El Paso Field Services Matagorda Gas Plant - Executive Summary

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The Matagorda Gas Plant (MGP) has a long-standing commitment to worker and public safety.  This commitment is demonstrated by the resources invested in accident prevention, such as training personnel and considering safety in the design, installation, operation, and maintenance of the facility's processes.  The El Paso Field Services (EPFS) policy is to implement reasonable controls to prevent foreseeable releases of substances. 
El Paso Field Services (EPFS) operates the MGP located in Matagorda County, Texas. The MGP is a cryogenic natural gas processing plant designed for the extraction of liquid (raw mix) Y-grade product from a gas stream.  The cryogenic extraction plant recovers ethane in addition to propane, butanes, and gasoline from the inlet gas.  Mixed liquids product from the liquids extraction plant  
is delivered via a pipeline that has the capacity to transport liquids containing ethane.  As evidenced in the ensuing text and based on criteria outlined in the Accidental Release Prevention Program (ARP), the MGP is a Program 1 process. 
The MGP was evaluated to determine if any regulated flammable or toxic substances were present in the process and if they exceeded the threshold quantity.  All equipment and vessels are considered to be connected and/or co-located; consequently, for the purposes of the ARP Program, all equipment and vessels are part of a single process.  The identification of even a single vessel exceeding threshold quantities of a regulated flammable or toxic substance would subject a process to threshold determination for offsite consequence analysis. 
EPFS identified one regulated flammable substance (flammable liquid mixture) and zero regulated toxic substances along with the largest vessel quantity stored.  It was determined that a single vessel of the flammabl 
e liquid mixture exceeded threshold quantity.  The identification of this exceedance indicates that MGP is a covered process with a regulated substance subject to offsite consequence analysis.   
Worst-Case Scenarios (WCS) 
The Environmental Protection Agency (EPA) has defined a worst-case release as "the release of the largest quantity of a regulated substance from a vessel or process line failure that results in the greatest distance to a specified endpoint."  A WCS analysis is required for each toxic substance in a covered process while only one WCS analysis is required for regulated flammable substances as a class.  WCSs are assumed to occur at ground level with meteorological conditions defined as atmospheric stability class F (stable atmosphere), wind speed of 1.0 meters per second (2.2 miles per hour), and an ambient air temperature of 25:C (770F).  Topography for WCSs are distinguished between rural and urban. 
Flammable Substances 
lated flammable substances or mixtures containing flammable substances include both gases and volatile liquids.  The WCS assumes that the total quantity of a vapor cloud is released and detonates.  The endpoint for a WCS involving a regulated flammable substance or mixture is an overpressure of 1 pound per square inch (psi).  For a regulated flammable substance, consequence distance determination is based on its heat of combustion.  For regulated flammable mixtures, the component with the highest heat of combustion can be used for consequence distance determination or, if preferred, directly calculated using equation C-1 of the Offsite Consequence Analysis Guidance Document (OCAGD).  The quantity of a regulated substance/mixture used to determine distance to endpoint is the largest quantity of a regulated substance/mixture in a single vessel.                                                                      
The flammable liquid mixture evaluated at the MGP is a mixture of methane, 
ethane, propane, butanes, pentanes, and hexanes contained in a pressurized storage vessel in liquid form.  The largest vessel quantity of the flammable liquid mixture at the site was contained in the Product Surge Tank having the capacity of approximately 30,000  gallons (119,682 lbs.). Using the criteria outlined in the ARP rule, this mixture represents the facility's WCS for flammables. 
Although methane was identified as having the highest heat of combustion (50,029 kjoule/kg) while acting only as a minor component (<3% liquid volume), it was determined that its use would provide a more conservative consequence distance.  The distance to the endpoint of 1 psi overpressure for the worst-case release of the flammable liquid mixture was calculated and determined to be 0.41 miles.  The distance to endpoint is less than the distance to any public receptors- - a mandatory criterion for all Program 1 processes. 
Toxic Liquids and Gases 
Toxic gases include all regulated toxic substances  
that are gases at ambient temperature (temperature 25: C, 770F), even if they are stored as liquids under pressure or refrigeration.  For the consequence analysis, a gaseous release of the total quantity is assumed to occur in 10 minutes, regardless of storage conditions.   For toxic liquids, the total quantity in a vessel is assumed to be spilled onto a flat, non-absorbing surface.  For toxic liquids carried in pipelines, the quantity that might be released from the pipeline is assumed to form a pool.  Passive mitigation systems may be taken into account in consequence analysis for both toxic gases and liquids. The endpoint for air dispersion modeling to estimate the consequence distance for a release of a toxic gas and liquid is presented for each regulated toxic gas and liquid in Appendix B of the OCAGD in Exhibits B-1 and B-2, respectively. 
The MGP does not have any toxic substances held above the threshold quantity.  Analysis of one worst-case release scenario for all regulated t 
oxic substances is, therefore, not required. 
Alternative Release Scenarios (ARS) 
An ARS describes an incident that is more likely to occur than those cited for a WCS.  One ARS analysis is required for each listed toxic substance in a Program 2 or 3  process that exceeds threshold quantity while only one ARS analysis is required for all flammable substances as a class in a Program 2 or 3 process.  For ARSs, both active and passive mitigation systems can be considered. 
Alternative release scenarios for flammable substances should have the potential to cause substantial damage, including on-site damage.  Alternative release scenarios for toxic substances should be those that lead to concentrations above the toxic endpoint beyond the process fenceline.  Those releases that have the potential to reach the public are of the greatest concern. 
MGP Alternative Release Scenario for Regulated Flammables 
The MGP is a Program 1 process and is, therefore, not required to complete an ARS for r 
egulated flammables. 
MGP Alternative Release Scenario for Regulated Toxics 
The MGP does not have any toxic substances held above the threshold quantity and, in addition, is a Program 1 process.  Analysis of each regulated toxic substance is, therefore, not required. 
MGP has not had any accidental releases during the past five years which meet the criteria for an accidental release as discussed in 40 CFR 68.42. 
The MGP has coordinated its emergency response activities with the local emergency planning and response agencies as required in 40 CFR 68.12(b)(3).
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