The UARL formula: unavoidable annual real losses

Even a brand-new, perfectly managed network leaks. Joints weep, fittings seep, and there is always some background leakage below the threshold of detection. The unavoidable annual real losses formula estimates that floor for your network.

Where the coefficients come from

The coefficients were derived by Allan Lambert and colleagues from background leakage rates, reported burst frequencies and average burst run times in well-managed systems, all at a reference pressure and normalised per metre of pressure. They are 18 litres per km of mains per day per metre of pressure, 0.8 litres per connection per day per metre, and 25 litres per km of service pipe per day per metre.

Service pipe length matters more than people think

If customer meters are at the property boundary, Lp is zero and the third term disappears. If meters sit at the house — common in the United States, the United Kingdom and much of Europe — an average of 10–15 m of private pipe per connection can add a third or more to the UARL. Getting this number right is the difference between ILI 2 and ILI 3.

Adjusting for supply time

Leakage only happens while the system is pressurised. For intermittent supply the UARL and the current real losses are both scaled by hours of supply per day divided by 24, so the ILI is a fair comparison regardless of supply regime.

Worked example

A system with 1,800 km of mains, 86,000 service connections, 6 m of service pipe per connection (516 km in total) and 43.5 m average pressure has UARL = (18 × 1,800 + 0.8 × 86,000 + 25 × 516) × 43.5 = 4.97 million litres per day, or about 4,970 m³/day. If its real losses are 15,000 m³/day, its ILI is 3.0 — band B.