Ayala Land Statement on Glorietta 2 Incident
A gas explosion resulting from the combustion of either methane or diesel vapor formed in the basement of the MSC building at Glorietta 2 mall has been highlighted in the media recently as a possible cause of the explosion last October 19. Ayala Land has consulted several local and foreign experts including Dr. Stephen Etheridge, a specialist on waste water and effluent treatment with special expertise on biogas production, and Burgoynes, an international consulting firm specializing in the forensic investigation of fires, explosions, and engineering failures. Their findings are as follows:
1. It is highly unlikely that methane would be produced in the Glorietta 2 basement sump pits because the conditions for the production of methane in substantial quantities were not present.
The generation of methane from organic materials is a process that occurs as a result of microorganisms feeding on and digesting waste. This process results in biogas, which is composed of methane (approx65%) and carbon dioxide (approx.35%) with trace amounts of other compounds, including hydrogen sulfide.
Microorganisms take a long time to grow and generate biogas, requiring approximately 8-10 days in an undisturbed condition to multiply to sufficient quantities. The length of time that waste water stayed in the basement sump pits is not sufficient for this process to take place, as the water in the sump pits was discharged several times per day.
2. Accumulation of biogas in the basement is unlikely because it would have vented via the large open stairwell leading to the delivery bay.
Biogas is about 20% less dense than air depending on the methane content of the gas. It is likely for a gas lighter than air to have vented via the open stairwell entrance leading to the delivery bay that is open to the street.
3. It is unlikely that biogas was present in the basement since personnel working in the basement did not smell hydrogen sulfide, which smells like rotten eggs and causes nausea and eye and skin irritation.
Hydrogen sulfide, a toxic chemical, would have been present even with small amounts of biogas. At very low concentrations, hydrogen sulfide gives off an offensive odor similar to rotten eggs. At slightly higher concentrations it can cause symptoms such as headaches, nausea, throat, and eye irritations, even eye injury. No such smells or symptoms were observed by the various personnel working in and out of the basement on the days preceding the explosion as well as on the actual day of the explosion.
4. It is unlikely that a gas explosion in the basement would have caused the severe damage observed in the loading bay area because at the time of the incident the loading bay was open to the street.
5. The diesel tank suffered an internal overpressure during the incident which resulted in the roof of the tank deforming and tearing along a weld. This is relatively minor secondary damage, and is entirely consistent with it having been caused as the result of the primary explosion. The overpressure in the diesel tank could not have caused the damage observed in Glorietta.
6. The diesel fuel in the day tank has been tested by a major oil company in their laboratory and the flash point result was found to be normal at 76 degrees celsius and within specification. The flash point is the temperature at which sufficient vapor from a liquid is formed for combustion to occur. Furthermore, no evidence was found to indicate any contamination with a volatile liquid.
SCIENTIFIC ASSESMENTS ON METHANE
3 Nov. 2007
A. Methane production
1. Explosive combustion of methane formed in the basement of Glorietta 2 mall has been highlighted as a possible cause of the explosion last October 19. We have consulted several local and foreign experts among whom is Dr. Stephen Etheridge, a specialist on waste water and effluent treatment with special expertise on biogas production, to evaluate the conditions at the Glorietta 2 basement and enlighten us on this matter.
2. The generation of methane from organic materials is a process which occurs as a result of the action of microorganisms in the absence of air (anaerobic digestion). Effluents such as sewage under the right conditions can produce biogas. Biogas is composed of methane (approx. 65%) and carbon dioxide (approx. 35%) with trace amounts of other compounds such as hydrogen sulfide. Carbon dioxide is an inert gas and can act to reduce the explosive impact of biogas when compared to pure methane.
3. To produce biogas at a substantial rate, the following conditions need to be present
3.1. The presence of micro-organisms to digest waste and produce biogas.
The specific micro-organisms that produce the methane component of biogas are methanogens and can only live in anaerobic conditions, meaning oxygen is absent. Oxygen is toxic to methanogenic microbes.
3.2. Producing biogas is a result of the microorganisms feeding and digesting waste. The methane-producing microorganisms are slow growing. In order to generate biogas, the waste must remain present for a long enough time for the microbial organisms to break the waste down to produce biogas. In typical agricultural biogas plants in the Philippines, the average time the waste stays in the tank is 30-40 days (hydraulic retention time = volume/flow rate). A minimum retention time in the order of several days is required for the reliable generation of biogas.
4. Hydrogen sulfide
Hydrogen sulfide is a toxic chemical that is present in biogas. Even at very low
concentrations, it gives off an offensive odor similar to rotten eggs.
The effects of hydrogen sulfide on humans are as follows:
o At over 3ppm there would be offensive odors like rotten eggs.
o At over 10ppm victims experience headache, nausea, throat and eye
irritation.
o At over 50ppm, it would cause eye injury.
B. Methane explosive limits
1. For methane to create an explosion, the amount of methane in the air should be within the lower and upper explosive limit.
