On August 7th, 2023, a very powerful explosion rocked the Turkish Grain Board (TMO) grain terminal located in the port of Derince Turkiye, injuring ten employees and killing two. While the investigation continues, it is believed that a grain dust explosion was the cause of this devastating event. Just about two weeks before this event, on July 27th, another grain dust explosion occurred at a grain handling facility located in Southern Brazil where at least eight people have been killed and 11 others injured. Within the same time frame, a grain terminal fire occurred on August 11th, 2023 in the French port of La Rochelle which is believed that a fire started on a grain conveyor which then created a much larger fire inside four grain silos. These recent events have raised concerns on the safe handling of grain dusts against dust explosions and fires.
According to Purdue University Agricultural Particulates research, there were 9 grain dust explosions reported in the U.S. in 2022 and a ten-year average of 7.8 explosions per year. The 2022 explosions occurred in; 1 ethanol plant, 2 feed mills, 2 grain elevators, 2 rice mills, and 2 grain processing plants. These recent events indicate that grain dust explosions continue to be a major problem for the global grain industry.
Table 1: Reference explosibility and ignitability values for various grain dusts. Values taken from NFPA 61 and 68. These values are for indicative purposes only. The actual values would change with the chemistry, moisture content and particle size of the material thus needs to be tested.
When all five of these elements are present, then a dust explosion would occur. In many cases, an explosion starts inside process equipment, then quickly propagates to interconnected vessels or to the production area via conveyors, process openings or connecting pipes creating much more damaging secondary explosions. As the recent events suggest, grain terminal operators and would need to take sufficient measures to minimize explosion risks at their grain handling operations.
WHAT TYPES OF MATERIALS ARE SUSCEPTIBLE TO GRAIN DUST EXPLOSIONS?
Grain is a combustible material thus its dust becomes an explosible dust. Particle size and moisture content are major determinants of the explosion severity of such dust. As the moisture content and particle size get smaller, the ignitibility of the dust becomes easier, thus increasing the dust explosion risk. Some of the common grain dust explosibility values are provided as a reference below, however, it is recommended to have such values determined via standard testing methods .
A grain dust explosion can reach up to 8 bar which is 10 -15 times higher than the typical mechanical resistance of most process equipment thus explosion protection methods are often used to limit damage to grain handling equipment from dust explosions.
WHAT ARE THE MOST THE PROBABLE IGNITION SOURCES FOR GRAIN DUST EXPLOSIONS?
According to EN1127-1, there are potentially 14 different ignition sources in an industrial process. It is crucial for plant operators to conduct ignition source assessments to ensure they are either not present or there are measures taken to control such sources. Based on USDA grain dust explosions research, below are some of the probable ignition sources found in grain handling operations:
WHICH TYPE OF EQUIPMENT DO GRAIN DUST EXPLOSIONS OCCUR?
Grain terminals consist of large-capacity conveyors, bucket elevators, storage silos and bins along with dust collectors. Grain is transferred between equipment via transfer chutes. Combustible dust clouds are expected inside such equipment during operations making them hazardous locations (ATEX ZONE21, 20). It is also possible for the grain dust to accumulate inside conveyor tunnels, elevator pits and other production floor areas, which can then trigger secondary dust explosions. Based on the published grain dust explosion data , bucket elevators, storage equipment, mills and dust collectors are some of the equipment susceptible to grain dust explosions as shown in the table below:
Table 2: Probable ignition sources (data from 2006-2014, Source: Sanghi and Ambrose, JASH, 2016)
|
Equipment
|
Number of
incidents
|
% of
incidents
|
|
Unknown
|
146
|
41%
|
|
Bucket Elevators
|
105
|
30%
|
|
Storage bins or silos
|
19
|
5%
|
|
Mills and other
grinding eqp.
|
18
|
5%
|
|
Dust Collectors
|
14
|
4%
|
|
Other areas inside
elevator
|
12
|
3%
|
|
Other areas inside equipment
|
11
|
3%
|
|
Headhouse
|
9
|
3%
|
|
Grain Dryer
|
3
|
1%
|
|
Other equipment/
locations
|
18
|
5%
|
Table 3: Probable Location of Primary Dust Explosions in the Grain Industry
WHAT ARE SOME OF THE PREVENTION AND PROTECTION MEASURES AGAINST DUST EXPLOSIONS?
