Boundaries vs. Design Requirements
Boundaries
Points in a model that physically force a given parameter.
A piping network can be considered to have three variables - flow, pressure, and the system. For analysis, it is required that the user fully define the system. The system is a collection of components that affect the flows and pressures - pipes, pumps, valves, etc. Because there are still two variables - flow and pressure - the user must specify at least one of them as a boundary somewhere in the system.
Note: Due to the nature of fluid analysis, if there is only one boundary it must be a pressure type boundary. See Role of Pressure Junctions.
For a model to be accurate, its entire definition must reflect the system it is representing. This includes both the system and the boundaries. It is obvious that a 120 foot, 12" diameter pipe should be represented in the model with the same length and diameter. However, ensuring the correct selection of boundaries is often less intuitive.
Specifying any given boundary as a fixed pressure will result in some calculated flow at that boundary. Changing this pressure boundary to a flow boundary with the calculated value will not change any of the pressure or flow results in the model. However, this has important physical implications.
For example, consider a pipe bounded by two pressures. Doubling the pipe size will significantly increase flow. However, if a boundary was instead a fixed flow, doubling the pipe size will not change the resulting flow but instead change the pressures at the flow boundary. If the boundary in the real system is a large fixed pressure tank, it should not be represented as a fixed flow, because changes to the system will effectively change the pressure required in the tank. In other words, changing the system changes the physical meaning of the boundary. If the tank is represented as a fixed pressure, as it behaves in the field, this is not an issue.
This is important in ANS because ANS is changing the system as part of the sizing process. Using the incorrect type of boundary can present sizing solutions that would not work in the real system.
Note: It is not uncommon or incorrect to use boundaries to represent a requirement in an analysis. This is a common design tool to determine what is required of a boundary. In the above example, it may be desired to know the required tank pressure to drive the required flow. What is important is to recognize whether a junction is representing a requirement or a physical boundary that cannot be changed.
Design Requirements
Design Requirements are limits on allowable behavior.
The distinction is important because Design Requirements are limits rather than imposed values. For example, a Design Requirement may state that a flow must be at least 100 gal/min. The design is considered acceptable if the flow ends up as 120 gal/min. On the other hand, using a boundary of 100 gal/min forces the model always to maintain 100 gal/min.
In this manner, Design Requirements are less restrictive than boundary conditions. Generally, the less restrictive the problem is, the more flexibility is available to obtain a better sizing result.
Even if it is known that there must be some fixed flow into a tank, using an Assigned Flow boundary at this location may not be the best choice. If the pressure in the tank cannot be changed to the value calculated based on the sizing result, the use of an Assigned Flow here would be incorrect. Instead, it would be appropriate to use an Assigned Pressure and two Design Requirements on flow - a minimum and a maximum, to limit the solution to an acceptable range.
