APPENDIX C—A SCREENING APPROACH TO DISPERSION OF HYDROGEN SULFIDE

Note: The exposure radii shown in Figures C-l through C-4 represent esti­mates developed by API's Air Modeling Task Force (AQ7) using simple screening models and modeling techniques. These models should be reason­ably accurate for low velocity releases of neutrally-buoyant mixtures of hy­drogen sulfide and carrier gas. Figures C-l through C-4 are useful as a conservative screening tool for high velocity releases and for light hydrogen sulfide carrier gas mixtures. Figures C-1 through C-4 are not recommended for low velocity releases of heavier-than-air hydrogen sulfide/carrier gas mixtures or of potential aerosol-generating mixtures, since these illustra­tions sometimes will underpredicl exposure radii for these mixtures. Site specific conditions should be assessed to determine the need for additional, more rigorous modeling techniques. Users should evaluate their operations and select proper modeling applications for their specific emergency plan­ning purposes.

C.1 Introduction

The material presented in Appendix С is generic in nature and is intended for emergency response planning purposes to arrive at conservative hydrogen sulfide dispersion estimates. Figures C-l through C-4 present the screening-level, model-predicted radius of exposure (ROE) for atmospheric concen­trations of hydrogen sulfide at 10,30,100,300, and 500 ppm for both continuous and puff (instantaneous) releases of pure hydrogen sulfide. The ROE represents the distance from the emission source to the concentration of interest measured along the plume's centerline at ground level. Equations were developed for predicting the ROE as a function of the quan­tity/rate of hydrogen sulfide released for each of the hydro­gen sulfide concentrations modeled and the type of release (continuous and puff). The equations and corresponding co­efficients are presented in Par. C.8 and Table C-l. Meteoro­logical conditions typical of worst-case daytime and nighttime conditions were modeled.

Various regulations dealing with hydrogen sulfide opera­tions prescribe a method(s) or technique(s) for ROE predic­tions. Such methods must be taken into account because specific compliance actions may require use of a method(s) specified by the regulation, unless use of other methods are allowed.

C.2 Methodology

The ROEs shown in Figures C-l, C-2, C-3, and C-4 were predicted using standard EPA-approved modeling procedures based on Gaussian dispersion theory. The ROEs shown in Fig­ures C-l and C-2 were predicted by modeling a continuous, steady-state point source release of 100 percent hydrogen sul­fide. The ROEs shown in Figures C-3 and C-4 were predicted by modeling an instantaneous hydrogen sulfide release. Both hydrogen sulfide release types were modeled as releases of a neutrally-buoyant material under steady-state meteorological conditions. An effective plume height (release height plus plume rise) of 10 feet was used in all the modeling work. It was assumed that the predicted ROEs do not vary significantly with effective plume height in the range of 0-50 feet.

Table C-1—Linear Regression Coefficients for

Mathematical Predictions of ROE as a Function of

Downwind Hydrogen Sulfide Concentration and

Release Quantity/Rate

	Type of	Concentration,	Coefficients
Time*	Release	ppm	A	В
Day	Continuous	10	0.61	0.84
Day	Continuous	30	0.62	0.59
Day	Continuous	100	0.58	0.45
Day	Continuous	300	0.64	-0.08
Day	Continuous	500	0.64	-0.23
Night	Continuous	10	0.68	1.22
Night	Continuous	30	0.67	1.02
Night	Continuous	100	0.66	0.69
Night	Continuous	300	0.65	0.46
Night	Continuous	500	0.64	0.32
Day	Puff	10	0.39	2.23
Day	Puff	30	0.39	2.10
Day	Puff	100	0.39	1.91
Day	Puff	300	0.39	1.70
Day	Puff	500	0.40	1.61
Night	Puff	10	0.39	2.77
Night	Puff	30	0.39	2.60
Night	Puff	100	0.40	2.40
Night	Puff	300	0.40	2.20
Night	Puff	500	0.41	2.09

*Day Meteorological Conditions: Stability Class PG D (Neutral)—5 mph Wind Speed.

*Night Meteorological Conditions: Stability Class PG F (Stable)—2.2 mph Wind Speed.

For the purposes of dispersion modeling, the amount of turbulence in the ambient air is categorized into defined in­crements or stability classes. The most widely used cate­gories are the Pasquill-Gifford (PG) Stability Classes A, B, C, D, E, and F (Pasquill, F., Atmospheric Difusion, Second Edition, John Wiley & Sons, New York, New York, 1974). PG Stability Class A denotes the most unstable (most turbu­lent) air conditions and PG Stability Class F denotes the most stable (least turbulent) air conditions. PG Stability Class D denotes neutral atmospheric conditions where the ambient temperature gradient is essentially the same as the adiabatic lapse rate. Under neutral conditions, rising or sink­ing air parcels cool or heat at the same rate as the ambient air, resulting in no enhancement or suppression of vertical air motion.

