Category: Hazardous Area Classification
Anniversary of the Imperial Sugar factory dust explosion-CSB releases a safety video message
On Feb 4th, 2009, on the first anniversary of the dust explosion that took place at the Imperial Sugar factory, the US Chemical Safety Board released a video message. In the video (see below) the CSB chairman John Bresland asks federal regulators and businesses to increase efforts at preventing combustible dust fires and explosions.
For those who remember, the incident was one of the worst dust explosions ever, with 14 fatalities.
Just a few days ago, a coal dust explosion rocked the Oak Creek We Energies power plant. It took place in a silo and injured 6 contract workers. Fortunately there were no fatalities. One of the workers described a loud explosion and a ball of fire came rolling down at them inside the 65 foot high silo, making them scramble to safety, reports the Journal Sentinel of Milwaukee.
What is the cause of so many dust explosions? The CSB completed a major study of such combustible dust hazards and has urged the Occupational Safety and Health Administration (OSHA) to develop a comprehensive regulatory standard designed to prevent dust explosions. OSHA has not issued a standard but has developed a program to increase enforcement of existing regulatory provisions.
One of the causes of these dust explosions is a possible electrical ignition source, due to using non-dust ignitionproof motors, switches and other non-protected electrical and instrument equipment in hazardous areas. Most industry people think of hazardous areas as those that contain flammable solvents and vapors, but fail to remember that hazardous areas also cover dust prone areas. These include grain silos, coal processing and storage plants and yes, even sugar factories. (Under the North American codes, these are classified as Class II hazardous areas and under the IEC codes, these areas are classified as Zone 21 and Zone 22)
Once a dust explosion takes place, it causes severe damage and the only way to safeguard lives and machinery, is by prevention. Use of explosion protected equipment that is certified for use in dust hazardous areas is a must.
(You can learn more about hazardous areas and dust explosion protection in Abhisam Software‘s e-learning program on Hazardous Area Instrumentation).
Comments are welcome, as usual.
Explosion and fire at Buncefield Oil Storage Depot – Five companies to face prosecution
It is now almost three years since the Buncefield oil storage depot explosion took place and finally the authorities have declared, that they will be pressing criminal charges against five companies, ostensibly who have been found guilty of acts of omission.
For those of you who do not remember the case, here is a short overview. There were a number of loud explosions ( I mean really really loud-reportedly people in Netherlands and France heard it and it was recorded also a seismic event! ) and a massive fire at the Buncefield Oil Storage Depot in Hemel Hempstead, Hertfordshire, UK. Over 40 people were injured in the accident, fortunately there were no fatalities. Following the explosion, a Major Incident Investigation Board (MIIB) was established by the Health and Safety Commission, supported by the Board of the Environment Agency, UK.
There were a series of investigations and reports being published from time to time by this board and some of the significant findings were as follows -my summary of a rather long series of detailed reports:
a) No consequence analysis was done by any of the design engineers or safety experts, as to what could be the severity of the possible explosions of the flammable vapors generated from the petroleum storage tanks.
b) The level control loop, (that is supposed to control the level in the tank and prevent overfilling) on one of the storage tanks failed. It consisted of a Servo tank gauge connected to a series of valves. This failure led to overfilling and spillage of massive amounts of petroleum into the dikes surrounding the storage tank. Petroleum was being pumped in at a rate of about 550 m3/hr for more than three hours, yet the servo level gauge indication, failed to record any change at all!
However the DCS trend records could be salvaged and the above information was gleaned from them. Apparently the CCTVs were working and the footage showed petroleum overfilling and flowing into the tank dikes, but nobody was watching it at the time.
c) The overfill protection was provided by a point level switch which was supposed to be independently connected to an alarm/annunciator panel (separate circuit from the DCS loop). The panel had an override switch and it may be that the interlock was bypassed (no conclusive evidence since everything got burned in the subsequent fire, this may never be known). However it is warning to design engineers who think that by merely having a redundant level switch is good enough. Were there any common cause failures that both the continuous indication, as well as the interlock failed? Not known for sure.
d) The operators apparently did not notice anything amiss and neither was the control system very sophisticated, to tally the pumping rate into the tank to the rate of change of the level. Now here’s the cake. The pumping rate now increased to 890 m3/hr leading to the petroleum overflowing from the tanks, filling up the bunds and secondary containment areas and forming large vapor clouds. It seemed this occured because the inlet lines were common to all the tanks and the other tanks level indications were working, so the system diverted their inlets into this tank that appeared about half full (due to the faulty level indication). There must have been thousands of gallons of the stuff overflowing from all directions and nobody could notice anything! (Yes, it was about 3:00 am in the morning-but so what- were there no operator rounds of the premises or anything like that?- or it doesn’t happen on the night shifts at all?!)
e) Apparently the hazardous area classification which may have been done during the initial stages, may not have considered wind directions. The entire vapor cloud was carried across the road from the tank farm to an emergency generator building,about 100 meters away, where it is thought to have been ignited. The building apparently was not a classified (hazardous) location.
