Year 2003 – a dust explosion occurred in a dust collector at an aluminium processing company, which resulted in destruction of the entire production hall and caused the death of one person.
Year 2007 – due to lack of proper grounding, the explosion in the pneumatic system for flour transfer from a tanker truck to a silo completely destroyed the mill building and contributed to death of 5 people.
Year 2017 – a small explosion occurred in a mill at a corn processing plant, leading to total destruction of the plant, death of 5 people and a conviction verdict obliging the company to pay damages, which amounted to PLN 6.5 million.
Different equipment, different dust types. Common denominator: no explosion isolation
“Different dust was ignited in each of these cases. There is more. Each time the explosion occurred in a different machine. Nonetheless, they have a common denominator that was ultimately the cause of extensive destruction. What was it?” – Zbigniew Wolff, explosion protection expert at the WOLFF GROUP, asks and immediately provides and answer:
“Lack of the so-called explosion isolation (or decoupling). In each of this cases, the explosion, was able to spread unhindered to other equipment and to other rooms, causing disastrous secondary explosions as a result.
“However, one can ask a contradictory question – why is this isolation used, if these three explosions occurred over a period of nearly 20 years and in different parts of the world? Is this not simply a risk inherent to the nature of industrial production, one that must be accepted?”” – Wolff continues – “Absolutely not.. These types of incidents are not as rare as is usually thought… Our service teams are called out after the explosion protection system is activated even dozens of times a year. However, such information is not attractive to media. Yes, there was an incident involving an explosion initiation, but correctly selected safety features (including explosion isolation) activated, providing protection against more severe consequences.
It should also not be forgotten that the use of explosion protection, including explosion isolation, is a legal requirement.”
Photo. 1: Data from dynamic pressure increase sensor of the MEX3.2HT type
Poland: 3 different major explosions in 6 months. Common denominator: no explosion isolation
If we looked only at the information in the media on events which, due to their scale, had to be presented, we would have recorded three serious explosions over the period of the last six months. The installations where the explosion occurred were unprotected or the protection was incomplete.
The most common mistake in such situations is exactly the lack of the so-called “explosion isolation”, i.e. a system which cuts off the explosion from the remaining parts of the installation at the moment the explosion occurs inside a device. This cutting off includes physically closing all channels, pipelines or transfers acting as potential routes used by the explosion to spread to the neighbouring equipment. The lack of insulation meant that explosions could propagate from one location to another, yet another and so on. This led not only to the destruction of equipment but also to extensive fires.
These explosions which occurred recently in Poland caused great damage, but fortunately no one died as a result. Issues were discovered however, which certainly had an impact on the image of the companies, and also – to put it mildly – on the assessment of the work of people responsible for safety at these sites.
If the installations used at these plants had been properly protected, we would never have heard about these explosions. The entire event would end with a “reboot” of the system. The scale of destruction reached millions of zlotys in each of the cases instead.
Components of a correct explosion protection system
But what does correctly mean? In a nutshell, the idea is that a device exposed to explosion hazards must always be provided with two components:
- a system which reduces the explosion pressure to a level safe for the device (pressure release or explosion suppression);
- a system isolating the device in which the explosion occurred from the rest of the installation.
This is the core of proper equipment protection against explosion effects: it is necessary to reduce the explosion pressure inside the device below its design strength – to protect the equipment from uncontrolled rupture – and to prevent residual pressure and flames from reaching the adjacent equipment – to protect the rest of the installation against secondary explosions. I had the opportunity to talk about the reasons why secondary explosions are extremely dangerous and where their unparalleled strength comes from for the purpose of a paper comparing an explosion relief valve with a decompression panel.
A download link to a case study concerning an aluminium dust explosion in a dust collector, cited in the introduction to this paper, may be found below. The study will be informative for everyone using a dust extraction system at their plant (even if it is not aluminium dust that is being extracted). In the box below you will find an introduction to the study and a button that will allow you to download the case study
Passive explosion suppression systems
To explain how explosion isolation works, we should begin with a basic distinction between passive and active systems – as this division determines how will the isolation system work and protect.
