilizing reaction quench technology or dump tanks. It should be
noted that in North America and some countries subscribing to
ASME codes, containment or instrumented protective systems may
not be allowed;
o If water-based solvents or solvents with lower volatility can be used,
VOC discharges can generally be reduced significantly;
o Subject to considerations of safety, cross-contamination and plant
layout, a number of stock tanks can sometimes be connected to a
common venting system to reduce the overall volumetric flow rate.
This is particularly effective when transfers are made between the
tanks in question;
o Similarly, the vent on a road tanker or other transportable container
that is being loaded or unloaded to a stock tank should, wherever
possible, be connected (i.e. back-balanced) to the stock tank vent
system;

o Where it is necessary to exclude oxygen or moisture by using
nitrogen, this should be achieved by means of a pressure-
controlled nitrogen supply and a pressure-controlled vent rather
than a continuous nitrogen sweep in order to minimize the volume
of gas vented. Furthermore, the nitrogen inlet and the vent outlet
should be located close to each other in order to minimize the
concentration of VOCs in the vent. Disturbance of the vapor space
should be minimized by connecting the nitrogen flow via a large
nozzle thus reducing the gas velocity;
o It is claimed that floating-roofs can reduce evaporative losses from
stock tanks by up to 90% compared to conventional fixed roof
tanks. Multiple and secondary seals also reduce evaporative
losses;
o The liquid inlets to stock tanks should, wherever possible, be below
the liquid level in order to minimize the disturbance of the vapor
space. This reduces evaporative losses;
o Hydraulic and pumped liquid transfers, rather than pneumatic
transfers, can significantly reduce VOC losses as vapor and mist in
the vent at the end of the transfer;
o The charging of material through an open lid or charge port into a
vessel containing VOCs usually results in VOC losses to
atmosphere;
o If the vessel is at or above atmospheric pressure, the losses occur
locally. If the vessel is under some vacuum, there will be an ingress
of air which could result in a VOC discharge to atmosphere
remote from the charge point. Furthermore, air sucked in could
result in fuel-rich mixtures becoming flammable in the vessel or in
downstream vent collection pipe work;

o Ιf the material to be charged is a liquid or can be dissolved in a
liquid, a closed charging system should be used. Where this is not
possible, a charge hopper should be considered with a narrow
entry point and a rotary, ball or slide valve into the vessel;
o As a general rule, the flow rate of inerts that come into contact with
VOCs should be minimized. Unnecessary purging and draughting
should be avoided. Attention should be paid to poorly designed or
faulty pneumocators, valves on nitrogen blowing or blanketing
systems that are passing or left open, etc. Correct location of
nitrogen blanketing on the vent line to the thermal oxidizer can
reduce vapor losses, but in some cases it may be necessary to
sweep the vapor space (e.g. if corrosive gases are evolved from
the liquid);
o High quality maintenance can reduce fugitive losses from poorly
seated relief valves, pin holed bursting discs, flanged connections,
control valve stems, pump glands, etc.. Fitting bursting discs to
relief valve inlets may eliminate fugitive emissions but their effect
on the relief stream capacity should be checked;
o Alternative process equipment may reduce fugitive losses e.g.
glandless or canned pumps, soft seat relief valves, bellows sealed
valve stems and improved gasket materials.

3.2 End-of-pipe Treatment
Some possible types of end of pipe treatment are:
 Condensation;
 Adsorption;
 Absorption;
 Thermal oxidation;
 Catalytic oxidation;
 Biological filtration;
 Membrane separation.
End-of-pipe solutions should always be regarded as a last option in view
of their capital and operating cost. Destruction systems can also have
inherent problems of statutory authorization and social pressures which
invariably take a significant amount of time, effort and money to overcome.
The additional cost to the business of these factors should not be
underestimated. The overall energy and environmental impact balance
should be considered carefully before selecting the appropriate, if any,
vents destruction system. The impacts of such things as additional support
fuel usage, discharges to atmosphere of thermal oxidizer flue gas,
discharges to water of scrubbing liquor blowdown or waste solids disposal
of spent adsorbent should be addressed opposite the environmental
improvement of treating the vent gas in question. This exercise is required
by statute under Best Practicable Environmental Option (BPEO)
assessments in the UK and under Best Available Control Technology
(BACT) assessments in the USA.
The above principal end-of-pipe treatment options are described in more
detail in GBHE-PEG-015 which also provides guidance on the selection of
the appropriate option together with names and addresses of suppliers. It
may be advantageous to use a combination of techniques such as
refrigerated condensation, adsorption or membrane separation in order to
concentrate or reduce the amount of VOCs prior to destruction by thermal
oxidation. This will result in a smaller and thus cheaper destruction unit.

