How to Assess a Powder's Fire and Explosion Hazards

Please take the quiz below to start assessing the fire and explosion hazard for your powder materials:

Do you know if your powder material is explosible?

❏ Yes ❏ No ❏ Not Sure

Do you know how easily your powder can be ignited?

❏ Yes ❏ No ❏ Not Sure

Do you know how strong the explosion could be and how much damage can result?

❏ Yes ❏ No ❏ Not Sure

Do you know if and what safeguards are in place for dust explosion hazards; what threshold values they are designed for; and if they are adequate?

❏ Yes ❏ No ❏ Not Sure

If your answer to any of the questions is ‘No’ or ‘Not Sure’, your personnel and business may be at a risk of a combustible dust fire and/or explosion.

This article describes the basic step in assessing fire and explosion hazards, provides some testing guidelines and then discusses four types of tests for determining your powder’s fire and explosion properties.

Introduction

A chemical powder product manufacturer only realized the potential explosion hazard when they finally decided after 35 years to perform a screening test on their powder material. It was truly an eye-opener for them.

In a recent dust explosion, a chemical manufacturer was milling an intermediate powder when a dust explosion occurred, causing serious injuries to two workers and severe damage to the plant. In another incident, an explosion occurred in a fluid bed dryer processing a food grade material. The explosion and ensuing fire resulted in significant damage to the equipment and the plant, even though the dryer had been equipped with an explosion protection system.

In both incidents, the plant personnel incorrectly interpreted the fire and explosion test results used to assess each powder’s safety hazards. In the milling explosion, the test results were more than 10 years old; even though the process had changed, the safety data were not updated. In the fluid bed dryer case, the explosion severity test results used for designing the dryer’s explosion protection system were taken from a published literature source rather than from tests using a representative powder sample and so the explosion protection system was incorrectly and inadequately designed.

Assessing and Addressing Fire and Explosion Hazards

Addressing the explosion hazards associated with powder materials is a multi-step process. However, testing to characterize the powder’s fire and explosion properties is the first and basic step. The test results are vital to ensure the safety of a process. For example, the results can be used to:

* Determine the location and extent of flammable atmospheres

* Identify potential ignition sources

* Assess the risk and consequences of igniting the powder

* Establish a basis of safety

* Eliminate fire and explosion hazards by defining or designing specific safety measures

Without adequate and up-to-date flammability data the above actions can never be tailored to the plant and the process. Large safety margins would have to be applied, without being certain that the worst case scenario has been covered. This article concentrates on the basic step of testing the powder to characterize its fire and explosion properties.

Important Testing Guidelines (Some Don’ts and Do’s)

Before discussing specific tests, it’s important to consider some important guidelines to ensure that you get meaningful results.

Don’ts:

* Don’t assume that your powder is non-explosible. If in doubt, test a representative sample to determine and document the explosible characteristics of your powder.

* Don’t use old test results. Old test results may not reflect any recent changes in a powder and/or process. For example, it is well understood that the fine particulates of a powder material could exhibit different explosion characteristics from the large particles of the same material. Relying on the old test results and failing to acknowledge a subsequent change in your powder could lead to serious consequences.

* Don’t rely solely on published results. If you read the fine print, most published data comes with a disclaimer warning the reader against using that data for designing safety measures.

* Don’t use results whose sample origins and test methods aren’t defined or don’t precisely match yours. Other test results may be for a powder with a different particle size or shape, moisture content, chemical composition, or trace solvent level, and the test methods and apparatus could have been non-standard. Results from such tests often won’t provide the accurate results you need to correctly design the safety measures for your process.

