1. A Safety Assessment of Hydrogen Supply Piping System by Use of FDS
Kyushu Univ.
Dept. of Earth Resource Engineering
○Toshimitsu TANAKA., Masahiro INOUE.
Hello everyone, my name is Toshimitsu Tanaka. I am a student at Kyushu University.
I came from Japan.
Today, I would like to talk about a safety assessment of hydrogen supply piping system.

2. Outline
１. Introduction ２. Safety Assessment Method ３. Description of CFD Code ４. Simulation Models ５. Simulation Results ６. Conclusion
This is the outline of my presentation.

3. Hydrogen as Energy Carrier
Introduction
Hydrogen as Energy Carrier
Renewable Energy
Consumption
Fuel-cell Vehicle (1)
Wind H2
Let’s begin with background of study.
Hydrogen has small impact on environment and it is expected to be widely adopted in our civil life.
One of remarkable feature of hydrogen is that it can be used as an energy carrier.
Surplus electric power could be converted to hydrogen and it would be consumed by FC vehicle and FC Systems.
FC vehicle uses hydrogen as a fuel, and FC systems generate heat and electricity by chemical means.
Solar
FC Systems (2)
(1): TOYOTA MOTOR CORPORATION.
(2): TOSHIBA FUEL CELL POWER SYSTEMS CORPORATION.

4. Increase of Residential FC System in Japan
Spread of
H2 Utilization Equipment
0.15
mil.
5.3 mil.
Number of Sales (millions of units)
Increase demand of
stable H2 supply
Focusing on residential FC Systems, 0.15 millions of units have been introduced in houses and apartments in Japan.
Japanese government tries to increase the number to 5.3 million by 2030.
As hydrogen utilization equipment like FC systems are adopted in society, the demand of stable hydrogen supply will increase.
Hydrogen piping system is one of powerful option, to meet increasing demand.
Hydrogen Piping System
is a powerful option
Sales of Residential FC cogeneration system and expectation（Advanced Cogeneration and Energy Utilization Center JAPAN）

5. Issue: Mixing of gases in the piping
Outlet (Open) H2
Air
Two Methods : Use inert gas to prevent mixing
Fully Replacing Method
Partially Replacing Method
However, there is an issue to be solved.
Air remaining in a constructed piping should be replaced with hydrogen to start to supply it to each houses.
To conduct this replacing operation safely, two methods are under discussion.
First one is to purge the air in the entire piping with inert gas before hydrogen supply.
In this method, safety is surely guaranteed but this method is not economical because it needs a large amount of inert gas especially for long piping and time consuming.
To overcome this problem, Partially Replacing Method is proposed.
In this method, a certain amount of inert gas is supplied beforehand and it is put between air and hydrogen.
Thus, mixing can be prevented by a small amount of gas compared with Fully Replacing Method.
Inert Gas
Inert Gas

6. Partially Replacing Method
Air – Nitrogen – Hydrogen Sandwich Structure is…
Maintained
Not Maintained
Sequence of the procedure
Gas Flow ?
The slide explains how to purge air from piping by using Partially Replacing Method.
In my study, I focused on nitrogen as an inert gas to use.
Please take a look on the left figure.
Initially, air presents in entire piping.
In next phase, a certain amount of nitrogen is supplied, then hydrogen supply is started.
Now, air – nitrogen – hydrogen sandwich structure is formed.
If supplied nitrogen is enough, hydrogen does not mix with air and purging operation can be conducted safely.
However, if the amount of nitrogen is not enough like on the right figure, hydrogen will mix with air here.
：Gas Supply Part
：Air
：Nitrogen
：Hydrogen

7. Explosion Limits of Gas Mixture
Safety in piping was judged based on Explosion Limits
Danger Judging Standard
H2 ≧４％
and
O2 ≧５％
Hydrogen [vol%]
Let me explain how the safety standard to judge the state of gas mixture was decided.
Explosion limits of gas mixture was considered.
Left figure shows explosion limits of hydrogen – nitrogen – air gas mixture.
Area surrounded by red colored solid line shows the state that gas mixture is inflammable.
Considering this explosion limits, gas mixture is not safe if hydrogen concentration is 4% or more and oxygen is 5% or more in the gas mixture.
H2 = 4%
O2 = 5%
Explosion Limits of
H2－N2－Air Gas Mixture and
H2－CO2－Air Gas Mixture
（Book for Hydrogen Energy）

8. from inlet to the cross-point
Safe Pipe Length 2
Layer-H2 … Conc. of H2 = 4%
Layer-H2
Layer-O2 … Conc. of O2 = 5%
Layer-O2
Dangerous
(H2≧4% and O2≧5%)
Purging Process can be
safely conducted
from inlet to the cross-point
Layer-H2 is contour that the concentration of hydrogen is 4%, and Layer-O2 corresponds to 5% of oxygen. Layer-H2 and Layer-O2 are located as shown in the figure, having nitrogen between two layers.
These layers will get closer with time.
Luck of nitrogen will make layers cross, then overlapped and dangerous area will appear.
Red triangle area is judged as dangerous.
The replacing operation cannot be conducted no longer once this area appears.
In other words, we can consider that purging process can be safely conducted until this cross-point.
Thus, I defined the length between inlet and the cross-point as Safe Pipe Length, SPL.
Safe Pipe Length
（SPL）

