How long is the tube?  If you want a quick absolute maximum limit, assume energy conservation (Bernoulli) and convert all the pressure to velocity.

Matt

In answer to mbt22
The pipe is 10cm long. I am not sure what you mean with converting all the pressure to velocity.

Base on critical velocity (which you don't want to pass)
The max flow is ~3lb/hr
BTW most of the pressure drop is probably at your valve.
 

What equation or reference did you use? 3 lbs of hydrogen gas (H2) occupies a volume of about 15,000L. One Mole of H2 = 2g, which will occupy 22.4 liters as stp. One pound = 0.453592kg
Doesn't temperature AND pressure play a role if you go from a compresed gas @ 60 psi (gauge) to atmospheric pressure?

Assuming choked flow at your valve the mass flow rate will be a function of the upstream pressure only (you said the upstream temp is room temp).

Which function or reference would you use to calculate the mass flow rate at constant pressure but different orifice sizes?

Choked flow

M=C*A*{k*ρ*P(2/k+1)^[(k+1)/(k-1)]}^0.5

K(H2)=1.410

Minimum P/Pd  required for H2 choked flow = 1.899

M= mass flow rate, kg/s
C = discharge coefficient, dimensionless (~0.62)
A = discharge hole cross-sectional area, m²
k = cp/cv of the gas
cp = specific heat of the gas at constant pressure
cv = specific heat of the gas at constant volume
ρ = real gas density at P and T, kg/m³
P = absolute upstream pressure of the gas, Pa
Pd = absolute downstream gas pressure

good luck