The safety and environmental aspects of thermal oxidation are discussed
further in Section 6 of this guide.
4 METHODOLOGY FOR COLLECTION & ASSESSMENT OF PROCESS
FLOW DATA
4.1 General
Vent gas collection and destruction systems are complex plants in their
own right. Hence, in order to ensure a safe design, a methodical approach
to the design basis and basis of safety is essential. This Section provides
a framework methodology which can be adapted to specific project
requirements.
Considerable effort is required to collect the information on flows,
compositions, component data, flammability data and scenarios which is
needed to produce the basis of safety for the system and the Hazard
Study. The use of a spreadsheet will assist in this process. This process is
especially difficult for batch plants where flows are intermittent and highly
variable.
For existing plants and processes it is essential to obtain the full co-operation of
the plant personnel in the information gathering process since they will have
experience of many of the possible deviations from normal operation which can
occur. It should be noted that some possible occurrences may never have been
experienced in the life of the plant due to their extremely low potential frequency.
The range of possible scenarios should be established by consultation with the
plant operations team and by examination of the Hazard Study records for the
project. If necessary, further Hazard Studies may be required to establish a
range of worst cases. Full transmittal of this information from the plant to the
project (or between members of the project team for new plants) is essential. For
new plants, all possible operating scenarios should be identified at the design
stage, again using information from the Hazard Study process. Other useful
techniques for hazard assessment and reduction are fault tree analysis, process
hazard review, failure mode and effect analysis and consequence analysis [Ref.
17].

The use of a standard proforma may be helpful in allowing clear and concise
collation of the data. It is essential to ensure that the person responsible for
completing the proforma is aware of the importance of the data being supplied.
One-to-one discussions are invaluable to avoid confusion. The proforma should
be comprehensive in the information requested. An example proforma is shown
in Appendix D. If the information supplied on the proforma is incomplete or
incorrect, it will have serious consequences for the design of the system,
possibly even making it unsafe. If errors are discovered in the information on
vents flow and compositions the rework required will almost certainly be costly in
terms of both man hours and new equipment. There are examples where VCDS
have been grossly undersized or there have been fluctuations of the composition
into the hazardous region due to a failure to identify the maximum short term
flows.
If possible, the vent collection system should be installed at least a year before
final design of the destruction system in order to provide time for comprehensive
monitoring of the flows and compositions in the header system under operational
conditions. This has benefits to the project in that the data collected during this
period enables a more efficient destruction unit to be designed with consequent
savings in design and operational costs. Regulatory authorities, however,
generally require the collection and destruction systems to be installed
simultaneously.
The proposed methodology for safe design consists of the following steps:
 Identification of vent sources;
 Characterization of vents;
 Quantification of process vent flows;
 Component flammability data collection;
 Identification of operating scenarios;
 Quantification of flammability characteristics for combined vents;
 Identification and quantification of possibility of air ingress;
 Tabulation of data;
 Hazard Study assessment.

The processes involved at each step are described further in Sections 4.2
to 4.9 (inclusive):
4.2 Identification of Vent Sources
It is essential that all vent sources should be identified before starting the
design of the header system. These may include:
 Tank breathing vents;
 Relief and breather valves;
 Tanker loading points;
 Reactor vents;
 Vacuum pump exhausts;
 Lute pots and siphon breakers.
It is important that all sources are identified, as the number and location
will have an impact on the size and complexity of the collection system. It
may be possible to identify a number of vents which could be eliminated,
recycled economically or minimized by other means at this stage. Any
existing vent or flare header systems should also be identified (e.g.
common purging of tank farms), and a strategy for dealing with these
included.
During this part of the project, the plant engineering line diagrams (ELDs)
should be updated for existing plants and vent sources for new plants
clearly marked. This information should also be carried over onto site plot
plans and general arrangement drawings and will aid both estimation of
project costs and mechanical design of the header system.
4.3 Characterization of Vents
The results from this part of the design process will have major
implications on the number, type and size of headers, the conditions in the
system and ancillary equipment needed. Vents may be characterized in
several different ways. Typical characterization groupings are:

 Fuel-rich, fuel-lean or flammable;
 Continuous or intermittent;
 Condensable or non-condensable;
 Corrosive;
 Toxic;
 Wet/dry;
 Mixed or variable properties.
This activity will indicate which of the vent headers is the most appropriate
to use for each vent stream and any treatment which is needed to make
the header safe if it would otherwise operate within the flammable region.
During the characterization process, the effect of any interactions between
vent compositions should be evaluated to ensure that the flows and
compositions in the system do not operate in the flammable region and
that there are no undesirable chemical reactions between the different
materials. This is particularly important where there may be polymeric
material which can clog the system. Any base load of inerts, support fuel
or dilution air should be included. An interaction matrix should be used to
ensure that all possible combinations are identified and assessed.
Interactions should also be examined between the VOCs and the
materials of construction of the header system. An example of this is
shown in Ref. 17.
Certain conditions such as fire relief and other types of emergency vent
may be exempt from treatment on the basis that they are likely to occur
extremely infrequently and have such large flow rates that they would
need the construction of a much larger destruction unit. Such matters
should be assessed during the quantification of process vent flows and, if
appropriate, discussed with the local regulatory authorities.
Vents often have varying compositions depending on the particular
operating scenario at the time; hence the "mixed or variable properties"
heading. These may need special consideration if they can transit from
fuel-rich to fuel-lean or vice versa. Similarly, consideration may be
required if the composition in the vent can change drastically or if a
material with extreme combustion properties such as hydrogen or a
material with an unusual flammability diagram such as ethylene oxide can

