Elevators are used in many industries to convey large quantities of bulk materials vertically. Organic bulk materials can form hazardous explosive atmospheres in the enclosures, which can lead to a dust explosion in the presence of an effective ignition source. This article provides an overview of the measures required for comprehensive explosion safety concepts.

Conveyor systems are machines used to move bulk materials in many industries. Examples of mechanical conveying elements (continuous conveyors) are belt conveyors, chain conveyors, vibratory conveyors, screw conveyors or bucket elevators. Bucket elevators, also known colloquially as elevators, are conveying systems that convey bulk material vertically at high transport speed up to a height of about 60 metres. This type of conveyor has been selected as an example in this article, as high hazards can emanate from this type of equipment. Screw and chain conveyors are considered "less at risk of explosion" during normal operation due to the high material concentrations inside the machine, the horizontal position and the lower conveying speeds [1].

If the bulk materials are flammable and thus explosive, an appropriate safety concept must be drawn up for the plant. Examples of such bulk goods are grain, malt and sugar (food industry), coal (cement industry, power plants) and wood (chipboard plants). Mineral bulk materials such as cement or sand are not combustible and therefore not explosive. 

A dangerous explosive atmosphere inside an elevator cannot be excluded - even during normal operation as intended. If an effective ignition source, such as a glowing ember introduced by the upstream processes, hot surfaces due to belt and bucket friction, hot bearings or foreign bodies, is present in time and space, this can lead to a dust explosion (see Figure 1).

In the case of elevators, the risk of explosion depends to a large extent on the bulk material conveyed and its dusting behaviour. Particularly at loading, transfer and discharge points, it is to be expected that the dust concentration of a combustible dust will exceed its lower explosion limit. In addition, an accumulation of particularly fine dust is to be expected in the foot and head section of the elevator. This dust can be much more sensitive to ignition and react more violently than the total fraction of dust conveyed in the elevator. The risk assessment should also include idling, start-up and shut-down operations as well as malfunctions and maintenance [2].

Preventive explosion safety

In explosion safety, the following methodical order of priority has been proven:

Preventing the formation of combustible dust deposits or explosive dust/air mixtures and 

Avoiding the ignition of combustible dust deposits or explosive dust/air mixtures (preventive explosion safety)

Reducing the effects of a fire or explosion event (explosion protection)

Safety concepts and measures can be both technical and organisational. Organisational measures include, for example, a ban on smoking, the use of prescribed equipment, cleaning procedures at the workplace, preventive maintenance of equipment and regular instruction of operating personnel [3]. 

Preventive technical measures include, for example, substitution of explosive substances, inerting, reduction of dust concentration and avoidance of ignition sources. 

The following are to be considered in particular as potential ignition sources for elevators [2]:

Hot surfaces due to heating of the belt, the buckets, the belt pulleys on the housing

Belt slippage

Overheating of bearings

Friction and impact processes due to introduced foreign bodies

Friction and grinding sparks due to the impact of buckets 

Electrostatic discharge processes with insufficiently grounded conductive parts

Tearing of additionally conveyed smouldering material or glowing embers when scooping or dropping the product

The following measures have proven effective as protective measures to avoid effective ignition sources [2]:

Installation of speed monitor and belt misalignment monitoring

Use of anti-static belts 

External bearings 

Monitoring of the bearing temperature

Avoidance of non-conductive coatings 

Gap width > 25 mm between moving and stationary parts

Limitation of the conveying speed

Whether an elevator can be secured exclusively with preventive protective measures depends on various boundary conditions, e.g. safety-related parameters and design, operation and arrangement within the production process. 

The risk of explosion depends on how frequently explosive atmospheres occur in the long term. The probability of the occurrence of explosive atmospheres and the extent of the protective measures to be taken are based on a classification of potentially explosive atmospheres into Zone 20 (frequently), Zone 21 (occasionally) and Zone 22 (normally not) [4]. The design and selection of electrical and non-electrical equipment of elevators is based on the zone classification. 

Explosion prevention is useful, but often insufficient as a protective measure in the case of elevators, since hazardous explosive atmospheres are difficult to prevent and ignition sources cannot be excluded.

