kev scenario 3

8/7/2019 Kev Scenario 3

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SCENARIO 3- Ethylene Oxide Vessel Rupture

Incident

One of three identical ethylene oxide storage tanks (E-107) suffered corrosion at a certain point

which developed into the formation of a hole. Despite the use of stainless steel 304 in

construction, there existed poorly welded seams at the offtake that were not properly

radiographed. The material inside the tank was under a constant pressure of 2.43 bar and a

temperature of 350C. The corroded points succumbed to the elevated pressure in the tank and

toxic gas escaped.

Incident Outcome

The release of ethylene oxide through the hole resulted in the accumulation of the toxic

compound in the air and an explosion of the gas upon exposure to an ignition source. Ethylene

oxide is a highly reactive and toxic chemical.

It is classified by the National Fire Protection Association (NFPA) as a class 1A flammable liquid,

and it is listed by EPA as an extremely hazardous substance. This grouping includes other

substances whose characteristics include a flash point less than 73F and boiling point less than

100F (National Fire Protection Agency, 2011).

Source Model

A model will be used to describe the behavior of the material after it has been released from

containment. Under ambient conditions (250C and 1 atm) ethylene oxide is a vapor. It is

however processed and stored as a liquid. This can be accomplished through one of two ways.

Ethylene oxide can be cooled and processed at a temperature below its boiling point (10.70C, or

it can be processed under pressure greater than its vapor pressure (>2.432 bar).

The application of pressure is done by the addition of nitrogen gas to the cylinders or by use of

a methane blanket (Othmer, 2004). In this case, nitrogen gas is used as it is readily available on

the plant.

For the purpose of modeling, the chemical is assumed to exit the tank while remaining in its

liquid state.


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Further assumptions and reasoning made:

o Dimensions of the tank:y Height: 7.95my Diameter: 3.98m

o The tank was 75% full of ethylene oxide, thus giving the height of liquid in the tank to be0.75 x 3.98 = 2.99m.

o The line leading out of the tank was a standard 1 inch pipe, so its removal from the tankresulted in the 25.4mm hole.

o Density of ethylene oxide remained constant during discharge (850 kg/m3).o Frictional flow due to the holes entrance and exit.o No external heat transfer and shaft work.o Fully developed turbulent flow.


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Liquid Discharge through a Hole in a Tank

Since the density of ethylene oxide remained constant during discharge, a mechanical energy

balance gives the following equation:

0m

Wevv

g2

1zz

g

gPP.

sf

2

1

2

2

c

12

c

12 !

V

Since there is no shaft work and flow is frictional, the above equation is simplified to give:

0egvv2

1zzg

)(g

c

2

1

2

21212c !

V

Where;

P1 = tank pressure above the liquid (1.419 barg)

P2 = pressure outside the hole (0 barg)

= density of the fluid (850 kgm-3

)

g = acceleration due to gravity (9.81 ms-2

)

gc = gravitational constant (force/mass-acceleration)

z1 = reference height i.e. at ground level (0 m)

z2 = height of liquid in tank (2.99 m)

fe = frictional loss term

The 2-K method was used to determine the frictional components. Since it was assumed

that fully developed turbulent flow would exist, then Reynolds number would be greater than

10,000, hence the head loss factors are as follows:

exitthefor0.1K

entrancethefor5.0K

f

f

!

!

V


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Using the data and equation, the pressure term (1st

term), height term (2nd

term) and

the velocity coefficient (last term) were calculated and the exit velocity was then determined

using the following equation:

11.1225.1

)7.14()94.166( !

!

!

ms

tcoefficienvelocity

termheighttermpressurevelocityexit

Where

Pressure Term = P/

Height Term = g (z1-z2)

Pressure in Pa, density in kg/m3

Diameter of hole, D = 25.4 mm

=> Cross-sectional area,24

2

1007.54

mD

Av!!

T

The corresponding mass flow rate = 5.19 kgs-1


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Possible Outcome Cases

Case 1 Liquid Trajectory:

The first case of this scenario is the projection of the ethylene oxide liquid from the hole

through some horizontal distance before it reaches the ground. This would happen as a result

of the pressure exerted by the liquid head plus nitrogen pressurizing gas above the level of

puncture, converting the liquids potential energy into kinetic; thereby giving it sufficient

energy be projected a distance away from the tank. The exit velocity of the liquid from the hole

The liquid trajectory from the hole was calculated using the following equation:

tvsdistance,Horizontal 2!

