experimental study of effective water spray curtain...

Experimental Study of Effective Water Spray Curtain Application in

Dispersing LNG Vapor Clouds

Morshed A. Rana, Benjamin Cormier, Jaffee Suardin, Yingchun Zhang, M. Sam

Mannan*.

Mary Kay O’ Connor Process Safety Center, Artie McFerrin Department of Chemical

Engineering, Texas A & M University, College Station, TX 77843.

[email protected].

Abstract

The installation of new LNG storage facilities in the US to meet the demand of natural

gas has brought increasing attention to LNG safety issues. Because of its highly

flammable nature, LNG poses several hazards to workers, properties, and the surrounding

communities. One of the major hazards is the formation of a flammable vapor cloud from

any accidental LNG release, which may result in a massive fire. The safety measures to

prevent and mitigate an accidental LNG release are essential and critical to protect the

employees and the public from injury or harm.

The water spray curtain is currently recognized as a promising technique to control and

mitigate many toxic and flammable vapors. Much theoretical and experimental work has

been carried out to determine the effectiveness of the water spray curtain in dispersing

heavier vapor. However, LNG vapor dispersion behaves differently from other dense

gases due to its low molecular weight and extremely low temperature. In this context, it is

essential to carry out research to understand the effects of water curtain on LNG vapor

clouds. There have been a very limited number of publications on the use of a water

spray curtain to mitigate LNG vapor clouds. These previous studies show that water

spray curtains can enhance LNG vapor dispersion from small spills. However, in order to

develop comprehensive and structured engineering guidelines for the design of an

effective water spray curtain for controlling LNG vapor many key questions still remain

to be answered.

An experimental methodology to study the LNG vapor dispersion behaviors with the

application of water spray curtain is presented in this paper. This field experiment

involves the fundamental study of forced dispersion, dilution due to air entrainment, and

heat exchange to determine the effectiveness of water spray in reducing the LNG vapor

“exclusion zone”. This paper discusses and outlines the experimental method and some

results based on gas concentration data analysis to emphasize the observed effectiveness

of water spray curtain on LNG vapor dispersion.

Keywords: LNG spill, vapor control, forced dispersion, spray nozzle.

1 INTRODUCTION

Liquefied Natural Gas (LNG) refers to natural gas converted into liquid state by super

cooling to 111K (-162.2°C). LNG commonly consists of 85%-98% methane with the

remainder as a combination of nitrogen, carbon dioxide, ethane, propane, and other

heavier hydrocarbon gases. LNG is usually stored at 111K and atmospheric pressure in

heavily insulated tanks. As forecasted by the U.S. Department of Energy, demand for

natural gas in US is increasing by 20% over the next 25 years because of its clean-

burning characteristics [1]. Due to this increasing demand for natural gas, more LNG

receiving terminals are proposed to be constructed in the US. The massive volume of

LNG storage tanks poses fire and explosion hazards due to its highly flammable feature.

Barriers such as dikes or impounding walls are usually built around the tanks to hold

accidentally spilled LNG to protect adjacent properties.

The vapor cloud formed from a LNG liquid spill disperses in the prevailing wind

direction and is diluted. When the gas concentrations are between the lower and upper

flammability limits (LFL and UFL), 5-15% by volume, the vapor cloud becomes

flammable. Safety standards require LNG facilities to have a “dispersion exclusion zone”

so that flammable vapor cloud from an accidental release will not propagate beyond the

plant boundaries [2]. This exclusion zone is defined from the LNG spill source to the

predicted distance at which the average vapor concentration is one-half of the LFL, i.e.

2.5% volume. This distance is estimated with various dispersion modeling techniques

incorporating appropriate control and mitigation measures.

Water spray curtains are recognized as an effective engineering measure to mitigate

various types of hazards in the industries because of its availability, simplicity of use,

efficiency and adaptability for different hazards such as gas dispersion, absorption, and

fire inhibition [3-4]. Generally, water curtains can be classified as full cone, full square

cone, hollow cone and flat-fan characterized by water droplet sizes and flow patterns [4].

