Prediction of Performance of Multi-Stage Orifice Assembly Using CFD Code

A multistage orifice assembly is often used in fluid systems in nuclear plants in order to achieve a specific flow rate for a large pressure drop. Such an assembly is usually designed with a number of orifices installed in series. Apart from the performance parameters, an important design consideration is to guarantee absence of potential for cavitation inside the assembly. An improperly designed multistage orifice assembly may create cavitation at the outlet of some stages of the assembly when the pressure at the vena contracta reaches a pressure close to the vapor pressure of the fluid at the operating temperature. The cavitation condition makes not only the operation noisy but also results in a significant reduction of equipment life. It is a challenge to the designer to select a set of orifices so that the cavitation condition is simultaneously met at all stages of the assembly. On the other hand, it is also a challenge to design each individual orifice in the assembly so that its influence to the next orifice is minimal. These challenges have been met in design of a replacement multistage orifice design for the centrifugal charging pump mini-flow line of a nuclear plant for a pressure drop of 2,595 psi (17.89 MPa) at 60 gpm (227 l/min) flow-rate. The orifice assembly has eleven stages accommodated within a pipe of 24 inches (600 mm) length, and each stage consists of a 3-dimensional compound orifice. The fluid analysis is performed using the CFD code FLUENT/UNS. The code numerically solves the Navier-Stokes equations of fluid motion to obtain the velocity field and pressure distribution within a modeled 2- and 3-dimensional geometry. Turbulence effects are modeled by relating time-varying Reynolds stresses to the mean bulk flow quantities using the standard k-epsilon model. The design was optimized by fine tuning the geometric parameters of the orifice units to minimize the potential for cavitation and at the same time maintaining a high accuracy in the flow-rate. The full-scale prototype was successfully tested to confirm the performance. Actual flows in the prototype tests were observed to be within a fraction of a gpm of the design flow-rate and pressure drop. After orifice assembly was installed in the plant system, there was absolutely no operating noise, as opposed to the original unit that was extremely noisy. A number of other multistage orifice assemblies have subsequently been designed for different power plants using similar computational technique with great success.