Boundaries

A Boundary serves as the start or end point of the hydraulic model. It defines the known fluid condition at a specific location, such as pressure, flow rate, or fluid level. Boundaries establish how the network interfaces with the external system and are essential for determining flow direction and overall network balance.

FlowDesigner has many types of boundaries available. Generally, at a network boundary, you will either know (or have as a design goal) either the pressure or the flow rate. The available boundary components are tabulated below:

Table 2.2. Boundary icons available in FlowDesigner and the parameters they define.

Component Name	FlowDesigner Component Icon	Parameters Defined
Assigned Pressure		Pressure (Stagnation or Static)
Assigned Flow		Flow
Reservoir		Surface Pressure
Open Pipe		Exit K value (Pressure = 1 atm)
Sprinkler Nozzle		Sprinkler Model, Exit Pressure
Open Resistance		To be added

Calculation Method

2.2.1 Assigned Pressure Calculation Method

FlowDesigner does not perform specific calculations at Assigned Pressure boundaries.

Instead, the software calculates the flow rate that will occur at the known pressure and determines the flow direction:

• Flow into the network: If the flow is directed into the network, the fluid properties, including temperature and fluid, are taken directly from the values specified in the Input Editor.

• Flow out of the network: If the flow is directed out of the network, the fluid temperature and fluid are determined using the conservation of mass and energy principles; the inputs in this component are not considered in the calculation.

Implementation Note: In FlowDesigner, an Assigned Pressure component can be placed in the boundary to define the target pressure. This allows the software to calculate flows and fluid properties consistently at that point.

2.2.2 Assigned Flow Calculation Method

FlowDesigner does not perform specific calculations at Assigned Flow boundaries.

Instead, the software calculates the pressure that will be achieved at the specified flow rate and flow direction.

• Flow into the network: If the flow is directed into the network, the fluid properties, such as temperature and mass composition, are taken directly from the values specified in the Input Editor.

• Flow out of the network: If the specified flow direction is out of the network, the fluid temperature and mass composition are determined using the conservation of mass and energy principles; the inputs in this component are not considered in the calculation.

This approach ensures that known flow boundaries are handled consistently while allowing FlowDesigner to solve the network hydraulics accurately.

2.2.3 Reservoir Calculation Method

There are no specific calculations performed at reservoir boundaries.

FlowDesigner determines the flow rate that will occur at the reservoir and evaluates the flow direction for each connected pipe:

• Flow into the network: If the flow is directed from the reservoir into the network, the fluid properties—such as temperature and mass composition—are taken directly from the values specified in the Input Editor.

• Flow into the reservoir: If the calculated flow direction is into the reservoir, the fluid temperature and mass composition are determined using the conservation of mass and energy principles.

This approach ensures consistent and accurate handling of reservoir boundary behavior within the model.

2.2.4 Open Pipe Calculation Method

For all phase states, the pressure loss across an open pipe is given by:

ΔP = 0.5 K ρ v² where K=1

The loss coefficient K is independent of fluid properties, making this approach applicable to all fluids and all phase states.

For further explanation and discussion on the universality of loss coefficients, refer to Miller.

2.2.5 Sprinkler Nozzle Calculation Method

For all phase states, the pressure loss across a sprinkler is calculated using the standard orifice flow relationship. Instead of specifying the physical area and discharge coefficient directly, sprinkler manufacturers typically provide a nominal size and a discharge coefficient (Ks). This Ks value should not be confused with the pressure loss coefficient K. The relationship between volumetric flow rate and pressure is:

Q = K_s  P^0.5

where:

• Q — flow rate (L/min)

• Ks — manufacturer-specified nozzle discharge coefficient

• P — pressure at the nozzle (bar)

FlowDesigner also supports sprinkler data defined as a table of flow versus pressure. When a sprinkler is specified in this format, FlowDesigner interpolates between the tabulated values using the sprinkler equation shown above.