Model the water distribution from a loop

Water Distribution Model spreadsheet

Spreadsheets are handy for solving engineering problems. In this case, the client installed a new USP Purified Water distribution system in his pharmaceutical facility. Questions flew: How many use points can we open simultaneously? Should we actively control the speed of the variable speed pump? What is the flowrate at each use point, and how will the rate change when we open multiple use points? How should we design the control of the hot sub-loop? What Cv should we size the backpressure valve for?

The model that I implemented using Excel and Visual Basic for Applications answers these questions I built the model using actual piping isometrics, valve characteristics, and pump curve; it could have used good guesses for these parameters if the design and procurement had not already been completed. You can adjust variables in the finished model to test the system’s response to the tank level, water temperature, number and location of open use points, travel stop settings on the use-point valves, and set-points of the control valves.

The VBA subroutines solve the pressure drop equations and perform some iterative calculations that determine the flow rate through the loop and use points while maintaining the pressure set point at the end of the loop. Worksheets within the spreadsheet contain all of the assumptions, pump curves, calculations for equivalent length of each of the piping segments, etc.

The client obtained answers to all of his questions and was left with a customized tool that he can use to evaluate changes that might be proposed in the future.