Standard Pasquill-Gifford (PG) dispersion coefficients for flat, open grassland were used in the continuous hydrogen sulfide release model. The Slade (refer to NTIS-TID 24190: Slade, D. H., Meteorology and Atomic Energy, 1968) disper­sion coefficients for flat, open grassland were used in the puff (instantaneous) release model. When modeling instan­taneous hydrogen sulfide releases it was assumed that the downwind (x) and the crosswind (y) dispersion coefficients

 

 

 

HYDROGEN SULFIDE RELEASE RATE (SCFHJ
8.9 89.5 895 8952

HYDROGEN SULFIDE RELEASE RATE (LB/HR)

Figure C-1—Radius of Hydrogen Sulfide Exposure Continuous Daytime Hydrogen Sulfide Releases [PG D (Neutral)—5 MPH Wind Speed]

 

 

LEGEND:

- 10 ppm (8-HR AVG) + 30 ppm (10-MIN AVG) + 100 ppm (10-MIN AVG) о 300 ppm (10-MIN AVG) * 500 ppm (10-MIN AVG)

 

 

HYDROGEN SULFIDE RELEASE RATE (SCFH)

8.9 89.5 895 8952

HYDROGEN SULFIDE RELEASE RATE (LB/HR)

Figure C-2—Radius of Hydrogen Sulfide Exposure Continuous Nighttime Hydrogen Sulfide Releases [PG F (Stable)—2.2 MPH Wind Speed]

 

 

Recommended Practices for Oil and Gas Producing and Gas Processing Plant Operations Involving Hydrogen Sulfide

33

R Ю4

0

E,

10

10²

10³

10⁴

LEGEND:

— 10ppm(10-MINAVG) + 30ppm(1-MINAVG)

* 100ppm(1-MIN.AVG)
⁰ 300ppm(1-MINAVG)

* 500 ppm (1-MIN AVG)

QUANTITY OF HYDROGEN SULFIDE RELEASED (SCF)
0.09 0.89 8.9 89.5 895

QUANTITY OF HYDROGEN SULFIDE RELEASED (LB)

Figure C-3—Radius of Hydrogen Sulfide Exposure Instantaneous Daytime Hydrogen Sulfide Releases [Slade A (Slightly Unstable)—5 MPH Wind Speed]

R Ю

101

10

10°

10°

10²

10³

10⁴

LEGEND:

^10ppm(10-MINAVG) ⁺ 30 ppm (1-MIN AVG) ♦ 100 ppm (1-MIN AVG) ⁰ 300 ppm (1-MIN AVG) ^x 500 ppm (1-MIN AVG)

QUANTITY OF HYDROGEN SULFIDE RELEASED (SCF)
0.09 0.89 8.9 89.5 895

QUANTITY OF HYDROGEN SULFIDE RELEASED (LB)

Figure C-4—Radius of Hydrogen Sulfide Exposure Instantaneous Nighttime

 

were equivalent. This assumption results in conservative (worst case) estimates of the ROEs. The following meteoro­logical conditions were assumed to be representative of worst case daytime and nighttime conditions. For continuous daytime releases a neutral Stability Class (PG D) and 5 miles per hour wind speed were chosen. For continuous nighttime releases, a stable Stability Class (PG F) and a 2.2 miles per hour wind speed were chosen. For instantaneous (puff) day­time releases, a slightly unstable Stability Class (Slade A) and a 5 miles per hour wind speed were chosen. For instan­taneous nighttime releases, a neutral-to-stable Stability Class (Slade B) and a 2.2 miles per hour wind speed were chosen.

The ROEs for continuous hydrogen sulfide releases at 30, 100, 300, and 500 ppm are valid for averaging times of 10 minutes to 1 hour. The ROEs shown for 10 ppm (continuous hydrogen sulfide release) are based on an 8-hour average concentration, since 10 ppm represents the 8-hour time weighted average (TWA) for hydrogen sulfide. To obtain the 8-hour/10 ppm average concentration a factor of 0.7 was used to convert the 1 -hour concentrations (refer to EPA-450/4-88-009: A Workbook of Screening Techniques for As­sessing Impacts of Toxic Air Pollutants). The ROEs for the puff (instantaneous) hydrogen sulfide releases at 30, 100, 300, and 500 ppm are valid for averaging times of 1 to 10 minutes. EPA's 0.7 conversion factor was used to obtain the 10 minute/10 ppm time averaged concentrations from instan­taneous peak concentrations predicted by the model. For continuous releases, the EPA considers 10-minute and 1 -hour averaging times to be equivalent. The modeling re­ported herein assumed that an instantaneous release would be of a very short duration (10 to 15 minutes maximum).

Brief descriptions of the models used to predict the ROEs for both continuous and puff (instantaneous) hydrogen sul­fide releases are presented in Par. C.13.