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.
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 Professional 12 has a Sizing Valve Device that can be used to model a pressure independent control valve. The remainder of this knowledge base article describes how to model a Pressure Independent Control Valve in a PIPE-FLO Professional 12 model.
The primary purpose of the PICV is to regulate flow, when modeling this type of valve in PIPE-FLO use the Sizing Valve Device. In PIPE-FLO control valve calculations, the program will determine the differential pressure (dP) across the control valve needed to maintain the flow rate. Each manufacturer of PICV’s lists a range of dP 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 Sizing Valve Property grid, PIPE-FLO will issue a message when the differential pressure is too great across the valve. This information is useful, because often a different set of internals hardware can be inserted in the pressure regulator portion of the valve that will make it accommodate greater dP.
One final point, since the internal operation of a PICV has two controls mechanisms in series; we cannot insert the Cv data of the valve to determine the valve position.
Below is a screen shot of a model with PICVs.
In this model, Sizing Valves were used to represent PICVs and one special loop was added around a Sizing Valve that has a Fixed dP device inserted. The Fixed dP device is added to offset the Overcontrolled message one may encounter when using all Sizing Valves in a model.
Following are steps used to model the PICV.
- Model the system using a Sizing Valve set the design flow rate as a PICV. If you get the Overcontrolled message, set one Sizing Valve 100% open.
- Determine the most hydraulically remote loop (MHRL) by sorting the Sizing Valves by Pressure Drop in the Listview window. The Sizing Valve with the lowest dP signifies the most hydraulically remote loop. The MHRL is sometimes called the Critical Path.
In this screen shot, you can see FCV-4 has the lowest dP of 5 psi. This indicates that the path with FCV-4 installed is the MHRL.
- Place a virtual loop around the Sizing Valve in the MHRL and place a Fixed dP device set to the pressure drop across the PICV. The virtual loop pipe lengths should be short enough so that their affect on the system is negligible. In this example the pipes in the virtual loop are set to a length of 0.001 ft. It is not necessary that the virtual loop be located at the MHRL. The virtual loop essentially serves as a dP transmitter and the VFD pump will adjust speed to maintain the dP set at the Fixed dP device. The virtual loop can be setup wherever the dP transmitter actually exists in order to better simulate the system.
In this screen shot, you can see that the Fixed dP loop and the pipes connecting to and from the FCV-4 Sizing Valve, have pipe lengths of .001 ft. All the virtual pipes have the same size, type and fluid zone. No elbows or other valve and fittings are added.
- Set the Sizing Valve in the MHRL to the design flow rate.
- If you are using a Sizing pump you are done.
- If you are using a Centrifugal pump, set the pump to Variable Speed Operation with a defined flow rate.
- Create lineups and simulate different operating conditions
Note: If you get error messages saying “Set flow rate cannot be achieved, a pressure gain across the valve was calculated”, and the Sizing Valves have a (-) dP, (shown below), this means the Sizing Valve is acting like a pump.
To solve this message, add .0001 to your pumps design flow rate. Example, if your Centrifugal Pump is set to a Variable Speed Operation with a flow rate of 200 gpm, change the flow rate to 200.0001 gpm. Below is an example.
This will allow the software to converge to a solution and eliminates the error message. The flow in the virtual loop will be .0001 gpm which is meaningless.
Pressure Independent Control Valves using in conjunction with Variable Speed Drives allows for the operation of HVAC chilled water system under a wide range of flow. By using the PIPE-FLO Professional 12, you can easily design a piping system model using these innovative control devices.