Abstract
This thesis describes an investigation into the use of small pressure-difference devices for flow measurement at the low flow rates encountered in process pilot plants. To the best of the author's knowledge, it is the first major study of devices of such a small physical size (less than 1 mm diameter) and has been made possible by the use of specially shaped synthetic rubies and sapphires as the primary elements. The work includes a review of previous experimental investigations and a theoretical study of the factors which affect the discharge coefficient of an orifice or nozzle. After an initial anemometric study of the effect of installation conditions on upstream velocity profile and the onset of turbulence, the discharge characteristics of four different orifice and nozzle designs are examined. One of these, consisting of an upstream bevel section followed by a short parallel portion, is selected for further study. Firstly, an examination of the effects of entrance length, of type and size of pressure tappings and of diameter ratio on the discharge characteristics of this design is made. The possibility of a scale effect is also investigated. Secondly, the geometry of the device itself is studied. Results with jewels of the slightly different bevel angles and parallel portion lengths occurring within a batch of nominally identical jewels are obtained. These are used to formulate a multiple linear regression equation relating jewel dimensions to discharge coefficient, and the results suggest that the use of any jewel from a batch without prior calibration will result in an accuracy acceptable for most pilot plant applications.