Relief systems / vent systems
This Technical Measures document refers to codes and standards applicable
to the design of relief and vent systems.
Related Technical Measures documents are:
The relevant Level 2 Criteria are 220.127.116.11
(29) a, 18.104.22.168 (35) a, 22.214.171.124
(38) e and 126.96.36.199(54, 59, 60).
Process plant can be subjected to excessive overpressure or underpressure
- External fire;
- Process abnormality or maloperation;
- Equipment or service / utility failure;
- Changes in ambient conditions;
- Excess chemical reaction.
To achieve a more inherently safe design, and to arrive at the most
economical solution overall consideration should always be given to:
- Can the overpressure or underpressure hazard be eliminated by changes
in process or plant design?;
- Can the overpressure or underpressure hazard be reduced by reducing
inventories or changing process conditions?;
- Can the overpressure or underpressure be contained by designing
equipment to withstand maximum feasible pressure?;
- Can alternative protection to a relief system be considered?;
- Can the required relief system be minimised by mechanical or
Explosion Relief is considered in a
separate Technical Measures Document. Relief systems considered in this
document are based on systems where pressure rise occurs over several
seconds or longer, and there is no reaction front. In these cases we may
- Safety valves can open in time;
- Piping is adequately sized to provide pressure relief;
- Relief flow may be determined by steady-state flow equations;
- Conditions are approximately uniform throughout each phase at any
- Further pressure generation by reaction in the relief piping is
General principles applicable to relief systems include:
- In all cases, relief devices must be selected and located to minimise
disturbance to plant and environment;
- Relief devices must not be isolated from equipment they protect while
the equipment is in use;
- The discharge from a relief device should pass to a safe location
which may be:
- A dump tank;
- Upstream in the process;
- A storage tank;
- A quench vessel or tower;
- A sewer;
- The atmosphere;
- A knockout drum;
- A scrubber;
- An incinerator;
- A flare stack.
Design basis and methodology of all relief stream packages must be
documented, and incorporated into plant modification and change procedures
to ensure that relief stream invalidation does not occur.
Sizing of vents (especially exothermic reactions, storage tanks)
One of the biggest problems in sizing vents is the availability and
accuracy of physical property data for the reaction components. It is good
practice when sizing a relief system to utilise several design methods to
achieve consistency in design.
When sizing pressure / vacuum relief systems for storages, if several
tanks are connected up to a single relief system the relief device should be
capable of accommodating the simultaneous vent loading at a relieving
pressure less than the lowest tank design pressure.
Venting can either be normal or atmospheric venting or emergency venting.
Different measures may be adopted to provide protection for the vessel or
tank in each case. The worst case scenario is generally experienced when
tanks are exposed to fire.
Normal venting requirements may be met by installation of pressure-vacuum
relief valves. Emergency venting may be accomplished by installation of a
bursting or rupture disc device. Depending upon the tank contents and the
physical characteristics of these contents consideration should be given to
the vent discharge point and configuration. Guidance is provided in
recognised industry standards.
There are various recognised methods for sizing vents. These include:
- API Methods;
- NFPA Methods;
- Vapour / Gas Only method;
- Leung’s method;
- Level Swell method;
- Stepwise method;
- Nomogram method;
- Fauske’s method;
- Two-phase method;
- DIERS method;
- Huff’s method;
- Boyle’s method.
The use of DIERS (Design Institute for Emergency Relief Systems)
methodology is becoming increasingly widespread. Detailed analysis of relief
systems using this methodology, together with experimental testing, is now
the accepted practice.
Flame arresters are commonly installed on the vent outlet of tanks
containing liquids with flashpoints below 21°C, generally where
pressure-vacuum vent valves are not in use. Their prime function is to
prevent the unrestricted propagation of flame through flammable gas or
vapour mixtures, and secondly to absorb heat from unburnt gas.
Flame arresters should be designed for each specific application, and due
to the likelihood of progressive blockage a rigorous inspection and
maintenance schedule should be in place.
Relief valves are characterised by:
- Slow response times (tenths of a second up to > 1 second);
- Risk of blockage;
- Trace leakage.