Where

s = distance away from tank where the liquid stream will impact the ground

v2 = discharge velocity (12.1 ms-1

, from above)

t = time for liquid to fall the distance h

The time taken for the liquid to fall the distance h, height of the hole from the ground, is

given by the acceleration due to gravity, g,

sx

t

g

ht

714.081.9

5.22

2

!!

!

Hence, s = 12.1 x 0.714 = 8.64 m

Therefore the liquid will be discharged approximately 8.64 m away from the tank.


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When the liquid is discharged it is contained in a dike (a supportive barrier for leakages)

which surrounds the tank, however the minimum size of this dike should be determined in

order to implement measures to prevent any liquid discharge outside of the affected zone. This

was done by finding the maximum discharge distance when the hole is located at some height

above the ground.

In order to determine the maximum discharge distance, h, the location of the hole is

given by:

!

Vg

PgH

2

1h

gc

Where;

H = total liquid height above the ground (or the maximum liquid height in the tank (4.5 m)

The maximum discharge distance, s, is given by:

Where;

Kf = excess head loss (1.5 m)

P = Gauge Pressure in Pa

gc = Gravitational Constant (1)

The maximum discharge distance was found to be 10.76 m with a maximum height of the hole

is 13.61 m. Therefore, the dike should have a radius of about 11 m.

!

f

gc

K

gPgHs

1

/ V


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Case 2 Pool Boiling:

Now it is known that the discharged liquid is being contained in a dike, where it will be

exposed to atmospheric conditions. Since the ethylene oxide was stored below its vapour

pressure, exposure to atmospheric conditions now will cause it to boil in the dike, thereby

releasing ethylene oxide gas into the atmosphere. So the second case of this scenario becomes

a boil pool vapourization. As such, the evaporation rate of this pool of liquid ethylene oxide

must be determined.

Prior to that, however the pool area must be determined, and is done via calculating the

size of the dike based on whatever design features was applied to it. It is assumed that this dike

is circular in nature and bounds three (3) storage tanks. It is also assumed that only one tank

can fail at a time so. It should also be noted that for safety considerations, the dike is designed

with a capacity of 150% of that of the storage tank.

Further assumptions and reasoning:

Pool growth was radial and uniform from the point of spill along a flat surface. As a worst case, maximum possible vapourization will take place and this will occur due

to maximum spread (area) of the pool, i.e. the area around the tanks bounded by the

dike.

Vapourization will begin at the same time and only occur when the dike is fully coveredby a layer of liquid ethylene oxide 1cm in depth. It should be noted that this assumption

may be unrealistic since ethylene oxide should evaporate on breaking containment and

immediately undergo phase change. However to estimate the maximum possible

vapourization rate the ethylene oxide should vapourize at the same time, when the area

is the largest.

As calculated before the dike should have a minimum distance of 11 m around the tank. Since

the dike bounds three storage tanks, and taking into consideration of the dimensions of the

tanks, it was assumed that the dike should have a diameter of 196 m.


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Cross-sectional area of dike,23

2

1017.304

mD

Ad

v!!T

Cross-sectional area of tank,264.31 mLengthDiameterAt !v!

Resultant pool area of ethylene oxide, 2xAAAtdp

!

2310075.30

64.31330170

mx

xAp

!

!

Pool volume, depthxAV pp !

337.30001.010075.30 mxxVp !!

kg

volumexdensity

255638

7.300850

izationforvapourpoolofmassRequired

!

v!

!

Mass flow rate calculated = 5.19 kgs-1

Time taken for ethylene oxide to cover the ground,holethroughrateflowmass

poolammoniaofmasst g !

s4925619.5

255638!!gt

= 13.7 hrs

Hence it will take approximately 13.7 hours before it is assumed that vapourization will begin.

The heat flux from the ground is given by the following:

21

s

gs

g

t

TTkq

TE

!


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Where;

gq = heat flux from the ground (J(m

2s)

-1)

gT = temperature of the ground (308 K)

T = temperature of the pool (283 K)

sk = thermal conductivity of the soil (0.92 W(m-K)-1

)

sE = thermal diffusivity of the soil (4.16 x 10-7

m2s

-1)

t = time heat is transferred from the ground (90 s)

Therefore,

12

21

721

2120

901016.4

28330892.0

!

!

