Process Safety Testing: Laboratory Data and the Basis of Safety
The process industry manufactures many types of unique compounds and substances using a select range of processing equipment for the manufacturing of end products. The storage, processing, handling, and packaging procedures of materials can create potentially hazardous conditions in the workplace with the potential risk factors being fire and explosion.
To mitigate the risk of fire and explosion, the industry is required under different Health and Safety legislation, for example, DSEAR/ATEX in the UK/EU and in some cases with the added regulatory compliance and requirement of COMAH/Seveso to undertake an adequate risk assessment and identify hazardous areas.
To perform risk assessments adequately a Materials Safety Data Sheet (MSDS) is used to understand the critical physical properties of the material being processed. Subject to the Basis of Safety (which is often generated from a generated from hazard identification (HAZID) exercise, an MSDS may not provide all necessary data needed and relying on this data alone could result in expensive over-engineering of the levels of protection within a site.
Typical equipment and processes used in processing manufacturing conditions are:
• Pneumatic Conveyors
• Dryers/ Granulators
• Tabletting Machines
• Sachet Filling
• Rip & Tip Operations
• Grinders and mills
• Dust Collectors
• Mixers / Blenders
• Hoppers and Silos
• FIBC Handling (charging & discharging)
• Dust / Vapour hybrid atmospheres
Sigma-HSE has made process safety testing as compact and comprehensive as possible by breaking it down into three main Basis of Safety approaches.
1. Avoidance of Ignition Sources
This approach looks at testing of materials to determine how sensitive they are to specific ignition sources such as electrostatic discharges, mechanical sparks and hot surfaces. This Basis of Safety is often used when charging vessels from sacks, IBCs or FIBCs, pneumatic conveying, milling and tabletting operations.
Many materials and hazardous substances that can create a flammable atmosphere are found to be insensitive to ignition and therefore, it is easy, practical and cost-effective to ensure that an ignition source capable of initiating a reaction is not present.
A sub-group of avoidance of ignition sources is the electrostatic properties of products, processes and materials being used to ensure process safety conditions are met. This may vary from the electrostatic properties of the materials being processed, the electrostatic properties of containers/transfer mediums and even Personal Protection Equipment (PPE).
2. Explosion Prevention and Protection
Explosion prevention and protection accept that an ignition is possible. If potential ignition sources cannot be controlled, then explosion prevention, either ensures that airborne levels of material are kept to a level below the lower explosive limit (if a flammable atmosphere is not present) or reduces oxygen concentration, by using inert gas blanketing to a level where combustion will not occur.
For explosion protection, test data determines whether containment, venting or suppression systems can be applied safely.
3. Thermal Decomposition
If materials are used at elevated temperatures, such as those experienced in drying operations, then it needs to be determined whether they can undergo an exothermic or self-heating reaction close to the dryer operating temperature.
As an example, in API manufacturing process operations, solvent evaporation is the main application for the need to increase the temperature. As these operations are performed at low temperatures, circa 60°C, thermal stability is not an issue.
However, for the manufacture of an incipient or bulk intermediate, higher temperatures may be encountered and therefore data on self-heating is required.
Based on the three above groups, we have created comprehensive testing capabilities that are tailored to address each area within the industry.
Avoidance of Ignition Sources
The tests to be considered are:
Minimum Ignition Energy (MIE) (with & without inductance) – Standard: BS EN ISO / IEC 80079-20-2: 2016
The Minimum Ignition Energy test is conducted to determine the lowest spark energy that will ignite a powder when dispersed in air, as a dust cloud. Looking at electrostatic and mechanical spark discharges as potential ignition sources.
Minimum Ignition Temperature (MIT) – Standards: BS EN 50281-2-1 & BS EN ISO / IEC 80079-20-2: 2016
The Minimum Ignition Temperature test is conducted to determine the lowest temperature at which a hot surface will ignite a powder when dispersed in air, as a dust cloud.
Layer Ignition Temperature (LIT) – Standards: BS EN 50281-2-1 & BS EN ISO / IEC 80079-20-2: 2016
The Layer Ignition Temperature test is conducted to determine the lowest temperature at which a hot surface will ignite a powder when settled as a dust layer.
MIT and LIT data are used for the determination of the maximum permitted surface temperature of electrical and non-electrical equipment.
This test data is used where explosion prevention or protection cannot be applied such as charging/discharging of vessels, some conveying systems, milling/sieving operations or tabletting.
