22-27 Blasts/Explosions

Can an industrial accident anywhere on the power plant site trigger a subsequent or chain reaction at the compressor station site? Please explain.

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:

 

  1. Natural gas:  The natural gas piping systems and components are separated from the other sections of the Project (to the extent possible) and all areas where natural gas systems and components are located are designated with an area classification that requires special design features that include explosion proof electrical components, gas detectors that are linked to automatic isolation systems and fire detection and suppression systems. Should a leak occur, the gas detection sensors are set to detect the gas before a concentration level is reached that would be capable of creating an explosion that could impact a larger area of the plant. For these reasons, the amount of any gas that could leak is limited such that it would not spread to an ignition source.

The CREC fuel gas system will be equipped with automatic detection and emergency shutdown systems, including the following:

 

  • The natural gas will be odorized for detection.

  • A network of low concentration natural gas detectors will be installed to monitor for fuel gas leaks in the gas yard and within all areas where fuel gas equipment is located, both indoors and outdoors. The detectors will be set to alarm in the facility main control system (“DCS”).  The custom-designed fire alarm and detection system will be in accordance with NFPA 72.

  • In accordance with NFPA 850 the plant will include emergency shutdown systems to isolate the gas piping, stop equipment and safely vent station gas. The natural gas supply pipeline will include an emergency shutoff valve (ESV) at the outlet of the metering yard and the ESV will automatically close in the event that a fire is detected.

  • Individual unit shutdown systems in case of mechanical or electrical failure of a compressor unit system or component.

  • Main line isolation valves will be fire safe, as defined by API 607.

  • Nitrogen hose connections and vent lines will be provided between all isolatable sections of the fuel gas piping to allow nitrogen purges and inerting for maintenance activities.

  • The fuel gas piping will be cleaned and purged in accordance with NFPA 56.

  • Pressure control devices to maintain the operating pressure at or below the maximum allowable operating pressure.  In addition, overpressure protection devices with sufficient capacity and sensitivity will be installed to ensure that the maximum allowable operating pressure of the station piping and equipment will not be exceeded by more than 10 percent (10%) in the case of a malfunction of the pressure control equipment.

  • All electrical equipment will be explosion proof.

  • System design to accommodate changes in gas quality, periodic maintenance (e.g., filter change-out), redundancy, separation of ignition sources (e.g., National Electric Code compliance), combustion controls and hardened to resist impacts.

  • Prevent damage to pipe by as-built mapping, below-grade flagging (above grade) and clear labeling of gas-bearing components.

  • Flame detection that uses ultraviolet sensors.

Safe operating practices will include the following at a minimum:

 

  • Periodic walk-through surveys of pipeline systems with hand-held gas detectors at all flanges, valves and other fittings; this is particularly important in the Gas Yard at filter, dewpoint heater equipment, pressure control valves and metering runs where many fittings and gas state changes occur that may contribute to leakage events.

  • Strong operating and maintenance procedures, including use of inert gas purging, maintenance of coating and cathodic protection systems, dewpoint heating, filtration and verification of valve and instrument functionality.

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.

 

  1. Hydrogen: Modern utility generators larger than about 300 MW are hydrogen or hydrogen and water cooled. Hydrogen has safely been used as the coolant medium in utility generators for over 70 years. General Electric (“GE”) estimates that there are more than 2,400 hydrogen cooled GE designed generators in service today. The generator and associated hydrogen cooling system include a number of features to ensure the safe operation of the equipment:

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.

 

  1. Main Transformer: The potential for an explosion is remote, its causes include lightning strike or transformer fault. The design features fire detection and suppression systems, location within a three sided concrete wall structure to protect immediately adjacent equipment systems and buildings and such that the open side has adequate space separation for protection for adjacent transformers and other equipment. Given the small impact area and the three sided walled enclosure, this scenario was ruled out as having any potential to impact Spectra’s Burrillville station. 

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.