There are seven industrially recognized methods for dust explosion prevention and protection. The most commonly utilized methods are:
- Control of ignition sources (prevention),
- Combustible dust reduction (e.g. cleaning, dedusting, replacement of the material with coarse ones),
- Inerting (oxygen reduction),
- Containment (pressure shock resistant process design),
- Explosion venting,
- Explosion suppression,
- Explosion isolation.
The choice of the appropriate risk reduction measure should be based on explosion risk assessment and by expert advice from the system manufacturers. Below you will find several commonly used explosion prevention and protection methods used in the grain handling processes.
Example 1: Dust collector inlet pipe spark detection and extinguishment application. The main components are 1- Pair of IR spark detectors (for detecting sparks and hot particles), 2- Water extinguisher at high pressure, 3- Signal router/local controller, 4- Control panel, 5- Alarm and flash light, 6- Programmable relay, 7- Linear heat detector, 8- Pressure booster controller, 9- Pressure booster.
Spark Detection Systems (explosion prevention): The goal is to identify sparks, hot particles or ambers within a pneumatic conveying system or within a transfer chute and extinguish it via atomized water extinguishers before ignition source reaches a combustible volume thus reducing the risk of fires and explosions. Typical applications in the feed and grain industries include dust collector/aspiration duct spark detection and extinguishment, mill outlet spark detection and extinguishment, bucket elevator feed/drop chute spark detection and extinguishment.
Example 2: Feed mill spark detection and extinguishment application.
Explosion Venting and Isolation (explosion protection): The goal is to relieve the explosion pressure and flame to a safe outside location and simultaneously initiate an explosion isolation system to prevent the propagation of dust explosion to inter-connected vessels. If the protected equipment is located inside a production building, the use of standard explosion relief vents would not be possible. In such cases, flameless vents where the explosion flame arresting material surrounding the vent panel absorbs the heat from the flame within milliseconds and extinguishes the flame while relieving only the gasses and dust to the area. When flameless vents are not suitable, then explosion suppression systems are utilized. Note that flameless vents can come in multiple forms, the one shown below is an EVN-type flameless vent which has a spring-loaded relief mechanism.
Example 3: Explosion protection of inside located bucket elevator using EVN type flameless vent.
Additional explosion isolation is needed at the inlet and outlet of the elevator to stop explosion
propagation to connected equipment.
It is critical that all the systems and components used as a part of an explosion protection system should be third-party tested and ATEX approved. Explosion protection and prevention systems are life-critical systems and thus should be designed by the manufacturer’s qualified design engineers as well as should be maintained by manufacturer-approved service personnel.
Example 4: Explosion venting and active isolation of a dual-leg bucket elevator. This design assumes elevator is located outdoors or close to an outside wall with vents directed to a safe outdoor location. Explosion isolation is provided using eSuppressorTM type high rate discharge suppressors. Explosion detection is done by SmartDS rate of rise explosion pressure detectors along with IR flame detectors. Vent quantity and size should be determined by a relevant standard such as NFPA68, NFPA61 or VDI2263.
A properly designed and protected grain handling facility would provide a safer work environment for the employees while minimizing unplanned downtime caused by destructive and costly dust explosions.
About the Author
Emre Ergun is the Managing Director of IEP Technologies Türkiye. He has more than 20 years of experience in the industrial explosion protection field with focus on risk management, product development, R&D, systems application engineering, standards development and business development. He holds BS Chemical Engineering from Middle East Technical University, Ankara, MS Engineering Management from Northeastern University Graduate School of Engineering, Boston MA and MBA from Babson College, F.W. Olin Graduate School of Business. Mr. Ergun is a certified safety auditor by the National Association of Safety Professionals, NASP. www.ieptechnologies.com.tr