Minimum night flow: finding leaks with data
Why the small hours give leaks away, what you need to measure each zone, how to separate legitimate night use from losses and what to do when the minimum flow goes up.
During the day, the water entering a distribution zone blends in with everyone’s consumption, and a leak hides in the middle of it. In the small hours, almost everyone is asleep. Much of the water that keeps flowing into the zone at that time is leakage. That is why minimum night flow is the most widely used indicator for finding physical losses with data.
What you need to measure it
- A closed zone: an area of the network with known inlets and boundary valves shut, known as a district metered area (DMA).
- A bulk meter at the inlet, with enough resolution at low flow. A meter that is only accurate at peak flow gets it wrong precisely in the small hours.
- Frequent readings, every few minutes, to catch the night minimum, which does not last long.
- History, because what matters is the change relative to the zone’s own baseline.
Separating legitimate use from losses
Not all the water used in the small hours is leakage. There is legitimate night consumption: toilet flushes, household storage tanks refilling, industry, hospitals, night irrigation. The usual practice is to estimate this consumption, by connection type and from the known large consumers in the zone, and treat the rest as probable leakage.
The most useful thing is not the absolute value but the trend:
- The minimum flow jumped from one day to the next and stayed there: a new leak.
- It crept up over several weeks: small leaks adding up, or higher pressure.
- It dropped after a repair: the data confirms the fix worked.
The role of pressure
At night, consumption falls and network pressure rises. With more pressure, the same hole leaks more. A pressure reducing valve programmed to lower pressure at night cuts both leakage and pipe bursts, and the minimum night flow shows the effect right away.
From the data to the hole in the pipe
The minimum flow tells you which zone the leak is in, not which street. From there, field surveying takes over: listening sticks, ground microphones, leak noise correlators. The gain is sending the crew to the right zone instead of sweeping the whole city.
What to do when the alarm goes off
- Check whether there is a known cause: valve operations on the network, a large consumer, a reservoir filling.
- Compare with the zone’s pressure on the same night.
- Send the leak survey crew to the zone.
- After the repair, confirm that the minimum flow is back to its usual level.
Where IoTData comes in
Bulk meter reading by pulse, reservoir level and the network that carries the data, with the DG satellite network delivering it in minutes, are already available, along with the history of each zone. The automatic balance for each zone and Modbus reading are in development. See bulk metering.
Sources: International Water Association water balance and district metered area methodology; loss control practices in water distribution systems.