Calculating Maximum Nitrogen Flow

Maximum flow means maximum pressure drop also. What is the pressure drop it can with stand? Flow can not be calculated without considering velocity. Normal velocity and normal flow are adequate for most applications.

in general (approximately) , what will be the velocity of Nitrogen gas used for purging will be. with that Xm/s velocity can you please tell me how to calculate flow.

A simple estimate of the maximum ideal flow rate is to:

1) assumed choked line without shocks such that the maximum velocity is the speed of sound. [ as = sqrt(gamma*R*T) ]

2) The maximum ideal volume flow rate is then [ Q = A*as ] where A = flow area

For the maximum ideal mass flow rate, continue with

3) calculate the static pressure and static temperature from the totals using compressible flow relations. For sonic flow, [ Pt / Ps = (1+(gamma-1)/2)^(gamma/(gamma-1)) ] and [ Tt / Ts = (1 + (gamma-1)/2) ]

4) calculate the static density from ideal gas law (rho = Ps / (R*Ts))

5) The maximum ideal flow rate is then simply [ W = rho*A*as ]

Note that all temperatures should be calculated in Kelvin.

The actual maximum flow rate, volume and mass, will be less than the ideal due to pressure losses and boundary layer effects. If you have more information available for estimating the flow rate, i.e. pipe length, pipe wall roughness, inlet and exit pressures, etc, check out a copy of "Internal Flow Systems", D.S. Miller, or "Applied Fluid Dynamics Handbook", Blevins, for equations and charts for compressible flows in piping.

As others have stated, max flow would be a choked flow condition although with that initial pressure I am not sure that is a concern in a 2" pipe. If you are talking about a continuous system, follow PW's guidelines while if you are talking about purging a system, and depressurizing an N2 pressurized piping system or vessel, the maximum velocity would be at time 0 when the system is at 7 bar and you are emptying the system to atmosphere.