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ELEMENT 8 HYDROGEN SULFIDE INSTRUCTOR & TECHNICIAN COURSE HSI008

ELEMENT EIGHT

Radius of Exposure

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ELEMENT 8 RADIUS OF EXPOSURE

8.1 Introduction:

Hydrogen Sulfide is a very toxic gas in small concentrations. The concentration that is currently considered immediately dangerous to life and health is 100 parts per million. H2S will paralyze your respiratory system rapidly if you are exposed to 1000 parts per million.

The purpose of this element is to introduce methods to determine how far an H2S release may spread. This will enable us to develop emergency action plans.

First, we need to know who may be affected by a sudden or continuous release. Second, we can control the risk of exposure for personnel who work within the exposure radius.

Gas dispersion modeling is a tool that can be used to describe how far a gas release may spread. This element is intended to introduce you to some of the types of models, and provide further information on where you could find more effective models.

We will take a look at early "radius of exposure" equations, and describe where computer simulated software can be found. It is important to understand that the early models make assumptions. We will see why early models are limited as far as accuracy, and should be used as rough guidelines.

The actual radius of exposure will depend on site-specific considerations.

8.2 H2S Release Dispersion Models:

When considering application of an H2S dispersion model, the user should examine the type of release which might be expected, for example, will the gas be dense or buoyant; will it be dry or wet; etc. Application of dispersion must consider if the release may have a vertical or horizontal plume configuration. See page 15 plume configurations. The prevailing wind and environmental factors such as humidity and temperature variables will influence the dispersion. Engineering calculations to determine H2S absolute open-flow rates based on pressure and hole size are necessary to ensure relative accuracy of the dispersion radius of exposure and travel distance.

Historical incidents of H2S accidental releases have led to the development of computer models to simulate the dispersion of the gas or vapor cloud over a given area. The importance of such modeling becomes apparent in planning for emergency evacuation of the affected population.

The Pasquill-Gifford equation is widely used by the Texas Railroad Commission in the Statewide Rule 36 to address the protection of the general public.

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This equation can be found in the Texas Rule 36 under section (c)(2) (A) and (B) and corresponds to a nomograph contained in the rule to identify a radius-of-exposure for 100ppm and 500ppm H2S. It is pointed out that for the rule; public protection begins at 100ppm.

8.3 Texas Rule 36

When considering application of an H2S dispersion model, you must examine the type of release that might be expected. For example, will the gas release be dense or buoyant; will it be dry or wet; etc. Bear in mind that H2S is heavier than air, but may be mixed with lighter than air gases. The actual vapor density of the plume or release may be lighter, heavier, or equal to that of air.

The release may have a vertical or horizontal plume configuration.

The prevailing wind and environmental factors such as humidity and temperature will influence the dispersion.

Engineering calculations to determine H2S absolute open-flow rates based on pressure and hole size are necessary to ensure relative accuracy of the dispersion radius of exposure and travel distance.

The Pasquill-Gifford equation is widely used by the Texas Railroad Commission and can be found in Statewide Rule 36 to address the protection of the general public. This equation can be found in the Texas Rule 36 under section (c)(2) (A) and (B) and corresponds to a nomograph contained in the rule to identify a radius of exposure for 100ppm and 500ppm H2S.

This rule requires contractors to provide different levels of emergency planning based on the expected radius of exposure to Hydrogen Sulphide.

For example, if the stream concentration is less than 100ppm H2S, contractors are exempt from the rule.

If the 100ppm ROE is less than 50 feet, contractors are exempt from most requirements except training. The exception is with storage tanks, where if the concentration of vapor is 500ppm or higher, you must install signs and possibly fences around the tank. This is called Case 1.

If the ROE is greater than 50 feet and less than 3000 feet, but does not include a public area and a the 500ppm ROE does not include a public road then there are set requirements. A limited amount of personal protective equipment must be used, operation equipment must conform to the National Association of Corrosion Engineer standards, and administrative controls are required (signage). This is called Case 2

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If the 100ppm ROE is greater than 50 feet and includes a public area or if the 100ppm ROE is greater than 3000 feet regardless of public areas, then the controls required are greater. At this point, gas detection systems will be needed, contingency plans implemented, and set procedures for reporting leaks are required. Rule 36 suggests contractors use the American Petroleum Institute's recommendations as a source of additional. This is Case 3.

A quick overview of the compliance requirements is shown in the following table reproduced from the 1990 Rule 36, page 7.

Provision Case 1 Case 2 Case 3

H2S concentration test X X XH-9 or compliance certificate X X XTraining X X XGovernment office notification X X XDrill stem tests restricted X* X* XBOP test X* X* XMaterials meet NACE stds. X XWarning & marker X XSecurity X XContingency plan XControl & safety equipment XMonitors X XMud (pH control or scavenger) X*

Wind Indicators X XProtective Breathing Equipment X XChoke manifold, secondary remoteControl, and mud/gas separator XFlarestacks X*

The asterisk(*) indicate exactly which district is affected. Districts 5 and 6 only.

