Calculation of the flow coefficient for control valves
Thread Content
According to the Petrochemical Automatic Control Design Manual and process control, whether a gas flow is in a choked state is determined by whether X is greater than XT*FK; however, according to the Calculation and Selection of Control Valves compiled by Ming Cidong, this becomes whether △P is greater than P1. Which of these two is more suitable? Is it correct to use these two formulas for the determination?
To add, the second discriminant is incorrect; it should be based on whether △P is greater than P1/2
Both formulas are correct. Whether X is greater than XT*FK is a precise calculation ； △Whether P is greater than P1/2 is a matter of empirical judgment. Since there are deviations between the actual operating conditions and the theoretical ones, these deviations are smaller compared to those resulting from the two calculation formulas, whose discrepancies can be ignored. In fact, the calculation results become significant only when the operating conditions are near the critical point. A mature process design will avoid this issue.
XT*FK takes into account calculations for different gases (differences in adiabatic indices), and it is recommended by the IEC and ISA formulas.