! sJmxxxt

TTkq

s

gs

g

TTE

Heat of vapourization, L= 5.99 x 105

Jkg-1

The evaporative flux, 125

00354.01099.5

2120 !

v

!! smkg

Hm

The total evaporation rate for the entire pool area = 30075 x 0.00354= 106.4 kgs-1


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Case 3 Plume Release:

Ethylene oxide has undergone vapourization at a particular rate, and the resultant

vapour was dispersed into the atmosphere. Since this dispersion occurs for more than 10

minutes, it is considered to be a plume release, if the release was less than 3 minutes it would

have been classed as a puff release. Ethylene oxide, from the material safety and data sheet

(MSDS), has a Threshold Limit Value (TLV) of 1 ppm, which is the maximum concentration of gas

which an employee can withstand for one hour without recognition of an odour. The odour

associated with it is sweet and the gas is colourless.

It also has an Immediately Dangerous to Life and Health (IDLH) value of 800 ppm.

Operators are required to wear self breathing apparatus at the gas house where ethylene oxide

is being stored.

It should be noted that the concentrations predicted by Gaussian models are time

averages. Thus, it must be considered that the local concentrations might be greater than this

average. This result is important since ethylene oxide is highly toxic and any fluctuations in the

local concentrations can have a significant impact on the consequences.

Assumptions

o The release is done atmospheric conditions (temperature, 300 K, and pressure, 1 atm)o Scenario occurs on a clear night, where the wind speed is approximately 1.67 ms-1.o The atmospheric stability class is Fo It is a ground level release, hence the release height is zero (i.e. H = 0)o The area in which the release occurs is ruralo Cross wind direction (y), is zeroo The location of the release is on the ground (i.e. z = 0)


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This plume model is intended to describe the continuous release of material and its

solution depends on the rate of release, the atmospheric conditions, and the height of the

release above the ground, and the distance from the release. In this scenario the wind is

moving at a constant speed in one direction and there are no cross winds (y=0). Since the

release is at ground level (z=0), the maximum concentration occurs at the release point. Hence,

the concentration of ethylene oxide gas will decrease as the distance downwind increases. At a

particular distance the concentration will be at IDLH value.

Hence in modeling this plume release, the primary step is to determine what will be the

minimum distance downwind required, based on the release rate, molecular weight of ethylene

oxide and ambient conditions (temperature, pressure and wind speed), whereby the IDLH is not

exceeded. This will, in the event of such a scenario, give any idea of the minimum safe distance

downwind an individual will be from the source.

The average concentration, C , is given by the following equation:

v

!

222

2

1exp

2

1exp

2

1exp

)2(,,,

ZZYZY

HzHzy

u

GzyxC

WWWWWT

Since the release is at ground level and no cross winds present, this equation simplifies to:

u)2(

G,z,y,xC

ZYWWT

!

(Lees, 2005)

ZY

WW , are the dispersion coefficients in the x and y directions

G = the continuous evaporation rate (106.4 kgs-1

)

u = the wind speed

x = is the distance downwind (m)


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Now using the equations for the Pasquill Gilford dispersion coefficients for plume dispersion

for different stability classes and both rural and urban terrain;

Input Data:

Release rate: 106.4 kg/s

Molecular weight: 44.05

Temperature: 300 K

Pressure: 1 atm

Release height: 0 m

Distance downwind: 50 m

Distance off wind: 0 m

Distance above ground: 0 m

Sample Calculation:

For stability class A, Rural Conditions

3

09.30.1x10]10.97)2(

4.106

,,,

!! kgmxxzyxC T

This concentration is 3090 mgm-3

. To convert to ppm, the following equation is used;

ppmmgmC

M

mole

gm

LatmC

ppm1725000)(

08206.0 3 !v

!

The concentration downwind was calculated 50m away. It was also calculated at the distance

downwind where ethylene oxide is 800 ppm and under.


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When x=50 m

RURAL Conditions

P-GS

tability A B C D E FAssumed wind speed: 0.1 0.1 2 3 2 2 m/s

Dispersion Coefficients:

Sigma y: 10.97 7.98 5.49 3.99 2.99 2.00 m

Sigma z: 10.00 6.00 3.98 2.89 1.48 0.79 m

Downwind concentration: 3.09E+00 7.07E+00 7.76E-01 9.78E-01 3.83E+00 1.08E+01 kg/m3

3086612 7073487 775502 977860 3829114 10769384 mg/m3

PPM: 1724999 3953124 433401 546491 2139958 6018631 PPM

URBAN Conditions

P-G Stability A-B C D E-F

Assumed wind speed: 0.1 2 3 2 m/s


Sigma y: 15.84 10.89 7.92 5.45 m

Sigma z: 12.30 10.00 6.53 3.86 m

Downwind concentration: 1.74E+00 1.55E-01 2.18E-01 8.06E-01 kg/m**3

1738585.14 155478.08 218339.08 806015.51 mg/m**3

PPM: 971634.26 86891.24 122022.05 450453.80 PPM

Now,

Trial and error was also used to check and see the distance downwind where the concentration

of the ethylene oxide will achieve the acceptable limit of 800 ppm and under. This distance was

worked out to be x=2449m under rural conditions and a wind speed of 0.1 m/s.


