A constructive technique of protection of process equipment and systems against the effects of an explosion caused by the presence of flammable and combustible dust, vapours and gases using explosion suppression and isolation systems is widely used in the process industry.
Introduction
The main advantage of a suppression technique is the identification of an explosion by means of a dynamic MEX pressure sensor (and in some cases by means of IR optical sensors) at the initial stage of explosion propagation and the direct suppression of an explosion at its initial stage by injecting suppression powder from the installed cylinders into the protected volume. Explosion suppression is used not only to protect process equipment, but also to isolate explosive apparatus(es) from the rest of the process plant.
This involves installing additional cylinders with damping powder or quick-action gate valves on the process piping at the inlet/outlet of the unit(s). This eliminates the possibility of explosion transmission and propagation.
Malt storage facility
A diagram of the malt reception, purification and storage system to be safeguarded at the brewery is shown in Fig. 1. It consists of the following components:
- the malt loading station from the truck to the intermediate hopper,
- transport of malt to the first bucket elevator (height 17 m) by a scraper conveyor,
- from the first bucket elevator to the station for cleaning the malt from small stones, pieces of metal and other impurities,
- the clean malt is then gravity-fed to a second bucket elevator (height 32 m) and further transported by a scraper conveyor to a battery of storage silos,
- the malt is further transferred from the battery of silos via scraper feeders to a third bucket elevator and then to the brewhouse for further processing.
Explosion suppression and isolation system on a third bucket elevator
The foot and head of a bucket elevator are equipped with two MEX dynamic pressure sensors and two IR optical sensors. The MEX pressure sensor is responsible for identifying a pressure increase caused by an explosion of malt dust.
A sudden increase in pressure, due to the formation of the so-called “fireball,” is related to the value of the Kst [bar m s-1] constant, which characterises the dynamics of an explosion development. It should be noted that this sensor will not react to possible fluctuations in the operating pressure in a bucket elevator.
The MEX dynamic pressure sensor is in fact equipped with two independent measuring cells (and when necessary also with a temperature measuring cell) and the system will only be activated if both sensors (cells) simultaneously identify a sudden increase in pressure caused by an explosion of malt dust. The MEX sensor sends a signal to the FAB signal converter, which sends if further to a single-zone (in this case) control panel.
The control panel decides, depending on the location of the MEX sensor, whether all or some of the installed suppression powder cylinders will be activated.
In a typical situation, a suppression system does not have an IR optical sensor. However, in certain process situations, experience recommends the use of a MEX pressure sensor and an IR optical sensor. An example of such application is a bucket elevator, more specifically its foot and head. An IR sensor provides the ability to identify the stream of sparks/flames in the foot or feeder head, as well as when they appear in the conveyor channels.
The reason for using an IR sensor in a bucket elevator is the potential for the so-called slow bursts, which can lead to a situation where the MEX sensor does not react fast enough. In this case, the optical sensor will give an independent signal to the control panel and ensure that the explosion suppression and isolation systems are activated.
The foot and head of the bucket elevator analysed was independently fitted with an HRD cylinder with damping powder. Additionally, due to the height of the elevator (32 m), two sections were separated along the length of the transport channels, which were secured with four HRD cylinders. This ensures protection of the head or foot of the elevator from possible propagation of an explosion through the malt transport channels. In addition, two HRD cylinders were installed to protect the inlet and outlet of the bucket elevator from potential transfer of the malt dust explosion to the rest of the process plant, Figure 2.
Other bucket elevators at the malt purification and storage facility as well as at the brewhouse were secured in a similar manner.
Explosion on the third bucket elevator
The cause of the explosion in the bucket elevator head was damage to the main bearing of the belt pulley, causing the belt to move and rub against the elevator housing, with intense heating of both components. The feeder was equipped with a belt travel sensor and a belt slip sensor. Neither was activated.
Intense heating, caused by friction, and an increase in temperature caused ignition and a strong explosion of the air and malt dust mixture in the elevator head.
A dynamic MEX pressure sensor installed in the elevator head identified the explosion (in practice, the rate of pressure build-up over time characteristic of malt dust – constant Kst) and sent a signal through the FAB signal transducer to the single-zone control panel. The control room activated all HRD cylinders with suppression powder, as well as two cylinders at the inlet and outlet of the feeder, to cut off possible explosion propagation.
As a result of the correct operation of the explosion suppression and shutdown system, the bucket elevator was not destroyed and, more importantly, the malt cleaning and storage facility were not damaged. The only consequence of the explosion was the need to replace both belt run control sensors and the HRD cylinder.
The course of the explosion (dependence of pressure changes in time) is shown in Fig. 3. The system recorded the following pressure changes over time:
- dP: sudden pressure increase over time (58.3 mbar in 50 ms, characteristic value for a malt explosion) which caused the MEX sensor to send a burst signal to the control panel in order to activate the suppression system
- Pto: pre-set pressure value (1037 mbar, for malt) at which the MEX sensor reacts and activates the system in case it has not measured dP before (a “free” explosion case),
- Pred: measured reduced burst pressure value, after the suppression system has been activated.
Possible consequences
The possible consequences of an explosion on the bucket elevator in question are difficult to assess conclusively. Securing the bucket elevator itself could protect it from possible damage. However, only the use of an explosion suppression and isolation system provides an effective means of protecting the entire malt cleaning, storage and brewing facility.
Explosion suppression and isolation systems for the protection of apparatuses and process plants are very modern constructive protection systems. They have a number of advantages over alternatives based on blast venting. To date, our company has secured a number of process devices (e.g. filters) as well as systems in this way.
Bibliography:
- Sam Mannan, Lee’s Loss Prevention in the Process Industries, Hazard Identification, Assessment and Control, 3rd edition, vol. 2, 2005,
- J.Barton, Dust explosion Prevention and Protection, IchemE, Warwickshire, UK, 2002,
- A. Laszuk, A. Wolff, Systemy zabezpieczeń przeciwwybuchowych w przemyśle chemicznym, Inżynieria i Aparatura Chemiczna, 2003, 6, 19-24,
- P.E. Moore, D.J. Spring, Design of explosion isolation barriers, Trans IchemE, part B, Process Safety and Environmental Protectrion, 2005, 83 (B2), 161-170,
- Kees van Wingerden, Auslegung von Explosionsentkopplungenssystem, TU Bd. 46 (2005), Nr. 9 – September.