If the expected concentration of H2S is unknown then according to Rule 36, a 100ppm ROE of 3000 feet must be assumed.

As you can see, the ROE calculation is an important tool for planning whilst working in an H2S environment in Texas. These calculations are also used in international areas, specifically Italy, or where a contractor requests guidance.

A scientific calculator is required to determine the radius of exposure using the Pasquill Gifford equation. You also need to know the production rate of the well and the concentration of H2S in the well. The H2S concentration used in the equation is the mole fraction of H2S and can be calculated by dividing the parts per million by 1,000,000. The flow rate is in cubic feet per day.

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The oil and gas industry applies this equation to pipelines or facilities within gas processing plants. Other applications include an uncontrolled release of gas due to loss of well control.

In the drilling industry, the flow rate used should either be the well's adjusted open-flow potential, or if the operator feels this is too high, the well's capacity to flow against a zero backpressure is acceptable for the equation.

The equation as originally formulated, is a three-plane coordinate system. The X-axis is that direction in which the wind would be blowing at a continuous rate. The Y-axis is the vertical coordinate and the Z-axis is the horizontal coordinate.

There are quite a few assumptions made. These also limit the reliability or accuracy of the results.

The equation assumes that the emission will be constant such that some distance downstream there is a point at which the 100ppm concentration will also be constant over the 24-hour period.

Normal Gaussian distribution of the Y and X planes are assumed; that is, there is less concentration in ppm of H2S on the sides of the release and the highest concentration is at the centre.

It is also assumed that the emission point will be at or near ground level. It eliminates a very complex component that is used to determine stack height. The Environment Protection Agency, as well as the Texas Natural Resources Conservation Commission uses this equation.

The original Pasquill-Gifford equation took several meteorological parameters into consideration. These parameters are classed into Categories A,B,C,D,E, and F.

Class A is the unstable condition, with stronger wind velocity, more radiation, uneven terrain, etc.

Class F is the most stable and is used in the equation to represent the most conservative stability condition. A one-mile per hour wind velocity is assumed to represent the least amount of air mixing.

This accounts for the equation providing the largest radius of exposure, thus the more conservative approach. The equation is not so accurate that if you calculated a 100ppm radius of 1500 feet exposure that the gas will not go beyond it.

The Pasquill-Gifford method of calculating radius of exposure is not infallible. The field data collected by the Texas Railroad Commission on a 10-minute sampling time indicate that for ground level H2S concentrations, a safety

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factor of 2 or 3 is present. The calculation is based on a worst case scenario to create the maximum radius of exposure.

This approach was selected to provide oil and gas operators with a uniform method of calculation. It should be noted that most states consider this method acceptable and some have adopted it or use variations thereof.

In high-density population areas, where the radius of exposure includes a large area, a more sophisticated model that takes into account better-defined parameters should be considered. Approval from various state regulatory agencies may be required to approve lesser radii of exposure that result from more technically developed dispersion models.

Here are the equations used in Rule 36:

X or 100ppm ROE = [(1.589)(mole fraction H2S)(Q)]0.6258

X or 500ppmROE = [(0.4546)(mole fraction H2S)(Q)]0.6258

Where: X = radius of exposure in feet

Q = maximum volume determined to be available for escape in cubic feet per day

H2S = mole fraction of H2S in the gaseous mixture

ROE = radius of exposure

Fortunately, if you do not have a scientific calculator available, the equation results have been tabled in the form of what we call a nomograph. Let's take a moment to look at the ones available. There are two here, one is used to quickly find the 100ppm H2S radius of exposure and the other is used to find the 500ppm ROE.

On the left side of the graphs is a scale where you can choose the quantity of H2S in parts per million or percent H2S.

On the right side is a scale where you can choose the flow rate of the well and is labeled as Q. It is important to note that the Q scale is in million cubic feet per day. On the top of the scale it says Q in (m2cfd). M in this case represents 1000, so m2 is 1000 times 1000 or 1,000,000.

The middle scale represents the radius of exposure in feet. Simply locate the flow rate on the right, the H2S concentration on the left and hold a straight edge between the points. Where it intersects the middle scale is the expected radius of exposure.

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Here is one example using the radius of exposure monographs:

Sample problem:A pipeline has a flow of 25 M2CF/D of gas that contains 2000ppm H2S.

Refer to the 100ppm ROE. Graph and locate the 25 M2CF/D and the 2000ppm H2S points. By connecting the points with a straight edge, read the 100ppm ROE which is 1200 feet.

Refer to the 500ppm ROE. Graph, and by using the same method above, read the 500ppm ROE which is 550 feet.

8.4 American Petroleum Institute Recommended Practice 55

Another equation used to determine the radius of exposure can be located in appendix C of API RP 55. This is a recommended practice for safe operations whilst producing wells containing H2S

The equations used here take into consideration some additional variables.

Is the release continuous or a puff (short-term release). Is the release100 % H2S or not? What is flow rate of well? Did the release occur in the day or night? Different weather

conditions were considered depending on the time of day.

The ROE equation = Antilog (A X log (H2S) + B); where,

A and B are constants taken from table C1 in API RP 55.

