Use of critical flow Venturi nozzles with steam

This paper describes the results of a study into the use of critical flow nozzles with steam. Previous studies have only considered using steam with high levels of superheat where it behaves more like an ideal gas. This present study considers steam with low levels of superheat and at wet saturated conditions. The isentropic real gas flow coefficient, which is needed for the calculation of the flow through a sonic nozzle, has been obtained using the equations of state for steam. Tables of the flow coefficient for both superheated and saturated steam have been produced. It is shown how, at pressures above 7 bar, superheat is required to prevent condensation of the steam before the nozzle throat. Experimental calibration tests have been carried out using superheated steam with pressures up to 10 bar; these suggest that the nozzle can be used, in conjunction with the tables produced herein, to measure the flow rate of steam with an accuracy of ±2.5, although some of this is due to the uncertainty in the calibration rig and in the variables in tile nozzle equation. The nozzle was then calibrated in wet saturated steam with dryness fractions down to 84. The nozzle can still be used with inlet pressures up to at least 10 bar by including a wet steam correction factor in the flow rate calculation or a more practical approach might be to precede the nozzle by an efficient steam/water separator, assume the steam to then be dry saturated and accept an uncertainty in the mass flow of 3.