Determination of Steam Quality Using an Orifice Meter

Abstract

Abstract Steam generators used to inject steam into petroleum reservoirs are designed to produce wet steam, usually about 80 percent quality. For economical operation the steam quality should be continuously monitored, which can be done using an orifice meter. In addition to using data for flow of the generator effluent through an orifice, the method uses the flow rate of water to the generator as measured independently, such as by a positive displacement meter. An improved calculation method has been demonstrated at pressures of 129 psig and 300 psig with qualities, of 32 to 100 percent. The calculation method is based on a literature correlation that includes data at pressure up to 929 psia. The method is expected to be useful at higher pressures as long as the volumetric gas-liquid flow ratio and the ratio of mass flow rate to pressure are above specified minimum values. Introduction Many references have mentioned or discussed the problem of two-phase flow through an orifice. However, most of the literature has been inaccurate or has not considered all of the important variables in this operation. Calculation methods for wet steam shown in two well known orifice meter handbooks are not accurate. For example, Diehl and Beck use the steam coefficient factor F. and state that the orifice differential is approximately linear with quality at a given pressure and constant total mass flow rate of wet steam. This is incorrect. The square root of the differential is approximately linear with quality. An accurate and detailed treatment of measuring two-phase flow with an orifice was made by Murdock. He published experimental data that both he and W. H. Osborne had obtained, as well as a correlation of the data, which has a standard deviation of only 0.75 percent. The data included two-phase systems of steam-water, air-water. natural gas-water, natural gas-salt water and natural gas-distillate at pressures of 15 to 929 psia. Two other recent references give data that are reasonably consistent with Murdock's correlation. However, the data in these references are not complete enough to permit exact comparisons. This paper illustrates the application of Murdock's correlation to measuring steam quality in thermal recovery operations, and further defines the limitations of the method. Experimental Procedure To evaluate the orifice method for measuring steam quality, orifice data were obtained at various qualities. The qualities were measured simultaneously by an independent method, and the values obtained by the two methods were compared. The effect of varying the total mass flow rate of water plus steam was studied. The equipment used is shown in Fig. 1. The following procedure was used at each total mass flow rate. At a measured rate, water was heated to within 3 F of the saturation temperature. Steam was passed through a cyclone to remove water. The steam from the cyclone was mixed with the preheated water and flowed to the orifice, flow control valve and condenser. The total liquid flow rate from the condenser was determined by weight. The feed rate of saturated water was determined by the calibrated water tank gauge, and the dry steam rate was calculated by difference. The orifice was calibrated by flowing dry steam at an orifice differential of 100 in. of water. Data were then obtained at various water and steam flow rates, with the total mass flow rate of water plus steam being the same (within about 1 percent) as the flow rate observed at 100 in. differential at 100 percent quality. Calculation Method The flow rate of saturated or superheated steam can be calculated from orifice data using the equation (1) The term whg is expressed in lb/hr. The subscript h denotes hourly rate, and the subscript G is gas phase. JPT P. 587ˆ