be present. The effect of such changes will have an impact on the design of the
headers in dictating the type of ancillary equipment or systems needed (e.g.
flame arresters or inert gas provision). This is of particular importance in batch
process where several reaction steps or unit operations may be carried out in a
single vessel.
4.4 Quantification of Process Vent Flows
Vents collection and destruction systems can only be designed safely with
full knowledge of the range of flows and compositions which may be
encountered not only during normal operation but also in abnormal
conditions (e.g. relief valve operation, process deviations etc.). For most
processes, whether batch or continuous, both the vent flows and
compositions are likely to be highly variable. Typically, the following
operations should be considered:
• Flowsheet (normal operation);
• Batch operating cycle;
• Tank breathing as a result of thermal expansion and contraction,
pumping etc.;
• Process deviations;
• Relief situations;
• Maintenance purging of some or all plant items;
• Start-up, shut-down and stand-by modes;
• Other abnormal operations.
Where possible, monitoring of flows and compositions should be carried
out over an extended period of time where applied to existing plants to
ensure that all normal situations are covered. Where this is not possible,
soundly based estimates should be made. It is unlikely that worst case
conditions will be seen during the monitoring period since the frequency of
combined events occurring may be very low. A judgment should therefore
be made as to the worst credible case, taking into account equipment
failures, process deviations, operator error, etc. Some of this information

may be carried over from pressure relief documentation, especially relief
philosophy and bursting disc/relief valve data sheets and. If possible, the
vent collection system should be installed prior to the destruction system
in order for performance monitoring to be carried out. This will yield
valuable design information for the destruction unit.
In-depth plant knowledge will be needed to fully identify all the possible
deviations and resulting vent compositions and flow rates. Once again,
any base load of inerts, fuel or dilution air should be included. As stated
above, a proforma may be useful for the transferral of information from
plant and operations personnel to the project team, although this is no
substitute for face-to-face discussions with plant personnel and should not
be used in isolation from other information sources. The data can be
classified into a number of flow rate/composition scenarios such as:
 Zero;
 Normal / flowsheet;
 Minimum flowsheet;
 Maximum flowsheet;
 Maintenance condition;
 Maximum plus over-design allowance.
It may be impracticable to install a vent gas collection and destruction
system that can cope with the simultaneous occurrence of the "worst
case" flows from all vent sources. The likely frequency and duration of
deviations from flowsheet should, therefore, be estimated in order
to determine which combination of vent flows will be accommodated and
which will be dealt with by other means. Common cause events should be
identified as these often lead to comparatively large vent flows e.g. power
failure. When calculating the flows due to relief valve operation, the relief
stream capacity should be used rather than the required relief rate.
A spreadsheet may be helpful to correlate the data in order to identify
those scenarios which would cause operational difficulties or process
hazard.

The quantification of vent flows may be particularly difficult for batch
processes which, by their nature, have intermittent flows and
compositions. In this case it is sensible to consider the maximum possible
flows from the process and the full range of flows from zero to the
maximum. For batch processes, consideration of the possibility of process
deviation and cross contamination is especially important. The reduction
of emissions from batch processes is discussed in Ref. 1.
It may be advisable to carry out a Hazard Study on the upstream process
plant at this stage to consider the feasible deviations which could occur
resulting in different emissions to the vent collection system. When
applying the Hazard Study guide words, consideration should be given to
the special cases which may be generated (e.g. more fuel, more air, less
fuel etc.).
Typical deviations which should be considered for all process plant, but
especially for batch processes, are:
 Charging wrong reactants (other materials stored in area or wrong
materials delivered);
 High or low process temperatures;
 High or low pressures;
 Overfilling of tanks, reactors or distillation columns;
 Purging, venting or pressure letdown;
 Agitator failure;
 Heating failure;
 Cooling water failure;
 Instrument air failure;
 Power failure.
Overfilling can be a major problem as it may result in liquid entering the
vent gas collection header system. This should be avoided as it can cause
a number of hazards as described in Section 5.3. Frothing of reactor or
tank contents may also result in liquid entering the header system.

Suitable precautions should be taken to prevent this situation occurring,
including the provision of liquid interceptors (knock-out pots) or liquid level
alarms if appropriate. Temperature, pressure, bubble and dew points for
each component and composition may be needed if there is a possibility
of flashing liquid entering the header and also to evaluate any possibility of
volume shrinkage of the gas on cooling or condensation after entering the
header. (Shrinkage may cause air to be drawn into the header giving rise
to a flammable mixture). This will also give an indication of whether
lagging or heat tracing of lines is needed and whether there are any
potential solidification or icing problems.
Incomplete quantification of data is likely to result in incorrect specification
of equipment including the vent collection pipe work, safety equipment
such as flame arresters, KO pots and downstream plant such as a thermal
oxidizer. It is therefore vital that the quantification process is carried out in
full. This can only be achieved by appropriate allocation of resources and
time in the overall project program (see Section 6.7). Particular regard
should be paid to the presence of more hazardous components such as
hydrogen, acetylene, ethylene oxide etc..
Chemical interactions should also be quantified at this stage using the
interaction matrix developed in Section 4.3. Undesirable reactions may
occur when mixing vent streams causing, for example, polymerization,
condensation or exothermic reaction. Such situations should be avoided.
4.5 Component Flammability Data Collection
Flammability data, particularly LFL, UFL and MOC, is required for each of
the components in the vent system in order to construct the flammability
diagrams for the different compositions and scenarios which may occur
(see Section 5.2). If possible, experimentally determined flammability
diagrams should be used. If flammability diagrams are not available then
they may be constructed for each of the worst case compositions for each
of the vents. For further explanation of flammability diagrams see
Appendix C.
In some systems there is synergy between the more reactive and less
reactive components of the gas mixture, hence relatively small amounts
of, for example, hydrogen have a disproportionate effect on the
flammability characteristics. If there are multiple components or significant
quantities of reactive gases present then experimental determination of
flammability characteristics should be considered.

