For compressible gas flow calculations, the Pipe Flow Wizard software converts the ‘K’ factor of the fitting to an equivalent length of pipe, which will result in the same friction loss. This is done automatically during the calculation.


A ‘K’ factor can be converted to an equivalent length of pipe when the diameter of the pipe is known.


Equivalent length = (K factor * Diameter) / Friction factor


The Pipe Flow Wizard software allows fittings to be placed at both the start and the end of the pipe, and with compressible flow, when pressure loss occurs along the pipe, the gas will ‘expand’ (become less dense), hence it’s velocity will increase, to maintain the same mass flow rate.


The ‘K’ factor method for calculating head loss across a fitting requires that the density of the fluid entering the fitting is known, since the result is a head loss that is relative to this density, and with a compressible gas flow calculation, where there are fittings at the start and end of the pipe, the density of the gas is different at these points. In addition, the starting pressure of a gas flow calculation affects the overall friction loss results (for the same flow rate) and therefore technically if there are fittings at the start of the pipe then the loss through these must be accounted for and the pressure immediately after the fittings would then need to be used as the start pressure for the friction loss calculation.


Using the equivalent length method for compressible gas flow calculations simplifies the problem. Once the overall loss through the pipe has been calculated (including the additional equivalent length for the fittings) then the amount of loss through the fittings at the start and the end of the pipe can be apportioned appropriately to break out the results in to the fittings losses and the pipe friction loss.