Criticality results give an indication of the importance of the shutdown of a segment in terms of the amount of demand met. There are several different indicators depending on the type of analysis selected.

In some cases, especially when EPS runs are being made, the system that results during a segment shutdown may be one that can't be solved hydraulically because large numbers of nodes are disconnected from the system. In that case, the Is Balanced check box will not be checked. Users should look carefully at those segments to determine the importance of such an outage.

The key indicator of the importance of shutting down a segment is the System Demand Shortfall (%). When it is large (and the system is balanced), outage of the segment will have serious impacts. The results will be different depending on the type of analysis and:

• Whether the scenario uses Pressure Dependent Demand (PDD) or non-PDD calculation options
• Whether the results are based on connectivity only (Run hydraulic engine not checked), a steady state scenario or an EPS scenario

It is generally advisable to use PDD-based scenarios for criticality. Otherwise demands will be met regardless of the pressure as long as the pressure exceeds Minimum Pressure Required to Meet Demand (displayed at the top of the right pane). With PDD, a continuous relationship between demand met and pressure is used.

While actual water users are located along pipes, the model represents them as being located at nodes. Segments which are located entirely within a single pipe element in a looped system will have no shortfall even though there may be water users along the pipe.

The user-defined Maximum Allowable Demand Shortfall field is used to indicate whether the System Demand Shortfall criteria are satisfied. When Maximum Allowable Demand Shortfall is larger than the System Demand Shortfall, and Minimum Pressure to Supply Demand is smaller than Pressure Supplied at Worst Node, the "Are all demands met?" property will be checked (True).

Interpretation of results also depends on the type of run:

• Connectivity only - In this case, demand will not be met only when the nodes are isolated from the source. Otherwise it is assumed that demand is met when a node is connected.
• Steady-State run - With steady-state runs, the shortfall is based on calculated pressure and is useful for identifying the results of outages which are not particularly long (such that the tanks drain). The shortfall includes demands that are not met because the nodes are isolated plus demands that are not fully met because pressure drops.
• EPS runs - With EPS runs, the effects of tanks draining are also determined. With EPS runs it is much more likely to have nodes that become disconnected such that the hydraulic calculations will not balance. While the connectivity only and steady state runs are snapshots which give shortfall in flow units (e.g. gpm), the EPS runs give results in volume units (e.g. gallons).

To compare between scenarios, the user should pick the Criticality Studies level of the left pane and view the bottom half of the right pane. The Average System Shortfall is a good indicator for comparisons but is based only on segments for which the hydraulic calculations are balanced.

Individual values in the criticality results are described below (in general, results from a steady run will be given as Flow while results from an EPS run will be given as Volume; hence Flow/Volume is listed below):

• Are all demands met? - This is checked (True) only if the percent demand shortfall for this segment is less than the Maximum Allowable Demand Shortfall in %. This will generally be unchecked because most segments will have a node with a demand and the node is isolated from the system. When the default value for Maximum Allowable Demand Shortfall is 0, then any segment that sees any drop in supply when closed will fail to meet demands (and hence this box will be unchecked). This property may be checked if the demand inside the segment is 0 or if the Maximum Allowable Demand Shortfall is set greater than 0. If the pressure at the node with the lowest pressure is below the Minimum Pressure to Supply Demand, then "Are All Demands Met" will be unchecked.
• Is balanced? - This is checked if the hydraulic calculations are solved. For some segments, removing the segment may affect the network so severely (e.g. disconnecting all the sources) that the calculations cannot be run. These are usually segments that seriously affect the reliability of the network and the user should inspect these manually. If "Is balanced?" is not checked, many of the results fields are N/A (not applicable.
• Maximum allowable demand shortfall (%) - This value defaults to 0%. However, for non-PDD runs, the user can override this value by entering a value in the "Maximum allowable demand shortfall" field.
• System Demand (Full)/System Demanded Volume - This is the total of system demands when there are no segment outages. It is given in flow units for steady runs and volume units for EPS runs.
• System Demand (Met)/System Supplied Volume - This is the total water supplied when the segment is out of service in flow units for steady runs and volume units for EPS runs.
• System Demand Shortfall (%) - This value is calculated as 100%*[1-(Supplied/Demanded)].
• Node with Largest Percent Demand/Volume Shortfall - This is the node label for the node with the maximum percent demand shortfall defined below. If there are no nodes with a shortfall, then this value and the next field are set to (N/A).
• Flow/Volume Demanded at Worst Node - Demand - Supplied at node from previous field.
• Flow Supplied at Worst Node - Flow supplied at node identified in the previous field.
• Node with Largest Pressure Shortfall - Node with largest value of ("Min Pressure to Supply Demand" - Pressure). This field is only used for non-PDD runs because pressure is handled differently in PDD. When the scenario calls for PDD, the "Minimum Pressure to Supply Demand" property is ignored. If the value of Min Pressure to Supply Demand is 0, then this value is not calculated and is set to (N/A).
• Pressure Demanded at Worst Node - Minimum pressure to supply demand at the worst node.
• Pressure Supplied at Worst Node - Actual pressure at Node with Largest Shortfall at the worst node.

In the case of non-PDD demands for steady runs, there are two situations for a given node that fails to meet demands.

1. Nodes that are disconnected by the segment outage in which case the demands are not included in the simulation.
2. Nodes that fail to meet minimum pressure in which case the demands are included in the simulation.

For the case of an EPS with Non-PDD demands, when choosing to "run hydraulic engine", the program checks the pressure at each node at each time step, and identifies nodes that fall below the desired minimum pressure at any given time. For criticality purposes, the program then assumes these nodes supply zero demand. Without PDD, the program cannot determine the exact shortfall. However, the criticality results in this case will still be useful, as they will identify nodes that have insufficient pressure.

In the criticality results, the "Node with largest percent demand shortfall" and "Node with largest volume shortfall" will show the node that had the highest demand during the time when the pressure was below the desired minimum pressure.