Brian Venn, TBE, CxA, Mechanical Testing, Inc. and Bob Walker, Belimo published “How to Select A Pressure Independent Control Valve” in HVAC Plumbing Product News, July 2021.
Why use a PIV in a system? PIVs provide a specific flow for each value of the control signal regardless of the pressure variations in the hydronic system. Eliminating the flow variances due to pressure fluctuations ensures control stability at part loads and prevents overflow in the hydronic loop reducing pump energy and low-differential temperature issues. A PIV replaces the existing control valve and balancing device at the load.
Mechanically regulated PIVs are the marriage of a control valve and a mechanical pressure regulator in one device.
An Independent Test and Balance Field Technician would verify the differential pressure across the valve. As stated earlier, this is generally accomplished when 5-50 psi is read across the PICV.
Once confirmed that the correct pressure exists, then the field verification is complete at the load. If the technician cannot verify pressure between the valve differential pressure tolerances, then traditional troubleshooting methods must be fallen back on, like verifying pressure drop across the coil or load is within or close to the rated drop of the coil. An alternative method would be to verify Delta T of Air across the coil, but design conditions must exist for this to give reliable data for troubleshooting.
Properly selected and field-verified PIVs are the hydronic equivalent to pressure independent VAV boxes. PIVs provide both installation and energy savings for the hydronic system reducing both capital and operational expenditures. Flow enters through the mechanical regulator followed by the control valve. All PIVs have a minimum (and maximum) differential pressure required to maintain pressure independence. The mechanical regulator is adjusting based on the varying pressures across the valve.
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