Water spray nozzles can be directed downward and upward, vertically or inclined. The

types and application methods of water sprays have significant effects on the efficiency

and effectiveness in mitigating different hazards.

Water curtain has also been suggested as one of the most economic and promising LNG

vapor mitigation techniques, which can reduce the size of the “exclusion zone”

effectively, by forced dispersion. The effectiveness of water curtain in controlling LNG

vapor depends on a number of parameters. These parameters can be divided into two

groups:

i. Parameters of its own characteristics: water droplet distribution, nozzle type and

size, direction, width and height, water pressure etc.; and

ii. External parameters: vapor cloud features, LNG properties, wind speed,

atmospheric stability etc

The paper presents the experimental methodology and some results of gas concentration

data of the first phase of a research program to study the effectiveness of water spray

curtain to disperse LNG vapor clouds.

2 BACKGROUND AND PREVIOUS WORK

The physical processes involved in using a water curtain to reduce the gas concentration

of a vapor cloud include forced dispersion, dilution, and heat transfer. A water spray

directed vertically through a gas cloud will have four effects: (i) mechanical effects of

creating a barrier to the passage a gas cloud, imparting momentum to the gas cloud to

disperse it, moving gas cloud upwards, (ii) dilution of the gas cloud by air entrainment;

(iii) thermal effects between the gas cloud, water, and entrained air; and (iv) physico-

chemical effects of absorbing gases with or without chemical reaction [5]. The actions of

a water spray in mitigating a vapor cloud may consist of all or any combination of these

mechanisms, if properly designed. Water curtains enhance LNG vapor cloud dispersion

mainly through two mechanisms: mechanical effects (forced dispersion and dilution) and

thermal effects. As the solubility of natural gas in water is minimal, thus the physico-

chemical effects can be ignored.

The interaction among vapor cloud (gas/air mixture), air and water droplet of a water

curtain is a sophisticated phenomenon to model and predict. Several models, based on

macroscopic or microscopic approaches, can be used to predict gas concentration

reduction with the application of a water curtain. The macroscopic models are semi-

empirical and can estimate the concentrations downstream of a water curtain and the

dilution factor of the water spray [5-7]. The microscopic models, based on fluid

dynamics, require accurate information on hydrodynamics and can only predict the

mitigation ability of water curtain without considering the subsequent dispersion of the

gas cloud. An easy-to-use model with a minimum requirement of inputs is essential to

estimate the downstream concentration reasonably well [3-5, 8-9].

Apart from theoretical modeling, much experimental work has been carried out to assess

the effectiveness of the water curtain in gas dispersion. Most work was conducted to

determine the optimum configuration of water sprays to mitigate dense gas clouds like

NH3, Cl2, HF, N2O4, CO2, etc [5, 10-15]. It has been proved that water curtains can be

effective in facilitating the dispersion of toxic and flammable vapor clouds evolved from

accidental spills.

There have been two published studies on the use of water curtain to control LNG vapor

clouds. Experiments on using water curtain for small LNG spills by University Engineers

for US Coast Guard in the late 70’s [16-17], and wind tunnel and small scale spill tests by

Factory Mutual Research Corporation for Gas Research Institute in the early 80’s [18-19]

are the major experimental works so far. The former one simulated spills on LNG ships

and used fan-shaped sprays. The experimental results showed reduction of methane

concentration by the water spray. Two mechanisms, mechanical turbulence and heating

by the sprays, were suggested as the cause of concentration reduction [16-17]. However,

as the temperature of the vapor cloud was not measured, the effect of heating on vapor

dispersion was not clearly identifiable from the experiments.

The later one simulated LNG spill contained in a dike and performed small scale tests

using both downward and upward solid cone sprays, to determine the effectiveness of

water spray curtain. Results showed that water sprays dilute the concentrations to about

1-2%. Air entrainment and heating of the cloud by the spray were considered to enhance

the vapor dispersion. The analysis also suggested that the vapor plume was lofted above

the diked area by the sprays, producing a low ground level concentration downwind with

some vapor potentially escaping between adjacent full cone sprays [18-19].

