PSI - Issue 48

Miroslav Gojić et al. / Procedia Structural Integrity 48 (2023) 334 – 341 Gojić et al / Structural Integrity Procedia 00 (2019) 000 – 000

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6

rooms or enclosures. X crit is an arbitrary value below LFL, e.g., the value at which a gas detector is set to alarm, usually 10÷25% of LFL. A low background concentration does not mean that the whole room is a non-hazardous area. The larger part of the room may be considered non-hazardous but the area near the source of the release is still a hazardous area until the release is sufficiently dispersed. The factor f in equation (6) is a measure of the degree to which the air in the enclosure outside of the release zone is well mixed and can be considered as follows: f =1; the background concentration is essentially uniform and the outlet is distant from the release itself, so that the concentration at the outlet reflects the mean background concentration. Factor f is usually between 1.5 for mildly inefficient mixing and 5 for very inefficient mixing. 3. Results and discussion Calculations of hydrogen release characteristics have been done using equations (1) to (5) with assumed conditions described in chapter 2. The results are summarized in Table 1.

Table 1. Results of hydrogen release characteristics under different release conditions

Parameter/Scenario

#1

#2

#3

#4

#5

#6

p , Pa p a , Pa T a , K

5·10 6

10·10 6

0.1·10

6

293

R , J/(kg°C) M , kg/kmol c p , J/(kgK)

8,314 2.016

14,310

, -

1.405

p c , Pa

0.192·10 6

C d , -

0.5

0.75 0.25

0.75

0.5

0.75 0.25

0.75

S , mm 2

0.025

1

0.025

1

Z , -

1

W g , kg/s ρ g , kg/m 3

3.9·10

58.8·10 − 5

233.9·10 − 5

7.8·10 − 5

116.9·10 − 5

467.7·10 − 5

− 5

0.084

k , -

1

0.8

0.5

1

0.8

0.5

LFL , -

0.04

W g /( ρ g · k · LFL ), m

3 /s

0.012

0.218

1.395

0.023

0.436

2.789

In all cases p>p c thus hydrogen release is sonic. Based on calculated hydrogen release characteristics criteria of dilution is analysed for less favourable conditions (pressure of 100 bar, scenarios from #4 to #6) using nomogram shown in Fig. 1. The results have shown that in the case of gas discharge from an opening with a diameter of S =1 mm 2 (scenario #6), it is not possible to achieve high dilution scenario and avoid the Zone 2 area. The same conclusion can be drawn for gas release at 50 bar and opening diameter of S =1 mm 2 (scenario #3). In the case for gas release at 100 bar with opening diameter of S =0.25 mm 2 , C d =0.75 and k =0.8 the air velocity due to the operation of the ventilation system should be at least 6 m/s to achieve high dilution (scenario #5), which can be achieved by the proper selection of the ventilation system in closed spaces. In all other cases considered in the paper (scenarios #1, #2 and #4), the required ventilation air speeds are lower, so conventional ventilation systems (such as mechanical supply and extraction ventilation or local extraction ventilation) can be applied in order to prevent hydrogen explosive atmospheres. Another nomogram (Fig. 2) from the standard is used to access hazardous distances from the release source for high velocity jet release, typical for hydrogen release at high pressure. The curves are based on a zero background concentration and are not applicable for indoor low dilution situations. Where a zone of negligible extent (NE) is suggested then the use of this nomogram is not applicable. For this reason hazardous distances are determined for