Pro-Science 4 th International Conference of Hydrogen Safety, September 12-14, 2011, SAN FRANCISCO,...

4th International Conference of Hydrogen Safety, September 12-14, 2011, SAN FRANCISCO, USA

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EXPERIMENTAL STUDY OF IGNITED UNSTEADY HYDROGEN RELEASES FROM A HIGH PRESSURE

RESERVOIR

Grune, J. 1, Sempert, K.1, Kuznetsov, M. 2, Jordan, T.2

1Pro-Science GmbH, Germany

2Karlsruhe Institute of Technology, Germany


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Structure of the presentation

Introduction

Objectives

Experimental

Test results

Summary


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Hydrogen is successfully used as energy carrier in many different applications.

Introduction

Accidental hydrogen releases from pipe systems are one of the main hazards that occur in the handling of pressurized hydrogen.

The accidental H2 release from the system should be detected fast and as safety

consequence the main supply tank should close. So the released amount of hydrogen is limited and the release conditions are unsteady.

The generated hydrogen cloud can be ignited subsequently by an external ignition source or in case of sudden hydrogen release from high pressure state the possibility of a self ignited jet fire is present.

Currently no systematic studies are available on the hazard potential of transient releases of small amounts of hydrogen from a high pressure reservoir.


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Objectives

Goal of this work is to quantify the possible hazards arising from spark ignited or auto ignited free transient hydrogen jets.

To study the nature of transient hydrogen jets and their combustion behaviour a simulation of an accidental hydrogen release from a high pressure pipe system was performed with two different start-up release conditions.

I. A fast valve opening to produce the free gas jet. This jet is ignited with a forced spark.

II. Additionally a rupture disc is installed in the exhaust pipe, which leads to a sudden hydrogen release with the possibility of an auto ignited jet fire.

- Circular release opening: 3 mm, 4 mm and 10 mm (forced ignition test). 4 mm (auto ignition test)- Initial reservoir pressures of up to 200 bar. - Reservoir volume of 0.37 dm3.

The main parameters in these experiments are:


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ab c

d e

fg

h

P

H2

a c3.70 dm3

tube nozzle

tube nozzle with rupture disc

Experimental set-up

da

b cd e

fg

h

P

H2

a c3.70 dm3

tube nozzle

tube nozzle with rupture disc

Experimental set-up

d

Experimental set-up

(pneumatic needle valve DN 3 mm, driven with pressurized helium, valve opening time < 2 ms)

The small volume between the rupture disc and the leak valve was filled with H2 at 1 bar.

The nozzle downstream of the rupture disc is open to the atmosphere.

The burst pressure of the used aluminium rupture disc was clearly lower than the overpressure in the reservoir.

Schematic of the experimental set-up.

Forced ignition test:

Auto ignition test:

Fast leak valve opening produced the unsteady free jet.

Additionally a rupture disc holder is installed behind the fast leak valve.


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Ignition source Positioned along the jet axis on a lanceHigh frequency electric spark (20 kHz, ~ 60 kV) with an electrode distance of ~ 6 mm.

Five dynamic pressure sensors: PCB (Type 113A31)

Pressure sensor line was used in parallel (50 cm) to the jet axis.

(reflected pressure measurement)

Integral heat flux sensorsPositioned on lance along the jet axis.

Unobstructed length in space of 4 m. Distance to the floor and the nearest wall was 1.6 m.

Jet axis

Experimental set-up

Experimental set-up

Optical observation By high speed camera in different configurations.


Pro-ScienceDynamics of H2-releases from pressurized vessels

Pressure decay in the vessel

180

182

184

186

188

190

192

194

196

198

200

202

0 0.001 0.002 0.003 0.004 0.005 0.006 0.007 0.008 0.009 0.01

t / s

P /

bar

4 mm calculated

4 mm valve and rupture disc4 mm valve

0

100

200

0 0.25 0.5t / s

P /

bar

Pressure decay in the vessel

180

182

184

186

188

190

192

194

196

198

200

202

0 0.001 0.002 0.003 0.004 0.005 0.006 0.007 0.008 0.009 0.01

t / s

P /

bar

4 mm calculated

4 mm valve and rupture disc4 mm valve

0

100

200

0 0.25 0.5t / s

P /

bar

Comparison of measured and calculated pressure decay inside the vessel:

- Burst pressure of the rupture disc ~ 160 bar. - 4 mm nozzle.

