Using Two Control Curves in SWMM 5 to Simulate a Head Difference Rule

Using Two Control Curves in SWMM 5 to Simulate a Head Difference Rule

In SWMM 5, Real Time Control (RTC) rules can't directly use the head difference across an orifice as a control parameter. However, you can simulate this behavior by using node depths, which are available as control parameters. Here's how to set up a system where orifice operation is based on the head difference indirectly:

Methodology:

1. Setup with Two Orifices:
   • Instead of one orifice, use two orifices (as shown in Figure 1 of your example), one for each flow direction or condition.
2. Control Curves and Rules:
   • Define two control curves (RuleOrf1 and RuleOrf2) that dictate how each orifice's setting changes based on node depth. These curves control the opening/closing of each orifice.
3. Rules for Control:
   • Use two separate rules to govern the behavior of each orifice:
   • Rule for Orifice1:
     RULE Orifice1
     IF  NODE UPNode  Depth >= 0 
     THEN ORIFICE ORIFICE1 SETTING = Curve RuleOrf1
     PRIORITY 10

     • This rule means Orifice1 starts open but will close (or adjust) based on the depth at UPNode. The curve RuleOrf1 should define how the orifice setting decreases as the depth at UPNode increases, effectively closing when the head difference becomes negligible.
   • Rule for Orifice2:
     RULE Orifice2
     IF  NODE DNode  Depth >= 0
     THEN ORIFICE ORIFICE2 SETTING = Curve RuleOrf2
     PRIORITY 10

     • Here, Orifice2 starts closed but will open or adjust based on the depth at DNode. The curve RuleOrf2 should specify how the orifice setting increases with the depth at DNode, opening up when there's enough head difference to allow flow in the opposite direction.

Explanation:

• Orifice1 Control: This orifice is designed to allow flow from UPNode to DNode when there's a positive head difference. As the depth at UPNode increases (indicating a positive head difference), the orifice gradually closes according to RuleOrf1.
• Orifice2 Control: Conversely, this orifice facilitates flow from DNode back to UPNode when there's a negative head difference or when the head difference is close to zero but with flow from the downstream side. As the depth at DNode increases (indicating a potential backflow), Orifice2 opens according to RuleOrf2.

Variations:

• You could reverse the control nodes for each orifice, using DNode to control Orifice1 and UPNode for Orifice2, depending on your specific hydraulic needs or the behavior you wish to simulate.

This setup effectively mimics a head difference rule by using node depths as proxies, allowing for sophisticated control of flow through orifices based on dynamic head conditions in the system.

 

Figure 1.  Two Orifice Solution

 

Figure 2. Two Orifice solution to have control over the Orifice(s) at both the upstream and downstream nodes.