Design considerations for relief valves include:
- The pressure drop before the safety valve must be low to avoid
- The design must take into consideration differences between gas and
- Back pressure can affect opening / closing pressures, stability and
- The relief valve usually solely determines relief capacity if
appropriate piping is used.
Regular proof checks are required to check lifting pressure, particularly
if located in a corrosive environment. Also valve seating checks should be
undertaken to ensure that the valve is not passing.
Bursting discs are characterised by:
- Very fast response times (milliseconds);
- Less risk of blockage than relief valves;
- Cheap to install and maintain;
- Available in a wide range of materials;
- No leakage;
- Non re-closing hence may allow large discharges even when pressure
falls below relieving (rupture) pressure;
- Potential for premature failure due to pressure pulsation, especially
if the rupture pressure is close to the operating pressure;
- Rupture pressure affected by back pressure;
- Risk of incorrect assembly.
Design considerations for bursting discs include:
- Protection against reverse pressure (vac dials);
- Differences between disc temperature and vessel temperature;
- Main factor affecting relief capacity is piping configuration;
The rupture pressure of a bursting disc is a function of the prevailing
temperature. It is common practice for an operator to specify the required
rupture pressure at a specific operating or relieving temperature however,
if the temperature cycles or changes during the process operation the degree
of protection of the vessel can be compromised. This is because as the
prevailing temperature decreases the rupture pressure of the bursting disc
will increase potentially resulting in the rupture pressure at temperature
being greater than the design pressure of the vessel. Thus if the pressure
increases at this condition, vessel failure will occur. The converse case
can also apply if the rupture pressure is quoted for ambient temperatures,
since the actual rupture pressure will decrease under normal operating
conditions which can cause premature failure of bursting discs.
The surrounding vent pipework should be adequately sized to accommodate
relief flows in the event of bursting disc failure.
Bursting discs are a common method for fulfilling emergency venting
requirements, although a routine maintenance programme should cover bursting
Bursting disc installations should incorporate vent pipework that is the
same diameter as the bursting disc itself.
Combinations of bursting discs and relief valves are occasionally
employed for specific applications. Double bursting discs (back to back
arrangements) are often provided with a pressure indicator/alarm between
them in aggressive environments where failures of the initial disc may
occur. In such instances the second bursting disc is reversed to withstand
the initial shock pressure.
Scrubbers (design for maximum foreseeable flow)
In many installations, scrubbing systems provide one of the major lines
of defence against release of toxic gas. Several key factors must therefore
be taken into consideration when designing and sizing the scrubbing system.
- The composition of the gas load must be known with respect to:
- Solids loading, particle size distribution and chemical
- Water vapour loading;
- Toxic gas loading;
- Inerts loading.
- Variations in gas load;
The basis of the scrubber design should take into consideration the
peak gas loading, the minimum gas loading and the mean gas loading in
addition to corresponding variations in inert gas loading.
- Depletion / saturation of scrubbing liquor;
Analysis of the reaction stoichiometry between the gas and the
scrubbing liquor will give some indication of the minimum scrubbing
liquor strength at which the absorption process can occur for a
recirculatory system. A methodology must be in place that ensures
replenishment of the scrubbing liquor at an appropriate point. Hence
monitoring of depletion of scrubber liquor and modelling of
breakthrough concentrations is critical. Furthermore, the process may
specify a maximum concentration of absorbed gas in the scrubbing
liquor at which the scrubber liquor should be replenished.
- Provision of Back-up systems;
In the vent of scrubber failure, it is sometimes possible to isolate
plant and process to prevent toxic gas emission by implementation of
appropriate interlocks and control systems. However, if temporary
isolation of plant and process is unfeasible a back up system should
- Control systems;
The control system for the scrubber operation should be interlocked
with the plant and processes that the scrubber services such that in
the event of scrubber failure process operations can be isolated and /
or suspended. The control system should feature scrubber diagnostics
that verify and indicate that the scrubber is healthy and working.
- Monitoring and instrumentation;
Typical instrumentation on a toxic gas scrubbing system should
- Stack gas analyser;
- Scrubbing liquor flow indicator;
- Scrubbing liquor tank level indication;
- Flow indication or DP instrumentation across scrubbing fan;
- Process interlocks for event of scrubber failure.