These diagrams can also be used to assess the possible consequences of
air ingress into fuel-rich systems. The flammability characteristics for
mixtures should be estimated using Le Chatelier's rule (see Appendix C).
A spreadsheet may be useful for this. Critical flammability estimates
should be backed up with experimental data.
4.6 Identification of Operating Scenarios
The range of operating scenarios which are appropriate to the individual
process sources should be identified. The scenarios to be considered may
include:
• Start-up from cold;
• Re-start after trip;
• Shut-down;
• Stand-by;
• Normal operation;
• Low rate operation;
• VOC/fuel excursion;
• Oxidant excursion;
• Inert excursion;
• Commissioning standby equipment or after maintenance;
• Depressurizing or venting down;
• Vacuuming down;
• Purging.
Any other possible scenarios should be identified as part of the individual
project.

There may be differences in the way that the system performs or in the
conditions of each vent under the different operating scenarios which lead
to differences in the flow rate or composition of the vent. The range of
operating scenarios identified depends on the operation of the plant, for
instance the conditions for a cold start up may differ from those which
occur after a plant trip. The identification process is intended to detail the
full envelope of operating conditions which can be generated by the plant.
The list produced should include those scenarios which would be
generated by failures of trips or controls. Obviously a full working
knowledge of the plant and its associated control systems and safety trips
is required to identify all the possible scenarios.
4.7 Quantification of Flammability Characteristics for Combined Vents
A brief description of flammability diagrams and associated terminology is
given in Appendix C. The flammability characteristics for the possible
combinations of vent sources under each of the possible operating
scenarios should be calculated. The compositions calculated can be
placed into one of the following categories:
• Fuel-lean;
• Fuel-rich (oxidant lean);
• Inerted;
• Flammable.
Fuel-lean vents are those which have fuel concentrations below the LFL
and which are therefore safe under all air ingress conditions. Fuel-rich
vents have compositions above the UFL which could, in theory, enter the
flammable region in the case of air ingress whether they are oxidant lean
or inerted. Flammable vents are those operating inside the flammable
region. As stated previously in Section 4.6, excursions should be
considered which could change the composition of the vent.
Flammable vent compositions should be avoided if at all possible or
treated to take them out of the flammable region (e.g. by inerting). If they
cannot be avoided, a full risk assessment of the likelihood and
consequences of incidents should be carried out.

Design of the system to cope with overpressure due to deflagration or
detonation may be necessary in exceptional circumstances (see Section
5.2.6).
It should be noted that air is not the only source of oxidant. In particular
chlorine and oxides of nitrogen may act as oxidizing agents. These may
originate in the upstream process.
4.8 Identification, Quantification and Assessment of Possibility of Air
Ingress Routes
Obviously, if the vent collection header is operating under positive
pressure at all times then air cannot be sucked into the system from the
atmosphere. Hence, it may be possible to eliminate all or the majority of
possible air ingress routes in the header system by operating at positive
pressure. However, this may not be possible for all vents or for upstream
process equipment operating under vacuum. Therefore it is essential that
all possible upstream air ingress routes are considered as well as those
relating directly to the header system. There may also be a number of
cases where failures mean that a nominally positive pressure system may
become negative pressure.
There may be a number of possible routes for air ingress into fuel-rich
vent headers. For each source, all possible openings or paths for air
leakage into the system should be identified and the potential ingress
rates estimated. It is important to include all flanged joints, instrument
connections and also possible failures of the header pipe work. Some
typical situations and operations which may lead to possible air ingress
routes are:
• Maintenance operations involving removal of equipment such as
isolation, control and relief valves, instruments, blank ends, flanges,
slip plates, etc;
• Failure of seal liquid supply to, or failure to top up, lute pots or
leakage of seal liquid. This may lead to the seal running dry thereby
opening up a route direct from atmosphere;
• Accidental damage to pipe work (e.g. vehicle damage to exposed
lengths of header adjacent to roadways);

• Corrosion of pipe work and fittings including cracks and weld
defects;
• Manual operations involving breaking/making connections such as
road/rail tanker purges to vent collection headers, opening of
inspection and charging ports, sampling operations, physical
changeover of batch unit operations, etc.;
• Process equipment operating under vacuum;
• Failure to adequately purge equipment prior to start-up, causing air
to be displaced into the header;
• Failure to completely purge headers and laterals from process
vents through to vents treatment unit;
• Inadequate isolation e.g. leaving sample points open, failure to
blank off, passing valves (including thermal oxidizer bypass valves);
• Oxygen generation by process.
It may be possible to eliminate a number of air ingress routes by
minimizing the number of flanges, equipment connections etc.. Similarly,
purge and sample points may be equipped with “dead man’s handle” type
valves to prevent them being left open inadvertently. Other operations
may also be modified to reduce the possibility of air ingress.
Where stacks are involved, many nominally positive pressure systems
may in fact be under a slight vacuum due to the chimney effect. This is
particularly apparent where the vent gas is above ambient temperature or
the molecular weight is significantly less than that of air. This
effect is more pronounced at low flow rates and can result in air ingress
causing a flammable mixture. The stack may be fitted with a liquid seal at
the base to prevent the header system operating under negative pressure.
Even systems that are nominally under positive pressure may in fact be
under negative pressure due to the chimney effect where no seal pot is
installed.

