Fixed and Variable Area Discharges

Pressure influences some Leak Flow Rates more than others

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Japanese and UK research data in 1980 on pressure/leak flow rate relationships were reconciled with other test data worldwide using the Fixed and Variable Area Discharges (FAVAD) concept, proposed by John May in 1994. The velocity of flow of a leak varies with a Coefficient of Discharge (Cd) and the square root of pressure; but John May recognised that the area of some leakage paths may also vary with pressure, and influence these relationships. The simplest version of the concept is the FAVAD NI equation:

Leakage Rate L (volume/unit time) varies with Pressure P to the power NI

Small background (undetectable) leaks at joints and fittings, and splits in flexible pipes, are ‘Variable area’ leaks sensitive to pressure, and typically have NI values around 1.5; research continues into why background leakage has such a high NI value. Detectable leaks and bursts on rigid pipes (‘Fixed area’ leaks) normally have NI values close to 0.50.

The FAVAD NI equation is usually shown as in the graph below, using pressures at the Average Zone Point (AZP). If Po and Lo are the initial AZP pressure and leakage rate, then at a different pressure P1, the leakage rate L1 can be predicted if the exponent NI is known, or can be estimated.

The FAVAD N1 equation is usually shown as in this graph, using pressures at the Average Zone Point (AZP).

Average NI values tend to be quite stable for larger systems, but vary for smaller zones depending on variations in the types and sizes of leaks on different pipe materials that are occurring from time to time.

NI values can be calculated from step-tests on Zones at night. Values derived for sectors in the UK, Japan, Brazil, Cyprus, USA, Australia, New Zealand and Malaysia show that NI usually varies between 0.50 and 1.50, with occasional values up to 2.5.

Favad N1 values can be calculated from step-tests on Zones at night

For large zones, or where no specific evidence exists, a linear relationship (NI = 1.0) is usually assumed; i.e., an X% reduction in average pressure will reduce leak flow rate by X%. Where a better estimate is required, NI can be assessed using empirical predictive equations

a) For low (close to Unavoidable) Background Leakage, NI = 1.5 – (1 – 0.65/ILI) x p/100

b) For higher Background Leakage,                       NI = 1.5 – (1 – 0.667 x ICF/ILI) x p/100

where ILI is the Infrastructure Leakage Index, and p is the % of detectable leakage on rigid pipes (2005U). and ICF is the Infrastructure Condition Factor (the ratio of actual background leakage divided by Unavoidable Background Leakage).

If the precise relationship is considered to be critical for a particular Zone, an NI night step test can be carried out to establish the NI exponent.

The FAVAD NI and Component Analysis concepts are widely used together in many of the Free Papers & PPTs’ and software for modeling and understanding relationships between pressure and leak flow rates.