RRS/Schirmer recently developed an H2S Contingency Plan for a refinery in New Mexico which complies with the New Mexico Oil and Gas Division Title 19 Chapter 15 Part 11 and API RP 55, Recommended Practice for Oil and Gas Producing and Gas Processing Plant Operations Involving Hydrogen Sulfide.
Hydrogen sulfide is a colorless, flammable, extremely hazardous gas with a “rotten egg” smell. Industrial activities that can produce the gas include petroleum/natural gas drilling and refining, wastewater treatment, coke ovens, tanneries, and kraft paper mills. Hydrogen sulfide is both an irritant and a chemical asphyxiant with effects on both oxygen utilization and the central nervous system. In addition, hydrogen sulfide is a highly flammable gas and gas/air mixtures can be explosive. The immediately dangerous to life and health (IDLH) concentration for hydrogen sulfide is 100 ppm.
Radius of Exposure Calculations
The first step in developing a Hydrogen Sulfide Contingency Plan is calculation of radius of exposure (ROE) for the “worst case release scenario” of hydrogen sulfide gas in a facility. The radius of exposure is defined to be the maximum downwind distance that could be exposed to one specific H2S concentration as a result of a release of a material containing hydrogen sulfide.
The radius of exposure calculations can be performed using Pasquill-Gifford derived equations or dispersion modeling programs such as PHAST, DEGADIS, ALOHA etc. A complete list of proprietary and publicly available dispersion models suited for ROE calculations is provided in the API RP 55.
The New Mexico Oil and Gas Division regulations document was originally developed for mitigating potentially dangerous hydrogen sulfide situations during drilling operations in oil wells and was later modified to include refineries. The ROE equations provided in the New Mexico Oil and Gas Division regulations document are based on maximum daily rates (24 hours unmanned operations) of hydrogen sulfide gas, appropriate for well operations but unrealistic for refineries. RRS/Schirmer dealt with the challenge of defining appropriate realistic modeling techniques to evaluate ROE for a refinery.
RRS/Schirmer selected the “worst case release scenario” based on requirements found in the Risk Management Program Guidance for Offsite Consequence Analysis document published by the United States Environmental Protection Agency (EPA). According to that document, one can describe a “worst case release scenario” as release of the entire contents of the largest storage vessel over duration of ten minutes.
RRS/Schirmer then adopted a calculation procedure from the API RP-55 to estimate radii of exposure associated with the worst-case release scenario for the refinery in New Mexico. The equation for predicting ROE for H2S releases was taken from the Appendix C of API RP 55:
where ROE is the H2S radius of exposure, A and B are coefficients contained in Table C-1 of the API RP 55, and [H2S] is the volume of H2S released. For a continuous release, the [H2S] must be specified in standard cubic feet per hour (SCFH); for a puff (instantaneous) release, [H2S] must be specified in standard cubic feet (SCF). The Table C‑1 in the API RP 55 document provides values for A and B for different hydrogen sulfide concentrations, continuous and puff releases, under both day and night weather conditions.
The New Mexico Oil and Gas Division regulations document defined “potentially hazardous volumes” as volumes of hydrogen sulfide gas of such concentration that:
- The 100-ppm ROE includes a public area. Public area means a building or structure that is not associated with the well, facility or operation for which the radius of exposure is being calculated and that is used as a dwelling, office, place of business, church, school, hospital or other similar area where public may reasonably be expected to be present
- The 500-ppm ROE includes a public road. Public road means a federal, state, municipal or county road or highway
- The 100-ppm ROE exceeds 3000 feet
If the ROE calculations identify that a well, facility or operation involves a potentially hazardous volume of hydrogen sulfide, then the facility shall develop a hydrogen sulfide contingency plan that the facility personnel will use to alert and protect the public.
Contents of Hydrogen Sulfide Contingency (H2S) Plan
The hydrogen sulfide contingency plan was developed by RRS/Schirmer with due consideration of guidelines in the API RP 55 and in accordance with the New Mexico Oil and Gas Division regulations document. The plan contained information on the following subjects.
- Emergency procedures for personnel to follow in the event of an release including responsibilities and duties
- Characteristics of hydrogen sulfide and sulfur dioxide
- Maps and drawings depicting the area of exposure and public areas and roads within the area of exposure
- Training in the responsibilities and duties of essential personnel and periodic on-site or classroom drills or exercises
- Coordination with state emergency plans
- Activation levels and the description of events that could lead to a release of hydrogen sulfide sufficient to create concentration in excess of the activation levels