Once again, this identification process requires a thorough knowledge of
the plant operation, layout and maintenance procedures if all possible
hazards are to be identified. Some knowledge of the proposed operating
pressure in the header system will also be needed to calculate the air
ingress rates from the various routes. This may mean taking some early
design decisions in order to get an early estimate of possible
consequential hazards.
The frequency and consequence of each possible air ingress combination
should be estimated. Air leakage into the system will alter the composition
in the header, possibly taking it into the flammable region. Additionally, the
flows from other sources and the pressure profiles in the header may be
affected by the leakage. The full possible range of operating scenarios for
other vent sources discharging into the same header should be
considered, including the effects of the air leakage. Similarly, the
interactions caused by air leakage should be identified. An interaction
matrix should be used for this evaluation process.
4.9 Tabulation of Data
A "control chart" should be created that lists the activities and events
(normal and abnormal) which would result in deviations from flowsheet
conditions whether resulting from process variations or by air ingress.
By inspection, those scenarios which would not result in a flammable
mixture occurring in the vent header system should be eliminated. It is
extremely important to identify the likelihood of any transient incursions
into or through the flammable region, as well as new steady state
flammable conditions.
The remaining scenarios are therefore the ones which would result in a
potentially hazardous situation.
The tabulation of data is an important aid to understanding the
complexities of the numerous operating scenarios and simplifies the
identification of potentially hazardous situations.

4.10 Hazard Study and Risk Assessment
An assessment of the potential for flammable mixtures and the likely
consequences should be carried out in the Hazard Studies. Whether or
not required in order to satisfy the Hazard Study criteria, the opportunity
should be taken to consider the possibility of making some of the
hazardous vents inherently safe or more safe or of reducing the duration
or frequency of potentially unsafe situations. It is most important that a
trained, accredited practitioner carries out, or at least coordinates, the
Hazard Studies and Risk.
4.11 Note on Aqueous / Organic Wastes
One way of dealing with aqueous effluent contaminated with organic
waste is to air strip it in a packed column. The air can then be used as
combustion air in a thermal oxidizer. There are, however, a number of
potentially hazardous situations that could arise including the following:
• Enclosing organic contaminated water in a tank may lead to a flammable
mixture arising in the vapor space of the tank. The Henry's law coefficients
of the contaminants should be examined to check for the possibility of a
flammable mixture above the liquid;
• If a large quantity of organic liquid gets into the aqueous waste stream
there is a risk of free phase organics getting into the stripping column. If
this occurs, then the air stream coming from the stripper may again be
flammable;
• The column may also be prone to clogging due to dissolved or suspended
solids. Reaction of scrubbing liquor with atmospheric gases or
constituents of the vent gas may also cause clogging (e.g. alkalis with
CO2);

• During start-up, shut-down or process interruption the vapor space in the
stripper column head may become flammable. On restart the flammable
mixture may be carried forward into the header system with consequent
risk of explosion hazard;
• Low air flow, however caused, can cause a flammable mixture to develop
in the air stripper (e.g. fan failure, damper failure, partial blockage).
4.12 Complexity of Systems
There may be considerable pressure both from environmental and
business sectors to increase the number of vents being treated by a single
thermal oxidizer. As the number of vents increases, the number of cases
to be considered may increase exponentially. This is reflected in the
increased amount of work needed during the design methodology
described previously.
Cost and business pressures often dictate that a single large thermal
oxidizer is installed rather than a number of smaller dedicated units. The
provision of two or more smaller VCDS may make the system inherently
safer due to the reduced complexity and lower number of possible failure
modes. When all factors including maintenance, availability and the cost of
down time are taken into account, the economics of a number of smaller,
independent systems may in fact be better than for a single large system.
4.13 Summary
The methodology described above is intended as guidance which can be
adapted to the particular requirements of any project. It does, however,
contain the basic steps which should be considered for the assessment of
potential incidents in the formal Hazard Study of a vent header system. It
is important that this work results in an auditable design trail.
It is essential that, wherever possible, vents should be eliminated at
source according to the principles of inherent SHE. This is not only
environmentally responsible but also good business practice as
vented material is wasted production which cannot be recovered and,
furthermore, end-of-pipe treatment is invariably expensive to design, build
and operate. If vents cannot be eliminated at source, they should be
reduced as far as possible or mitigated.

5 SAFE DESIGN OF VENT COLLECTION HEADER SYSTEMS
5.1 General
A typical vent collection and destruction system (VCDS), including a
thermal oxidizer, is shown schematically in Figure 1.
FIGURE 1 SCHEMATIC OF TYPICAL VENT COLLECTION AND THERMAL
OXIDIZER SYSTEM
All pipelines carrying potentially flammable liquids or gases have some risks
attached. These risks may stem from external factors such as corrosion or
impact damage or internal factors such as process composition changes or the
failure of a fan or pump. The risks can be minimized by good engineering design
as described in the following guidelines.
The quality of the design, maintenance and operation of the vent header system
is critical to the safety of the thermal oxidation unit, since many safety problems
with VCDS originate in the vent headers or upstream process plants.

