Issue
TAC Valve Body published Cv values may differ markedly to equivalent Kv values.
Environment
TAC Valve Body published data utilised when sizing and selecting S-E / TAC Control Valves.
Cause
TAC Valve Body published Cv values based on flow rates measured in U.S. Gallons per minute rather that U.K.Gallons per minute.,
Resolution
- Check published Cv value relative to published Kv value stated for TAC Valve Body.
- If published Cv value is within 3% of published Kv value then Cv value may be used for sizing / selection of Valve Body.
- If published Cv value is up to 17% higher than Kv value then Cv value should not be used for sizing / selection of Valve Body.
- Cv values published in older TAC Valve Body Data Sheets should be checked before considering use in U.K. valve sizing formula.
Numerous TAC Valve Body Data Sheets show that for a particular Kv value, the equivalent Cv value is approximately 17% higher than the Kv value.
For example, where TAC Valve Body Data Sheet shows Kv = 10, then equivalent Cv = 11.7 or in other words Cv = Kv x 1.17
Cv values published in Satchwell Valve Body Data Sheets however show the relationship between Kv and Cv as:- Kv = Cv x 1.03 or in other words Cv = Kv x 0.97
This difference in equivalent values is due to Cv values stated in some TAC Data Sheets being based on flow rates measured in U.S. Gallons per minute, while those Cv values stated in Satchwell Data Sheets however are based on flow rates measured in U.K. Gallons per minute.
If flow rates are measured in U.S. Gallons per minute, then the following conversion factor applies :- Kv = Cv x 0.86, as the U.S. Gallon is smaller than the U.K. Gallon.
Taking published TAC Valve Body Data Sheet information therefore, where the Cv value is stated as 11.7 and the Kv value is stated as 10, confirms that the conversion factor Kv = Cv x 0.86 ( 11.7 x 0.86 = 10 ) has been applied, further confirming that published Cv values are in fact based on U.S. Gallons and not U.K. Gallons.
Consequences of this are as follows : -
If it is assumed that the published Cv value is a U.K. Cv value, when in fact it is a U.S. Cv value, and the published Cv value is then used as a basis for valve sizing and selection, then this will result in a smaller Valve Body being selected than would otherwise be the case.
Consequently, once installed, the selected Valve Body will produce a much higher pressure drop and correspondingly higher authority for a given flow rate than would otherwise have been the case.
In practice the installation of an undersized Control Valve can result in starvation of flow to the item of plant being controlled and the potentially costly replacement of the Valve Body for another of a larger size.
Similarly, the installation of an undersized Control Valve will result in a greater pressure drop being applied to the hydraulic circuit than may be desirable and could lead to an adverse effect on the performance of the associated circulating pump.
It is important to ensure therefore that where both Kv and Cv values are published for a given Valve Body, the Kv value is utilised when undertaking valve sizing and selection. This should ensure that a Valve Body of the correct size is selected.
Measures are currently in place to actively remove all references to Cv values from both S-E / TAC Data Sheets and all future editions of the S-E HVAC Valves and Actuators Catalogue.