Explosion protection

Explosion protection measures must be implemented in this context if the above-mentioned prerequisites for an explosion cannot be safely avoided. The following measures will limit the effect of an explosion to a harmless level [5]:

Explosion-resistant construction

Explosion venting

Explosion suppression and additionally

Explosion isolation (from flame and pressure) 

As an example of a safety concept, this article presents explosion venting in combination with suppression as explosion isolation. Experience has shown that explosion-resistant construction is rarely used for elevators due to the material thickness (weight) required to withstand an explosion pressure at a typical height of 8-10 bar during a dust explosion.

Due to the geometric conditions of the elevators, the equations from the standards cannot be used for a calculation. Examples of the design of explosion venting areas are given in VDI Sheet 8.1. [6]. The fundamentals are based on practical large-scale tests carried out within the framework of a research project under the auspices of the Research Centre for Applied System Safety and Industrial Medicine (FSA) in the period 2007-2010 [7].

Explosion venting neither prevents nor extinguishes an explosion, it merely limits the explosion overpressure. Flames and pressure effects outside the elevator as well as ejected parts must be expected and appropriate protective measures must be considered. Fires can also break out inside the housing [8]. As an alternative to a "free" venting of the explosion pressure with explosion vents, flameless explosion venting systems have proven themselves in practice. 

Figure 3 shows an explosion vent and a flameless explosion venting device (Q-Ball E). The Q-Ball E flameless venting system was specially developed for elevators and has an integrated explosion vent for explosion venting and a special metal mesh that cools the flame gases, which can reach temperatures of up to 1500°C, so that neither flames nor pressure escape. Both protective systems have a signalling system that provides reliable monitoring and can force an emergency stop of the production line in the event of an explosion.

The explosion protection measures (flameproof design, explosion venting as well as explosion suppression) must be combined in each case with explosion isolation from upstream and downstream plant components. If present, pipework to dust collection systems should also be considered. 

The infrared sensors of the explosion isolation system Q-Bic register and extinguish an incipient explosion with extinguishing powder within milliseconds (chemical barrier). The special nozzle system SJX distributes the extinguishing powder optimally in the housings. The triggering mechanism in the extinguishing powder bottle works without explosive ignition, unlike other systems.

The new generation of the EXKOP controller offers the possibility of remote diagnostics of the protective system. Data transmission - after approval by the customer - can be carried out via the GSM module, Internet or classically via USB connection. A visualisation of the flow diagram of the process plant can be stored in the touch screen display to enable intuitive operation. Numerous languages are already stored in the factory. Another advantage is the bus system of the control system. In older versions, classic relays were used, so that now only a single-core cable is required for the systems, which in practice can be several hundred metres long. 

Summary

Using the example of bucket elevators, also known as elevators, a methodical approach was presented to design a ”state of the art” safety concept. Explosion protection measures must be taken if explosive atmospheres are present inside elevators during operation and effective ignition sources cannot be excluded. As an example, the flameless explosion venting device Q-Ball in combination with explosion isolation Q-Bic (explosion suppression) was presented. Due to its architecture, the new generation of the EXKOP controller in the active protective system offers the possibility of remote diagnostics and works with a modern bus system.

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This article was published under the same title in the conference proceedings of the 24th Fachtagung Schüttgutfördertechnik 2019 "Digitalisierung in der Schüttgutfördertechnik" in Magdeburg under ISBN No. 978-3-947068-06-7 published by LOGiSCH-Verlag, pp. 205-214.

[1] Associations DRV, VDM, DVT, BVA: Guide to Explosion Protection in the Grain and Feed Industry based on the Ordinance on Industrial Safety and Health, Bonn 2005

[2] ISSA, Collection of examples “Dust explosion prevention and protection for machines and equipment”, ISBN 978-92-843-7182-2, Mannheim 2014

[3] VDMA, Guide on dedusting systems – Fire and explosion protection, Frankfurt am Main, 2019

[4] DGUV 113-001, Explosion protection rules, item number 41257031, 2019

[5] TRBS 2152 Part 4, Explosive atmosphere - Constructive protection measures, which limit the impact of an explosion to an acceptable level, February 2012

[6] VDI guideline 2263 Part 8.1, Fire and explosion protection on elevators; March 2011

[7] Research Centre for Applied System Safety and Industrial Medicine FSA, Constructional Explosion Protection for Elevators, Project No. F05-0701, Mannheim 2010

[8] DIN EN 14491, Dust explosion venting protective systems, October 2012