Pressure independent control valves are the latest evolution of hydronic technology that offer the best opportunities for controllability and energy efficiency.
The pressure independent balancing valves (PIBV) discussed in the previous section allow a specific flow rate to be maintained over a wide range of differential pressures. However, they do not have the ability to start and stop flow, or the ability to vary the flow rate through a heat emitter or cooling coil.
The latter functions are often desirable to regulate the rate of heat transfer from a heat emitter or from the cooling coil within an air handler or fan-coil.
This functionality can be added to the basic operating characteristic of a pressure independent balancing valve. The result is known as a pressure independent control valve (or PICV).
PICVs are useful when the flow rate to a heat emitter or cooling coil needs to be regulated by a building automation system or other electronic controller in response to changes in the heating or cooling loads.
Figure 5-1 shows a Caleffi 145 Series FLOWMATIC® PICV valve. With the green manual adjustment knob as shown, this valve is an adjustable PIBV.
Figure 5-2 shows the cross section of this valve.
The lower portion of the valve (within box A) senses the pressure differential between the higher pressure (p2) and the slightly lower pressure (p3). The combined action of the diaphragm and spring in the lower portion of the valve maintain a nearly constant differential pressure (p2-p3) across the flow regulation chamber of the valve. This enables the valve to maintain a set flow rate over a wide range of differential pressure as shown in Figure 5-3.
The upper portion of the PICV valve (within box B in Figure 5-2) combines a disc that moves up and down as the valve stem moves, with a tapered slot through which flow passes. These details are shown in Figure 5-4.
As the disc moves up and down, it acts like a “shutter” over the tapered slot, controlling the gap through which flow can pass.
For proper operation, the orientation of the tapered slot must be adjusted based on the maximum flow rate (G max) that will pass through the valve. This is easily done by loosening the locknut at the top of the valve and rotating the disc marked with number 1 through 10. Doing so rotates the shutter as required by the maximum flow rate.
A table that shows how to select the setting number is provided later in this section. Once the setting number has been adjusted, the lock nut is retightened. This adjustment does not affect the stroke of the stem, only the size of the flow aperture when the valve is fully open. This is an important feature because the proportional control signal to the valve's actuator is always fully utilized for the full stroke of the shaft, even when the maximum flow rate through the valve is reduced.
Once the position of the valve’s disc relative to its tapered slot has been set, the valve can function as an automatic balancing device capable of holding a set flow rate over a wide range of differential pressures between its inlet and outlet.
The relationship between the flow rate setting and the differential pressure range across which that setting can be maintained is shown in Figure 5-5.
The valve’s ability to hold a set flow rate requires that the differential pressure between its inlet and outlet be within the “working range” as shown in Figure 5-5.
The minimum differential pressure varies from 3.6 to 4.4 psi depending on the flow rate setting of the valve, which is adjusted using the numbered disc and lock nut on the valve’s bonnet. The upper limit of differential pressure at which the valve can maintain a set flow rate is 60 psi.