Flow Coefficient (CV) and Calculation of Flow Through Valves
Flow Coefficient (CV) and Calculation of Flow Through Valves
Flow Coefficient (CV) and Calculation of Flow Through Valves
The flow coefficient - Cv - let us compare flow capacities of valves at different sizes, types and manufacturers. The flow coefficient is in general determined experimentally and express the flow capacity in imperial units - GPM (US gallons per minute) of water that a valve will pass for a pressure drop of 1 lb/in2 (psi). The flow factor - Kv - is also in common use, but express the capacity in SI-units. The flow coefficient - Cv - required for a specific application can be estimated by using specific formulas for the different fluids or gases. With the estimated Cv value - the correct valve can be selected from the manufacturers catalogues.
SG = specific gravity (1 for water) dp = pressure drop (psia) or alternatively in SI units: Cv = 5.32 w / (500 (dp SG)1/2) where w = water flow (kg/h) SG = specific gravity (1 for water) dp = pressure drop (kPa) Example - Flow Coefficient Liquid The flow coefficient of a control valve which in the full open position passes 25 gallons per minute of water with a one pound per square inch pressure drop can be calculated as: Cv = (25 gpm) (1 / (1 psi))1/2 = 25 (1d)
For non critical pressure drop the outlet pressure - po - from the control valve is greater than 53% of the inlet pressure - pi. The flow coefficient can be expressed as: Cv = q [SG (T + 460)]1/2/ [1360 (dp po)1/2] where dp = (pi - po) po = outlet gas absolute pressure (psia) (5b)
The specific gravity of some common gases can be found in the table below: Specific Gravity1) - SG Acetylene (ethyne) - C2H2 0.90 1) Air 1.000 Alcohol vapor 1.601 Ammonia - NH3 0.59 Argon - Ar 1.38 Arsine 2.69 Benzene - C6H6 2.6961 Blast Furnace gas 1.02 Butadiene - C4H6 1.87 Butane - C4H10 2.0061 1-Butene (Butylene)- C4H8 1.94 Isobutene - C4H8 1.94 Carbon dioxide - CO2 1.5189 Carbon monoxide - CO 0.9667 Carbureted Water Gas 0.63 Chlorine - Cl2 2.486 Coke Oven Gas 0.44 Cyclobutane 1.938 Cyclopentane 2.422 Cyclopropane 1.451 Decane 4.915 Deutrium - D2 0.070 Digestive Gas (Sewage or Biogas) 0.8 Ethane - C2H6 1.0378 Ether vapor 2.586 Ethyl Chloride - C2H5Cl 2.23 Ethylene (Ethene) - C2H4 0.9683 Fluorine 1.31 Helium - He 0.138 Heptanes 3.459 Hexane 2.973 Hydrogen 0.0696 Hydrogen chloride - HCl 1.268 Gas
Hydrogen sulfide - H2S Hydrofluoric acid Hydrochloric acid Illuminating gas Isobutane Isopentane Krypton Marsh gas Mercury vapor Methane - CH4 Methyl Chloride Natural Gas (typical) Neon Nitric oxide - NO Nitrogen - N2 (pure) Nitrogen - N2 (atmospheric) Nitrous oxide - N2O Nonane Octane Oxygen - O2 Ozone Pentane Phosgene Propane - C3H8 Propene (Propylene) - C3H6 Sasol Silane Sulfur Dioxide - SO2 Toluene-Methylbenzene Water gas (bituminous) Water vapor Xenon
1)
1.1763 2.370 1.261 0.4 2.01 2.48 2.89 0.555 6.940 0.5537 1.74 0.60 - 0.70 0.697 1.037 0.9669 0.9723 1.530 4.428 3.944 1.1044 1.660 2.487 1.39 1.5219 1.4523 0.42 1.11 2.264 3.1082 0.71 0.6218 4.53
NTP - Normal Temperature and Pressure - is defined as air at 20oC (293.15 K, 68oF) and 1 atm ( 101.325 kN/m2, 101.325 kPa, 14.7 psia, 0 psig, 30 in Hg, 760 torr) Since specific gravity is the ratio between the density (mass per unit volume) of the actual gas and the density of air, specific gravity has no dimension.