Passive systems are almost completely unobtrusive at first glance. They are just installed and wait passively. They are waiting for the explosion to reach the system, so that the energy of the blast forces a safety shut down.
Passive systems may be installed both in ducts through which the material is transported (check valves, Ventex valves), and may be used as correctly selected cell valves, the main task of which is to dose the material e.g. from a silo to a duct.
Cell valves
It is important to point out an extremely serious mistake often made in the case cell valves here. A valve that acts as explosion isolation must be provided with a document commonly referred to as ATEX certificate. Moreover, the valve must be provided with two such documents. The first document guarantees that the valve itself cannot comprise an ignition source. The other ensures that when an explosion occurs, the valve design will stop the explosion from spreading.
Depending on whether the dispenser is ATEX certified or to what extent the certification has been carried out, we can list three types of devices:
- dispensers in standard versions, which may under no circumstances be used in explosion hazard areas, and must not be used as decoupling systems;
- dispensers with ATEX certificate approving the devices for operation in potentially explosive atmospheres, which, if properly selected, are not an ignition source of a potentially explosive atmosphere. Additionally, cell valves of this type may not be used as explosion decoupling systems;
- ATEX-certified dispensers intended for use in potentially explosive atmospheres and as an autonomous explosion decoupling system (pressure rupture and/or flame resistant designs).
The last two listed device types are often mistaken for each other during selection of cell dispensers. For example, it is not uncommon in requests for quotations and technical specifications to find provisions stating that cell locks must be ATEX-certified for a specific explosion hazard area – e.g. 20 inside and 22 outside the device. However, this ignores the fact that the device will also be used as an explosion decoupling system. This results in the purchase of improperly sized locks. Their subsequent replacement can be difficult and expensive. This is because the manufacturer of the equipment purchased either does not offer dispensers with the required certification, or the capacity of a properly certified lock of the same dimensions is lower than required (this is due to the limitations in the certification for explosion decoupling).
Purchase of a larger device may provide a solution to this zugzwang. In this case, however, we may have to accept often costly installation modifications.
In some situations, an HRD-type explosion decoupling system may be a better solution than dispenser replacement. In most cases, this solution does not require additional space for installation and can be integrated directly into an existing duct or hopper (downstream of the dispenser).
Just like every explosion-proof system, ATEX-certified cell feeders also require regular servicing and inspections. In this case, however, manufacturers usually specify the maximum intervals in the technical documentation, also noting that the exact frequency depends on the operating conditions of the device and should be determined by the user of the installation according to their practical experience.
This means in practice that these inspections must be carried out even several times a year in certain situations, under the pain of losing validity of the ATEX certificate (the certificate is valid for the maximum permitted gap between the rotor blades and the valve body).
Return flap valve
The principle of its operation is extremely simple and widely used even in the most common home ventilation systems – these are check valves in fans. In the industry, a return flap valve is a high-pressure resistant hinged flap able to open freely to one side. Thus, the flap can let the pneumatically transported material pass within the air stream present. At the same time, when the explosion occurs, the pressure directed opposite to the flow of material closes the flap, preventing the explosion from spreading. There are also solutions in which a movable flap opened by the pressure of the air flow is replaced with a flap mechanically locked in the open position. This solution has the advantage that it can be installed not only horizontally but also vertically. A flap of this type is presented in the two following figures.
Diagram 1: Principle of operation of a new type of return flap valves eliminating the problem of installation within vertical ducts
Ventex valves
These valves are usually installed in the ducts of dust collection systems, central vacuum cleaning, positive and negative pressure transport, drying or granulation systems. It is important that they can be installed horizontally, as well as vertically. It can also provide single or double action. The valve is closed by the explosion pressure propagating within the pipeline.
A notable difference compared to return flap valve lies in the special design of the pear-type valve. It results in low flow resistances generated by the valve. This solution is approved for use with flammable and explosive gases, dusts and hybrid mixtures. However, when dusts are present in the stream, it must be taken into account that their levels must not exceed the limits specified in the valve documentation. This is possible thanks to the design of the valve, in which the “pear” element is pressed against a special seal when closed. The tolerances are very small in this case, ranging from 0.1 to 0.15 mm. Excessive dust levels result in a risk of fouling or rubbing off the valve seals, potentially leading to malfunctions.
