Question

how to calculate nitrogen requirement for inert grinding

Answer 1

The start-up of process plants containing hydrocarbon feed streams are usually preceded by creating an inert atmosphere within the system.  One of the options of creating this inert atmosphere is via pressure purging, using nitrogen.

Pressure purging is based on using nitrogen to inert a system to low oxygen concentration below which a flammable atmosphere is not sustainable.  To achieve this, nitrogen is used to raise the pressure of the system (e.g vessel, heat exchanger, piping etc) from initial condition, which is usually atmospheric, to a chosen value (x barg).  With availability of pressure control from the nitrogen header into the system, the pressure of the system does not have to reach the nitrogen system pressure.

After pressurizing (to x barg), the system is vented back to the initial (atmospheric) condition.  This pressure-venting cycle is repeated until the required inert condition is achieved within the system.

The following steps are used to determine number of pressure-venting cycles and nitrogen requirement for pressure purging to achieved required inert O2 concentration

Choose or calculate inert O2concentration required (vol %).

Calculate number of cycles (pressure-venting) based on initial & inert (final) oxygen concentration, initial system pressure and chosen nitrogen pressure.

Calculate quantity of nitrogen per cycle to determine total nitrogen required.

Step 1. Choose or calculate inert O2 concentration required (vol%)

The first stage is to determine what level of O2 is acceptable within the system for the hydrocarbons being introduced.  As a rule of thumb 9 vol% oxygen is below the Minimum Oxygen Concentration (MOC) vol% required for complete combustion of hydrocarbons, whilst 6 vol% is still sufficient for incomplete combustion.  As such most systems are assumed to be safe at approximately 4 – 5 vol% O2.

If the required inert O2 concentration is not known, then the MOC for the hydrocarbon stream must be calculated.  A value below the MOC is then selected as the inert O2 concentration.

The minimum oxygen concentration required by a gas can be calculated from the equation: