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 (29) a, (35) a, (38) e and, 59, 60).

General principles

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

To achieve a more inherently safe design, and to arrive at the most economical solution overall consideration should always be given to:

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:

General principles applicable to relief systems include:

  • 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:

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:

Design considerations for relief valves include:

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:

Design considerations for bursting discs include:

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:

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:


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:

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:

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

Further reading material

Case studies illustrating the importance of relief systems / vent systems