When performing a lineup calculation in PIPE-FLO, you may occasionally encounter a "Percent Tolerance not achieved" error message.  In most cases,
this problem can be eliminated by adjusting PIPE-FLO's default convergence criteria.

When you perform a calculation in PIPE-FLO by clicking the Calculate button on the toolbar, PIPE-FLO uses default convergence criteria when solving the system.  The defaults are:

     Percent tolerance: 0.01%
     Maximum iterations: 100
     Linear switch point: 10%
     Laminar cutoff Re: 2100

If the system's calculated %Deviation is greater than the percent tolerance at the end of 100 iterations, the following error message is displayed:



Most of the time you can get the system to converge by increasing the maximum number of iterations.  To adjust this convergence criteria, choose the System / Calculate menu item.  The System Calculation dialog box is shown below:

 
Initially you might want to try doubling the maximum number of iterations.  When you click the Calc button, the Iteration number and the %Deviation for each iteration are listed on the right side of the dialog box.  If the system doesn't converge, try increasing the maximum number of iterations again and recalculating the system.

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.