No significant difference for the pressure decay inside the vessel.


Pro-ScienceDynamics of H2-releases from pressurized vessels

Hydrogen release via valve.Start up of the flow is controlledby the valve opening behavior.

Sudden hydrogen release via valve and rupture disc(burst pressure ~ 160 bar).

30 cm

(jet is ignited!)

4 mm nozzle, 200 bar


Pro-Science Hydrogen release via rupture disc.

t

60 mm

Auto ignition in 2 mm glass tube:

Auto ignition in 5 mm square glass tube:

A view inside the release nozzle downstream the rupture disc:

The auto ignition in the tube in a complex process!

An extension tube downstream the rupture disc is necessary to reach auto ignition.

200 bar, 180000 f/sBurst pressure of the rupture disc ~ 120 bar

220 bar, 450000 f/s


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Critical conditions for spontaneous ignitions inside the extension tube downstream the rupture disc.

Hydrogen release via rupture disc.

In this configuration all spontaneous ignition events lead to a jet fire.

4mm nozzle

0255075

100125150175200225

0 20 40 60 80 100 120 140

Length of extention pipe / mm

P0

[ b

ar]

ignition (jet fire) no ignition

Length of extension pipe / mm


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Maximum overpressure: P0 = 160 bar, d = 4 mm; auto ignition

0

10

20

30

40

50

60

70

80

0 50 100 150 200

distance from the nozzle / cm

P /

mb

ar

overpressuremembrane ruptureoverpressurecombustion

P2P1

P4

P3

P5

-50

-30

-10

10

30

50

70

0.019 0.0195 0.02 0.0205 0.021 0.0215 0.022

t / s

P /

mba

r

pressuregauge P2

Maximum overpressure: P0 = 160 bar, d = 4 mm; auto ignition

0

10

20

30

40

50

60

70

80

0 50 100 150 200

distance from the nozzle / cm

P /

mb

ar

overpressuremembrane ruptureoverpressurecombustion

P2P1

P4

P3

P5

-50

-30

-10

10

30

50

70

0.019 0.0195 0.02 0.0205 0.021 0.0215 0.022

t / s

P /

mba

r

pressuregauge P2

Hydrogen release via rupture disc.

Example for the combustion pressure amplitude of a spontaneous ignition test.

4 mm nozzle, 200 bar

The small amount of burned mixture released from the nozzle ignites the following hydrogen flow jet fire.

-Fist: shock wave from the ruptured disc leaves the nozzle exit.

-Second: combustion of the turbulent free jet.

Two pressure peaks were observed:


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P0 = 200bar; 4mm nozzle

Ignition distance 50 cm

0

20

40

60

80

100

120

140

160

180

200

20 30 40 50 60 70 80ignition delay time / ms

Pm

ax /

mb

ar

gauge P3

Ignition distance vs. maximum combustion overpressure

0

50

100

150

200

250

300

0 50 100 150 200

ignition distance / cm

P /

mb

ar 4 mm [100 bar] 3 mm [100 bar] 3 mm [200 bar]

4 mm [200 bar]10 mm [200 bar]

time

high reactive mixture cloud

jet

time

high reactive mixture cloud

jet

Spark ignition: maximum pressure

Due to the start-up process of the head of the flow in combination with the transient release conditions a highly reactive hydrogen / air cloud inside the turbulent jet is formed.

The volumetric size and the position of this highly reactive hydrogen / air cloud are changing during the transient release.

For every configuration a distinct ignition time and ignition position exists for the generation of a maximum pressure wave due to a "local explosion" in the free jet.

-Variation of the ignition time. -Variation of the ignition distance.


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Nozzle: 4 mm, P0 = 200 bar, ignition time and distance optimized.