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When x=2449m

RURAL Conditions

A B C D E FAssumed wind speed: 0.1 0.1 2 3 2 2 m/s


Sigma y: 482.89 351.19 241.44 175.59 131.70 87.80 m

Sigma z: 489.80 293.88 160.51 67.97 42.35 22.59 m

Downwind concentration: 0.00 0.00 0.00 0.00 0.00 0.01 kg/m**3

1432 3282 437 946 3036 8539 mg/m**3

PPM: 800 1834 244 529 1697 4772 PPM

URBAN Conditions

A-B C D E-F

Assumed wind speed: 0.1 2 3 2 m/s


Sigma y: 556.99 382.93 278.50 191.47 m

Sigma z: 1091.56 489.80 118.67 90.63 m

Downwind concentration: 5.57E-04 9.03E-05 3.42E-04 9.76E-04 kg/m**3

557.05 90.29 341.59 975.91 mg/m**3

PPM: 311.32 50.46 190.90 545.40 PPM


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Toxic Gas Effect

The toxic gas effect model serves to simulate the effects of a toxic release at known distances

from the release point. In addition to assumed distances, it is based on concentration readings

which are corresponded in a Probit equation which gives the likely fatality outcome at the

respective geographical locations. Additional factors affecting the model are the rural and

urban conditions, stability class and wind speed.

To calculate the percentage fatalities from a fixed concentration-time relationship, the

following Probit Equation is used:

Y = k1 +k2 Ln (Cn. te)

Where, k1, k2 and n are constants.

According to the CPD Green book (1992b); deWeger, Pietersen and Reuzel (1991):

K1= -6.8 k2= 1.0 n = 1

(deWeger, Pietersen, & Reuzel, 1992)

The tables below summarize the concentrations previously calculated along with the

corresponding Probit variable and fatality outcome

50m RURAL

Stability class Concentration

(mg/m3)

Probit Variable (Y) % Fatalities

A 3086612 8.14258461 99

B 7073487 16.7718641 100

C 775502 14.5612658 100

D 977860 13.7931218 100

E 3829114 15.158144 100

F 10769384 16.1922179 100


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50m URBAN


(mg/m3)


A B 1738585.14 7.568582202 99

C 155478.08 5.154260036 50.6

D 218339.08 5.493804546 60.9

E F 806015.51 6.799858264 90.6

2449m RURAL


(mg/m3)


A 1432 0.466827348 0

B 3282 1.296208272 0

C 437 -0.7200668 0

D 946 0.052242569 0

E 3036 1.218296139 0

F 8539 2.252399184 0

2449m URBAN


(mg/m3)


A B 557.05 -0.477345 0

C 90.29 -2.29697329 0

D 341.59 -0.96638881 0

E F 975.91 0.083370369 0


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DISCUSSION

Physical Effects

The physical effects of a liquid discharge of ethylene oxide can be detrimental to health of

personnel and the facility. The extreme level of toxicity of ethylene oxide has been shown to killmost or all people within a 50km radius of the leak (99-100%). This is the proof that the NFPA

classification of 1A is reasonable. Realistically, upon a leakage of ethylene, its reaction to

atmospheric pressure will immediately convert it to gaseous phase. Whether liquid or gaseous,

however, ethylene oxides flash point of 230C will make it combust rapidly on exposure to an

ignition source. This was the case in the ACCRAPAC facility in Elkhart, Indiana in September

2000. (Environmental Protection Agency, 2000).

Ethylene oxide is Flammable; toxic can cause burns when trapped by clothing; affects multiple

organs and is a suspect carcinogen (Zabetakis, 2000). As a result, self breathing apparatus

accompanied by an oxygen pump is required of an operator working in the gas storage area,

since leaks are surprisingly common in such a facility. Leaks occur even when filling of a tanker

takes place therefore a leak from a disconnected pipeline or a hole is definitely significant and a

one inch hole is shown to have a release of 5.19 kg/s. This forms a pool on the ground of radius

8.64 m and has an evaporative rate of 106.3 kg/s.

