Additional Considerations

The modeling work presented in Appendix С assumes a neutrally-buoyant, gaseous hydrogen sulfide release in flat, rural terrain under steady-state meteorological conditions. Also, the ROEs shown in Figures C-l through C-4 are for a generic class of hydrogen sulfide releases covering a wide range of site and release conditions. Actual ROEs will be de­pendent on the specifics of the type of release, release con­ditions, and release site. For instance, the ROEs for a release in a more urban setting where structures, buildings, etc. are present will be reduced significantly due to structure-induced turbulence. Some other conditions that could significantly af­fect the actual ROE include: a liquid/aerosol release, dense cloud behavior, a buoyant cloud (plume liftoff), a jet release, time-dependent release (well blowout, pipeline ruptures, etc.), and complex terrain. If any of these phenomena are present, then more rigorous modeling may be necessary.

The ROE curves of Figures C-l through C-4 should not be used when the mixture of hydrogen sulfide and carrier gas being dispersed is significantly heavier than air and the mix­ture is released at a low velocity. If the hydrogen sulfide/car-rier gas mixture specific gravity exceeds approximately 1.2, Figures C-l through C-4 may not give conservative ROEs for all release rates and meteorological conditions. Hydrogen sulfide, as encountered in the petroleum industry, is usually

a minor constituent of a carrier gas, such as natural gas or carbon dioxide. Carbon dioxide has a specific gravity of 1.52. Dispersion predictions for hydrogen sulfide/carbon dioxide mixtures, using a dense gas model sometimes under-predict hydrogen sulfide ROEs for low velocity gas releases. Low velocity gas releases would include those with initial velocities less than 200 feet/second and releases greater than 200 feet/second involve impact of the gas jet from the leak with a nearby surface, thereby breaking the jet's momentum. Likewise, Figures C-l through C-4 should not be used with any hydrogen sulfide/carrier gas release that potentially could form an aerosol.

Figures C-l through C-4 can also substantially overpredict ROEs. In the case of hydrogen sulfide/carrier gas mixtures significantly lighter than air (i.e., specific gravity less than 0.8) released at low velocity, use of these illustrations may overpredict ROEs by a factor of 2 to 3. Use of these illustra­tions can result in overestimation of ROEs for high velocity hydrogen sulfide/carrier gas releases (i.e., gas release veloc­ities greater than 200 feet/second) regardless of the orienta­tion of the release. However, this overprediction is particularly significant in the case of vertical, high-velocity releases. In such situations, the overprediction can be two or­ders of magnitude. The user should consult more rigorous at­mospheric dispersion models.

When calculating the ROE for dilute concentrations of hazardous gases, a significant overestimation can result. For example, it would not be practical to expect higher down­wind atmospheric concentrations than are present in the re­leased gas stream. The user should consult more rigorous atmospheric dispersion models.

In summary, the composition of the hydrogen sulfide/car­rier gas and the velocity and orientation of the release are critical variables, dramatically affecting predicted hydrogen sulfide ROEs. Also, other variables, such as released gas temperature and flashing or aerosol formation involving liq­uid containing dissolved hydrogen sulfide, can have signifi­cant impacts on ROE predictions. Accurate atmospheric dispersion techniques are, of necessity, complex. Under some circumstances, such as those mentioned above, more rigorous modeling may be required.

References and models are available to address special re­lease scenarios. A partial list of models that may be used in such cases is shown in Pars. C.5 and C.6. API does not en­dorse any one particular model. Further guidance on appro­priate model selection and application can be obtained from the model developers as well as other individuals experi­enced in this field. A specific reference to address well blowout and pipeline ruptures is "Release and Dispersion of Gas from Pipe Line Ruptures" Wilson, D. J., Department of Mechanical Engineering, University of Alberta, Edmonton, Canada.

In the event that hydrogen sulfide release quantities calcu­lated by the user are below the ranges shown in Figures C-l

through C-4, extensions of the ROE curves are allowed to a minimum ROE of 50 feet. In some cases, ROEs of less than 50 feet may be inferred from extrapolation of the curves. Figures C-l through C-4 were developed using an assumed release height plus plume rise of 10 feet. Actual release heights of other than 10 feet will result in different ROEs.

Results

ROEs for atmospheric plume-centerline, ground-level concentrations of hydrogen sulfide resulting from instanta­neous and continuous hydrogen sulfide releases were pre­dicted and are presented in Figures C-l through C-4. Figures C-l and C-2 present the predicted ROEs for continuous hy­drogen sulfide releases during worst case daytime and night­time meteorological conditions, respectively. Figures C-3 and C-4 present the predicted ROEs for instantaneous hydro­gen sulfide releases during worst case daytime and nighttime meteorological conditions, respectively. The ROEs for con­centrations of 10, 30, 100, 300 and 500 ppm were modeled for both release types. The 10 ppm concentration ROEs rep­resent an 8-hour averaging time for the continuous hydrogen sulfide release and a 10-minute averaging time for the instan­taneous release. The 30, 100, 300, and 500 ppm concentra­tion ROEs represent a 10-minute averaging time for the continuous hydrogen sulfide release and a 1-minute averag-

ing time for the instantaneous release. A hydrogen sulfide re­lease rate range of 10 to 10,000 lb/hr (111.8 to 111,765 SCFH) was modeled for the continuous type release. For the puff (instantaneous) type hydrogen sulfide release, a release quantity range of 0.1 to 1000 lbs (1.1 to 11,177 SCF) was modeled. If the hydrogen sulfide release is based on pounds, standard cubic feet (SCF) can be obtained by multiplying pounds by a factor of 11.2.

