Demand area analysis

Purpose

Demand area analysis calculates an "unprofiled demand profile" within a demand management area (DMA). Typically this is domestic demand. The profile represents the hourly variation in demand for a single property. The profile is stored as a demand pattern in a demand diagram. The calculation of this profile is an essential part of the calibration process which precedes a simulation.

Required data

In order to calculate the unprofiled demand pattern you need a number of items:

• The boundary of the DMA, which will be defined by closed valves. Closure is set in the network’s control.
• Flow into or out of the demand management area. This is provided by telemetry data attached to meters at the boundaries.
• The number of properties at each node (zero or more). These is the weighting factors that will be summed and applied to the pattern, so the network must be set up with these property counts.
• Other known sources of demand can be specified. For example, hospitals or factories within the area will have a known consumption which can be handled separately to domestic demand. So a hospital may have its own demand pattern which will be attached to a relevant node in the network. A number of other demand categories will be prepared to describe the demand of hospitals and factories.
• Leakage is an additional source of demand which is estimated as explained below.
• A new demand category must be created in the demand diagram. Demand area analysis will place the pattern values into this category.

Estimation of leakage

Leakage is estimated as part of the software procedure. Leakage is assumed to be constant and is estimated by finding the minimum night flow (see below). At the time of minimum flow it is assumed that each property consumes water at a rate of 1.7 litres per hour. The residual flow is assumed to be leakage.

This estimate of leakage can be formed because we assume we know a minimum value for domestic demand. This value of 1.7 is an OFWAT standard but can be reset in the software.

Leakage is assumed to be constant throughout the day.

Minimum night flow

The time and value of minimum flow is calculated as part of the software procedure. This is the minimum demand within the DMA and there is an accepted OFWAT procedure for finding the time and flow, which attempts to avoid spikes in the data. First you move a 1 hour window of time incrementally through the data and find the average demand for each window. This should smooth spikes. Now choose the time window with the minimum average. Within this window choose the time that has the lowest flow: this defines the time and value of minimum night flow. It is assumed that this is the time when domestic demand falls to its minimum of 1.7 litres per hour per property.

Software procedure

Once the data is set up, the procedure is as follows:

• InfoWorks WS Pro traces outward from a selected point to find closed valves. This defines the limit of the DMA.
• Telemetry data is acquired and summed in order to provide a time profile of total demand in the DMA.
• Calculate the minimum night flow time and value as described above.
• Estimate leakage (see above) by taking total minimum night flow and subtracting (a) the known demand profiles (hospitals and factories) and (b) the assumed minimum domestic usage of 1.7 litres per hour per property. The residual is leakage which is assumed to be constant throughout the day.
• The unprofiled (domestic) demand can now be calculated for other times of the day by taking the total demand (telemetry) and subtracting from it (a) known profiles and (b) the estimate of leakage.

Output of the process

The key output for the DMA is (1) the estimate of leakage and (2) the new demand pattern representing unprofiled or domestic demand.