Because of low molecular weight and extremely low temperature, dispersion of LNG

vapor cloud behaves differently from other dense gases. Though previous work has

confirmed that water curtain represents one promising technique to mitigate the hazard of

accidental LNG release, it was not clear from such experimental work how to design an

effective water curtain. Indeed, it is probably true that there is no simple set of optimum

parameters, rather there will be a specific set of design parameters for each particular

type of circumstances [20]. Other key issues, like (i): the extent of LNG vapor

concentration reduction by water curtain; (ii) the effectiveness as a function of water

curtain configuration; (iii) the optimum distance between the water curtain and potential

spill site; (iv) the effective height and width of the curtain; and (v) the highest wind speed

for which water curtain remains effective; are yet remained undetermined [18].

.

3 EXPERIMENTAL

In the LNG industry, upward-pointing water curtains are mainly used to facilitate LNG

dispersion and prevent water from entering the LNG pool by inclining them from the spill

location. The mixing process for an upward spray is by the upward momentum imparted

to the cloud by the water spray. This upward momentum enhances air entrainment

through the lower surface of the cloud and thus better dilution is achieved. The

momentum also pushes the diluted cloud upward and facilitates dispersion [10].

It was the intent of this research to understand the underlying physical phenomena on the

interaction of water spray with LNG vapor as well as observe the effectiveness of the

water curtain to mitigate and control LNG release through multiple sets of experiments.

This paper is based on the first set of the experiment and the detailed experimental set-up

and procedure are described including different parts involved in the tests. The

experiment consists of the LNG source, the water spray nozzle and water curtain,

placements of the measurement points and meteorological measurements. The procedure

was to perform natural and forced dispersion of LNG to evaluate the effectiveness of the

water spray curtain in terms of concentration and temperature reduction.

3.1 Test Facility This set of outdoor spill experiments was conducted on November 16, 2007, at the LNG

Emergency Response Training Facility of the Brayton Fire Training Field (Emergency

Service Training Institute (ESTI)) in College Station, Texas, USA. The Texas

Engineering Extension Service (TEEX) of the Texas A & M University System provides

comprehensive firefighter training and emergency services at the Brayton Fire Training

Field. Hands-on spill control and fire suppression training are also provided to personnel

involved in LNG production, transportation, storage, and response activities at the LNG

training facility. The LNG facility is made of concrete and consists of three pits, as

shown in Figure 1 including:

i. An L-shaped pit to simulate the trenches used to divert any LNG spills into

containment pits (A).

ii. Two 1.22 meter (4 feet) deep pits with areas of 3.05m X 3.05m (10ft X 10ft) (B)

and 10.06m X 6.40m (33ft X 21ft) (C), respectively.

iii. One 2.44 meter (8 feet) deep pit with area 6.4m X 6.4m (21ft X 21ft) (D). This

pit includes a simulation of a high dike wall, typical of the containment facilities

used during LNG offloading.

The Brayton Field LNG facility is equipped with a 20.3cm (8") water pipe loop.

Sufficient water supply was available during the test, from the city water supply, with the

aid of a pump powered by a pumper truck. The pumper truck could deliver up to 3.8

m3/min (1000 USGPM) water at 130psi across an isolating block valve to supply the

water curtain units. The facility is installed with fire water monitors and removable water

spray heads.

D

C

B

A

52.43m long, 76.2 mm (3") OD, insulated LNG Discharge Pipe (Al)

LNG Truck Position

Water Curtain Test

Area

Pumper Truck& Pump LocationWater Drainage

20.3 cm (8") Water Line

Water Monitor/Hydrant

50.8 mm (2") Water Discharges, Valves & Water Spray Heads

N

safe area outside the boundary

LNG Discharge Line

Figure 1 Test facility and spill location

3.2 Spill Area LNG spill tests were conducted on a flat concrete pad of the LNG training facility. The

concrete spill area was enclosed with 1.52m × 1.52m (5ft × 5ft) inner and 1.83m × 1.83m

(6ft × 6ft) outer wooden frames. To minimize the LNG flow through the gap between the

wooden frame and the concrete ground, the space between the frames were filled with

wet sand during the tests. The intension of using wet sand was that when LNG contacted

wet sand it would freeze immediately and stop liquid LNG flow out of the spill area

through the gaps. Figure 2 shows the photograph of the enclosed spill area.