Note: The ROEs presented in Figures C-l through C-4 are plotted against the amount of hydrogen sulfide released. For the release of a multi-compo­nent gas stream, the actual amount of hydrogen sulfide released should be used to determine the ROE.

Equation coefficients based on linear regression for pre­dicting the ROE as a function of the release type (continu­ous/puff) and quantity/rate of hydrogen sulfide released for both daytime and nighttime meteorological conditions are presented in Table C-l. The equation is given in Par. C.8. The coefficients are applicable only over the ranges pre­sented in Figures C-l through C-4, and extrapolation could result in overly conservative estimates of the ROEs. Any re­lease lasting significantly longer than 15 minutes should be interpreted as a continuous release. The modeling work pre­sented in Appendix С assumes steady-state meteorological conditions. ROEs predicted for a long averaging time (8-hour) and long downwind distances are conservative because it is unlikely that the same meteorological conditions will persist during that time period.

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.

Respiratory Protection

Respiratory Protection

The National Institute for Occupational Safety and Health (NIOSH) has examined the criteria for respirator tests and sources of respirator leakage and recommends that positive pressure, either supplied-air or self-contained personal breathing apparatus, as appropriate, with a full face piece be worn by anyone exposed to atmosphere containing sulfur dioxide concentrations above OSHA's permissible exposure limit (PEL) (refer to 29 Code of Federal Regulations Part 1910.1000, Subpart Z, Table Z-l). Refer to Par. 6.4 for proper breathing equipment recommendations for oil and gas producing and gas processing operations involving sulfur dioxide.

 

	Concentration in Air
Percent by Volume	Parts Per Million By Volume	Grains Per lOOStd. Cubic Feet	Milligrams Per Cubic Meters
0.0001	1	0.12	2.71
0.0002	2	0.24	5.42
0.0005	5	0.59	13.50
0.0012	12	1.42	32.49
0.010	100	12.0	271.00
0.015	150	17.76	406.35
0.05	500	59.2	1354.50

0.10

1000

118.4

2708.99

Typical Characteristics Regarding Hydrogen Sulfide Exposure"⁷' Pungent odor, may cause respiratory changes. ACGIH TLV®"8>, and NIOSH REL.

Burning eyes, breathing irritation, and minor throat irritation.

Note: OSHA PEL (refer to 29 CFR 1910.1000, Table Z-l; ACGIH and NIOSH STEL as

averaged over 15 minutes.

Throat-irritating cough, constriction in chest, watering eyes, and nausea.

Concentration considered immediately dangerous to life or health (IDLH).fW Refer to DHHSNo. 85-114, NIOSH Pocket Guide to Chemical Hazards:№>

Extreme irritation. Can be tolerated for only a few minutes.

Causes a sense of suffocation, even with the first breath. Rescue promptly and apply artificial ventilation and/or cardiopulmonary resuscitation (CPR) techniques.

Death may result unless rescued promptly. Artificial ventilation and/or cardiopulmonary resuscitation (CPR) techniques should be immediately applied.

Note: Data in Table B-1 are approximate values for guidance. There are

published data that show slightly different values.

'"'Based on 1% sulfur dioxide - 1184 gr/100 SCF@ 14.696 psia and S9°F

(101.315 kPa and 15°C).

"⁷»Sulfur dioxide has physiological effects on humans. These effects vary

from person to person. FOR ADDITIONAL INFORMATION, CONSULT

WITH THE EMPLOYER AND RESEARCH THE MATERIAL SAFETY

DATA SHEETS (MSDS).

fWTLV is a trademarked term of American Conference of Governmental

Industrial Hygienists (ACGIH). Refer to Threshold Limit Values and

Biological Indices and companion documents available from ACGIH, 1330

Kemper Meadow Drive, Cincinnati, Ohio 45240.

"WIDLH means an atmospheric concentration of any toxic, corrosive, or

asphyxiant substance that poses an immediate threat to life or would cause

irreversible or delayed adverse health effects or would interfere with an

individual's ability to escape from a dangerous atmosphere (refer to 29

Code of Federal Regulations Part 1910.120). NIOSH considers 100 ppm or

more to be the IDLH concentration for sulfur dioxide (refer to NIOSH

Pocket Guide to Chemical Hazards).

«(^Available from Superintendent of Documents, U. S. Government

Printing Office, Washington, D. C. 20402.

