Difference Between Pressure and Flow Demands

In the PIPE-FLO model, boundary conditions can be established using Pressure Boundaries and Flow Demands. These can be used to set a specific total pressure or flow requirement or to evaluate if the system can meet these requirements.  

The model below shows a water distribution system being used to supply fire hydrants. The system requirements are that when two hydrants are opened anywhere in the system each hydrant must deliver at least 300 gpm with 20 psig at the outlet of the hydrant when fire hoses are attached.  In other words we need to meet both a pressure and flow requirement.   

In PIPE-FLO we can only:

  • Set a pressure at a node and calculate the resulting flow rate out of the node that gives us the set pressure
  • Set the flow rate at a node and calculate the resulting pressure at the node that gives us the set flow rate. 

At first look you may think PIPE-FLO is unable to make the necessary calculation.  But if we look at what PIPE-FLO calculates, we can see quickly if our customer's needs are meet.

If we set the pressure at the hydrants to 20 psig and run the calculations PIPE-FLO determines the flow rate needed to obtain the set pressure of 20 psig.  If this flow rate is above the required amount for the set pressure then the condition is met.




Now let's look at the calculated model with hydrants 1 and 6 open with the boundary condition of 20 psig set at the outlet of the fire hydrant.

After performing the calculations the calculated flow rate to hydrant 1 is 2,543 gpm and the flow rate to hydrant 6 is 657.3 gpm.  As you can see the system is able to supply at least 300 gpm per hydrant while maintaining 20 psig at the hydrant.   




Now we will replace the pressure demands with flow demands. The flow demand at hydrant 1 is set to 2543 gpm and the flow demand at hydrant 6 is set to 657.3 gpm, the calculated values when setting the pressures at the nodes to 20 psig.

After performing the calculations notice the pressure at hydrant 6 is 20 and hydrant 1 is 20.01 psig.  As you can see if you set the pressure and calculate the flow rate at a demand, or set the flow rate and calculate the pressure at the demand, the results should be identical. 

Lastly, we will take a look at what happens with the flow demands set to 300 gpm.  

Notice the flow rate out to each hydrant is 300 gpm and the pressures at both hydrants are above 56 psig.  As you can see the design condition is met under both cases. 

Using Lineups, further analysis can be done to determine how many hydrants can be open and still have the 20 psig requirement met.

In review, demands can be set either to a flow rate or to a pressure.

  • By setting the demand as a flow demand, you specify the flow rate out of the node and PIPE-FLO calculates the pressure at the node needed to maintain the set flow rate out of the node.
  • By setting the demand as a pressure demand, you specify the pressure at the node and PIPE-FLO calculates the flow rate needed to maintain the set pressure at the node.