Figure 2 Enclosed spill area

3.3 Source and Flow Measurements LNG was supplied by a tanker truck of 41.64 m3 (11,000 gal) capacity and discharged at

the spill location with 52.4 m (172 ft) long, 7.6 cm (3 inch) OD, 3.2 mm (1/8 inch) thick

insulated, fixed Aluminium pipe line and around 15.2 m (50 ft) long, 7.6 cm (3 inch) OD,

flexible discharge hose. The flow rate of the LNG from the tanker was manipulated by

manually controlling the opening of the outlet valve from the tanker to achieve a steady

flow rate. The flow rate (in GPM) was continuously measured with a cryogenic flow

meter connected between the pipe line and the discharge hose during the tests. The

cryogenic flow meter used was a 3-inch FTB-911 turbine meter with male NPT end

fittings.

3.4 Water Curtain and Flow Measurements Two types of portable spray curtains were employed and both were upward directed. One

was constructed of seven 1" TF 48 NN nozzles, connected to a 6.1 m (20 ft) long 5.1 cm

(2 inch) OD carbon pipe. These nozzles are manufactured by BETE Fog Nozzles and

produce a full cone, spiral, 60º spray pattern. According to the manufacturer, each nozzle

can throw water upward as high as 4.6 m (15 ft) at 100 psi and 0 wind speed. Nozzles can

create smaller droplets of sauter mean diameter 580 µm. Prior tests showed that the water

curtain made with the nozzles created 7 m (23 ft) wide, 3.7 m (12 ft) high coverage.

Figure 3 illustrates the nozzle and water spray curtain.

Figure 3 Photographs of 1" TF 48 NN nozzle and water spray curtain

The other spray instrument was the Hydro-Shield spray device available through the

Brayton Fire School. With this spray head water flows out through a pipe, which has half

circular end, and hits a flat plate at the exit. The obstacle created by the flat plate

produces 180º flat fan shaped spray pattern. Initially it creates a sheet of water at the pipe

exit and then breaks up as larger water droplets. This device is capable of projecting a

water curtain covering about 15.2 m (50 ft) wide and 7.6 m (25 ft) high according to

visual observation. Figure 4 shows the hydro-shield water spray.

Figure 4 Hydro-shield water spray curtain

During the tests, water was supplied from the water line to the water spray curtain with a

6.4 cm (2½") OD fire hose. A flow meter and a pressure gauge were connected at the exit

of the fire hose to measure the water flow rate and water pressure at the inlet of the

curtain during the tests. Water flow was turned on and off with the ball valve already

connected to the water supply line.

3.5 Concentration and Temperature Measurements 34 IR point gas detectors and 32 type K thermocouples were used and installed on 14

tripods at different heights to measure the methane gas concentration at 0.5m, 1.2m and

2.1m above ground and temperature of air-methane mixture at 0.5m, 1.0m and 2.0m

above ground. The heights were chosen based on the previous research. The tripods were

placed at different downwind distances from the LNG spill area to measure the

concentration and temperature both upstream and downstream of the water curtain. The

instruments were placed according to the predicted wind direction of the experiment day.

Table 1 shows the position of tripod poles and gas detectors and temperature sensors

heights on the poles.