Active systems on explosion isolation
Active systems are the second category of protection isolating the explosion from reaching other equipment and devices. Such systems no longer wait for the moment when an explosion reaches them to act, but actively prevent the explosion from spreading by closing any ducts, pipes or hoppers on which they are installed, as soon as possible, when the explosion is detected by special sensors.
Their operation can be compared to a well-known scene from many movies, when a detector system detects a thief inside a museum or a bank and immediately activates falling bars, tightly locking the thief in a trap without an escape option. And since the explosion is a fairly simple phenomenon as far as its physical nature is concerned, there is no fear of security being tricked by it, like by Tom Cruise in “Mission: Impossible.”
HRD type extinguishing agent cylinders
HRD cylinders can be used not only as an explosion suppression system, but also as an extremely effective active explosion isolation system. Installed in ducts, pipes or hoppers, HRD cylinders, when activated by sensors, they immediately inject suppressing powder into such ducts, thus immediately closing the ducts, preventing further spread of the explosion.
Quick release gate valves
A quick release gate valve is the most resistant explosion decoupling systems adapted to the most difficult situations. It works in a similar way to a typical knife gate valve, except that their reaction and closing time are extremely short. Moreover, the resistance of quick release gate valves to explosion pressure in the case of small diameters can reach even 50 bar (up to 30 bar in the case of larger diameters). These values can probably only be achieved in the event of an explosion of hybrid mixtures. In standard applications, quick release gate valves work with dusts, gases as well as with hybrid mixtures. Similarly to HRD systems, they are activated by dynamic pressure sensors and/or flame sensors, and the entire system is managed by a dedicated control panel.
Quick release gate valves can operate as a certified explosion decoupling system in installations designed to withstand the maximum explosion pressure (so-called 10 bar resistant designs). Obviously, they can be just as well used as a protection at lower explosion pressures.
The list of devices with which this type of solution can be used is limited practically only by the duct diameter – it should not exceed DN400. We talk about filtration units, cyclones, reactors, dryers, pressure vessels, silos, mills, etc. here. The only disadvantage of this solution is its very high price, especially in the case of bigger diameters. On the other hand, we get a system which may be operated independently upon explosion, without the need for a third party service intervention. This is critical for installations, where system recovery time is critical.
In the case of active systems, location of the protection is crucial for the explosion isolation to work
The key to detecting an explosion and preventing it from spreading through the installation lies in selection of the right place to install such a protection system. The system must be able to operate before the explosion reaches its installation point. It should be kept in mind that the primary explosion can occur in various parts of the equipment. The explosion can occur near the pipeline, inside the device, and on the other side of the device. This means that pressure and flame waves will travel a different distance during the same time, depending on the location of the explosion.
How should we protect the equipment using active systems?
Let us explain this by taking HRD cylinders as an example. Below, we are going to use terms related to the distance between two protected devices connected via a duct. These terms are not precise, because diagnosing which of the following situations occurred requires individual calculations in each case, taking into account the diameter of the protected duct and dust parameters related to the explosion.
Short distance (up to 6-7 meters) = as one device
In the case of small pipeline distances between devices, it is generally impossible to use explosion decoupling solutions. Both devices should be regarded as a single system in terms of protection, and protection should be activated on both devices, regardless of the device the explosion occurred in.
Slightly longer distance = 2 cylinders
In the case of slightly longer pipeline distances between the units, 2 cut-offs are possible. The logic of operation is such that if an explosion occurs in one device, the cylinder that adjacent to the other device is activated, such that the suppression powder has enough time to be forced into the pipeline before the explosion reaches the cut-off point.
Even greater distance = 1 cylinder
A single decoupling between devices is used in the case of greater distances, but smaller than the distance posing a detonation hazard within the pipeline.
Pipeline detonation hazard distance = 2 cylinders
In the case of long distances, bigger than the distance posing hazard of detonation within the pipeline, two pipeline cut-offs are used. The cylinder closer to the device where the explosion occurred is then activated. This protects not only the other device, but also the pipeline itself.