5.2 Process Design of Vent Headers
5.2.1 Basis Of Safety
The principal requirement of the basis of safety for vent headers should be
control of the vent gas composition such that the header does not operate
in the flammable region during normal operation or abnormal situations.
This can be done properly only after systematically identifying,
characterizing and quantifying the vent streams using a rigorous
methodology such as that described in Section 4. This approach is based
on inherent safety and is important since there may be possible sources of
ignition in the vent header system itself (fans, pumps etc.) and there is, of
course, a permanent source of ignition in the thermal oxidation unit. Even
in the absence of obvious ignition sources in the header system there is
still a possibility of static electricity discharge, especially in non-conductive
or mixed conductive and non-conductive pipe work systems. The
probability of an ignition occurring may be low but cannot be assumed to
be zero. Operation in the flammable region could therefore result in an
ignition in the header leading to deflagration or detonation.
It can be difficult to design vent headers to have a sufficiently low
frequency of deflagration or detonation, particularly if the consequences of
such an event would be the rupture of a long vent header. Where a
header passes through a number of different plant areas, the domino
effects from the rupture of a header are potentially serious.
Notwithstanding the precautions taken to prevent vent headers operating
in the flammable region, process deviations, equipment failures or other
unforeseen circumstances may arise which result in the formation of a
flammable mixture within the system. Typical of such events are leakage
of air into the system from maintenance activities, process deviations on
start-up or shut-down or failure of instruments. These failures or
deviations, however unlikely, will have a finite potential frequency. Since
there is also some finite probability of ignition sources being present, it is
prudent to consider installing a second form of protection to further reduce
the possibility of a flame front propagating into the header pipe work or
other pieces of equipment with consequent hazards.
Secondary protective systems are not designed to provide continuous
protection against the permanent or extended presence of a flammable
mixture in the header but do provide protection for a limited period
enabling the system to be shut down safely or the flammable condition to
be removed.

Typical secondary protection systems are flame arresters or flame
suppression systems and shutdown or diversion systems actuated by
oxygen analyzers.
Flame arresters are designed to prevent the propagation of a flame along
a header from an ignition source. To be effective they should be placed as
close as possible to the source and should be considered for fitting in the
vent headers both upstream and downstream of any potential ignition
sources in the header itself and also immediately upstream of the thermal
oxidation unit. Positions of flame arresters in a typical vent header system
are shown in Figure 1. All flame arresters, including those on diversion
stacks or vents, should be fitted with high temperature trips or alarms to
warn of an ignition occurring in the system.
Some additional protection from flashback from the thermal oxidizer may
be provided by the velocity of the gas through the burner nozzle if it is
greater than the turbulent burning velocity. However, it is extremely
difficult to estimate turbulent burning velocities. There is also some doubt
as to the possibility of flame creep back along the walls of the burner
nozzle and back into the header. Thus this approach cannot be used as
the basis of safety against flashback.
It may be tolerable to operate very short sections of the header system in
the flammable region, if this condition is unavoidable, depending on the
hazard consequences and the probability of an ignition occurring. In this
situation, the length of line operating in the flammable region and the
probability of an ignition occurring should be minimized. An example of
this is the section of line at the exit from a scrubbing system or reactor
where dilution air or inert gas can only be injected after leaving the
scrubber or reactor thus creating a small flammable region prior to dilution.
In some cases it may be possible to design the plant to withstand
deflagration or detonation where this condition is known to exist.
The consequences of a detonation occurring in a line should be
considered very carefully with particular emphasis paid to possible injury
or plant damage from missiles. Domino effects by missile impact into other
pieces of plant and equipment, e.g. tanks holding toxic or flammable
materials, should also be taken into account.

Requirements of the basis of safety in addition to the avoidance of
flammable mixtures, include protection against internal and external
corrosion, mechanical impact damage, liquor logging and back pressures
that could adversely affect upstream plant. It may be necessary
to segregate vent streams in parallel headers in order to manage these
issues or even to install a number of independent, smaller VCDS.
5.2.2 Process Design Basis for Vent Collection Header Systems
Vent header systems should be designed to avoid the possibility of
flammable mixtures as described above in Section 5.2.1. For the initial
design, three main types of header should be considered: fuel-rich, fuel-
lean and inerted. Several branches may connect into each header at the
process plant end. It is, therefore, important to ensure that a flammable
mixture cannot result from the mixing of a fuel-rich vent from one branch
mixed with a fuel lean vent from another branch.
The design basis should also take into account the quantity of material
vented and the design pressure of the upstream process equipment. For
example, low pressure storage tanks may not have a high enough design
pressure to provide the necessary driving force for the required flow of
material down the vent header and hence a suction fan may be needed.
This may, however, introduce new hazards from pulling air into the system
or sucking the tank in.
Complications may also be introduced into the design by the presence of
high and low pressure vents and high and low temperatures, especially if
venting into the same header. For high pressure vents the possibility of
back flow and over pressurization or contamination of low pressure
sources (e.g. stock tanks) should be considered. High temperature may
cause damage to headers or take the mixture above its auto-ignition
temperature. High temperature and high pressure may also affect the UFL
and LFL. For further information on the change of flammability limits with
temperature and pressure see list of Best Contacts in Appendix B.
Separate header systems or additional processing equipment may be
required to avoid these issues. Low temperature (e.g. from vaporization of
liquefied gases) may cause condensation in the line and liquid logging or
even freezing. In cold climates it is often necessary to lag and heat trace
headers to prevent condensation or icing.