Table 1 Location of 14 tripod poles and 36 gas detectors and 32 temperature sensors

positions on the poles

Pole no.

y m

x m

Gas Detector, GD Temperature Sensor, TS

z =0.5 m z = 1.2 m z = 2.1 m z = 0.5 m z = 1.0 m z = 2.0 m

1 1.2 1

22ª 21 --- 3¹ 4 --- 2 -1.2 24 23 --- 1 2 --- 3 3.7

3.3

25 26 36 5 6 7 4 1.2 27 28 35 8 9 10 5 -1.2 30 29 39 11 12 13 6 -3.7 38 37 40 14 15 16 7 1.2 5.5 --- --- --- 29 30 --- 8 -1.2 --- --- --- 31 32 --- 9 3.7

11.3

13 12 11 17 18 19 10 1.2 01 15 07 20 21 22 11 -1.2 19 18 17 23 24 25 12 -3.7 20 02 10 26 27 28 13 1.2 13.7 06 34 04 --- --- --- 14 -1.2 08 32 31 --- --- ---

x = downwind distance from the edge of the enclosed spill location z = height above the ground ª = gas detector identification number ¹ = temperature sensor identification number

Figure 5 and 6 shows the experimental set up and the sensors, detectors and water curtain

placements.

Figure 5 Experimental set-up

Figure 6 Photograph of equipment placement

The type-K thermocouples, manufactured by Omega Engineering, were capable of

measuring temperature as low as 173K (-100ºC) up to 373K (100ºC) up to. The gas

detectors used were designed to detect methane and calibrated to measure the

concentration in % v/v unit. These electric powered IR point gas detectors were

manufactured by Honeywell Analytics. The detectors were not physically installed on the

tripods, but were installed in a wooden vacuum chamber. 30.5m (100 ft) long Teflon

sample tubes were used for the purpose of flowing methane from the field to the

detectors. One end of each tube was connected to a gas detector and the other end was

placed on the tripod pole. A vacuum pump was used to pull LNG vapors at 0.3 L/min to

the detector through the tubes and then released to the environment. Figure 7 and 8 shows

the gas detector connection and photograph of detector chamber.

Figure 7 IR Gas Detector-Teflon tube connection

Figure 8 Gas detector chamber

3.6 Other Equipment The meteorological conditions were measured during the experiments with three weather

stations. Two stations were placed in two separate positions of the test facility, 2.1m (7 ft)

above ground-level and the third was installed around 15.2m (50 ft) east of the test

ground, 9.1m (30 ft) above. Purpose of these stations was to measure the wind velocity,

direction, temperature, humidity and heat index.

Two hydrocarbon (H/C), one IR and two regular digital cameras were used to capture the

whole experiment. The hydrocarbon and IR cameras were used to observe the actual

dispersion of invisible flammable hydrocarbons (mainly methane). The H/C cameras

were placed away at a safer location from the visible cloud region and at 90º to each

other, so that they can capture all the moments.

Gas detector, temperature sensor, flow meter and pressure gauge data were acquired at an

interval of 1 second with a data acquisition system and recorded simultaneously in two

computers. Data receiving and acquisition systems were installed carefully to receive

signals, transform signals to data and record data successfully. The weather stations have

built-in data record system, which could save data for three days. The weather data were

transferred to computers after the experiments. All of the detectors, sensors and gauges

were calibrated and checked several times prior to the test day.

3.7 Experimental Procedure Four similar spill tests were conducted to evaluate the effectiveness of two types of water

spray curtains in controlling an LNG vapor cloud. Each curtain was tested twice with a

separate spill. The first two tests were conducted with a full cone spray curtain. The water

spray curtain system was then replaced with the Hydro-Shield flat fan spray and two

more similar tests were conducted.

During the tests, water curtains were directed vertically upward and positioned

perpendicularly to the prevailing wind direction, at a fixed downwind distance from the

spill location. All of the tests consist of two measurements, the natural and forced

dispersion. During the natural dispersion no action was taken to control the LNG vapor

flow. Forced dispersion of the LNG vapor occurred in the presence of water spray curtain

application.

Each test started with continuous release of liquid LNG onto the enclosed spill area.

Initially only LNG vapor started to emerge from the hose and within a couple of minutes

liquid stared to flow from the hose. As soon as the liquid touched the concrete ground it

vaporized and dispersed naturally towards the prevailing wind direction. Several minutes

after the start of the LNG release, the water spray curtain was turned on to disperse the

cloud forcefully. The flow of LNG was also continued for a few minutes during forced

dispersion. When the LNG pool reached about 5 cm to 8 cm (2" to 3") height, the LNG

flow was turned off while continuing the water curtain operation. The curtain was kept on

until the visible white cloud disappeared.

