While small percentages of undiluted propane and other LPGs may sometimes be injected into flowing natural-gas streams, peak shaving most often involves use of propane-air.

Propane-air is created by mixing approximately 55% propane and 45% air. The mix is injected into the natural-gas system as partial replacement for up to about 50% of the combined gas stream. The "right" mixture for a peak shaving site can reflect several factors, including the make up of the natural-gas and LPG streams and the interchangeability criteria to be met.

There are several approaches to mixing propane and air, and many blender (mixer) options can be considered. The mixer type influences selection of other equipment used in the system. Common types of propane-air blenders with common mixed-gas pressure ranges are shown in the diagrams below.

Note: Required controls and safety devices are not shown in detail.

A propane-air mixture containing 1,470 btu/cf has burning characteristics similar to natural gas containing 1,000 btu/cf with a specific gravity of .60.

See Table 4 for typical propane-air compositions and associated specific gravity and Wobbe values.

See Figure 2 for propane-air dew points.

One measure of interchangeability is the Wobbe Index. The Wobbe number of a fuel gas relates heating value and specific gravity. By maintaining a consistent Wobbe number when replacing or combining fuel-gas streams, acceptable end-use performance is generally assured. Of more narrow concern, some flame characteristics are not predicted by the Wobbe number for all fuel compositions. Of course, applications where 'chemical' use of 'methane' occurs (e.g., fertilizer production) are outside the simple scope of 'fuel.'