You must determine for night or day release.

You must determine for continuous or puff release.

You must consider ROE ppm H2S range you are mapping. There are 5 different ROE calculations for each consideration.

Example 1:Determine the ROE100ppm for a continuous release of 100% H2S at a rate of 11,170 SCFH (standard cubic feet per hour) in daylight hours. This assumes a 5-mile per hour wind.

Using Table C-1, the coefficients applicable to this scenario are:A = 0.58B = 0.69

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Using the equation:

The H2S used here is the mole fraction of H2S times the flow rate in cubic feet per hour. This would be 11,170 X 1,000,000/1,000,000 or 11,170 scfh.

ROE100ppm = Antilog ( A times log (H2S) + B) we arrive at:ROE100ppm = Antilog (0.58 X log (11,170) + 0.69 or 628 feet.

Note: We used 11,170 for our H2S because we are dealing with a 100% H2S release.

Example 2: Determine the ROE100ppm for a continuous release of 100% H2S at a rate of 11,170 scfh in nighttime. This assumes a 2.2-mph wind speed.Using Table C-1, the coefficients applicable to this scenario are:

A = 0.66B = 0.69

Once again, the H2S figure used would be 11.170scfh and the answer would be:

ROE100ppm = Antilog (0.66 X log (11,170) + 0.69 or 2300 feet.

To calculate the instantaneous release the H2S part of the equation equals the total cubic feet released in a specific period of time. Therefore, if the well contained 100% H2S, flowing at a rate of 11,170 scfh, then the total released in 10 minutes would equal (11,170 scfh divided by 60 minutes per hour) X 10 minutes.

Example 3:The above calculation would equal:

(11,170 scfh / 60) X 10 or 1862 scf

What happens if we have a well that does not contain 100% H2S as is commonly encountered. First we would have to determine the H2S portion of the equation as with the following example:

Example 4: We have a continuous release of natural gas containing 5,000,000 SCFD. The H2S concentration is 8,000 parts per million.Let’s say we have a gas well that contains 8,000 ppm H2S. The well is producing 5,000,000 SCF per day. How many cubic feet of H2S are released each hour.

First the H2S concentration must be written as a mole fraction. Divide 8,000 ppm by 1,000,000ppm.

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Mole fraction of H2S = 8,000/1,000,000 = 0.008 MF H2S.

The well is releasing 5,000,000 SCFD, of which 0.008 MF is H2S, so the well is releasing 5,000,000 X 0.008 or 40,000 SCFD H2S.

The well is therefore releasing 40,000 divided by 24 hours per day or 1666 SCFH H2S.

You would use this value to calculate a continuous release. If this well had an instantaneous release that lasted 10 minutes then find the amount released per minute or divide 1666 by 60 minutes per hour, which is 27.8 SCFM and multiply the result by 10 minutes or 278 SCF. Use this amount when calculating ROEs using Table C-1 puff models.

As we mentioned there are many assumptions with the Texas rule 36 equations. There are also assumptions made with the API equations.

Wind speed? 5 mph for days, 2.2 mph for nights. Stack height = 10 feet above sea level. There are no obstructions to flow, buildings, etc. The gas release is considered neutrally buoyant.

8.5 FACTORS AFFECTING DISPERSION OF HYDROGEN SULFIDE:

Weather conditions:

Poor dispersion Good Dispersion

Calm winds (0 mph) Fair winds (above 10 mph)

Stable Air Unstable Air

Cloudy skies Clear Skies

Winter season Summer season

Night-time Daytime

Fog No fog

Low temperature High temperature

(below 30 degrees ˚F) (above 90 degrees ̊ F)

Leak Characteristics:

Poor Dispersion Good dispersion

High H2S concentration Low H2S concentration

Large volume Small volume

High pressure Low Pressure

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Terrain Features:

Poor Dispersion Good dispersion

Low places, ravines, gullies Flat, open ground

Buildings and obstructions No buildings or obstruction

Trees Grassland

INSTRUCTOR EXPLAIN THE RADIUS OF EXPOSURE NOMOGRAPHS FOR 100PPM, AND 500PPM HERE.

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8.6 Further Information:

API RP 55 (American Petroleum Institute), Appendix C

In the appendix there are several proprietary computer models as well as publicly available ones. It is not meant in this element to promote any particular model. Personnel should carefully evaluate site-specific conditions to choose between any model.

Proprietary available models include but are limited to:

CHARM is a model developed by Radian Corp. in Austin, TX.

FOCUS was developed from Quest Consultants out of Norman, Oklahoma.

TRACE was developed by Dupont in Westlake Village, California.

WHAZAN is a model developed from Technica International out of Woodstock, Georgia.

Publicly available models include but are not limited to:

DEGADIS is a United States Coast Guard model and further information can be obtained from the National Technical Information Services (NTIS) out of Springfield, Virginia.

HEGADAS is a Shell research model and information can be obtained through NTIS.

SLAB is a model developed by the Lawrence Livermore National Laboratory in Livermore, California.

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