Explosion Safety in Powder Product Processes

All facilities handling powder products have distinctly different characteristics. Although the main components remain the same, they are arranged differently. These include bag discharge stations, conveyors, elevators, screens, dryers, mills, silos and dust collection filters. In this article, we will examine all components of a facility and define the explosion safety measures it requires. By progressing through a specific example, we will explain why a comprehensive explosion protection concept is more cost-effective than protecting each piece individually. Figure 1 shows an existing facility. [caption id="attachment_128289" align="aligncenter"] Figure 1. Examples of standard equipment in a powder product production facility[/caption]

1.1. Discharge and Mechanical Conveyors

Most production processes, for example in mills, animal feed plants and power stations, begin with "material discharge". After being transported by road or rail, the material is usually discharged into bunkers. Organizational measures can be taken, such as allowing vehicles to cool for at least 15 minutes before starting the discharge operation. This significantly reduces ignition hazards from hot brakes, hot exhaust pipes or catalytic converters. Grounding is also recommended as protection against ignition sources. When dealing with moving objects such as trucks and railway wagons, proceeding with extreme caution is vital. For this reason, personnel should be trained regularly. In our example, the supplied material is conveyed to the elevator via a screw conveyor. Conveyors have different design characteristics that will require different protection methods. All these methods—such as speed limitation, ensuring appropriate material combinations and safety-compliant installation—are designed to reduce or prevent ignition hazards that could arise from the conveyors themselves. Open conveyor belts are considered less hazardous than chain and screw conveyors because the transported material is generally not mixed and does not come into direct contact with hot surfaces. Depending on material fineness, moisture level and dust generation tendency, connected facility sections linked to the conveying principle and speed must be protected with explosion relief equipment based on explosion risk assessment. Any explosion in connected facility sections must not be allowed to propagate through conveyors, and isolation systems are therefore required. The general standard is to use ATEX-approved rotary valves, chemical barriers, extinguishing valves or quick-closing valves.

1.2. Elevators

Elevators are the most common conveying equipment used in powder product processes. They are typically used to transport large volumes vertically to heights of approximately 60 metres. At the same time, they are a special hazard source because their design is conducive to explosions ("explosive mixtures" and "ignition sources"). Moreover, if elevators are unprotected, pressure waves and flames can spread along the line. Due to this increased risk, the Association of German Engineers (VDI) established an expert committee consisting of explosion safety specialists that prepared a special VDI standard for facility operators and elevator manufacturers. VDI Standard No. 2263, page 8 (Sections 8.1 and 8.2), specifies explosion protection measures for elevators with recommendations on the sizing of pressure relief equipment and the design of explosion suppression systems. In addition to explosion flaps used as pressure relief equipment, the VDI standard also addresses flameless relief equipment. The reason for this is that if the elevator is located inside a building, pressure should not be relieved solely through explosion flaps. Based on the specifications of the VDI standard, an expert committee was established to develop a comprehensive protection system for elevators. ElevatorEX, developed by this working group, offers a suitable solution for all types of elevators. Hazard-prone parts of the system such as elevator feet and head should be equipped with low-maintenance pressure relief equipment such as Q-Box or explosion flaps. Elevator feet are isolated by extinguishing systems activated by spark detectors. This system is suitable for both newly installed and existing elevator systems. While the expert committee repeatedly discussed and examined the explosion characteristics of cylindrical-body elevators, it determined that cylindrical sections have greater explosion potential compared to rectangular sections. This results in increased explosion severity, meaning the explosion pressure rises by approximately a factor of 2. Of course, a number of measures such as using dust extraction systems to limit dust concentration are also recommended. These practices apply not only to elevators but to all parts of a facility.

1.3. Silos

Silos normally do not contain their own ignition sources. This means they lack significant components that could cause explosions (See Figure 2. Five properties required for an explosion). However, there is a risk that an ignition source could enter the silo from lines bringing product to it. While outdoor silos must be protected with explosion flaps, silos in enclosed spaces must have flameless relief or explosion suppression systems. Depending on the material conveyed by the system, preventative measures may also be taken. For example, as a prevention system, spark detectors or extinguishing valves can prevent ignition sources from entering the silo. However, the use of explosion prevention measures should always be discussed and evaluated by relevant specialists. [caption id="attachment_128293" align="aligncenter"] Figure 2. Five properties required for an explosion[/caption]

