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 5.2.1.3 (29) a, 5.2.1.5 (35) a, 5.2.1.6 (38) e and 5.2.1.10(54, 59, 60).

General principles

Process plant can be subjected to excessive overpressure or underpressure due to:

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 instrumented systems?.

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 assume:

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 moment;
Further pressure generation by reaction in the relief piping is negligible.

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

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

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 instability;
The design must take into consideration differences between gas and liquid duties;
Back pressure can affect opening / closing pressures, stability and capacity;
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

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 disc installations.

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. These include:

Composition of gas load;

The composition of the gas load must be known with respect to:
- Solids loading, particle size distribution and chemical composition;
- 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 be provided.
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 include:
- 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.

Stack heights

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:

For a given stack height, the effective height of the emission can be determined by employing an appropriate plume-rise equation;
Application of atmospheric dispersion formula enables the downward path of the emission to be modelled. Various formulae may be employed. These include:
- Bosanquet-Pearson model;
- Gaussian model employing Briggs formulae;
- Wilson model
- Pasquill-Gifford model;
- Sutton model;
- TVA model.
Various software models are available to undertake these calculations. The most widely used in the UK is the ADMS model.

Factors affecting stack design include:
- Composition of waste gas (and changes in composition);
- Physical and chemical properties of waste gas;
- Topography (buildings, hills, lakes and rivers etc.);
- Seasonal changes in weather;
- Prevailing winds (direction and speed);
- Humidity;
- Rainfall

Flaring

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 dilution)

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 considered.

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.

Spot ventilation

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 extraction inlet;
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 pressure relief.
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 installations.
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 Association, 1998.
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, 1997.
API RP 526 Flanged Steel Safety Relief Valves, Fourth Edition, 1995.
API RP 527 Seat Tightness of Pressure Relief Valves, Third Edition, 1991.
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 Institution.
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 Institution.
Part 2 : 1984 Specification for safety valves for compressed air or inert gases.
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.