Preventing Backflow in a Pipe
A selected Pipe can be tagged to prevent backflow when the system is solved. A pipe in which backflow is not permitted is identified with a double arrow marker, 
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When solving a system if backflow occurs in the pipe the flow rate will be set to zero.
Use the Pipe pop-up menu option to toggle the status of Pipes tagged with the prevent backflow symbol.
Note:
After the system is first solved, if any pipe has a negative flow rate, then the pipe is temporarily removed from the model, and the system is recalculated without these pipes. If a solution is found, then the pipes that previously were found to have backflow will have their flow rate set to zero, which is effectively the result of the backflow valve operating successfully (preventing flow in the reverse direction) in the real-world scenario. This is then presented as the final solution.
If the system cannot be solved after removing all pipes with a negative flow in the initial solution, then the software will report that the system cannot be solved.
It is possible for a system to have multiple pipes set to prevent backflow, and if in the first solution some of these pipe have negative flow rates, then after temporarily removing them, and resolving the system for a second time, this ‘updated’ system may have new negative flows in some of the remaining pipes that have prevent backflow set. When this happens, the software does not continue to remove these pipes and does not look to try and find a third solution. It will simply report that the system cannot be solved with the prevent backflow applied. Use of the prevent backflow option on a pipe should therefore be used sparingly and the engineer should understand the limitations of applying the prevent backflow option, only using it for appropriate scenarios.