9. Important Factors on Safety Assessment
Amount of Nitrogen
Flow Velocity
Piping Structure
The amount of nitrogen, flow velocity and piping structure are important factors on the safety assessment.
If the amount of inert gas is too small to prevent mixing, hydrogen would mix with air easily.
Flow velocity effects on how the gases flow.
The impacts caused by the differences of flow type, laminar or turbulent should be discussed.
How about piping structure?
Focusing on horizontally bended part, the effect of piping structure was examined.
Safety
(SPL)

10. 3.Description of CFD Code
・FDS is a open-source simulation software developed by
　NIST (National Institute of Standards and Technology)
・Mainly used to analyze the behavior of smoke
and heat transport from fires
To analyze the behavior of gases such as nitrogen and hydrogen and estimate SPL, CFD software called FDS was used.
FDS is open-source simulation software developed by NIST and it is mainly used to analyze the behavior of smoke and heat transport from fires.
In addition to these analyses, the behavior of gases such as hydrogen is also able to be calculated by FDS.
・The behavior of hydrogen is also calculable by FDS

11. Basic Model（Linear-pipe Model）
・Model of Piping System (Length:10m, Diameter:36mm) H2 N2
Air
：Hydrogen Supply Part
As for the simulation models. I-shaped model, has only straight pipe and is the most basic model.
It was modeled after a piping of 10m length and 36mm diameter.
In Blue colored area, nitrogen is filled with atmospheric pressure as initial condition.
To examine effect of the amount of nitrogen, the length of this area was changed as: 1m, 2m and 3m.
Five different flow velocity was used in the simulation.
They are 100, 200, 300, 400, 500 expressed in Reynolds number at Hydrogen Supply Part.
Re(H2) was calculated based on the physical properties of hydrogen.
The cross-section of the pipe was set as rectangular by the restriction of the software.
100
200
300
400
500
1m, 2m, 3m
(3 patterns)
Re(H2) =
(5 patterns)

12. Simulation Models with Bended Part(s)ZN
(Filled with N2)
One bend pipe model
To examine the effect of piping structure, two bend models which have horizontally bended part were used.
They are L-shaped model which has one horizontally bended part and…
・1 horizontally bended part, L-shaped
：Gas Supply Part
：Air
：Nitrogen
：Hydrogen

13. Two Bends Pipe Model (U-Shaped)
・Two horizontally bended parts. ZN
(Filled with N2)
U-shaped model which has two bended parts.
：Gas Supply Part
：Air
：Nitrogen
：Hydrogen

14. Flow Velocity Piping Structure Re(H2) = Amount of Nitrogen Gas 100
Linear-pipe (I-Shaped)
One bend pipe (L-shaped)
Two bends pipe (U-shaped)
ZN = 1m　→　Volume of N2 = 1.3L
ZN = 2m　→　Volume of N2 = 2.6L
ZN = 3m　→　Volume of N2 = 3.9L
100
200
300
400
500
Making a summary of simulation conditions, three patterns were considered to the amount of nitrogen, five patterns to flow velocity, three patterns to piping structure.
Combining these patterns, totally 45 simulations were conducted.
Re(H2) =

15. 3m 2m 1m Length of ZN （Volume of Nitrogen） U-shaped ( ) L-shaped ( )
(　　　)
L-shaped
(　　　) 3m
(3.9L)
I-shaped
(　　　) 2m
(2.6L) 1m
(1.3L)
This graph shows the result of simulations.
The vertical line represents SPL and horizontal one represents flow velocity.
SPL becomes longer as the amount of nitrogen increases.
For the piping structure, the horizontally bended parts seem to have almost no negative influence on SPL, because SPL obtained by L-shaped and U-shaped are almost equal or somewhat longer than those of I-shaped model.
Green line indicates that SPLs are almost constant when Reynolds number is less than 300 and they increase as Reynolds number is more than 400.
695
1390
2085
2780
3475
4170
Re [-]

16. Re(H2) = 300 Re(H2) = 400 Layer-H2 Layer-O2 Layer-H2 Layer-O2
These figures visualize the flow of gases.
When flow velocity is 300, both of hydrogen and oxygen flow is laminar flow.
However, when flow velocity is 400, their flow state is turbulent.
These differences of gas flows may cause the rapid changes of SPL in the previous Figure.
The important point is that whether the gas flow is laminar or turbulent flow may have a large influence on SPL.
Layer-O2

17. 6. Conclusion
Adopting partially replacing method, the consumption of nitrogen is averagely reduced by 66.2%, compared with fully replacing method.
The horizontally bended part(s) seems to have almost no negative influence on the safety.
The safe pipe length seems to be affected by the type of gas flow: laminar or turbulent.
Let me summarize the points of my presentation.
First point is that adopting partially replacing method, the consumption of nitrogen is averagely reduced by 66.2%, compared with fully replacing method.
Second point is that the horizontally bended part(s) seems to have almost no negative influence on SPL.
Third one is that the safe pipe length seems to be affected by the type of gas flow: laminar or turbulent.