Water Utilities - Understand & mitigate risk for dryers in your process

Through the thorough investigation of multiple industrial accidents and near misses over the course of the last few decades, a larger focus has been applied to mitigating potential hazardous risks when utilising dryers in processes. Although a potentially hazardous operation for multiple industries, drying is an important operation in many processes. Drying processes alternate between industries, but it largely focuses on washing, drying, cooling, heating, evaporation and desalination. These actions are then utilised by the likes of conditioning systems, food processing plants, chemical plants and water and utility plants.

Dryers are subject to high levels of potential risk, as explosions and fires in dryers and other associated processing plants have been a constant global occurrence for multiple decades. When considering dryers in plant design, the potential for combustible dust and powder fires and explosions, and/or the drying material itself to potentially lead to ignition in dryers cannot be overlooked. What’s more, dryers are subject to high levels of dust, water and particulate emissions, which can also lead to hazardous conditions. Materials can also self-heat leading to ignition and slow burns which may accelerate due to the addition of air into the plant. According to the Dust Safety Science 2021 Combustible Dust Incident Report Summary, ‘Dryers had the largest number of injuries in the first half of 2021 with four injuries in a pharmaceutical dryer explosion, one injury in a grain dryer explosion and one injury in a wood pulp and flower dryer explosion.’ Furthermore, according to The Accident Database, 89 dryer related accidents between 1965-2000 accounted for 415 injuries and 16 fatalities. 

Although the use of dryers is vital in the production of chemicals, pharmaceuticals, and other products, they continue to create potentially hazardous incidents unless they have been properly mitigated. This article discusses a few key components that can be used to develop a strategy to mitigate dryer hazards and minimize the potential risk for fire and explosion hazards that pose a significant threat to business, people, workplace and the environment.

What are industrial dryers? (A very basic overview)

There are a large variety of dryer types that are used in varying processes by multiple industries. In water treatment facilities, for example, sludge dryers are used to dry water treatment sludge in order to remove all of the contaminants. The material to be dried is mixed with air or other gaseous materials for drying. The dryer may consist of an engine-driven blower and fans that blow heated air through the mix, thereby removing moisture from it; this process converts bond to solution (to form a slurry) which dries the material. The dryer may also use heat pipes to remove moisture from the materials in question. Sometimes there are optional drying units such as vacuum ovens that further dry materials after they have passed through the blower system. 

There are two main variations of batch dryers – continuous or batch. Batch drying is a system at which a certain volume of material is dried at a time. The volume is fixed by the holding capacity of a dryer and dried to the required moisture. The material is removed before the next batch of material can be dried. Batch drying, therefore, requires constant loading and unloading. Continuous drying, as the name suggests, continuously flows material through the dryer without stopping. In most cases, large quantity operations require the use of continuous dryers and the large rates of the solid dry product create the potential for both explosion and fire. 

The most common types of continuous dryers include the following: 

  • Spray dryers
  • Fluid Bed dryers
  • Tray dryers
  • Vacuum dryers
  • Flash dryers
  • Rotating / drum dryers
  • Filter dryers
  • Belt dryers
 

Different drying machinery, material uses and practices present varying hazards, so, for both operational reasons and to help produce quality end-products, distinctive safety directives must be installed to ensure that no accidents or near-misses occur during production processes.

What are the hazards accociated with drying processes? (A very basic overview)

Drying processes are generally hazardous and failure to correctly ensure the safety of dryers in operations may lead to catastrophic events such as loss of life, damage to business and the environment via fire, explosion, and air pollution. Although a large chain of unforeseen circumstances may lead to an industrial-related accident, two major areas of focus for any business is the improper handling of drying materials and related industrial equipment. These two focus areas can then be split into the below sub-categories, of which we have provided a few examples of potential deviations:

  • Fire hazards – industrial dryers utilise flammable gases, oils and solvents to displace oxygen in storage during the drying process. This may cause fires during the application of combustion products or when backup power is accidentally activated.
  • Explosion hazards – when drying material is exposed to high temperatures, it may undergo chemical reactions that produce heat or gas pressure that can lead to an explosion.
  • Chemical hazards – during the unloading of drying materials, solvents may be released as a result of chemical decomposition or degradation.
  • Release of flammable vapours/solvents and organic heat transfer fluids
  • Overheating – the creation of spontaneous heating and autoignition of the solid being processed.
  • Sparks – electrostatic, friction or electrical.
  • Discharge of hot product to downstream processes or storage
 

Air pollution hazards – dryers emit significant amounts of particulate matter (PM) into the atmosphere such as combustible dust and powders, gas or combustion products. During application processes, these materials often result in the release of harmful pollutants such as sulphur dioxide (SO2), nitrogen oxide (NOx) and carbon monoxide (CO).

Understanding Materials - Combustible Dust and Powders

The potential for fire and/or explosion when processing potentially combustible powders and dust, especially in drying processes, have been the cause of fatal and potentially fatal near-miss events. Although these hazardous events are preventable through meticulous plant designs and process operations, rigorous safety parameters can only be achieved through thorough the implementation of standardised risk assessments and an in-depth technical knowledge of the product and by-product materials. 