Powder Volume Resistivity – Standard: BS EN ISO / IEC 80079-20-2: 2016
This test is performed at a controlled relative humidity condition, of 25 %. The volume resistivity of a powder dictates how efficiently charge migrates through a material by electrical conduction. The higher the volume resistivity value, the more resistive the powder is.
High-resistivity powders will accumulate and retain the charge presented to them in all situations.
Low and most mid-range resistivity materials (conductive and static dissipative) will dissipate charge, providing it has a good path to Earth.
This can be achieved by handling these powders in a well-earthed environment (earthed conductive or static dissipative containers, silos, hoppers, and plant equipment). The build-up and retention of charge on powder or equipment creates a hazard only if the charge is suddenly released in the form of a discharge which can cause an ignition of a flammable atmosphere.
Charge Relaxation Time – Standard: BS 7506: 1996
This test is performed at a controlled relative humidity condition, of 25% and compliments the Powder Volume Resistivity measurement, as it indicates how long a material can retain its electrostatic charge and helps in giving clearer resolution of electrostatic classification, where resistivity testing gives a borderline result. This test can show which side of the “dissipative fence” the material lays.
It is useful for operations that involve powder movement or storage such as pneumatic conveying, blending, milling, big bag storage, etc.
Explosion Prevention and Protection
The tests to be considered are:
Minimum Explosive Concentration (MEC), Standard: BS EN 14034-1 2006+A1 2011
The Minimum Explosive Concentration test is conducted to determine the minimum quantity of powder dispersed in air, as a dust cloud that will form an explosive atmosphere. Thus, keeping below this concentration can prevent an explosive atmosphere from forming.
Explosion Severity (Pmax & Kst) (20L), Standards: BS EN 14034 1&2 2006+A1 2011
The explosion severity test is conducted to determine the maximum pressure (Pmax), Maximum rate of pressure rise ((dP/dt)max) and Kst of an ignited powder, dispersed in air, as a dust cloud and used to calculate a material “Dust Constant” or Kst (K for constant and “St” short of Staub – German for dust)
Limiting Oxygen Concentration (LOC) Standard: BS EN 14034- 2006+A1 2004
The Limiting Oxygen Concentration test is conducted to determine the minimum quantity of oxygen, within an atmosphere, that will enable a powder dispersed in the air, as a dust cloud to ignite. Keeping below this determined oxygen level will often prevent fires or an explosion from occurring.
This test is performed using nitrogen as the inert gas, but other gases can be used if nitrogen is not the inerting gas of choice.
Thermal Stability Testing
To determine whether safe drying conditions apply, it is necessary to select the correct test and understand how to apply the data. Instead of using ‘basic’ screening methods such as the Grewer Oven, the following three tests consider the critical operational safety conditions that apply to specific drying operations.
Air Over Layer Test
A thin layer of powder, normally to a depth of 15mm is exposed to flowing heated air. The temperature of the surrounding airflow and 3 measurements in the sample are then measured and recorded. It replicates thin layer deposits in any drying situation i.e. drier walls and roofs where hot air rushes over its surface
A 30°C factor of safety is applied to the results.
Bulk Powder Test
This test is used to define safe processing conditions where bulk powder is present with limited air availability.
Typical applications for this test would be the base of large spray dryers, fluid bed dryers where the flow through the air has stopped or the storage of big bags.
A 50°C factor of safety is applied to the results.
Aerated Cell Test
This test uses the above bulk powder test cell, but a lid is attached with a sintered glass base. Pre-heated air is then pushed through the sample.
This will mimic conditions where there is a bulk of powder with a large amount of air availability such as a fluid bed dryer or a rotating dryer.
Benefits of Sigma-HSE's Solutions
The comprehensive testing services offered by our experienced team at Sigma-HSE are much more than a standalone test result.
• Combined testing and process safety consultancy experience to ensure that our accurate and methodically generated test data is applied to your process safety applications correctly to protect your staff, site and brand reputation.
• Free technical advice to ensure the correct test package is selected for your needs.
• Complementary post-project support, to ensure you are happy with the data and understand the implications for your project.
Sigma-HSE’s testing laboratory has state-of-the-art equipment and is operated by senior laboratory staff with decades of thorough knowledge of the process industry.
Our ISO 17025-accredited quality systems ensure that all results are accurate with traceable test data. Internationally recognised standards are followed by such international standards as those adopted by the EU (BS & EN standards) and ASTM standards as adopted within the USA.
However, this is where Sigma-HSE’s team of process safety experts and consulting engineers can bring a wealth of expertise from years of providing expert analysis and process safety solutions to national, local and international standards for multinational companies.