4 RESULTS AND DISCUSSION

A total of four tests were completed in a 1½ hour period. During the tests, the wind speed

averaged 2 meters per second (4.5 miles per hour) and blew mainly from the south-west

to south-east region. The average ambient temperature during the tests was 295.4K and

humidity was 25.5%.

Table 2 includes the average flow rates and total flow time of LNG and water and

average pressure of water at the curtain inlet for each test.

Table 2 Average flow rate and pressure

Test

Water Curtain LNG Flow Water Flow

Spray Flow

Pattern

Avg.

Rate

m3/s

(GPM)

Total

Duration

min

Avg.

Rate

m3/s

(GPM)

Avg.

Pressure

psi

Total

Duration

min

1 BETE

Fog

Full Cone

Spiral 2.5×10-3

(39.3) 9.0

10.1×10-3

(160.8) 40 9.8

2 BETE

Fog

Full Cone

Spiral 3.8×10-3

(60.7) 8.2

15.5×10-3

(245.8) 45 5.8

3 Hydro-

Shield Flat Fan 3.5×10-3

(55.9) 5.9

11.3×10-3

(178.6) 79.5 4.3

4 Hydro-

Shield Flat Fan 3.4×10-3

(54.8) 3.5

15.1×10-3

(240.1) 99.7 3.4

All tests were video taped using digital, hydrocarbon and Infrared (IR) cameras. The

hydrocarbon and IR cameras were used to differentiate between the dispersion of

flammable hydrocarbon and the visible condensate cloud. The following, Figures 9–11,

show clips from the videos of the tests captured with the regular and hydrocarbon

cameras, almost at the same time.

(a) (b)

Figure 9 Spill and natural dispersion: (a) regular image (b) hydrocarbon camera

image

(a) (b)

Figure 10 Forced dispersion with full cone spray curtain: (a) regular image (b)

hydrocarbon camera image

(a) (b)

Figure 11 Forced dispersion with flat fan curtain: (a) regular image (b)

hydrocarbon camera image

In Figure 9 the regular image shows a smaller visible condensate cloud but hydrocarbon

camera shows actual size of the flammable cloud. In Figure 10 and Figure 11, the regular

camera image shows that white condensate cloud becomes invisible after it crosses the

spray area. The black areas of the hydrocarbon camera image show the presence of colder

hydrocarbon in the gas-air-water vapor cloud. The light darker area after the spray

indicates that still there are some colder hydrocarbons (which are not visible) and the

concentration/temperature is lower.

The gas detectors were calibrated with methane gas (50-50% CH4-Air Mixture) to

measure methane concentration in % volume/volume unit. However, during the tests one

gas detector (GD 18) malfunctioned and did not show any reliable readings the entire

time. The other six detectors (GD 7, 8, 11, 34, 2 and 17), provided reliable data for tests 1

and 2, for a portion of test 3, but failed during test 4. A potential cause for this failure

might have been caused by water going in the Teflon tubes of the detectors; thus,

blocking the path of methane from flowing to the detectors. Figure 12 shows some gas

detector readings at different locations.

Concentration Reading 0.5m Above Ground and Different Downwind Distances

Test 1

0.0

2.5

5.0

7.5

10.0

12.5

15.0

17.5

20.0

22.5

25.0

27.5

30.0

0 60 120 180 240 300 360 420 480 540 600 660 720 780 840 900 960time, sec

CH

4 co

ncen

trat

ion,

% v

/v

GD24GD30GD19

Figure 12 Gas detector reading at different downwind locations [Position of gas

detector 24 ,GD 24 ≡ (x, y): (1m, 1.2m); GD 30 ≡ (x, y): (3.3m, 1.2m); GD 19 ≡ (x,

y): (11.3m, 1.2m)]

The fluctuations in the methane concentration readings from the gas detector were caused

by the wind gustiness and crosswind turbulence. More fluctuation was observed close to

the spill location due to the turbulence created by the momentum of the spill. The water

curtain was positioned 7m (23 ft) downwind from the spill location. The wind’s frequent

change of direction sometimes allowed the LNG vapor to disperse without passing

through the water spray region and caused fluctuations in the methane concentration

measurements. Therefore, to analyze the gas concentration data, the average of

concentration readings of all the detectors placed at certain downwind distance and

heights are considered. Figure 13 illustrates the method for averaging gas concentration

data.