Although many dusts and powders used in processes may not be combustible, it is important to have this validated. If a material has the potential to be combustible, then it may ignite in the presence of an ignition source. An ignition source can be unintentionally introduced both externally or internally via chemical reactions present in the dust or powder. 

For these materials to combust specific conditions must be met, including:

  • specific particle size and distribution.
  • a dust cloud present that is dispersed in the air whereby there is sufficient oxygen to support combustion.
  • the dust cloud must be confined in a way that supports the propagation of an explosion.
 

It is important to understand the properties of combustible dust and powders to fully comprehend the actual conditions required to be combustible and then cause a fire or explosion. The below tests, performed by an accredited laboratory, can determine the potentially combustible properties of the materials:

 

Not all powders will exhibit combustible properties, but understanding the properties of any materials in use will help assess the potential for fire or explosions within a dryer. The best way to ensure that operational dust and powders are combustible is to have them rigorously tested by an accredited process safety laboratory. The test results will enable the formulation of a ‘basis of safety’, resulting in a thorough process safety strategy to be established.

Formulating a 'Basis of Safety'

The formulation of a ‘basis of safety’ for hazards relating to drying is another important step in protecting business, people, workplace and the environment. Organisational safety should therefore be based on two principles. 

Firstly, businesses should look at preventing explosions (Explosion Prevention) or, they can accept that an explosion can occur so should look to ensure that no one is injured and the plant is not damaged (Explosion Protection). With these two options in mind, an organisation, when formulating the ‘basis of safety’, must remember that it is vitally important to look at the plant and operations holistically, rather than investigating and detailing a specific piece of equipment i.e. a singular dryer.

Explosion Prevention

  • Inerting – materials must be “inert” to prevent sparks etc that may result in injury or disturbance of the dryer and combustion. If the material is combustible it will produce heat during drying. If air mixes with an explosive mixture this can then cause ignition that may subsequently lead to an uncontrolled fire. By inerting materials, it is possible to reduce flammable gas concentrations by lowering the amount of oxygen available for internal reactions and decomposition. As a result, less oxygen product is produced from burning chemicals which therefore reduces ignition sources.
  • Elimination of Ignition Sources – The purpose of elimination methods is to prevent the occurrence of fire and explosions by reducing or eliminating ignition sources. The three major sources are the removal of all potential ignition sources (such as surface fires, fuel spills and oil leaks), the elimination of flammable atmospheres that may exist within the dryer and the removal of environmental hazards (such as arcing between electrical components).
 

Explosion Protection

  • Containment – contains the explosion
  • Venting – relieves the pressure of the explosion
  • Suppression – smothers the explosion
 

It must be noted that only the data obtained from the thorough testing of, dust, powders and other materials should be used to formulate a concrete ‘basis of safety’. 

Only after fully understanding the chemical properties of materials and their by-products may the ‘basis of safety’ be used to formulate specific safety limits for different operational conditions. This may include fire-retarding agents, ventilation system design criteria and operating requirements (such as oxygen concentration). If an accident subsequently occurs this information can then be used to support the determination process by providing evidence of why something caused harm (rather than just ‘something’ was present). 

The ‘basis of safety’ formulation process is governed by European regulatory standards and the following principles – fully referenced in the legislation: these can be found in Annex II (INTRODUCTORY PROVISIONS) – Part B General Clauses EN14978-2.0 2017/11,1120,.39. It must also be noted that if a specific plant modification is undertaken, then the ‘basis of safety’ will need to be reviewed. 

Process Safety Analysis

A risk assessment aims to identify risks that are inherent to an organization’s work activities, such as accidents, and those that are likely to be caused by a project or program’s work activities (such as injuries due to machinery), with the results assessed for severity on a scale. In most cases, it is prudent to plan to review risk assessments at key stages throughout the feasibility, design, commissioning and operation for the entire plant. This includes the operability of both pre and post dryer equipment. 

A Process Safety Analysis tool, such as a HAZOP should be carried out on the entire plant, including drying operations. Standing for Hazard and Operability, a HAZOP involves the detailed methodology of investigating hazard and operability issues in a processing facility when process parameters encounter any deviation from design intent. Employers are obliged to protect employees during routine operations and a HAZOP can be used to address safety concerns during the siting of a new facility and throughout the modification of an existing plant or its equipment. HAZOPs can also be performed at various lifecycle stages of a process and during any modifications throughout a plant’s operational lifespan. All necessary due diligence is addressed in detail throughout a HAZOP, making it popular with many regulatory bodies. 

When considering a dryer, deviations might include both higher and lower temperatures, increased pressure or flow rates that have deviated from the norm. If a deviation has a credible cause, the consequences are considered by the team and safeguards can be designed or are retained for further investigation (consequence analysis), before potentially being implemented.

For HAZOP results to be useful, it is necessary for the team members responsible for developing and implementing this specific risk assessment to have a detailed knowledge of their respective scientific disciplines. It is particularly important that those working on the design and implementation fully understand what they are dealing with in terms of safety and operability issues (including being aware of any regulatory constraints), so as not only to reduce re-work but create risks and issues during later operational life.

