Modeling a Circuit Setter Balancing Valve

PIPE-FLO® Professional does not have a native device for a Circuit Setter that is commonly used in many cooling water systems. However, it is possible to simulate the Circuit Setter operation in PIPE-FLO® Professional using the Control Valve device. The Control Valve device requires the Cv data as a function of valve position as a key piece of Design Data for the valve.

PIPE-FLO® Professional users who wish to model circuit balancing valves in PIPE-FLO® Professional should first obtain the valve's hydraulic performance data from the valve manufacturer. The hydraulic performance data may be in the form of a Flow Coefficient vs. Valve Position data table or a Flow Rate vs. Head Loss (or Pressure Drop) curve. The hydraulic performance curve of Flow Rate vs. Head Loss for a 4" Bell & Gossett Circuit Setter Plus balancing valve is shown in Figure 1 as an example for this article.

The Cv data for each valve position can be read from the curve by reading across the Cv line which is drawn at 2.31 feet (1 psi pressure drop across the valve). The value of flow rate at the intersection of a valve position line is the value of the Flow Coefficient (Cv) at that position, which can be seen by using Equation 1 for SG=1 and dP=1.

C_v = {Q} \sqrt{ \frac{SG}{dP}}

Equation 1:  General Flow Coefficient equation.

For example, at 10% open the flow rate and Cv is about 28. At 100% open, the Cv is approximately 330. Note that the performance curves are on a Log-Log scale.

Obtain the Cv data at each 10% increment and enter the values into the Control Valve Data Table in the PIPE-FLO® model, shown in Figure 2, to model the Circuit Setting.

Two other critical parameters that may not be readily available from the valve manufacturer are the Liquid Pressure Recovery Factor (FL) and the Critical Pressure Drop Ratio Factor (xT). These parameters influence the valve's tendency to cavitate and experience choked flow. FL is required for calculations involving liquids and xT is used for gases. If not available, the values of these parameters can be approximated by comparison of the internal flow passages to those of typical valves such as globe valves, butterfly valves, and ball valves. Typical values can be found in the ISA 75.01 Standard for Sizing Control Valves (IEC 60534-2-1 equivalvent) and range from about 0.5 to 0.98 with the lower the value corresponding to greater susceptibility to cavitate or experience choked flow.

Figure 1: 4" Bell & Gossett Circuit Setter Plus® balancing valve performance curves.

Figure 2: Control Valve performance data table in PIPE-FLO® Professional.

Application of Circuit Setters: Modeling and Balancing Closed Loop Cooling Water Systems

A typical application of Circuit Setters is in the balancing of flow rates to heat exchangers in a closed loop cooling water system with fairly constant heat transfer loads. Many of these types of systems have a very large number of loops and users and because of the steady state nature of the thermal loads, controlling the flow rates to each user with a fully automated control valve is not economically justifiable. Instead, the flow rates are set by establishing appropriate differential pressures across branches, headers, and loops by setting the position of the Circuit Setters to fixed positions. 

The process of determining the fixed positions of the Circuit Setters to balance the system is iterative and very time consuming when done in the field. Modeling the system in PIPE-FLO with Circuit Setters and using the process described by the Bell & Gossett Instruction Manual (V1000187C), determining the valve position to set each Circuit Setting is a quick and accurate process.

To balance these types of systems, model the system using Control Valves for the circuit setters, riser balancing valves, and the triple duty valve by obtaining the Cv profile from the Performance Curves. Start the balancing process by setting all of these valves 100% open, as shown in Figure 3.

Figure 3: Closed loop cooling water system with all circuit setters at 100% open.

Once all of our valves are set to a manual position of 100% open, we can determine which valve is the Most Hydraulically Remote in each branch by finding the Circuit Setter with the least amount of differential pressure across it. This can be done using values on the FLO-Sheet, but some may be very close due to rounding. Use the List View Window to sort the Control Valves by dP to find the Most Hydraulically Remote. Starting with Branch 1, the circuit setter with the least amount of differential pressure across it is CV 9:

After the most hydraulically remote circuit setter has been established, we then want to leave the most hydraulically remote circuit setter set to a manual position of 100% open while we set all the other circuit setters in that branch to the required flow rate. For the purpose of this example, we want all the flow rates through the branches to be 20 gpm.

These steps will need to be repeated for all branches in the system. Below is a screenshot of our example system with all the branches with one circuit setter set to a manual position of 100% open and all the other circuit setters set to 20 gpm.

Now that all the branches have been specified, we need to move to the circuit setters on the risers. The process to balance these circuit setters on the risers will be the same as the branches, we need to establish what valve is the most hydraulically remote. For this example model, that would be valve CS 5.

Since CS 5 is the most hydraulically remote, we want to leave it set to a manual position of 100% open and then set all the other riser circuit setters to their required flow rate in the branches. Meaning, for our example, the other circuit setters in the risers would all be set to a flow rate of 100 gpm.

The last step to balancing our circuit setters is to adjust the pump's flow so that all the circuit setters are receiving their required flow rate. This can be done by either adjusting the Triple Duty Valve downstream from the pump or setting the pump to variable speed mode.

Triple Duty Valve set to FCV 400 gpm

Pump set to VFD 400 gpm

As we can see, without our pump being set to a variable speed operation, all of our differential pressure is focused at the Triple Duty Valve. The focus of differential pressure at the Triple Duty Valve has the potential to cause the valve to be out of specification and is something that we need to be aware of.

Summary of steps:

  1. Set all Circuit Setters to a manual position of 100% Open
  2. Find most hydraulically remote Circuit Setter and leave at 100% open
  3. Set all the other Circuit Setters in the branch to desired flow rate
  4. Repeat steps 2-3 for the riser Circuit Setters
  5. Adjust the pump flow rate
    1. Set to variable speed operation, or
    2. Set flow rate at the Triple Duty Valve

For reference to older versions of PIPE-FLO® Professional see Modeling Circuit Balancing Valves in PIPE-FLO old.pdf
*Process was based off of Bell & Gossett "Circuit Setter ® Plus Calibrated Balance Valves" Instruction Manual.