In this issue
		Energy Services Bulletin home page
		Recycling promotion highlights energy efficiency, local resources
		MHA Nation tackles wind development one step at a time
		ACORE committee provides renewable energy forum for utilities
		Schools, forests, taxpayers all win with Fuels for Schools
		Utilities join projects to promote energy-efficient homes
		SOLAR 2006 focuses on renewable energy's role in climate recovery
		Automated meter reading spreads through Western territory
		Nebraska utility, university partner on energy research center
		Western plans workshops to demystify power factor
		New Equipment Loan manager brings experience, plans to job
		Topics from the Power Line: Powering irrigation with renewable energy
		Energy shorts
		Technology spotlight: Pressure-independent valves - Do they save energy?
		Calendar of events

This column features helpful information, innovative equipment, systems and applications utilities around the nation can use to save energy and improve service.

Pressure-independent valves: Do they save energy?

Two-way modulating valves are commonly used in variable and constant volume HVAC hydronic systems to control the flow of water through coils depending on room temperature requirements.

Various actions—such as adjusting a valve or changing the pump speed—affect pressure in the distribution system. These actions in turn affect flow through a conventional two-way valve even if the load does not change, and may change the temperature in the space. The thermostat responds and the valve modulates to bring the flow back to what is required to meet the load. Until the flow is corrected, the coil operates at an off-design point and will over- or under-condition the space.

Operating differences

With a “pressure-independent” valve this off-design point operation will not happen. The valve responds only to a signal from a thermostat indicating that the load has changed. A pressure-independent valve will not “feel” the operation of other equipment on the system. By responding only to the change in load, the pressure-independent valve is “self-balancing"—it delivers the proper amount of flow to meet the requirements of the load regardless of the state of other components of the system.

Possible benefits

The potential advantages pressure-independent valves offer over conventional two-way modulating valves include:

• Greater turn-down ratio may improve flow control both at part-loads and at high differential pressures.

• Ease in sizing and selection may reduce design costs and result in more reliable installations.

• The need to balance valves is eliminated, reducing installation costs and labor required for balancing.

• The automatic balancing characteristic may reduce overflows compared to oversized valves, saving pumping energy.

Potential energy savings

The following energy-saving claims have been made about pressure-independent valves:

• Pressure-independent valves stabilize flow and, with stable flows, heat transfer is better. The result is energy savings because less flow is required to achieve the same heat transfer.

• Because pressure-independent valves are self-balancing, they always deliver the right amount of flow to meet the load, and so maximize the temperature difference across the coil. In comparison, conventional two-way valves are often oversized. An oversized valve will tend to “overflow,” i.e. pass more water than is necessary to meet the load.

These energy savings claims have not been verified in an independent and controlled study that isolates the effect of the valve replacement from other modifications made at the same time, such as removing bypasses. Nevertheless, it is safe to say that, to the extent that its automatic balancing features reduce overflows, pressure-independent valve will save pumping energy compared to an oversized conventional valve. However, installing pressure-independent valves to replace properly sized and tuned conventional two-way valves in a stable system will likely have little benefits in energy savings, comfort, maintenance or equipment life.

Other considerations

The decision to replace existing valves with pressure-independent valves should be made carefully because they cost more and may not have significant energy or non-energy benefits in all applications. Particularly in existing systems with conventional two-way modulating valves, problems such as low-temperature differentials or flow instability should be confirmed before replacing them with pressure-independent valves. Likewise, several alternate strategies in common practice can be used to reduce flow instability.

The bottom line

The above cautions aside, pressure-independent valves may save energy in a system that is not properly sized and tuned, especially combined with other improvements such as removing bypasses. Also, the automatic balancing feature can potentially save on labor and design costs of new systems, in addition to energy savings in the right applications. The additional cost of pressure independent valves is at least partially offset by eliminating balancing valves.

Some valve manufacturers provide design and consulting services to customers. They can help facilities achieve significant energy savings with a variety of system improvements in addition to pressure-independent valves, such as eliminating bypasses, removing balancing and pressure regulating valves, placing differential sensors and optimizing fan energy.