DSEAR / ATEX Assessment

In the UK, the Dangerous Substances and Explosive Atmospheres Regulation 2002 (DSEAR) require plant operators to assess potentially hazardous explosive atmospheres. This risk assessment ultimately classifies potentially hazardous areas by allocating them as either Zone 20, 21 or 22 – depending on the frequency upon which an explosive atmosphere may be present. The identification of potentially hazardous zones will enable a plant to safely select, run and maintain specialist equipment, including dryers, within a plant. A HAC (Hazardous Area Classification) requires operators to assess where explosive atmospheres may form within a plant. This assessment, which makes up a part of a DSEAR, is undertaken to allocate zone size according to the company’s safety requirements. 

The HAC can then be utilised as a basis for selecting operational equipment. Good design can minimise the extent of any hazardous areas as once zones have been allocated, specialist equipment can be built, bought and installed to appropriate legislative standards. As mentioned in a previous blog on ‘how to avoid over-zoning’, most organisations err on the side of safety when undertaking their risk assessments. This over cautiousness can result in large hazardous areas being designated in higher than necessary zones (i.e. Zone 1/21 instead of Zone 2/22), which in turn, can cause a large amount of financial expense in terms of purchasing and maintaining ATEX or other certified equipment

In a sludge drying plant, for example, explosive atmospheres may arise in areas that have the potential to produce clouds of dust, carbon monoxide, methane, or contaminants like petrol. When taking dust clouds as an example, hazardous zones may be classified as a result of the amount of dust that escapes during normal operation or as a result of equipment or human operating error. If the amount of dust released from a dryer is small, and quickly cleaned up, then any areas outside its containment system may also be small or non-existent. However, an accumulation of dust through plant age or poor housekeeping can result in risks being generated in areas that otherwise would not be zoned. In this case, the dangers are considered to have a small probability, but with enough time and energy, it could have the potential to be catastrophic. 

An additional designation would then occur for an assessment of how and where plants can conduct their activities without breaching existing legislation or regulations. Once these risks are identified, operators must take steps to prevent a recurrence using design controls and other safety systems.

Managing Restarts

The implementation of appropriate process safety practices is a constant necessity across the industry and many organisations do not realise the increase in susceptibility of loss to fire and explosion when restarting operations that have not been active for an extended period. Recent studies have indicated that in the petrochemicals and chemicals industries up to 40 to 50% of process safety incidents and/or major losses have occurred during start-ups, shutdowns and other similar infrequent events (You can read more on Managing Safe Restarts in our blog via this link). Once in operation, dryers usually run at oxygen levels well below the MPOC (Maximum Permissible Oxygen Concentration) and after idle periods, the LOC (Limiting Oxygen Concentration) can exceed at that point in the process. For dryers whose basis of safety relies on depleted oxygen levels, this is the most hazardous part of the restart operation. Therefore, specific risk assessments must address such situations, no matter how infrequent they may be. 

All operational dryers must also be equipped with clear and comprehensive operating manuals that have been prepared by both the original equipment manufacturer (OEM) and operational management. The manual should be learned by the operators, who are to be trained in recognising hazardous situations, including the original and restart risk assessments and the material used throughout that particular process. When using such documentation, detail on the state of the current and previous state of drying processes are needed to ensure a safe restart. During this period, management must also consider what is different and what has changed since previous operations were last active. This should include equipment, operating conditions, layout and the storage of raw materials. Proper engineering design and installation methods must also be adhered to for dryers to operate safely under normal conditions. The materials being dried, the slurry type material, physical characteristics of the product/materials involved all affect how a dryer will react during a prolonged start-up or when restarting after an interruption.

Conclusion

All organisations, no matter how complex drying is in their processes, must be aware of the multitude of hazards present in this specific operation. As safety is the primary goal of any business, when it comes to the intricacy of industrial dryers, several issues need to be addressed. Different types of hazards can occur during the operation of dryers, so ensuring that all safety measures are taken to ensure safety for business, people, workplace and the environment is paramouunt. A comprehensive approach to process safety will ensure that you have an effective process for managing risk, which in turn will help protect your business from any potential harm.

Sigma-HSE are recognised process safety experts and have aided and optimised processes, including dryers in processes, across various industries. Our team will be happy to discuss your requirements and provide actionable safety solutions that are both cost and time effective.

Further Reading

https://www.hse.gov.uk/pubns/risks-from-sewage-sludge-drying-plants.pdf

https://www.process-heating.com/articles/93704-temperature-uniformity-optimization-in-process-ovens-and-dryers

https://www.aidic.it/cet/13/31/119.pdf

https://www.powderbulksolids.com/industrial-fires-explosions/dust-safety-science-releases-2021-mid-year-incident-report

https://dustsafetyscience.com/2021-report-summary/

https://www.slideshare.net/GerardBHawkins/fire-and-explosion-hazards-in-dryers

https://www.perforce.com/blog/qac/what-iec-61508-how-determine-safety-integrity-level-sil-basics

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