An objectionably low-pressure transient event, arising for example from a power failure or from an intermittent/interrupted supply, has the potential to cause the harmful intrusion of untreated, possibly contaminated groundwater into pipes with leaky joints or cracks as the risk of backflow increases significantly with reduced pressure. This is especially important in systems with pipes below the water table. Pathogens or chemicals in close proximity to the pipe can become a potential contamination source. In the event of a large intrusion of pathogens, the chlorine residual normally sustained in drinking water distribution systems may be insufficient to disinfect contaminated water, which can lead to damaging health effects.
InfoSurge automatically calculates the volume of intrusion due to objectionably low or negative system pressures. This will help you determine the extent of contamination and evaluate the most effective response strategies. After selecting pressure sensitive demand, two intrusion calculations are available (Leakage Factor and Constant). The "Leakage Constant" is defined using the relation Q(leak) = Lc*(pressure difference)^0.5. This is same concept as the orifice equation. For example, if Lc = 0.1, a leak of 0.1 gpm with a pressure difference of 1 psi occurs. The "Leakage Factor" is defined as the percent of leakage in the distribution system. For example, a leakage factor of 0.1 means that the leakage is 10% of the demand. From this factor, a leakage constant is calculated for each demand junction, which will discharge 10% of the flow at the initial pressure. For example, at a junction where the demand is 10 gpm at 64 psi (pressure difference) assume that 1 gpm (10% of the demand) is leaking through an orifice, the leakage constant is computed as Lc = 1/(64)^0.5 = 0.125.
The following presents the example of intrusion modeling. The intrusion (volume) is estimated based on either the leakage constant or the leakage factor assigned when the pressure gradient is negative (Pexit > Pline), where Pexit is the exit pressure and Pline is the pressure inside the pipeline system. The "Intrusion Integral" is defined as the negative area under the time plot of the curve of dP(Pexit – Pline)^0.5. For example, if a node experiences the negative pressure of 4 psi for 6 second (0.1 minute), the intrusion integral is 0.2 [= 0.1*(4)^0.5]. Therefore, the estimated intrusion volume is calculated by multiplying the intrusion integral by a leakage constant. For example, if the node has Lc = 0.1, the intrusion volume through the node is 0.02 gal (=0.1*0.2).
Example - Run intrusion modeling considering 10% of initial base demand is leak.
- Check the box of Pressure Sensitive Demand and assign Exit Head. Select a intrusion calculation type (leakage factor or constant) and assign the corresponding valve.
- After running a surge analysis, Output Report (see the following) presents two more report regarding intrusion (Surge Node Intrusion & Surge Intrusion Summary).
- The report of Surge Node Intrusion presents the resulting values of intrusion integral and intrusion volume in each node.
- The report of Surge Intrusion Summary presents the total volume of intrusion.