Do’s

To develop meaningful test data, consider the following factors,

* Use a representative dust sample

* Use a certified/accredited lab to perform the analysis

* Select appropriate tests

* Use standard test methods

* Keep test data updated

The finer the particle size, the more severe the explosion. Some powder handling operations may start with a relatively large particles, but fine particulates could be generated as a result of particle attrition or as a direct result of unit operation (such as milling). So, testing the finest particle size and the driest representative sample will provide you with worst-case data. Utilizing standard test methods and certified lab facilities also ensure the quality of the results that can be relied upon to design safety measures. It is also essential to keep the test data updated. Some industry standards and best practices require updating the safety test data every five years or so, provided that no process change is implemented. If a change is proposed or implemented, new tests may be necessary even sooner.

Dust Fire and Explosion Hazard Tests

There are several tests that are necessary to determine the fire and explosion properties of your powder. These tests are grouped into four types:

1. Dust cloud explosion characterization tests

2. Ignition sensitivity tests

3. Thermal instability tests

4. Explosion prevention tests

You may not need to run all the tests on your powder; the selection of appropriate tests depends on your powder and/or process characteristics.

1. Dust Cloud Explosion Characterization Tests

Explosion Screening Test: This is the first test performed for powders whose explosibility is not readily known. The test uses a modified 1.2-l Hartmann apparatus (in step 1) and 20-l sphere (in step 2) to attempt to initiate and sustain a controlled dust explosion. In other words, this is a Go/No-Go test.

More than 70 percent of powders used in industry are explosible or “Go”, ranging from foodstuffs such as sugar and wheat flour to chemicals and pharmaceuticals and even metal dusts. If the powder is not-explosible or “No-Go”, you probably don’t need to run further tests. However, some ‘No-Go’ powders can be explosible at high temperatures. If you know your suspended powder will be exposed to higher-than-ambient temperatures during processing, make sure the explosion screening test is conducted at your process temperature. It is also important to remember that some dusts pose a fire hazard even though they do not pose a dust cloud explosion hazard.

Dust Explosion Severity: Once you’ve determined a powder material is explosible or a “Go” material, the next step is to determine the dust explosion severity. Here, the tester suspends a powder sample in a 20-l spherical explosion chamber and an ignition source is introduced at the same time to cause the explosion. The sample size is varied to determine the optimal dust cloud concentration. The maximum pressure and rate of pressure rise are measured and used to determine the Kst value of the material (a measure of the explosion violence). These data can be used for designing dust explosion protection measures such as relief venting, suppression, or containment.

2. Ignition Sensitivity Tests

These tests provide the useful information regarding the ignition sensitivity of the powder material from potential electrostatic discharge, frictional sparks, and hot surfaces.

Minimum Ignition Energy (MIE): This test determines the smallest amount of electrostatic spark energy required to ignite your suspended powder so you can determine what static ignition risks exist in your process. The tester disperses the dust into a cloud in the presence of an electrostatic spark. If the powder ignites, the tester reduces the spark energy to a point where it no longer ignites the dust cloud. The lowest energy at which the spark ignites the dust cloud is the minimum ignition energy.

Based on the test results you might decide, for instance, that your powder’s minimum ignition energy is high enough to safely pneumatically convey the powder or package it in non-conducting plastic packages or containers without the danger of an electrostatic discharge igniting the powder.

Minimum Ignition Temperature-Cloud (MIT-C): This test determines the lowest surface temperature that can ignite a dust cloud. The test is carried out in a furnace apparatus where a dust cloud is created without any other ignition source present.

Minimum Ignition Temperature-Layer (MIT-L): This test determines the lowest surface temperature capable of igniting a powder layer.

The MIT-Layer is used together with the MIT-Cloud to define the maximum operating temperature for electrical and mechanical equipment used in dusty environments. In addition, powders with low MIT-C and MIE may present a particularly high risk of ignition by mechanical impact or frictional sparks.

