4. METHOD OF SOLUTION
The basic assumption in RAPVOID is that the emission can be treated as pseudo-steady state with the total discharge rate conserved. Inertial effects can be allowed for by calculating the additional pressure differential required to accelerate the entire pipework contents.
The flow in the pipes allows for friction and if no heat passes through the pipe walls, the flow in the pipework is adiabatic but not isentropic. Allowance can also be made for heat transfer through the walls.
At geometric discontinuities losses are allowed for by putting a frictional multiplier into the pipework to give an additional length of pipe equivalent to the estimated number of velocity heads lost.
First the total pressure is estimated at the outlet, then the discharge rate is derived by finding the static pressure at outlet, which gives the highest isentropic discharge rate. It is then possible to calculate the static and total pressures increment by increment up the pipework and to compare the total pressure at the entry to the pipework with the total pressure in the discharge vessel. The iteration on the discharge total pressure is then continued until a match is obtained between the inlet total pressure and the total pressure within the vessel.
If there are choke points within the pipework upstream of the final outlet, the code examines this possibility by comparing the mass flow at each change of section with the choked mass flow for the relevant total pressure and enthalpy. If the choked mass flow is lower, then the code iterates to obtain the converged mass flow at the most upstream critical choke position that it discovers. Having converged into this solution, it then examines the conditions downstream of this choke point for the derived mass flow.
The method of determining the critical flow for a given total pressure and enthalpy is to discover the static pressure which gives the maximum flow (AEEW M1364).