	ТаЫе В-2—Summary of	Occupational	Exposure	Values	for	Sulfur	Dioxide
	OSHA PELs«"	ACGIH TLVs«2>				NIOSH RELs'2-»'
TWA	STEL	TWA	STEL			TWA STEL
ppm mg/m3	ppm Ppm Pprn	mg/m3	ppm	mg/m3		ppm	mg/m' ppm mg/m3

14

N/A

13

13

N/A

PELs Permissible Exposure Limits.

TLVs Threshold Limit Values.

RELs Recommended Exposure Limits.

TWA Eight-hour Time Weighted Average (refer to specific

reference document for different methods of weighting used). STEL Short Term Exposure Limit averaged over a period of 15

minutes.

N/A Not Applicable.

tt/JRefer to 29 Code of Federal Regulations Part 1910.1000, Subpart Z,

Table Z-l.

miRefer to Threshold Limit Values and Biological Exposure Indices, 1993-

94 (check latest edition).

(l^lReScr to NIOSH 77-158: Criteria for a Recommended Standard for

Occupational Exposure to Sulfur Dioxide.

Physiological Effects

1 ACUTE TOXICITY

INHALATION AT CERTAIN CONCENTRATIONS CAN LEAD TO INJURY OR DEATH (refer to Table B-l). Exposure to concentrations below 20 ppm can cause eye ir­ritation, throat irritation, respiratory tract irritation, chest constriction, and some nausea. Exposure to concentrations above 20 ppm can result in marked coughing, sneezing, eye irritation, and chest constriction. Exposure to 50 ppm causes irritation to the nose and throat, running nose, coughing, re­flex broncho-constriction with possible increase in bronchial mucous secretion, and increased pulmonary resistance to air

"•""TLV" is a trademarked term of the American Conference of Govern­mental Industrial Hygienists (ACGIH). Refer to Threshold Limit Values and Biological Exposure Indices and companion documents available from ACGIH, 1330 Kemper Meadow Drive, Cincinnati, OH 45240 (check latest edition).

flow (breathing congestion) occurs promptly. This atmo­sphere (50 ppm or more) will not be tolerated by most per­sons for more than 15 minutes. Some reported acute reactions of exposure to high concentrations include, but are not limited to, inflammation of the eyes, nausea, vomiting, abdominal pain, and sore throat. These symptoms are some­times followed by bronchitis, pneumonia, and /or complaints of weakness for a period of weeks.

B.3.2 CHRONIC TOXICITY

It has been reported that prolonged exposures to sulfur dioxide may lead to increased risk of chronic nasopharyngi-tis, alteration in sense of smell and taste, shortness of breath on exertion, and a higher frequency of respiratory tract infec­tions compared to unexposed persons. It has also been pos­tulated that sulfur dioxide in the work environment "possibly enhances" the suspected carcinogenic (cancer) effect of ar­senic or other cancer agents'"'. No definite evidence is avail­able regarding co-carcinogenesis or promotion of cancer by sulfur dioxide exposure. A few persons apparently have or develop a hypersusceptibility to sulfur dioxide. Decrements in pulmonary function tests have been noted after both acute and chronic exposures.

B.3.3 EXPOSURE RISKS

It is not yet clear what concentrations of low level expo­sure or lengths of exposure increase the risks, nor by how much the risks are increased. Sulfur dioxide exposures should be minimized. Smoking by persons exposed to sulfur dioxide should be strongly discouraged.

Exposure Limits

The Occupational Safety and Health Administration (OSHA) has established a permissible exposure limit (PEL) of 5 ppm as an 8-hour TWA for sulfur dioxide (refer to 29) Code of Federal Regulations Part 1910.1000, Subpart Z, Table Z-l. The American Conference of Governmental In­dustrial Hygienists (ACGIH) recommends 2 ppm as an eight-hour TWA Threshold Limit Value (TLV)® and 5 ppm as a STEL averaged over 15 minutes for sulfur dioxide."'" Refer to Table B-2 for additional information on exposure values. CHECK WITH THE EMPLOYER CONCERNING EXPOSURE LIMITS FOR PARTICULAR CIRCUM­STANCES.

APPENDIX B—PHYSICAL PROPERTIES AND PHYSIOLOGICAL EFFECTS OF SULFUR DIOXIDE

Physical Data

Chemical Name: Sulfur Dioxide.

CAS Number 7446-09-05.

Synonyms: Sulfurous anhydride, sulfurous oxide.

Chemical Family: Inorganic.

Chemical Formula: SO₂.

Normal Physical State: Colorless gas appreciably heavier than air. Vapor density (specific gravity) at 32^CF (0°C) and 1 atmosphere - 2.26.

Boiling Point: 14°F(-10.0°C).

Flammable Limits: Non-flammable (produced from burn­ing hydrogen sulfide).

Solubility: Readily soluble in water and oil; solubility de­creases as the fluid temperature increases.

Odor and Warning Properties: Sulfur dioxide has a pun­gent odor associated with burning sulfur. It produces a suffo­cating effect and produces sulfurous acid on membranes of the nose and throat.