192 1. CAPACITY COEFFICIENT 1. 1 Cv VALUE The Cv value is a coeffi cient that expresses quantitatively (in a numerical value) how much fl uid fl ows through a specifi c value. The concept of the Cv value was fi rst developed by the Fluid Control Institution, USA, in 1958. There are other coeffi cients (absolute numbers) based on a concept similar to that of the Cv value. They are the Av value and the Kv value. On the mainstream in Japan is the Cv value. Since the Kv value was announced in Germany in 1960, there have been trends toward the international unifi cation of these values into a single one. The Cv value is becoming part of the mainstream not only in Japan but internationally as well, at ISO international conferences. The concept of the capacity coeffi cient can be expressed by the following equation： [General Equation] C ＝ Q 　ρ／△ P　　………………………(1) C;Capacity coeffi cient Q;Flow rate △ P;Diff erential pressure ρ ,G;Specifi c gravity Cv value ＝ 1 means that: The diff erential pressure (pressure loss) across the valve is 1PSI{1lb/in2}(about 0.07kgf/cm2) and clean water (concentration ρ＝ 1) at 60° F (about 15.5℃ ) fl ows at a fl owrate of 1USgal/min (about 0.2271m3/hr). That is,the relationship between fl ow rate and diff erential pressure (pressure loss) is determined by the Cv value. In Japan, diff erential pressure (pressure loss) is expressed in kgf/cm2 instead of PSI. When the fl owrate is expressed in m3/hr, Equation (1) for the capacity coeffi cient becomes something like the following： Cv ＝ 1.17Q G/ △ P　　………………………(2) Cv；Capacity coeffi cient Q； Flowrate (m3/hr) △ P； Diff erential pressure (kgf/cm2) G； Specifi c gravity 1. 2 HOW TO MEASURE Cv VALUE A method of measurement is now available as the standard applicable in JIS B 2005 and IEC 534. An example is outlined below. Set the diff erential pressure across the sample valve to 0.75kgf/cm2 and measure the rate of fl ow at that time. Then, adjust the diff erential pressure in step from 0.75kgf/cm2 to 0.07kgf/cm2 ( ＝ 1PSI ＝ 1lbf/in2), and measure the fl ow rate(in m3/hr). For large − diameter valves, it is very diffi cult (impossible) to set their diff erential pressure to 0.75kgf/cm2 ＊ 2, begin at the point where the pressure is adjusted progressively, and take measurements up to 0.07kgf/cm2. Substitute the value at that time in equation (2) shown on the last page, then determine the Cv value. This is a "table of Cv values when Asahi AV valves are fully open”Also shown is a "table of Cv values by fl ow qty％ us valve opening to" (and subsequent pages), which presents data necessary for valves used for control. The Cv value represent the fl ow characteristics of each valve. Water Storage Tank P Pump Upstream Throttle Valve Flow Meter Sample Valve Downstream Throttle Valve Pressure Tap Q P1 P2 TECHNICAL DATA FLOW CHARACTERISTICS