2. The lower explosive limit of methane is 5% (of the air in the basement). The volume of the whole basement is 412.19 cubic meters. In order to support an explosion, there should be a minimum of 29.44 cubic meters of biogas (assumed to be at 65% methane), which would have a calorific value of 163.8 MCal, equivalent to the calorific value of 18 liters of diesel.
3. The upper explosive limit of methane is 14% (of the air in the basement). Given the size of the basement, the maximum amount of biogas to support an explosion is 73.05 cubic meters (assumed to be 65% methane), which should have a calorific value of 406.5 MCal, equivalent to the calorific value of 44.6 liters of diesel.
C. Basement facts
1. Sump Pits
1.1 There are two small sump pits in the Glorietta 2 basement . A sump pit is a floor level receptacle which temporarily holds waste water which is pumped out 2-3 times a day into the city sewer system. It is not a septic tank but a transfer sump.
1.2 The kitchen sump pit receives waste water from the kitchens of 5 restaurants. It
measures 1.52m x 0.79 x 1.20 meters and is drained automatically about 3 times
per day (approximately 80 – 85% of the contents is pumped out).
1.3 The waste sump pit receives sewage waste water from a few comfort rooms in a
portion of Glorietta 2. It measures 1.52 x 1.70 x 2.25 meters. It is manually drained
2-3 times per day to a level of about 6 inches from the bottom by a submersible
pump. This is a transfer sump – NOT A SEPTIC TANK
Each sump pit has 2 pumps, 1 primary pump and 1 back up pump in case the
primary pump fails. In case both pumps in the waste sump pit fails, the overflow
of the waste sump pit will flow into the kitchen sump pit where it will then be
discharged into the city sewage system.
1.4 At all times, there was at least one pump working. This would be augmented by portable pumps if required. On the day of the blast, 3 sump pumps were working. This means that there was a regular pump-out process of the waste water into the city sewage system
2. Personnel movements in the basement
On a typical day, various people work in and out of the basement such as:
o Roving security guards check the basement every hour for 24 hours.
o There is a service entrance in the loading dock that is manned 24 hours a day by a security guard who stays at a desk. The service entrance is adjacent to the staircase leading to the basement.
o Maintenance technicians inspect and run the pumps at least twice a day, morning and afternoon, draining the transfer sumps of their contents.
o Janitors maintain cleanliness of the basement premises several times per day.
o The supervisors and engineers check basement operations from time to time.
Conclusions:
According to Dr. Stephen Etheridge, it is unlikely that biogas would be produced in the Glorietta 2 basement sump pits because the conditions for the production of biogas were not present:
7. It is highly unlikely that methane would be produced in the Glorietta 2 basement sump pits because the conditions for the production of methane in substantial quantities were not present.
The generation of methane from organic materials is a process that occurs as a result of microorganisms feeding on and digesting waste. This process results in biogas, which is composed of methane (approx65%) and carbon dioxide (approx.35%) with trace amounts of other compounds, including hydrogen sulfide.
Microorganisms take a long time to grow and generate biogas. The length of time that waste water stayed in the basement sump pits is not sufficient for this process to take place.
It is unlikely that the sump pits at the Glorietta 2 basement could have acted as effective biogas producing tanks as they were emptied up to about 80 to 85% about 3 times a day. Not only would this have exposed much of the liquid to air (oxygen is toxic to methane producing microorganisms) but would have resulted in the removal of most of the microorganisms present in the waste. The required hydraulic retention time for the effective production of biogas would not be met under these conditions.
8. Accumulation of biogas in the basement is unlikely because it would have vented via the large open stairwell leading to the delivery bay.
Biogas is about 20% less dense than air depending on the methane content of the gas. It is likely for a gas lighter than air to have vented via the open stairwell entrance leading to the delivery bay that is open to the street. The basement is at the bottom of an access stairway which has no doors, and is thus not sealed from the atmosphere.
9. It is unlikely that biogas was present in the basement since personnel working in and near the basement did not smell hydrogen sulfide, which smells like rotten eggs and causes nausea and eye and skin irritation.
Hydrogen sulfide, a toxic chemical, would have been present even with small amounts of biogas. At very low concentrations, hydrogen sulfide gives off an offensive odor similar to rotten eggs. At slighter higher concentrations it can cause symptoms such as headaches, nausea, throat and eye irritations, even eye injury. No such smells or symptoms were observed by the various personnel working in and out of the basement on the days preceding the explosion as well as on the actual day of the explosion.
If an odorous, lighter than air gas had been present at the basement it is likely that it would have vented via the stairwell and been detected by the users of the loading area and the guard stationed close to the stairwell entrance. Moreover, none of the people who entered or worked in the basement that morning nor in the days preceding the blast noted any such odors. It was noted that three maintenance technicians worked in the basement for about 3 hours (from about 4-7pm on October 18).
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