Consequential Analysis

The entire tank will leak out after 13.7 hours, thereby giving operators sufficient time to

evacuate the area before the stated evaporation rate occurs. As can be seen, the

environmental conditions at the time of release significantly influences the consequential effect

of the release, thereby the planning and hazard analysis team needs to consider the effects

under all wind speeds, ambient temperatures and whether it is a rural or urban setting.

Emergency response

The proposed location of the ethylene oxide plant is convenient in terms of the emergency

services available. Point Lisas Industrial Estate has a nearby fire station fully equipped to handle

industrial fires, and is situated minutes away. In addition to professional services, employees

are to be trained to handle certain manageable situations. There should be an ambulance ready

to use in the event of an accident. Also, everyone related to the process section of the plant

(operators, supervisors, engineers) are to be trained in cardiopulmonary resuscitation (CPR).


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Mitigation Measures

An inherent savior to such an incident is a proper shutdown procedure. If the process is not

terminated in a sensible manner, variables such as pressure and temperature can be

mismanaged and a very destructive explosion can occur, which is characteristic of ethylene

oxide. As a result, the ethylene oxide storage system should be located at a distance from other

process equipment in a gas house. This gas house should have proper stress withstanding walls

and should be sealed off from vapour escaping.

The operator assigned to handle the gas house should wear a suit isolated from the

atmosphere and connected to an oxygen pump. Also he should wear an ethylene oxide gas

monitor (suggestion: BW GAXT-E - Gas Alert Extreme ethylene oxide ETO gas detector)

(Optimum Energy Products, 2011)

As mentioned before, when ethylene oxide is being loaded onto tankers, the connection and

disconnection of pipelines and reinforced fiber hoses may sometimes encourage a leakage of

the tank contents. Therefore, the installment of a reclamation tank may be useful. The

reclamation tank functions to absorb leaked ethylene oxide gas and stores it separately from

the tank.

Likelihood of an Accident

As in every plant, the likelihood of an accident is always considered an imminent threat.

However, the extent of danger and the need for preparation is different for each chemical or

situation. In this case, the likelihood of an accident is very high considering the pressure of the

vessel holding the gas at liquid phase. In addition, the toxicity and the fact that it is a colourless

gas with a non-irritant smell (sweet) may lower ones guard against a possibility.


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RECOMMENDATIONS

y Plan and design a safe shutdown procedure in the event of an accident The facilityshould examine the compound and determine which systems require a quick shutdown

procedure for loss prevention. In the case of toxic releases, the valves should be

remotely operated so that the operators do not have to enter the area and put

themselves at risk

y Relocate Relocate certain equipment if they are in an area where they pose a high riskto personnel. This includes the reactor and the ethylene oxide storage system. Also, The

latter and its facility should be in accordance with NFPA 30, 5-3.2.2, where the blast

walls can withstand an explosion of ethylene oxide.

y Vent Provide adequate venting for any chemical that may leak and evaporate orundergo deflagration

y Electrical Equipment Limit the electrical equipment around flammable material tothose that meet the National Electrical Code (NEC) rating for the type of material being

handled. For ethylene oxide, this group is the NEC Class I Group B.

y Ensure adequate supply of breathing apparatus Ensure that the amount of oxygen inthe tank is always above specification and that the number of apparatus is sufficient in

case the operator requires assistance.


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REFERENCES

deWeger, Pietersen, & Reuzel. (1992). CPD Green Book.

Environmental Protection Agency. (2000). EPA 550-R-00-001 . Elkhart.

Lees, F. (2005). Lees Loss Prevention in the Process Industries (3rd Edition ed., Vol. 1). (S.

Mannan, Ed.) Elsevier.

National Fire Protection Agency. (2011). NFPA 560: STANDARD FOR THE STORAGE, HANDLING,

AND USE OF ETHYLENE OXIDE FOR STERILIZATION AND FUMIGATION. NFPA.

Optimum Energy Products. (2011). Canary Sense. Retrieved 2011, from www.canarysense.com:

http://www.canarysense.com/index.php?page=ethylene_oxide_eto_gas_detector

Othmer, K. (2004). Encyclopaedia ofChemical Technology(5 ed., Vol. 10). Wiley.

Zabetakis, M. (2000). Characteristics of Combustible Gases and Vapors. US Bureau of Mines , 1.


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APPENDIX


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kev scenario 3

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