Introduction

The use of Pressure Independent Control Valves (PICVs) has gained wide acceptance in many HVAC chilled and heating water applications. Quality PICVs with high turndown provide accurate control of flow rate regardless of the differential pressure fluctuations across the valve. This allows for high temperature differentials across the coil. Some manufacturers even guarantee design delta T performance or better. When used in conjunction with a variable flow control system and Variable Speed Drive technology, pressure independent control valves greatly reduce the energy consumed by HVAC chilled water systems, resulting in a lower operating cost.

Click here to learn more about Pressure Independent Control Valves.

The Pressure Independent Control Valve consists of a differential pressure regulator, a control valve with actuator, and a set of pressure taps allowing for connection to external pressure instrumentation. The purpose of the differential pressure regulator is to maintain a constant differential pressure across the control surface under all flow conditions. The differential pressure is typically set to 5 psid. By maintaining a constant low differential pressure across the flow control section, the valve can regulate the flow rate accurately regardless of differential pressure variations across the entire Pressure Independent Control Valve.

PIPE-FLO has a control valve component that can be set up as a pressure independent control valve. The remainder of this knowledge base article describes how to model a system using Pressure Independent Control Valves.

Modeling PICV’s in PIPE-FLO

The primary purpose of the PICV is to regulate flow, when modeling this type of valve in PIPE-FLO use the Flow Control Valve option. In all PIPE-FLO calculations the program will determine the differential pressure across the control needed to maintain the flow rate. Each manufacturer of PICV’s lists a range of pressures in which their valve will operate properly, for example in reviewing the literature this range is between 5 to 70 psid. By entering the valves maximum allowable dP in the control dialog, PIPE-FLO will indicate when the differential pressure is too great across the valve. This information is useful, because often a different set of internals can be inserted for the pressure regulator portion of the valve which will accommodate greater dp.

One final point, since the internal operation of a PICV has two controls in series; we cannot insert the Cv data of the valve to determine the valve position.

Using PICV’s in a PIPE-FLO System

Now that we have accurately modeled PICV’s as individual controls within PIPE-FLO we will see how to apply them in a PIPE-FLO analysis using both PICV’s and a variable speed pump. The steps involved include:

1. Determine the most hydraulically remote load
2. Set the most remote load to a constant dp
3. Select the centrifugal pump
4. Set the pump to Variable Speed Control
5. Create lineups and determine operating conditions

An example system has been included in figure 1. In addition, this completed system can be downloaded from this knowledge base article so you can view it using the PIPE-FLO or Flow of Fluids programs.

Determine the Most Hydraulically Remote Load

Once the cooling water system is entered into PIPE-FLO with the multiple parallel paths, and each FCV control is set to the set value of 50 gpm, the first step is to determine the most hydraulically remote load. This value will be used in the pump selection process. The easiest way to accomplish this is to:

1. Set the centrifugal pump to Pump Sizing and enter the design flow rate, in this example 200 gpm (four loads of 50 gpm each).
2. Set all the Flow Control Valves used to limit the flow rate in each circuit to their design flow rate (50 gpm).
3. Prior to performing the calculation we must leave one load open so as not to over control the program solution. To accomplish this we will open one of the controls in what we figure will be the most hydraulically remote load. This can be done by making an educated guess. Let’s assume in this example that we think FCV 2 is in the most hydraulically remote load, so set this control to Fully Open. Once the calculations are performed, notice FCV-3 and FCV-4 are red indicating a problem and the results are in brackets
4. Display the List view for control valves and sort the list on Control dP from lowest to highest. The valve with the lowest DP will be at the top of the list. If the valve that you set in step 3 as the most hydraulically remote is at the top of the list then you guessed correctly; proceed to step 6. If not proceed to step 5. In this example FCV-4 is on the top of the list, as a result this is the most hydraulically remote load.
5. If the list on Control dP list has any results in brackets (20.44), these controls are more hydraulically remote than the control you guessed. The most hydraulically remote control is the control with the greatest negative value. Proceed to step 3 and uses this control as the most hydraulically remote and set the previous guess to its set valve. Return to step 3 and set the FCV-2 set value to 50 gpm, and open FCV-4.
6. When the calculations are performed again, display the control list from lowest to highest and notice FCV-4 is the top valve with 0 psid.

The subject of determining the most remote load is presented in greater detail in the PIPE-FLO help file and other knowledge base articles.

Set Most Remote Load to a Constant value

When using PICV's in real system, a differential pressure indicator is set across the most hydraulically remote PICV and the speed of the pumps is controlled to maintain the pressure differential across the valve by the Variable Speed Drive attached to the circulating pump.

In this example, since we will want to simulate PICVs used with a VSD, we will need to set up PIPE-FLO model to maintain a constant 5 psid pressure drop across the most hydraulically remote control. To accomplish this we will replace the FCV-4 control with a component set to a fixed dP. To be able to switch between a standard control and a PICV in our most hydraulically remote load, we'll put the component in parallel with the control. Looking at figure 2 you can see the configuration.

To simulate the control to maintain 5 psid we'll close the Control FCV-4 and open the parallel circuit with the fixed dp component called FCV-4 as CP. Since we will be running the pump with a Variable Speed Drive to maintain a constant differential pressure across the most hydraulically remote load; we will install a component in parallel to the flow control valve. The component dP is set up as a fixed 5 psid regardless of the flow rate. By closing the control and opening the component, PIPE-FLO will account for the dP across the control valve.

Now, when performing the calculations using PIPE-FLO, the calculated total head for the pump includes sufficient head for the 5 psid pressure drop across the most hydraulically remote load. This ensures that all of the valves in the system have adequate differential pressure to operate pressure-independently.

Select the Centrifugal Pump

Once the design flow rate and the resulting total head value have been calculated, you can select the centrifugal pump for the application. The PIPE-FLO help file has a section on Selecting Pump that will provide detailed examples on the selection of centrifugal pumps. You must have pump operating data entered under the Data tab in the Pump dialog. Performance data is required in order to model the pump as a variable frequency drive.

The example included in this Knowledge Base article includes a pump selected.

Set the Pump to Variable Speed Control

Once the pump performance data is entered and the pump set to a given flow rate, the program will calculate the pump speed and head needed to achieve the flow rates set through each control valve. If all of the controls are set for a fixed flow rate, with the pump set as a variable speed to deliver a set flow rate, the system becomes over controlled.

Once again the most hydraulically remote control valve must be open so that PIPE-FLO can calculate the required head and resulting pump speed. This time, instead of opening the control (FCV-4), isolate the control and open the component set to a fixed dp (FCV-4 as CV). Now PIPE-FLO will calculate the pump head, the pump speed, and power needed to achieve the design conditions along with the resulting dp across each control.

Create Lineups and Determine Operating Conditions

The final step is to determine how the system will operate under different operating conditions. This is best accomplished using PIPE-FLO's Lineup feature. This major feature is covered in detail in the PIPE-FLO Help file. In the example there is a lineup entitled 160 gpm flow, in which the pump is set to deliver 160 gpm, and each of the three controls are set for 40 gpm each. The control set for 5 psid will control the pump to determine the speed needed to deliver 40 gpm to the most remote load with a 5 psid.

Conclusion

Pressure independent control valves using in conjunction with Variable Speed Drives allow for the operation of HVAC chilled water system under a wide range of flow. By using the PIPE-FLO program, one can easily design a piping system model using these innovative control devices.