Sizing control valves for non-turbulent flow by means of the loss coefficients K and K_L

摘要

There are a few methods available for calculating the flow through a control valve under non-turbulent conditions. The two receiving the stamp of official international recognition are those described in the IEC standard 60534'1'2 and the ISA/ANSI standard 75.01 (refs 1 & 2). Why then is it important to introduce another method to add to the process control engineers' libraries? The answer is that there is no call for another method but over many years the author has toyed with the idea that the valve's Reynolds Number Factor (F_R) can be calculated quite simply through the ratio of the turbulent and laminar loss coefficients (K/K_L)~(1/2). The Reynolds Number must be known but no other information is required. The method does not require any references to graphs. Unfortunately the method proved not to be quite so simple. Change in flow patterns at low Reynolds Numbers and some uncertainties about the values of the loss coefficients (K_L), also at low Reynolds Numbers, brought the work to a standstill until, with some reluctance, the problem was resolved by the introduction of that panacea for all ills, a modifying factor f. It is hoped that as flow technology develops, particularly in the role of loss coefficients for non-turbulent flow, these problems can be analysed and resolved technically, thus eliminating the empirical correction factor.