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Pressure relief

 

Definition

In process and mixing technology, pressure relief refers to a design-based explosion protection measure used to control and specifically vent any sudden build-up of excess pressure inside a piece of equipment – such as a mixer.

The aim is to preserve the mechanical integrity of the vessel in the event of a dust explosion or an impermissible rise in pressure, and to protect people, the plant and the surrounding area from the destructive consequences, without having to design the vessel to be fully pressure-resistant against the entire explosion pressure.

In practice, pressure relief is achieved via specialised relief devices such as burst discs, explosion dampers or flameless relief systems, which open when a defined set pressure is exceeded and divert the pressure and flame front to a safe area.

 

Key features

Structural, passive explosion protection

Pressure relief forms part of tertiary explosion protection: The explosion is not prevented, but its effects are limited to a controllable, reduced explosion overpressure p. The system operates automatically, without electronic triggering.

Pressure relief devices

Rupture discs are single-use, gas-tight membranes that rupture at a precisely defined set pressure and instantly vent the excess pressure to the outside. They are simple in design and require very little maintenance.

Explosion dampers or pressure relief dampers are resealable components that open in the event of overpressure and can usually be returned to their operating position afterwards. They are particularly suitable for standard-fit installations, such as those in the animal feed and agricultural industries.

Flame-less pressure relief combines a relief opening with a special filter, such as Q-tubes or flame-less relief valves. The pressure is relieved, whilst flames and hot gases remain inside. This is particularly important for installations in production halls, cleanrooms or wherever discharge to the outside is not possible or permitted.

Defined response pressure and design

Pressure relief elements open at a specified response pressure, typically in the range of approximately 0.1 to 0.5 bar gauge pressure, depending on the product and system design. The required relief area is calculated in accordance with DIN EN 14491 and the relevant ATEX requirements, based on:

  • vessel volume
  • dust parameters such as the Kst value and pmax
  • maximum permissible reduced explosion pressure pred,max

Compliance with Directives and Standards

In Europe, when processing combustible dusts, design in accordance with the ATEX Directives 2014/34/EU and 1999/92/EC, as well as the relevant standards, is mandatory. Pressure relief is a key element of a standards-compliant explosion protection concept.

Hygiene and GMP design

In the food and pharmaceutical industries, pressure relief systems are designed to prevent the formation of dead spaces where product residues can accumulate, and to ensure that surfaces are easy to clean and, in many cases, suitable for CIP and SIP. This ensures compliance with hygiene standards and supports adherence to GMP requirements.

Low-maintenance and combinable

Rupture discs and dampers are generally low-maintenance, but must be checked regularly for integrity and functionality. Pressure relief can be effectively combined with other explosion protection measures, such as spark detection and suppression, explosion decoupling via quick-closing valves or non-return valves, and inerting.

 

Significance for mixers used in powder blending

Mixers frequently process fine, dry powders such as starch, flour, sugar, milk powder, pigments, plastic additives, metal powders or active pharmaceutical ingredients. Many of these substances form explosive dust-air mixtures when combined with air.

Typical ignition sources include electrostatic discharges, mechanical friction, heated surfaces or metallic foreign bodies. If ignition occurs, the pressure inside the mixing vessel rises dramatically within milliseconds.

Special considerations for mixers:

  • a large free gas volume above the mixed material, in which dust clouds can form – this increases the risk of a dust explosion
  • complex internal components such as mixing blades, shaft seals and shredders, which can be severely damaged or destroyed in the event of uncontrolled overpressure
  • frequent installation indoors with staff movement and adjacent plant, which means that open flames must be avoided at all costs during conventional pressure relief

A professionally designed pressure relief system ensures that:

  • the mixing vessel remains structurally intact and does not rupture
  • the explosion energy is directed into a safe area, either to the outside or into a flameless relief system
  • expensive internal components and downstream plant parts are protected from severe damage
  • the reduced explosion overpressure (pred) remains within the permissible load limits of the vessel and steel structure

If flammable liquids, solvents or vapours are introduced into the powder, a hybrid mixture is present. The required pressure relief areas are then larger than for pure dust, and venting to the open air is often not permitted – flameless systems or inerting must be considered in this case.

 

Typical applications and industry examples

Food industry

When mixing flour, sugar, cocoa, starch or milk powder, pressure relief is practically standard, as these powders usually have relevant Kst values and thus a significant explosion potential.

Chemical industry

When homogenising plastic granules, pigments, additives or intermediate products, pressure relief protects both the operating personnel and the mixing and conveying systems in the vicinity.

Pharmaceutical production

In GMP-compliant mixers, pressure relief is frequently combined with flameless relief systems to prevent flames from entering cleanrooms and product discharge into the open air, thereby maintaining cleanroom classification.

Metal powder processing

In the case of highly reactive dusts such as aluminium, magnesium or titanium powders, pressure relief is often combined with inerting and additional monitoring systems to manage the high risk of explosion.

Animal feed and agricultural industry

Mixers for premixes and animal feed components are often fitted with explosion relief valves as standard to meet the requirements of the Industrial Safety Regulation and the rules of the employers’ liability insurance associations.

 

Customer benefits

  • Personal safety: Minimisation of the risk of injury to operators and those in the vicinity through limited pressure and flame effects.
  • Plant protection and operational continuity: In the event of an incident, damage is essentially confined to the pressure relief element, rather than destroying the entire mixer or the building. This significantly reduces downtime and facilitates a rapid return to operation.
  • Legal certainty and insurance benefits: Compliance with the relevant ATEX and standard requirements supports the legally compliant design of the plant and is recognised by insurers as an accepted explosion protection measure.
  • Investment security: Standard-compliant, hygienic and long-term, tried-and-tested pressure relief solutions increase the service life of the plant and protect the investment.

The specific design of the type, number and size of the pressure relief elements is always project-specific. It is based on the product characteristics, the process parameters, the installation conditions and the permissible loads on the vessel and building structure, and should be carried out by experienced explosion protection or process engineers.