Gas mixing occurs when streams of different gas qualities are united to a single stream. In pipeline systems this means that different gas sources meet in a T-type or Y-type of pipeline. A simple example of two streams of volume (V) with two calorific values (H) is given below (see figure 3). The resultant value depends on the product of volumes or flow (Q) and the amount of each calorific value according to:

V1*H1 + V2 * H2

Hi,2= v:+v

H Zi(V, * Hi) Zi(Qi * Hi)

Hs ZiVi ZiQi

H1,2 , Hs = resulting calorific value V1 , V2 = volume of stream #1, #2 H1 , H2 = calorific value of stream #1, #2 Qi = flow of stream i ( = dV/dt)

The mixing of the flows and thus the resulting value is not perfect in the vicinity of the mixing point. This is due to the pipe dimensions which may be large or different and the flow characteristic: laminar or turbulent. The mixing process is better and faster if the flow is turbulent. If the flow is laminar mixing may take a long way and time as even a layering effect may occur. In order to speed up mixing in such cases static mixer pipes, which have small obstacles inside to provoke little turbulence, will be built in the line.