Percent Tolerance Not Achieved Error Message

Background

When performing a calculation in PIPE-FLO, you may encounter the "Percent Tolerance Not Achieved" error message.  In many cases, this problem can be eliminated by adjusting PIPE-FLO's convergence criteria. The default convergence criteria are:

  • Percent Tolerance: 0.01%
  • Maximum Iterations: 100
  • Linear Switch Point: 10%
  • Laminar Cutoff Re: 2100

When the flow rates in each branch or loop are not specified by a pump, control valve, or flow demand, PIPE-FLO iterates to determine the flow rates that satisfy both the Conservation of Mass and Conservation of Energy in the system. The solution is displayed when the system's overall Percent Deviation is less than the Percent Tolerance. If the Percent Deviation is greater than the Percent Tolerance at the end of the specified Maximum Iterations, the following error messages are displayed:

Version 12 and above:

Version 2009 and before:

Adjusting the Maximum Number of Iterations

You may be able to get the system to converge by increasing the maximum number of iterations. The maximum number of iterations that can be entered is 999.

Version 12 and above: click on an empty spot on the FLO-Sheet to display and change the Calculation Settings in the Property Grid.

Version 2009 and before: choose the System / Calculate menu item.  The System Calculation dialog box is shown below. The number of Iterations and the %Deviation for each iteration are listed on the right side of the dialog box.

Adjusting the Linear Switch Point

If increasing the maximum number of iterations doesn't eliminate the convergence problem, you may need to adjust the Linear Switch Point. This is the percent tolerance where the program will switch from the Hardy Cross calculational method to the Linear Method.  The linear method is faster than the Hardy Cross method, but the initial flow rates must be fairly close to the solution or convergence problems may be encountered.  By default, the Linear Switch Point is set to 10%.  By lowering this number, the program can get closer to a solution with the Hardy Cross method before switching to the Linear Method.

Adjusting the Laminar Cutoff Reynolds Number

Another cause of not meeting the Percent Tolerance is a calculated Reynolds Number near the laminar cutoff, defaulted to 2100. Values close to the cutoff could result in a Darcy Friction Factor that goes from a high value to a low value then back to a high value from one iteration to the next. A check of the Reynolds Numbers for each pipe in the List View Window will indicate if this is the cause. If so, lowering the cutoff value slightly may be sufficient to eliminate the message with only a slight decrease in accuracy.

Too Many Fixed dP Devices

If the model contains too many Fixed dP Devices, especially in parallel paths without flow rates specified, convergence problems may be encountered. Converting the Fixed dP Devices to a Curve dP Device with a flow vs. dP relationship may eliminate the message. Alternatively, specifying the flow rates with Control ValvesVFD Centrifugal Pumps, or Flow Demands may resolve the message as well.