The concentration of waste gases at ground level can be reduced
significantly by emitting the waste gases from a process at great height,
although the actual amount of pollutants released into the atmosphere will
remain the same.
The basis for design begins with determination of an acceptable
ground-level concentration of the pollutant or pollutants. If the waste gas
is to be discharged through an existing stack, or the stack size is
restricted the ground-level concentration should be determined and if it is
unacceptable appropriate control measures should be adopted. Steps in the
design methodology include:
Flaring may be used to destroy flammable, toxic or corrosive vapours,
particularly those produced during process upsets and emergency venting.
Key design factors to ensure flare safety and performance include:
- Smokeless operation;
- Flame stability;
- Flare size and capacity;
- Thermal radiation;
- Noise level;
- Reliable pilot and ignition system;
- Flashback protection.
The major safety issues are the latter two items. BS
5908 : 1990 recommends that permanent pilot burners should be provided
with a reliable means of remote ignition. An additional means of ignition,
e.g. a piccolo tube should be provided, independent of power supplies. Flare
header systems should be provided with an inert gas purge sufficient to
provide a positive gas flow up the stack to prevent back diffusion of air.
Forced ventilation (especially to control direction of flow and
Non-pressurised systems in which fumes and vapours are generated should
have adequate ventilation to remove those fumes to a safe place. This may be
a scrubber or a stack for discharge. Consideration should also be given to
the venting of discharges from relief systems. Both dedicated enclosed
forced ventilation systems and area forced ventilation will need to be
A further purpose of ventilation is to dilute and remove the hazardous
substance to such an extent that the concentration in the protected space is
kept to acceptable levels. Ventilation rates are generally designed to
reduce the concentration to about one quarter of these levels.
The use of forced ventilation has an impact on the area extent and
classification of hazardous areas. The methodology for assessment of type
and degree of ventilation is covered in British Standards. Although mainly
applied inside a room or enclosed space, forced ventilation can also be
applied to situations in the open air to compensate for restricted or
impeded natural ventilation due to obstacles.
General ventilation is applied to the room or compartment as a whole (see
forced ventilation above). It may also be applied locally to the plant or
process as spot or local ventilation. Basic design principles include:
- Fume extraction inlet should be as close to the source of gas or
vapour as possible;
- The rate of extraction of fume should be greater than or equal to the
rate of generation of fume in the particular area;
- Air supply inlets should be located to provide ventilation for other
regions that may become contaminated;
- General air movement should be from areas surrounding the emission
source, across the contaminated zone and thence through the fume
- A velocity of 0.5 to 2 m/s is generally recommended (Lees 25.7).
Trunking is often used to allow operators to move the point of
extraction as required.
Special cases: chlorine, Lpg storage
In the event of overpressure in liquid chlorine storage tanks, the
discharge line from the pressure relief system should enter a closed
expansion vessel with a capacity of nominally 10% of the largest storage
vessel. This expansion vessel should then be manually relieved at a
controlled rate to an absorption system. Further information concerning bulk
chlorine storage relief systems is provided in HS(G)28.
In the event of overpressure of LPG storage tanks, the tank should be
fitted with a pressure relief valve connected directly to the vapour space.
Underground or mounded vessels affect full flow capacity of pressure relief
valves. Further information concerning LPG storage relief systems is
provided in LPGA Cop 1.
In the event of overpressure in anhydrous ammonia storage tanks, the tank
should be protected by a relief system fitted with at least two pressure
relief valves should be fitted. Further information concerning anhydrous
ammonia storage relief systems is provided in HS(G)30.
Status of guidance
Although existing guidance provides reasonably comprehensive information
for the sizing and design of basic relief systems, more complex relief
system applications – for example with polymerisers – are not
specifically covered by guidance.
Guidance and Codes of Practice relating to relief and vent systems
- HS(G)176 The storage of flammable liquids in
tanks, HSE, 1998.
Paragraphs 115 to 123 give guidance on venting, emergency venting and
- HS(G)50 The storage of flammable liquids in
fixed tanks (up to 10000 cu. m in total capacity), HSE, 1990.
