The capacity of control valves is generally represented by Cv coefficient which is determined for water. So that, it becomes essential to make a correction of Cv when the valve is intended to use for highly viscous fluid. Under the circumstances each valve manufacturer has proper Cv correction formulae and graphic chart for his own products. However, it is not seldom to see from many actual installations that such correction is not satisfactory in the light of the designed working conditions of the piping, i.e. major deviation from the designed flow quantity and pressure drop in the piping. In order to develop more reasonable Cv correction method the author made a study on the empirical formula for resistance coefficient on JIS valves intened for the range of lower Reynolds Number which had recently been published by JIS Flow Resistance Committee, and the author could set up a tentative C_V correction formula incorporating correction factor K. Subsequently the author carried out a series of test to corroborate the accuracy of the developed formula with a satisfactory result, and made up a graphic chart R_e-K for C_V correction for practical use. The C_V correction factor K values otained by the author's formula substantially lies half way as compared with those by valve manufacturs and this, the author considers, means the developed formula does not include safety margin of the capacity, giving critical values which satisfies various conditions.