If VOCs do condense in the header during cold periods and then vaporize
on warming, the destruction unit can be overloaded. There are recorded
instances where this has occurred. In carbon steel headers embrittlement
and damage may result from sub-zero temperatures. (see Section 5.3.3).
5.2.2.1 Fuel-Rich Headers
Wherever practicable, fuel-rich headers should be operated under positive
pressure rather than under suction since a leak of gas to the atmosphere
will usually be less hazardous than an ingress of air which could possibly
result in the mixture becoming flammable. The exception to this is where
the material in the line is not only flammable but also toxic or highly
damaging to the environment; in which case the consequences of a
release should be considered carefully against the consequences of air
ingress.
The pressure of a sub-atmospheric header will probably have to be raised
above atmospheric at some stage upstream of a thermal oxidizer.
Therefore, it is generally better to provide the boost in pressure as far
upstream as possible in order to minimize the length of header subject to
possible air ingress.
A major consequence of a leak of non-toxic gas from a fuel-rich header
operating under positive pressure is likely to be a torching fire which may
impinge on other adjacent equipment. The possibility of consequential
ignition or damage to other equipment in this event needs to be
considered. With vent systems it is unlikely that sufficient gas will be
released to cause a significant fireball or flash fire; however, if the release
occurs in a confined space there is a risk of a confined explosion.
Significant overpressure is only generated when the flammable cloud has
a degree of confinement. Most vent headers run in unconfined areas so
the risk of a confined explosion is generally small. For assistance with
explosion and consequence modeling see Best Contacts in Appendix B.
5.2.2.2 Fuel-Lean Headers
Fuel-lean headers can be operated above or below atmospheric pressure
without increasing the risk of generating a flammable mixture through air
ingress. If the vent gas is toxic or particularly damaging to the
environment, then consideration should be given to operating at
sub-atmospheric pressure.

5.2.2.3 Inerted Headers
Inspection of the operating position on the flammability diagram is
necessary to determine the effect of air ingress into an inerted header.
Where it is possible for air ingress to result in a flammable mixture, the
header should, wherever practicable, operate above atmospheric
pressure.
5.2.3 Modifying Composition of Vent Headers
Wherever practicable, flammable mixtures should not be sent to the vent
collection system. If the vents arising from a plant or process are in, or
very close to, the flammable region then they should be made safe prior
to, or immediately after, entering the vent header system. Similarly, if it is
possible for a flammable mixture to be generated within a vent header by,
say, the mixing of fuel-rich and a fuel-lean vent streams or condensation
of VOCs in a fuel-rich stream, the possible consequences should be
evaluated and, where appropriate, corrective action taken. This can be
done in a number of ways (see 5.2.3.1 to 5.2.3.4):
5.2.3.1 Enriching
The vent gas may be enriched by adding fuel gas to take the composition
above the UFL. Some of the thermal oxidizer support fuel can be added in
this way. The amount required to make the vent "safe" should be
calculated based on the variability of the composition and flow and
possible air ingress rates. For reactive gases, such as those containing
significant quantities of acetylene, ethylene oxide, hydrogen etc., it is
difficult to specify an upper "safe" limit because of the size of the
flammable region. Hence, the flammability characteristics of
the gas mixture should be taken into account when specifying the
appropriate amount of enrichment.
5.2.3.2 Diluting
The vent gas can be diluted with air to below the LFL. From NFPA 69 a
value considered "safe" for this would be LFL/4 without composition
monitoring or up to 60% of the LFL with monitoring, but other values may
be appropriate on consideration of the factors described above. Operation
above LFL/4 with or without monitoring, should be considered very
carefully. This value is chosen because of the variability of process flows
and the difficulty of estimating compositions accurately for upset
conditions.

Reasons for the choice of factor should be described in the basis of
safety. The amount of confidence in the accuracy of flow and composition
data should be considered when making the decision.
It should be noted that, whereas application of the NFPA standards is
mandatory in the US and Canada and may also be mandatory in some
other countries and is strongly recommended for use throughout the
Americas, it may not be accepted in others. It is necessary to check with
local regulatory authorities before making a final decision.
5.2.3.3 Inerting
Inert gas can be injected into the vent in order to reduce the concentration
of oxygen in the header to below the minimum oxygen concentration
(MOC) to sustain combustion. NFPA 69 suggests a limit of 60% of the
MOC with monitoring or 40% of the MOC if the MOC is below 5%. If not
continuously monitored, the oxygen concentration should be checked on a
regular basis (see NFPA 69). Again, the variability of the vent flow and
composition should be considered along with the measurement accuracy.
There may be circumstances where it is appropriate to use a larger safety
factor such as 25% of the MOC depending on the variability of vent flows,
process deviations and confidence in the data. The reasons for the
choice of dilution factor should be detailed in the basis of safety. As
above, it should be noted that application of the NFPA guides may be
mandatory in some countries.
5.2.3.4 Combination of Vent Headers
Combining vent streams should be considered very carefully. Although
mixing vent streams to ensure operation outside the flammable region is
possible, the various combinations of flow and composition should be
quantified in detail as deviations in one or other of the streams may result
in the header becoming flammable [Ref. 17]. This method of ensuring
operation outside the flammable region is not generally recommended
unless there is a high degree of certainty about vent flows, compositions
and equipment reliability.

