Hydrogen Safety

[octane]

At our new construction of hydrogen storage tank and associated piping, we have to introduce hydrogen for the very first time.
Is it necessary to take some specific precautions before doing it? Please comment.

[Qaiser]

Before introducing H2,
Purge the whole circuit with Nitrogen to displace all the Oxygen.
Pl keep following things in mind during N2 purging:
1. Connect N2 from the very 1st (upstream) available point in the circuit.
2. Pressurize the circuit to the N2 header pressure (around 4 to 8 bar) keeping piping design pressure in view, whichever is less. You may use N2 from a VIE (vacuum insulated evaporator)supplied by a gas marketing company.
3. Depressurize the circuit from the very last (downstream) available point.
4. Repeat pressurization-depressurization cycles for several times depending upon the pressure. (Given below is an example for calculating number of cycles)
Example:
(1 atm = 1.01 bar, so we take 1 atm = 1 bar for simplicity in calculation)
N2 Header pressure available = 4 bar (gauge) = 5 bar (a)
H2 circuit design pressure = 25 bar (gauge) = 26 bar (a)
H2 circuit pressure contains air and is on atmospheric pressure before purging, pressure = ~1 bar (a)
a) Before purging: Composition of gases in H2 circuit: N2 = 79%, O2 = 21% (in this case lets treet Argon as N2 being inert)
Pressurize the circuit with N2 to 5 bar (a).
Now Composition of gases in H2 circuit: N2 = 95.8%, O2 = 4.2% (by material balance)
Depressurize the circuit completely to atm pressure. composition remains same.ie,
b) After 1st purging cycle: Composition of gases in H2 circuit: N2 = 95.8%, O2 = 4.2%
Pressurize the circuit again & Depressurize.
c) After 2nd purging cycle: O2 = 0.84%
Repeat.
d) After 3rd purging cycle: O2 = 0.168%
& so on. On 4th cycle O2 conc. goes down to less than 0.1%.

If N2 header pressure is 8 bar (g), 3 purging cycles are sufficient to bring O2 <0.1%. You may go for more cycles if more dilution is required.

This is the practice in industry.
You may pressurize / depressurize from more than one points to speed up the process.
Don't pass nitorgen by connecting form upstream & venting from downstream at the same time. It may create pockets of O2.

I've attahced an excel sheet for purging cycle calculation. It contains very simple yet useful material balance.

Queries are welcome, if any.

[octane]

Thanks...
What other safety precautions we must take else purging the line. Personnel safety? Any barricade distance or anything else?

[Qaiser]

If you are done with pneumatic or hydraulic testing then there are much less chances of leakage.
So, operators roam around the circuit during gas introduction & operation, so no barrication etc required. But access control must be practiced. no trespassing etc.
Moreover, you should keep following things in mind:
1. No H2 venting other than thru steam snuffed vent stack at a safe height.
2. In case H2 leaks, it catches fire due to friction at leaking point & the flame is not visible in daylight. So be careful while going near flanges.
3. Keep an eye on pressurizing rate.

You may ask questions specific to your site too. I'll try my best to satisfy your queries.
ciao.

[octane]

Thanks...
It is of great help.
I would be contacting you for any further query.