Figure 13 Average concentration at different heights and downwind positions

As there was time lag between the LNG flow from the pipe and gas detector reading, all

the data were edited to synchronize between the videos, temperature sensor and gas

detector readings. The following charts summarize the average concentration reading:

3.3m (11 ft) and 11.3m (37 ft) downwind from the spill; before and after the water curtain

action; at different heights (0.5m, 1.2m and 2.1m) for two tests with two types of water

curtain.

Average Concentration vs TimeHeight (z=) 0.5m and Distance from the Release (x=) 3.3m & 11.3m

Test 1: Spray 7m from the release

0.0

2.5

5.0

7.5

10.0

12.5

15.0

17.5

20.0

0 60 120 180 240 300 360 420 480 540 600 660 720 780 840 900 960

time, sec

CH

4 co

ncen

trat

ion,

%v/

v Before the spray location

After the spray locationat x = 11.3 m

LNG release discontinued

Water turned on

Figure 14 Average gas concentration 3.3m and 11.3m from the spill, at 0.5m above

ground for test 1; (full cone water curtain position: 7m downwind from the spill)

Average Concentration vs Time

Height (z=) 1.2m and Distance from the Release (x=) 3.3m & 11.3mTest 1: Spray 7m from the release

0.0

2.5

5.0

7.5

10.0

12.5

15.0

17.5

20.0

0 60 120 180 240 300 360 420 480 540 600 660 720 780 840 900 960

time, sec

CH

4 co

ncen

trat

ion,

%v/

v


Water turned on

Before the spray location at x = 3.3 m






0.0

2.5

5.0

7.5

10.0

12.5

15.0

17.5

20.0

0 60 120 180 240 300 360 420 480 540 600 660 720 780 840 900 960

time, sec

CH

4 co

ncen

trat

ion,

%v/

v


Water turned on







0.0

2.5

5.0

7.5

10.0

12.5

15.0

17.5

20.0

0 60 120 180 240 300 360 420 480 540 600

time, sec

CH

4 co

ncen

trat

ion,

%v/

v


Water turned on




ground for test 3; (flat fan water curtain position: 7m downwind from the spill)



0.0

2.5

5.0

7.5

10.0

12.5

15.0

17.5

20.0

0 60 120 180 240 300 360 420 480 540 600

time, sec

CH

4 co

ncen

trat

ion,

%v/

v


Water turned on







0.0

2.5

5.0

7.5

10.0

12.5

15.0

17.5

20.0

0 60 120 180 240 300 360 420 480 540 600

time, sec

CH

4 co

ncen

trat

ion,

%v/

v


Water turned on





During natural dispersion, before the water curtain action, concentrations at 3.3m

downwind of the spill and 0.5m above the ground are higher than concentrations at 1.2m

and 2.1m height for both the tests. Horizontal and vertical momentum of LNG flow from

the pipe, flashing and evaporation from the LNG pool caused some turbulence close to

the spill area. Because of these data still shows fluctuation at this position. At 11.3m

downwind, concentration near the ground (0.5m) becomes less than the higher level

(1.2m and 2.1m) concentrations after a period of time. This clearly indicates that LNG

vapor naturally dilutes and disperses upward. This dispersion process illustrates that the

negative buoyancy of an LNG vapor cloud changes to neutral and then to positive

buoyancy as it travels downwind.

