Engineers put control valve to the test: Web exclusive
Pressure independent control valve solves instability problems, saves energy, and reduces equipment maintenance.
Timothy Vann, Rochester Institute of Technology, Rochester, N.Y.
In hydronic HVAC systems made up of traditional balancing and control valves, system pressure is affected every time a valve changes its position. When the system pressure changes, the flow through all of the valves reacts and causes the amount of heat transfer through each device to also change. Assuming that each device was at equilibrium to start, we can deduce that each device is now getting either too much or too little heat transfer. As a result, each device requests that its valve open or close to compensate for the change in flow induced by the change in system pressure.
Due to this chain reaction, HVAC systems with traditional valves can be inherently unstable, resulting in overflows and underflows at the terminal coils. In turn, this leads to wasted energy, suboptimal heat transfer, premature failure of equipment, and a less comfortable space for the building occupants.
The engineering team at Rochester Institute of Technology (RIT) wanted to investigate an alternate HVAC design that could improve the stability of its system. Its research suggested a relatively new technology called a pressure independent control valve (PICV), which is able to absorb system pressures to maintain a constant desired flow rate at all times. In theory, if a PICV functions properly, then changes in system pressure will not cause flow changes through the valves—thus solving the instability issues.
PICV testing setup
It selected a hot water supply line supplying 240 F to two plate and frame heat exchangers that make hot water for a 160,000-sq-ft academic building. This line has an existing two-way full port ball valve and a manual balancing valve on the primary side of the heat exchangers (see Figure 1). In the past, this line exhibited the classic characteristics of typical HVAC systems: instability issues, large swings in temperatures, and valve “hunting.”
For the purposes of the test, the engineering team replaced both the ball valve and the manual balancing valve with a single Danfoss PICV and actuator. The setup comprised the following items:
- One 4-in. Danfoss AB-QM PICV (167 gpm) with a matching Danfoss modulating actuator (2 to 10 V)
- The PICV installed on the primary side of a plate and frame heat exchanger using medium-temperature hot water (MTHW) at 240 F delivering hot water to the building
- A GE AT868 ultrasonic meter (with a rated accuracy of +/-2%) installed on the pipe to measure flow rates
- Automated Logic Controls WebCTRL System used to control the valve and record the flow meter measurements
It is important to note that the tests were conducted on a live system, not in a laboratory setting. The test PICV was used to control the temperature of the water to an occupied building under normal operating conditions.
Flow stability at standard system pressure
The first series of tests was conducted by sending the valve actuator different control signals and measuring the resulting flow on the GE ultrasonic meter using the normal operating system pressure.
Testing started with the valve completely closed (0%), and then the team began opening the valve in 5% increments until completely open (100%). Next, it reversed the procedure, starting with the valve completely open (100%) and closing in 5% decrements until completely closed (0%). As shown in Figure 2, at standard system pressure the flow had the desired linear response as the valve was opening or closing. However, since the team did not change the system pressure, it had not yet tested the pressure independence feature.
Flow stability at varied system pressures
In this test, the team again opened and closed the valve incrementally but this time varied the system pressure at the supply plant. Figure 3 shows the excellent linear response from the valve at almost all the test points.
The valve did not always deliver the expected flow when the differential pressure at the valve did not meet the manufacturer’s stated minimum pressure rating of 4 psi. For example, when the system pressure was 10 psi and the valve was 85% open, the differential pressure at the valve would fall to between 3.1 and 3.6 psi. This is below the minimum requirement for full linear control and caused some loss of valve authority. It is worth noting that this situation of high flow and low system pressure would be unlikely to occur in actual operation. During high demand periods, the plant pressure would be at least 15 psi or more. In all test points that reflected actual plant operation, the linearity and control authority were well maintained.
Constant flow maintained
In the final test, the engineering team kept the valve position constant while varying the plant pressure to see if the valve would compensate for the pressure changes and keep the flow constant. The valve was held at 40% open and the plant system pressure was varied up and then down (see Figure 4).
The PICV held the desired flow despite significant changes in system pressure. Only at the very low end of the system pressure range did the flow start to slightly drop due to the fact that the pressure at the valve was starting to approach the manufacturer’s minimum pressure rating.
The results of the team’s analysis showed that as long as the pressure across the valve body is within the stated pressure range and the valve position is more than 10% open, the flow through the valve is almost perfectly linear regardless of system pressure.
Several months after the valve had been installed in RIT’s academic building, the team reports that the performance has been excellent. The control stability has been outstanding as the valve maintains temperature very well with very little valve movement. Normal changes in the plant system pressure do not affect the valve operation; this is especially evident at minimal positions where traditional valves had been particularly unstable. Figure 5 clearly illustrates the difference in performance between the old ball valve and the new PICV. Furthermore, Figure 6 shows the actual performance of the PICV in terms of holding set temperature with very little movement of the valve, demonstrating the power of a pressure independent valve.
Ultimately, the result of improving the stability of the building’s hydronic system will be energy savings, better temperature stability for building occupants, and less valve and actuator maintenance as a result of less movement in these components. RIT is planning to make PICVs a standard in its future new builds and renovations.
Timothy Vann is a member of the Rochester Institute of Technology Facilities Management Services engineering team. He has a bachelor’s in mechanical engineering and is an experienced computer programmer, HVAC design engineer, and control system engineer. RIT student Taylor Osmonson assisted in gathering information for this project.