22-27 Blasts/Explosions
This appears to be the same question that was asked and answered in Invenergy’s responses to the Town of Burrillville’s (Town) Request No. 17-2, including the Exponent letter that was attached to Response No. 17-2.
To repeat, here is Invenergy’s Response to No. 17-2 (exhibit not re-attached):
We do not believe it is possible that a problem at the Clear River Energy Center (CREC) could cause an explosion at the Spectra/Algonquin compressor station. The codes and standards incorporated into the design and construction of the CREC and the physical separation of the Algonquin compressor station and the CREC minimizes the possibility of direct impacts to the Spectra/Algonquin compressor station in the remotely possible event of a fire or explosion at CREC.
The design of the CREC incorporates the requirements of dozens of industry standards including but not limited to American Society of Mechanical Engineers (ASME) Boiler and Pressure Vessel Code, National Fire Protection Association (NFPA), National Electric Code (NEC), American Petroleum Institute (API). Adherence to these standards minimize the likelihood that there would ever be a fire or explosion at the CREC.
In order to determine potential scenarios that should be examined, Invenergy examined the systems and associated design features at CREC. These systems and features are typical to gas fired power plants, and as such, the Project design will also include design features to mitigate consequential damage to other portions of the CREC facility and keep any impact area within the confines of the CREC property. The key systems that could have a potential to cause a fire or explosion are listed below and their associated specific design features include:
The CREC fuel gas system will be equipped with automatic detection and emergency shutdown systems, including the following:
Safe operating practices will include the following at a minimum:
The gas system design features include, controls utilizing gas detection, fire detection and suppression and when combined with regular inspections and proper maintenance of gas system equipment, limits this type of event to be confined within a smaller area thereby virtually eliminating the potential for undetected gas leaks that could lead to a fire or explosion.
The generator applied to CREC is hydrogen cooled, and as with the potential for a natural gas leak, there will be hydrogen leak detection sensors located on the generator which stringently monitor for potential leaks. These detectors will be set to monitor, alarm and take protective actions when hydrogen is detected at a level that is below the lower explosive limit.
The generator is equipped with end shields on each end, designed to support the rotor/bearings, to prevent gas from escaping, and to be able to withstand a hydrogen explosion in the unlikely event of such a mishap. In order to provide the required strength and stiffness, the end shields are constructed from steel plate and are reinforced. Horizontally split inner and outer oil deflectors are bolted into the end shield and provide sealing of the oil along the shaft.
Furthermore, the hydrogen systems and components will be located in areas that are designated with an area classification that requires special design features including explosion-proof electrical components, gas detectors that are linked to automatic isolation of systems and integrated with the fire detection and suppression systems.
The generator will have an internal volume of hydrogen that will be maintained in a sealed condition using multiple redundant seals. The seals will include mechanical seals and a seal oil system that uses pressurized oil barrier between the mechanical seals and the rotating shaft. The seal oil maintains an air-side seal and a hydrogen-side seal by forcing oil in both directions. The oil is monitored to detect any hydrogen that may get entrained into the oil and provide a means to scrub the hydrogen from the oil.
Hydrogen, like all flammable gases, is only reactive when it is present in concentration levels between the lower explosion limit and the upper explosive limit. That is, when there is sufficient oxygen present to sustain combustion. The generator will be equipped with a purity monitoring system that measures the quality of hydrogen in the generator. If the purity level begins to decrease toward the upper explosive limit, this system adds hydrogen to maintain purity.
The generator will also be equipped with an inert gas (one that does not react with hydrogen) purge system to purge the generator of hydrogen should generator maintenance be necessary. This system will also be used to purge and dilute the hydrogen to below the lower explosive limit if there is a leak. These systems are used throughout the power industry and have successfully controlled and prevented hydrogen explosions. Daily inspections and proper maintenance of equipment help to reduce this hazard.
While we believe that the impact of any conceivable event at CREC will not migrate to the Algonquin compressor station, in order to address the question on the likelihood of an explosion occurring, we contacted Exponent, Inc., who is an industry recognized expert in conducting the type of analysis that was requested and asked that they conduct an evaluation of the probability of either a natural gas explosion or a hydrogen explosion event and to determine the maximum impact radius of the worst case scenario, no matter how unlikely. Exponent performed the evaluation which is included as an attachment.
As can be seen in the attached study provided by Exponent, the likelihood of either the Algonquin Station or the CREC facility suffering a gas explosion event as described in the question is anticipated to be on the order of 10-5 to 10-6/yr, or once every 100,000 to 1 million years.
We also requested Exponent to describe what conditions, along with any assumptions and associated reasoning, would be necessary, no matter how unlikely, in order for such an event to occur and to determine the size of the impact radius that could result from such an event. Their inputs, assumptions and analysis are included in the attached report which concludes that even with postulating physically impossible scenarios like having the maximum possible volume of gas be released instantaneously and fill the largest contained area (the power block building) with a “stoichiometric natural gas/air mixture in order to maximize the confined volume of fuel involved in the explosion,” the resulting impact area does not impact the Spectra compressor station.
Also, as addressed in the response to question 17-4, Exponent determined the distance away from the source of a worst case hypothetical explosion, where the blast wave pressure threshold of 1 pound per square inch gauge could reach. This threshold is the lowest pressure criterion for damaging explosion effects described in the ALOHA technical documentation and the EPA Risk Management Program Offsite Consequence Analysis. At 1 psig of pressure, a blast wave could shatter glass windows, however much higher pressures are necessary to damage the buildings or equipment at the compressor station. The calculated distance to the 1 psig pressure threshold for the maximum postulated scenario (no matter how improbable) was found to be no more than 884 feet from the source on the CREC site which does not create any damage to equipment at the Spectra/Algonquin compressor station, please refer to the attached Exponent letter response for the details of this analysis.
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