Example "local explosion"Spark ignition:

0.6 m

Integral heat flux sensors Nozzle

Ignition source

H2 jet

Real and shadow image

Digitally processed(pressure wave visualization)


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Maximum peak overpressure, auto and spark ignition: nozzle 4mm

0

50

100

150

200

250

0 50 100 150 200 250

P0 / bar

Pm

ax /

mb

ar

25 mm, no ignition

25 mm, ignition

42 mm, no ignition

42 mm, ignition

60 mm, no ignition

60 mm, ignition

60 mm, ignition

80 mm, no ignition

80 mm, ignition

120 mm, no ignition

120 mm, ignition

subsequent spark ignition

posibility of eardrum rupture [7]

Hazard potential of pressure loads.

Maximum measured overpressure amplitudes from the 4 mm nozzle experiments, measured in reflected orientation in 50 cm distance to the jet axis.

For 200 bar of initial release overpressure reflected amplitudes of 220 mbar in a distance of 50 cm to the jet axis were measured, this value reaches the upper limit for possible irreparable ear injuries.


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160

180

200

220

240

260

280

300

25 50 75 100

Q /

[kJ

/m2 ]

Integral heat flux: P0 = 200 bar

0

50

100

150

200

250

300

350

0 50 100 150 200 250 300

distance to the nozzle / cm

Q /

[kJ

/m2 ]

nozzle 4mmnozzle 3mmnozzle 10mm (self ignition) nozzle 4mm

160

180

200

220

240

260

280

300

25 50 75 100

Q /

[kJ

/m2 ]

Integral heat flux: P0 = 200 bar

0

50

100

150

200

250

300

350

0 50 100 150 200 250 300

distance to the nozzle / cm

Q /

[kJ

/m2 ]

nozzle 4mmnozzle 3mmnozzle 10mm (self ignition) nozzle 4mm

Integral heat flux: self ignition; 4 mm nozzle

0

50

100

150

200

250

300

350

0 25 50 75 100 125 150 175 200 225 250

P0 / bar

Q /

[kJ

/m2 ]

50 cm distance to the nozzle


Integral heat flux: self ignition; 4 mm nozzle

0

50

100

150

200

250

300

350

0 25 50 75 100 125 150 175 200 225 250

P0 / bar

Q /

[kJ

/m2 ]



Thermal energies from ignited transient free jets

An early ignition of the released hydrogen close to the nozzle produces the highest thermal loads.

Measured maximum integral heat fluxes along the jet axis.

The maximum measured thermal loads decrease almost linearly with increasing distance to the nozzle.

Due to the fast process: Integral heat flux sensors were used.

Measured integral heat fluxes along the jet axis (auto ignition).

The case of the spontaneous ignition of the first released hydrogen inside the nozzle pipe is the earliest possible ignition time.

A conservative linearization of the maximum thermal loads along the jet axis is possible.


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Critical distance to the nozzle for possible second degree burns of human skin

0

50

100

150

200

250

300

0 50 100 150 200

initial release overpressure / bar

dis

tan

ce

to

th

e n

ozz

le /

cm

4 mm nozzle

Assumption: The characteristic release time of the transient hydrogen jets is the conservative exposure time for the integral thermal loads.

Hazard potential of thermal loads

Energy threshold: A.M. Stoll et al exposure time = characteristic release time


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In this study an accidental release of pressurized H2 from a small reservoir of 0.37 dm3 was investigated experimentally with two different procedures.

In the first procedure a fast valve opens and tubular release nozzles with diameters of 3, 4, and 10 mm produce a hydrogen free jet in air with subsequent spark ignition.

In the second release application an additional rupture disc was installed in the 4 mm exhaust pipe and the sudden release conditions initiated a spontaneous ignition of the jet.

A minimum reservoir overpressure of 25 bar for a spontaneous ignition event was observed.

The transition from the auto ignition inside the exhaust nozzle pipe to the full jet fire in the ambience generates a detectable overpressure blast wave due to a "local explosion" in the free jet.

For every configuration a distinct ignition time and ignition position exists for the generation of a maximum pressure wave due to a "local explosion" in the free jet.

The generated thermal loads were investigated systematically for both ignition scenarios.A free jet that is ignited early and close to the nozzle generates the maximum thermal load for its ambience. The spontaneous ignition of the first released H2 inside the tube behind the rupture disc is the earliest ignition situation of the released jet.

Summary

Pro-Science 4 th International Conference of Hydrogen Safety, September 12-14, 2011, SAN FRANCISCO,...

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