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Valves, such as common globe, butterfly, and ball valves, that performed satisfactorily for constant flow or variable flow primary-secondary systems generally are not satisfactory for primary-only variable flow systems because of the great variations in differential pressures across modulating control valves in direct return hydronic systems.
The valve requirements for primaryonly systems are more severe than those for primary-secondary systems because the pump head on primary-only systems often is 20% to 100% greater than the pump head of the secondary pump of a primary-secondary system. For example, in Figure 1 the coil valves must be able to operate properly under all conditions and with a pump head 40 ft ( 120 kPa) greater than the head required for a secondary pump on a primary-secondary system.
This article reviews the limitations of commonly used control valves for these systems and specifies the requirements necessary for valves to meet the high differential pressures prevalent in primary-only systems.
Two-way modulating valves have four operational ratings that should be considered when selecting the valve for these systems.
* Close-off: determined by the power of the actuator and the valve construction.
* Static: determined by the strength of the valve body and generally available with ANSI 150 or 300 ratings.
* Rangeability: At least 200 to 1. (The ratio of the maximum flow with the valve wide open to the minimum controllable flow.)
* Dynamic: the maximum differential the valve's wetted parts will withstand. This rating has not been defined by our industry. Manufacturers sometimes refer to it as the modulating rating. Dynamic rating is more descriptive and will be used in this article. A valve with a close-off rating of 100 psid (690 kPa) and a dynamic (modulating) rating of 25 psid (172 kPa) should not be used on systems where the valve differential pressure is ever greater than 25 psid ( 172 kPa) even though it will close off against 100 psid (690 kPa). Differential pressures greater than 25 psi (172 kPa) may damage the valve's wetted parts (erode or wire draw the plug or seat).
Valves with Flow Coefficients (Cv's) Greater Than 2
These valves are generally used on air handlers to maintain the design leaving air temperature, to isolate the chillers, to maintain the minimum flow...