CRITICAL PRESSURE POINT
In order to assess if pressure management will be suitable for
a particular system, it is necessary to identify the Average
Zonal Point (AZP) and Critical Pressure Point (CPP). The Critical
Pressure Point (CPP) is the point in the water network with the
lowest (hydraulic/available) pressure. Each pressure managed
area (PMA) has a corresponding location where the pressures
are the lowest. The critical point is often monitored when pressure managing an area. If the pressure is within an acceptable range at the critical point, it can be assumed through good network design that pressure levels in the remainder of the network are also acceptable.
Critical point locations are normally either the highest location, the furthest location from the inlet, or on pipe legs with high frictional losses. Pressure in pipes will decrease with increased elevation, thus customers at height can have reduced service pressure. Points remote from the inlet to a PMA will suffer from frictional losses and be affected by water demand along the network. The frictional losses are dependent on the flow in the pipe and the pipe characteristics. Increased demand will cause faster flows and increase flow related head loss. Customers at locations far from the inlet can suffer low pressure during peak demand. In these circumstances flow modulated control of PRVs can be used to counter this effect.
It should be noted that the critical part of the network can move during different conditions. For example, the furthest point may be the critical point during peak demand and during low demand the highest elevation can become critical. Also, large changes in seasonal use, such as agricultural practice or a large influx of tourists, can influence network demand.
Knowing where the lowest pressures levels are in the network will allow water utilities to monitor and intelligently manage pressures. If pressures at the critical point are acceptable, then pressures in the remainder of the network will also be acceptable. Logging the critical point will provide utilities with warning that pressures in the network are too low and can also indicate that pressures have been set too high.
Many water utilities have to maintain a minimum level of service to customers. Maintaining water pressure at the critical part of the network ensures that all customers have acceptable pressures, avoiding penalties for water companies failing to meet service levels.
There are a number of ways to locate the critical part of the network. Often it is the highest location in the network, which can be easily identified. If it is the part of the network that suffers from the highest flow-related head loss then it can be harder to locate. Undertaking a logging exercise combined with network modelling can reveal the flow-pressure relationships and identify the area with lowest pressures. The other indication of low pressure areas are when customers regularly contact water utilities to complain about low pressures.