The interactions of the components in each of the vent streams, along with
the possible range of compositions, should also be examined to ensure
that there are no undesirable reactions or other consequences of the
combination.
Chemical reactions may occur in the header causing, for example,
corrosion, condensation or polymerization of material in the line. Using the
combination of vent headers as the basis of safety will result in a
significant amount of additional work in order to provide sufficient
justification and hazard quantification. An interaction matrix should be
used to check for undesirable interactions between the streams being
mixed.
Complex vent collection systems connecting several plants or units to a
common destruction unit may cause the propagation of an incident from
one plant to the others. It may, therefore, be preferable to have several
smaller systems instead of one large system. A consequence analysis
should be performed to consider the options. The benefits of scale for a
single, large vent header and destruction system may not be significant
when considered against the additional burden of design engineering
needed to ensure the safety of the system.
5.2.4 Summary
The decision on which of the above methods to apply depends on the
starting composition of the vent. In order to decide the best route for
altering the vent composition, the flammability diagram for the vent
composition should be considered. Flammability diagrams are described
further in Appendix C.
5.2.5 Flame Arresters
5.2.5.1 General
Flame arresters are designed to prevent the propagation of a flame front.
They are classified as a form of secondary protection and are effective for
a limited period before burn through or overheating occurs. Each arrester
is designed for a specific duty based on the composition, flow rate and
operating conditions in the line.
The presence of a flame arrester can provide time for the plant to be shut
down or the fault condition to be rectified before an incident occurs.

The design of flame arresters is a complex subject. Only a short synopsis
of the main features is provided here. For further advice on design and
specification of flame arresters see the list of Best Contacts in Appendix B.
5.2.5.2 Types
The most common type of arrester in use is the conventional crimped
metal type (such as supplied by IMI Amal in the UK and Enardo in the
USA) but other types are available including flat or perforated plate and
liquid seal. Arresters are designed for a specific range of duties. An
arrester designed to cope with potential detonation will be designed to a
more stringent standard than one designed for deflagrations. There are a
number of standards applicable to testing of flame arresters including ISO,
BS, Canadian, Underwriters Laboratories [Ref. 14] and US Coast Guard
[Ref. 15]. The USCG tests are reckoned to be the most stringent. It should
be noted that suitable approval will be needed for flame arresters before
installation in the USA (Factory Mutual or Underwriters Laboratories) and
some other countries. Again, local authorities should be consulted.
Plate type arresters are less common in use than crimped metal types and
are limited to the less reactive gases and therefore are not suitable for
mixtures containing hydrogen or acetylene. This type is made by several
manufacturers, particularly in the USA, including Protectoseal and
Westech.
Liquid seal arresters are less common but are useful when dealing with
gases containing particulates or mists. There are no known published
design methods for this type; however, empirical design procedures have
been used in GBHE. Under conditions of high gas flow the seal may break
down and a gas path exist through the arrester. This type of arrester
should not be specified without reference to GBHE.
Pebble bed arresters are another example of a type which was used
extensively in the past but is little used today. Again there are no known
design criteria for this type of unit.

5.2.5.3 Specification
Most arrester manufacturers have their own specification sheets which
should be filled in as far as possible for the enquiry. Information is
required on gas composition, flow rate, materials of construction and
acceptable pressure drop. The key design parameter is the minimum
experimental safe gap (MESG). This may have to be determined
experimentally for gas mixtures although it is known for many single
component gases.
The location of the arrester in relation to the ignition source is also
important as it affects the flame velocity and whether there is likely to be a
deflagration or detonation. Based on this specification and the
manufacturer’s knowledge of the performance of their own designs, a
flame arrester will be proposed. Much of the design knowledge for the
arresters is based on performance of actual units in operation and is not
available in the public domain.
Flame arresters are designed to stop deflagrations or detonations. The
latter are significantly larger, stronger and more expensive than the
former. The two types are not interchangeable. Deflagration arresters are
intended to stop relatively low velocity flames whereas detonation
arresters are designed for supersonic flame fronts and shock waves.
5.2.5.4 Pressure Drop
There is always some pressure drop across the arrester which varies with
the type and duty. Crimped metal and plate type arresters are designed
with larger cross sectional areas than the pipe in which they are installed,
partly to minimize the pressure drop and partly to reduce the flame speed.
Where more reactive gases are present and the gas flow channels in the
arrester smaller, the pressure drop will be higher. It should be noted that
flow through the arrester is likely to be laminar due to the large diameter.
For liquid seal arresters the pressure drop is dictated by the head of liquid
needed to make the seal. Typically the liquid level is in the region 300-400
mm. Some liquid may be lost via the overflow due to level swell and liquid
can be lost by vaporization to the vent gas. An adequate source of make-
up liquid is therefore required.
For some low pressure vent collection systems, the pressure drop may be
critical and should be discussed with the arrester manufacturer.

DESIGN OF VENT GAS COLLECTION AND DESTRUCTION SYSTEMS

DESIGN OF VENT GAS COLLECTION AND DESTRUCTION SYSTEMS

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DESIGN OF VENT GAS COLLECTION AND DESTRUCTION SYSTEMS