No doubt this entire catastrophic incident and the consequent investigations will have a major impact on how instruments and controls are designed and maintained in petrochemical/hydrocarbon processing plants, how operator alertness and awareness is important and so on.
More details are available at the Buncefield investigation site.
Note: All images have been sourced from the Buncefield investigation site and all copyrights belong to that site.
Can using Gas detectors simplify hazardous area classification?
Is it possible that if we use gas detectors to continuosly monitor the explosive limits in a classified (hazardous) location, we could manage it better?
Of course! If you recall, the area classification concept itself is based on the amount of time in a year, an explosive gas mixture (or a combustible dust-air mixture) is likely to be present in a given hazardous area. Thus a Divsion 1 classified location will have more incidences of explosive mixtures being present in a year than Division 2. Alternatively in the IEC system we have three Zones, which are Zone 0, Zone 1 and Zone 2. If you recall the definitions, Zone 0 is a place where hazardous gas/vapor and air mixtures are likely to be present for more than 100 hours in a year.
Why I am I again drumming in definitions that all of you already know? Because if you read the above carefully, it talks of a gas or vapor-air mixture as “likely” to be present.
Well, if you install an explosive gas detector in the area, you could actually measure if any such mixtures are actually present. Not likelihoods, but real occurences, if any. This means that from the uncertainty of likelihoods we are now talking about actual hours of explosive gas or vapor mixtures being present.
This is a big leap from earlier years and could lead to a revolutionary change in how hazardous areas are classified in the future.
Read more about it here.
Proper Hazardous Area Classification can prevent accidents
Mention the words “Hazardous Area Classification” and most people who operate plants think, “Oh No! It means that I have to install explosionproof equipment.”
Well, nothing could be further from the truth. First of all all hazardous areas do not have explosionproof equipment (it is just one of the many ways in which one can implement protection in hazardous areas-other means like increased safety or purging can also be used-but that’s a matter for another post altogether!).
The point I am making here is that not all areas of a plant that process hazardous materials need be classified as hazardous. Hence, one must carry out a hazardous area classification exercise, not only during the design of the plant but also subsequently at intervals of one to three years.
This hazardous area classification exercise, if done correctly, can not only decrease the costs of operating plants that process solvents, explosive vapors and gases and similar materials, but also significantly reduce the chances of an accident from happening.
How can that be?
In any process plant that processes hazardous materials ( those that are potentially explosive or inflammable), there are some places, where these materials are very likely to be present and in other places, least likely to be present. Even in places where these materials are present in large quantities, it does not mean, that they can burst into flames anytime (for example, one may have thousands of gallons of Acetone, a highly inflammable liquid in a storage tank, but it may have a Nitrogen blanketing over it, so no chances of it ever catching fire). Thus, if one does a study of the various areas in such plant, that could be hazardous, one will find some areas with more likelihood of fire/explosion, some with lesser likelihood of the fire/explosion and other areas with a very very little chance of ever bursting into flames. Based on this, we can classify such areas as Division 1, Division 2 and Safe areas. If we use the IEC system then we have the classification as Zone 0, Zone 1, Zone 2 and safe.
(Note: I admit, the above is a very simplistic description of area classification. If you wish to know about this in more detail you can find a comprehensive training course on Hazardous Area Instrumentation here, that covers a lot of this stuff . Also you can take a look at the Hazardous Area Classification ebook here, a free trial can be downloaded from the link)
Nitrogen blanketing is one way of reducing the classification of an area. Another cheaper and far more simpler way for reducing the degree of classification, is the ventilation. For example if an area of a production building of a plant, does have some vessels having some hazardous liquids, but if the area is very well ventilated by having something like huge exhaust fans and blowers in it ( implying that the fumes that are explosive, will be easily dispersed outside), then the area classification can be reduced (say from a Division 1 to Division 2 OR from a Division 2 to a Safe Area). However, there is a catch. The conditions, under which this area classification was done, MUST NOT CHANGE and these conditions must be communicated to the personnel who operate the plant.
If this is not done, then disaster may loom. Have a look at a real life case, where a worker switched off a ventilation fan, in a plant building. The fan was very important in carrying away explosive fumes from inside the building to outside. The US Chemical Safety Board has made a video of this case study and posted it on their website as well as on YouTube.
You can view the video below and draw your own conclusions.
Our conclusion is simple. If the area classification has been done, with certain assumptions or conditions, then the shopfloor workers must be knowing about it. If these conditions are changed, then the original area classification done becomes NULL and VOID. If this is not understood by all and sundry, then disasters may occur.