1.4. Screens

The information we provided for silos also applies to screens. They are normally equipment without their own ignition sources, so explosion protection is generally required only in rare cases; such as when screening highly explosive mixtures. However, at dryer outlets, for example when spray dryers are used in the dairy industry, drum dryers in the wood industry and flash dryers in the starch industry, they can be very hazardous and must be subject to comprehensive review. The movements that cause a screen to vibrate and rotate can cause sparking of hot ash coming from a mechanical conveyor. The final stage is these hot ashes in the screen breaking down and subsequently causing an explosion. This occurred in 2012 in South America and resulted in the largest explosion possible at a wood chip mill, causing 6 deaths. In such cases, providing explosion protection for screens is very important, although it creates special challenges, particularly for occupational safety engineers. Equipment with vibratory motion requires special solutions. Moreover, such solutions require flameless relief equipment (See Figure 3). Protection methods such as suppression cannot be appropriately applied to some screens because, due to screen structure, they cannot be uniformly distributed. This must be verified. [caption id="attachment_128295" align="aligncenter"] Figures 3 and 4. Protection of a screen with Q-Box[/caption]

1.5. Mills

Creating an ignition source is inherent to the nature of a grinder that contains metallic parts colliding at high speeds. At the same time, since this system contains oxygen and high-density dust, it makes an explosion extremely likely. For this reason, many mill manufacturers produce their machines with an explosion-pressure-resistant design (up to 10 bar). Depending on the size of the mill, this can be very expensive for the buyer. As an alternative solution, flameless relief can be used. In either case, an isolation valve must be added both above and below the mill. Installing Q-Rohr LF on the mill air inlet would be an intelligent solution. Q-Rohr LF is a modified version of the widely known Q-Rohr. The difference between them is that Q-Rohr LF does not contain an explosion flap. As a result, it is possible to aspirate air under normal operating conditions and the air can subsequently be supplied from a stainless steel mesh filter without any problems. If an explosion occurs, this filter eliminates the heat in the environment, protecting the environment from flames and explosion pressure.

1.6. Dust Extraction Systems and Filters

The probability of an explosion in a dust collector filter is higher than normal. For this reason, many filter products use explosion prevention systems. The reason is that sparks and hot ash can enter the system along with dust from other parts of the facility. Particularly, self-cleaning of filter bags can create critical situations. This situation leads to excessive dust generation. For this reason, filters used in enclosed spaces are protected with flameless relief equipment, while filters used outdoors are protected with explosion flaps. If there are vehicle routes around the explosion flaps, the solution is to use special intelligent attachment modules on the explosion flaps to direct flames and pressure waves to non-hazardous areas. Individual protection is only possible through a comprehensive cost-benefit analysis. Based on the information above, all sections of a facility can be protected individually. However, this method is often not economical. Cost-effective solutions are possible through comprehensive examination of the entire facility; this process covers all interactions between components and specific arrangements in production operations. Of course, professional protection against explosions has a price, but the cost of inadequate non-professional protection is not merely financial. In the worst case, people pay this price with their lives. For this reason, independent experts recommend that facility operators always work with experienced professionals who employ a comprehensive approach and implement full-package explosion safety solutions. An efficient turnkey explosion prevention solution never costs a facility operator money. Therefore, logically, experts must be involved in the process.

1.7. Practical Examples

As mentioned above, randomly protecting all sections of a facility with appropriate explosion protection equipment is not the correct approach. True professionals always begin by evaluating what the actual need for explosion protection is. As we have seen above, not all systems use explosive mixtures, so explosion protection is not required. Still, it is good for explosion experts to separate the wheat from the chaff. For example, the length of carrier conveyors varies but their principles are always the same, and if they are used in various locations in a particleboard mill, it is important to question whether explosive mixtures and potential ignition sources are likely to occur. If only coarse, moist wood chips are conveyed before drying, explosion protection is not required. After passing through the dryer, the explosion hazard increases, so measures must be implemented. Such detailed process engineering analysis is equally valuable when evaluating elevator explosion protection in mills and animal feed plants. Particularly in inlet elevators, if an explosive atmosphere is likely to form due to high dust content and explosion hazard is high; by moistening the product and due to the presence of contaminants in unwanted places, the hazard that could occur in the elevator is generally significantly reduced. In such cases, necessary explosion safety can be provided through purely preventative or organizational measures. Even when it is clearly evident that explosion protection measures are unavoidable, examining details is absolutely necessary. For example, with small changes in distances between dryers, mills or cyclones, through cleverly engineered piping, and taking into account properties such as tested installation distances of decoupling systems, savings can be achieved in costs.