Short answer
Pressure drop is the loss of pressure between the meter outlet and an appliance inlet while gas is flowing. On low pressure installations designers commonly aim for no more than about 1 mbar from meter outlet to the furthest appliance at full load, though IGEM/UP/2 allows the designer to set the figure provided every appliance still receives at least its minimum inlet pressure.
What drives pressure drop
Flow rate, pipe length, internal diameter and fittings. Pressure loss rises roughly with the square of flow, and falls steeply as diameter increases — doubling diameter cuts loss dramatically. Elbows, tees and valves add equivalent length.
Measuring it
Run all appliances at full rate, measure working pressure at the meter outlet and at the appliance inlet, and take the difference. Do this at the furthest and largest appliances.
When it is too high
Symptoms include burners lighting but failing at full fire, flame lifting or yellowing, and output falling when other appliances start. The cure is resizing the pipework or reducing simultaneous demand, not adjusting appliances to compensate.
Figures are typical for UK natural gas. The current edition of the relevant IGEM standard and the manufacturer's instructions always take precedence on site.
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