The forced dispersion process occurs when the water curtain is turned on. As the LNG

release continued for some time after turning the water spray on, the average methane

concentrations at 3.3m expectedly continued to increase. However, the average

concentration readings at 11.3m downwind distance behaved differently, for both the

tests, during the water curtain action. For test 1, the full cone test, concentrations at 1.2m

and 2.1m heights started decreasing behind the spray position (11.3m) immediately after

the spray was turned on. The concentration reading at 0.5m height of this location

indicates a reduction in the rate of concentration increase. For the flat fan test,

concentration rate increase was reduced at 11.3m downwind distance during the water

curtain action when the LNG was still flowing from the pipe.

In both tests, the gas concentration continued to decrease at both distances during the

water curtain application after discontinuing the LNG flow. The differences between

3.3m and 11.3m downwind curves show the change in methane concentration behavior

due to the water curtain application for both tests. However, in all of the tests the methane

concentration at 11.3m downwind never reached below one half of the LFL, 2.5% (v/v),

by forced dispersion..

Tests with the full-cone curtain showed the average gas concentration at 11.3m

downwind of the release is less than the concentration at 3.3m downwind before and after

the curtain action for 0.5m and 1.2m heights. But, concentration 2.1m above ground at

11.3m downwind is higher than 3.3m downwind before the water action and decreases

after the curtain was turned on. During the tests, visually, one was able to observe that the

constructed full-cone-spiral-nozzle curtain created more turbulence near the spray region

than the flat-fan-spray curtain. However, the full cone spray curtain could not provide

enough crosswind and above ground coverage to interact completely with the incoming

vapor cloud. The crosswind coverage of full cone water spray was smaller than the flat

fan spray which allowed the LNG to pass around the curtain and disperse. For this test,

the methane concentration after the curtain region (11.3m) did not decrease significantly.

Again, the difference in LNG concentrations before (3.3m) and after (11.3m) the full

cone spray region at 2.1m high was not very significant as the vapor cloud could crossed

over the top of the water curtain without complete interaction. This result indicates that

this constructed water curtain provides more mixing effect than momentum effect.

Average gas concentration values at 11.3m downwind distance were observed to be

lower for the fan type spray than the full cone spray. Concentration profiles at 0.5m

above ground at 11.3m and 3.3m downwind show significant differences in the flat fan

spray test. At 1.2m above ground, average concentrations at 3.3m downwind are higher

than 11.3m before and during water curtain application. Concentrations at 2.1m above

ground and at 11.3m downwind are higher than at 3.3m, both before and during the entire

period after the water curtain application. This indicates that the flat fan water curtain

was dispersing the LNG vapor upward through momentum effect more than mixing and

diluting it with air. Figure 20 describes the visual coverage and interpreted interactions of

two types of water curtains with vapor cloud during the tests.

Figure 20 Spray-cloud interaction and water curtain coverage

5 CONCLUSION

This paper analyzed the experimental concentration data to determine the effect of water

curtains in changing LNG vapor concentration. The tests conducted in this research

indicate that water curtain is able to control LNG vapor cloud by changing the ground

level methane concentration. However, the tests could not show a significant change in

concentration at higher level than the ground level. The constructed full cone spray shows

more effectiveness in creating turbulence closer to the spray region than the flat fan

spray. The flat fan is more effective in reducing the ground level concentration by

creating a barrier in the path of the cloud and pushing the cloud upward. From the

analysis, it is clear that the action of water sprays reduced the methane concentration by

dilution and mixing with entrained air as well as pushed the vapor upward. More analysis

on the heat transfer effect of water spray on cold LNG cloud, which is ongoing, will help

reach more decisive quantitative results. Each test had to stop within a short time because

amount of the large amount of water used would cause problems to some of the

instruments located near the test area. This also indicates that to control a large spill the

required system has to provide sufficient water. In addition, draining the large amounts of

water can present problems. However, to establish a definitive engineering guideline on

water curtain design for LNG cloud dispersion, controlled experiments for longer times

and with different wind conditions are needed.

Acknowledgement

The authors are grateful to BP Global Gas SPU for their financial support of this study.

They would also like to thank the Brayton Fire Training School and their LNG

Emergency Response training team for their help and contributions to this research.

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experimental study of effective water spray curtain...

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