3. Thermal Instability Tests

Some powders in bulk, when subjected to heat can undergo an exothermic oxidation/decomposition and as a result their temperature will rise due to the heat released from the reaction. At the same time, some of the heat is lost from the powder to the surrounding environment. However, if the rate of heat generation exceeds the rate of heat loss, the temperature of the powder will increase and at some point the powder could spontaneously catch fire. You can use three tests to assess the fire and explosion hazards of your powder when it isn’t suspended in a dust cloud but is exposed to heat in the following conditions: bulk powder, aerated powder, and powder layer. Not all tests are required for all powders and the choice is dictated by the process conditions. Please seek expert advice on the selection of a particular thermal instability test.

Bulk Powder Test: If your powder is subjected to high temperatures (for instance, at the bottom of a dryer, in a storage hopper, or during milling), you can simulate these conditions using the bulk powder test (also called a diffusion cell test). In the test a powder sample is placed in a glass cell and is heated in an oven either isothermally (by keeping the oven temperature constant) or by ramping (by constantly increasing the oven temperature). The amount of air that can reach the powder is limited to natural diffusion through a sintered glass disc at the cell’s bottom and through the open top (hence “diffusion cell”). At a certain temperature, the powder will exhibit exothermic activity. The results can help you determine the maximum safe operating and storage temperature for your powder when it’s in bulk form.

Aerated Powder Test: If air flows through your powder during processing (for instance, as it dries in a fluidized bed dryer), you can use the aerated powder (also called air-through) test to simulate these conditions and determine the aerated powder’s minimum ignition temperature. In the test, the tester places a powder sample in a similar cell as before, but now enough air is passed through the cell to aerate the powder. The vessel is placed in an oven and heated until the aerated powder exhibits exothermic activity. This information can tell you the maximum safe operating temperature for your aerated powder.

Powder Layer Test: If your powder forms a layer and is exposed to heated air flowing over the layer’s surface (for instance, during tray drying or when your power deposits on a dryer’s walls or ledges), you can use the powder layer (also called air-over-layer) test to simulate this situation. In the test, the tester places a layer of room temperature powder in a tray in an oven. The oven blows heated air over the tray until the powder begins to exhibit exothermic activity. The results can help you determine the maximum safe operating temperature for your powder layer, or maximum allowable thickness, or cleaning cycles.

4. Explosion Prevention Tests

These tests provide the information to determine how to prevent the formation of an explosible atmosphere by either reducing the oxygen or by reducing the dust cloud concentration.

Limiting Oxygen Concentration (LOC): In this test, the tester disperses a powder sample in ambient air in the 20-l spherical explosion chamber and then ignites the cloud. In a subsequent series of tests, the tester displaces the oxygen in the chamber by increasing the nitrogen level each time, gradually creating a partially-inert atmosphere (the tester can also do this using other inert gases, such as carbon dioxide and argon). The results tell you below what oxygen level the powder can no longer be ignited as a dust cloud.

Minimum Explosible Concentration (MEC): If your dust is explosible, it creates an explosible atmosphere only at concentrations above a certain minimum. This concentration is the MEC. In the test, the tester disperses a powder sample in the 20-l spherical explosion chamber and gradually reduces the concentration until the powder cloud no longer ignites. By determining this minimum threshold value, you can assess whether the dust concentration in your process exceeds this level.

Conclusions

Following the information above will help you determine your powder’s fire and explosion hazards so you can establish a basis of safety for your process. Be aware, however, that ongoing, periodic tests of your powder’s fire and explosion properties are critical for determining how the properties may have changed over time. This can happen due to changes in the powder, your process, or your plant environment. For instance, your raw material’s mean particle size or moisture content can vary due to changes in the supplier’s processing methods. Your maximum operating temperature can increase if you replace an old mill with a new model. Or your plant’s humidity level can change after you install a new ventilation system. By keeping the test results updated based on your specific process parameters, especially when you know the powder or process has changed, you have the critical basic information that is needed for assessing and controlling fire and explosion hazards.

Muhammad Qureshi, Ph.D., is a process safety specialist at Chilworth Technology Inc. He provides consulting services in dust explosion and electrostatic hazard assessment and is also responsible for standard and customized electrostatic testing. For more information, visit www.chilworth.com.