Question continued to above. The pressure regulator inlet is 6 kg/cm2 with 10 Nm2/hr flow from N2 generator. Outlet of pressure regulator is 100 mmWC, so what is the flow of N2 at the outlet of pressure regulator.

Pressure-Flow Calculations

Why is the flow required? Could you just trap the blanket of N2 in the tank and keep the system closed once purged? I worked at a plant with an expansion tank of hot oil with a N2 purge, and used this technique rather that use a constant flow of purge gas.

A 20 or 25mm line would be adequate for that flowrate. Get the consultant to check it, and state why.

Interesting that the size of the tank hasn't been disclosed.

Seems to me the only reason you require nitrogen flow is to compensate for volume fluctuations related to temperature variation and leaks. Since we don't know the leak rate of your tank, we can't begin to estimate the pipe size required to maintain pressure.

Depending on how much the tank volume changes and whether the tank can withstand some pressure variation, the vent might be closed with a suitable rupture disk for over-pressurization. Then a continuous N2 blanket flow is not required, just the original N2 purge and blanket to reduce the flammable risk. Install a low pressure alarm to let you know when the rupture disk bursts and the tank needs attention. This might save the OP the cost of a constant N2 flow.

That is, if the OP is still around. And whether any of this discussion makes sense to him.