The Hardy Cross method for analysis of fluid systems assumes that in inlet flows and outlet flows for the system are known. However, when one constructs such a system in real life, how do we ensure that the flow entering and exiting the system will be exactly equal to the parameters set initially? e.g. if the system is connected to a water tank for input, and has a loop with 3 outlets, and we assume that the input water flow is 300 l/s and the outlet flows are 100 l/s each. We decide all lengths and diameters as part of stating the initial condition to apply the Hardy Cross equation, and the method tells us what the flow rates will be based on the analysis. How do I know that the system will obey these parameters, and that the flow rates will match the initial assumptions I made.
You design the system for a demand that it should meet, 3x 100l/s in your example. If the demand is higher, the sytem will likely fail to deliver the actual demand. If the demand is lower, no problem.
On the network side, you know the pressure you want to deliver the water at. You could supply valves at the tap that deliver only (say) 100l/s at 4 bar, but in most cases actual water flow has to be managed on the consumer side and there are countless ways to do that:
- manually turning down the tap (if the consumer is a shower or something like that)
- If the consumer needs a constand water flow, they can use a combination of pressure regulator, simple flow meter and manual valve.