Replaced by HS(G)176, 1998.
- HS(G)158 Flame arresters : preventing the
spread of fires and explosions in equipment that contains flammable
gases and vapours, HSE, 1996.
- HS(G)11 Flame arresters and explosion
reliefs, HSE, 1981.
Replaced by HS(G)158.
- HS(G)28 Safety advice for bulk chlorine
installations, HSE, 1999.
Paragraphs 120-132 provide guidance on relief systems for bulk chlorine
- HS(G)30 Storage of anhydrous ammonia under
pressure in the UK : spherical and cylindrical vessels, HSE, 1986.
Paragraphs 30 to 32 give guidance on pressure relief systems.
- LPGA COP 1 Bulk LPG storage at fixed installations. Part 1 : Design,
installation and operation of vessels located above ground, LP Gas
Supersedes HS(G)34 Storage of LPG at fixed
installations, 1987, HSE.
Part 1, section 3.1.10 gives guidance on the provision of pressure
relief valves on storage vessels.
Part 1, section 3.7.6 gives guidance on the provision of pressure relief
valves on vaporisers.
Part 1, section 3.2.4 gives guidance on the provision of hydrostatic
relief valves on pipework where LPG may become trapped ( e.g. between
shut-off valves and blank flanges).
- HS(G)34 Storage of LPG at fixed
installations, HSE, 1987.
Superseded by the above LPGA COP 1.
Paragraphs 60 to 65 give guidance on pressure relief systems.
- API RP 520 Sizing, selection, and installation of pressure relieving
devices in refineries
Part I – Sizing and Selection, 1993.
Part II – Installation, 1994.
- API RP 521 Guide for pressure-relieving and depressuring systems,
- API RP 526 Flanged Steel Safety Relief Valves, Fourth Edition, 1995.
- API RP 527 Seat Tightness of Pressure Relief Valves, Third Edition,
- API Std 2000 Venting atmospheric and low pressure storage tanks:
Nonrefrigerated and refrigerated, 1998.
- API RP 2521 Use of pressure-vacuum vent valves for atmospheric Loss,
First Edition, 1966.
- BS 2915 : 1990 Specification for bursting
discs and bursting disc devices, British Standards Institution.
- BS 5500 : 1997 Design of pressure vessels,
Appendix J, British Standards Institution.
- BS 5720 : 1979 Code of practice for mechanical ventilation and air
conditioning in buildings, British Standards Institution.
- BS 5908 : 1990 Code of practice for fire
precautions in the chemical and allied industries, British Standards
Section 7, Paragraph 43.3.3 Flare Stacks provides guidance on ignition
and prevention of flashback in flare stacks.
- BS 5925 : 1991 Code of practice for ventilation principles and
designing for natural ventilation, British Standards Institution.
- BS 6759 Safety Valves, British Standards
Part 2 : 1984 Specification for safety valves for compressed air or
Part 3 : 1984 Specification for safety valves for process fluids.
- BS EN 60079-10 : 1996 (ºIEC 60079-10
: 1995) Electrical apparatus for explosive gas atmospheres. Part 10 :
Classification of hazardous areas, British Standards Institution.
Further reading material
- Parry, C.F., 'Relief Systems Handbook', Institution of Chemical
Engineers, Reprinted 1994.
- Kumar, A., 'Design and Operate Flares Safely', The Sapphire Group,
Chemical Engineering Magazine, Environmental Manager, December 1998.
- ASME, 'Recommended Guide for the Prediction of Dispersion of Airborne
Effluents', ASME, New York, 1968.
- FPA, ‘Flammable liquids and gases: Ventilation’, FS6013, Fire
The data sheet indicates that ventilation rates should be calculated so
as to reduce concentrations to about a quarter of the lower explosive
- Singh, J., 'Sizing Relief Vents', Hazard Evaluation Laboratory (Fire
Research Station Site), Chemical Engineering, 97, 8, p104, August 1990.
- Lees, F.P., 'Loss Prevention in the Process Industries: Hazard
Identification, Assessment and Control', Second Edition, 1996.
Case studies illustrating the importance of relief systems / vent