Night-Day Factor NDF

What is the Night-Day Factor NDF and why is it important?

NDF converts Night Leakage Rate (per hour) to Daily leakage, and vice-versa

Leakage can be assessed either from Water Balance or from Minimum Night Flows. However, one of the most common errors in leakage calculations is to simply assume that daily leakage from a water balance can be converted to hourly leakage at night by dividing by 24 hours/day; or that night leakage per hour can be simply multiplied by 24 hours per day to assess daily leakage. These assumptions would only be correct if the average pressure in the Zone is constant for 24 hours per day, because leak flow rates vary with the average pressure and N1 exponent in the Zone, as described in FAVAD and N1 Update in the Influences of Pressure Info-Hub.

In practice, the multiplier in Hours/day, known as the Night-Day Factor NDF (or Hour-Day Factor HDF in the UK) can vary from less than 10 hours/day for gravity systems with high leakage and frictional losses, to more than 60 hours/day for pressure and flow modulated systems, so the assumption of a fixed NDF of 24 hours per day, can introduce large systematic errors to leakage calculations.

The method of assessing Night-Day Factors was improved and simplified in Nov. 2017.

The latest fast-track approaches to calculating Night-Day Factor which does not require a spreadsheet, can now be found in FAVAD and N1 Update in the Influences of Pressure Info-Hub.

The previous Night-Day Factor NDF webpage (July 2016) which required data to be entered into an NDFCalcs spreadsheet in the AZP&NDFCalcs free software, can now be found in the Archive.

The AZP&NDFCalcs free software has been discontinued as it is no longer needed. Information on how to determine the location of Average Zone Point can also now be found in the Influences of Pressure Info-Hub.

Allan Lambert
14th November 2017