Numerical prediction and experimental verification of cavitation of Globe type Control Valves

Globe valves are one of the oldest types of valve used for throttling applications for all sizes due to better controllability and wider range. One of the major limitations associated with the use globe valves in liquid application is cavitation and it takes place both in part open and in fully open conditions due to varied reasons. There are different designs of globe valves available but for control valve applications, cage and plug designs are widely employed. Cage and plug design consists of body, valve cage, plug and an actuating mechanism. Actuating mechanism is connected to the valve plug which is a moving part, through valve shaft. There are many investigations reported about the flow visualization and numerical simulation of normal type globe valves. But study on valves with cage and plug design are not reported in detail. The objective of the present study is to provide a three dimensional analysis of flow through a globe valve with cage and plug design with emphasis on the inception and development of cavitation in detail. Cavitation reduction is achieved by breaking the flow in the form of more than one liquid jet, thereby increasing the turbulence inside the valve flow path. The numerical simulation was done using GAMBIT to set up geometry and grid and FLUENT to solve difference equation postulated from the conservation of mass and momentum of the fluid in motion. The k-epsilon model was used for turbulence. Results of five configurations of the cage with constant flow areas and valve stroke are presented in this paper. The numerical results were verified with an experimental program employing total flow measurement and pressure drop created by the valve at full opening. The study was conducted for different jet configurations to generalize the results of the study. Experimental validation was done in the water test facility with an operating pressure of 1.6 MPa and flow rate of 0.05 m/s. In the study, total area of opening for the flow and the valve stroke were kept constant. Accelerometers and dynamic pressure sensors were employed to sense the severity of cavitation at different differential pressures across the test valve. INTRODUCTION Valves are widely used in irrigation, energy, water distribution networks and process industries and in many other areas. Among the different types of valves used in the process industry, control valves play a vital role in the functioning and profitability of the plant. Trouble-free operation of control valves in the piping network is essential to avoid a situation leading to the total closure of the concerned industrial activity. Further, their efficient working leads to an effective use of the available resources. The abundant improvements in the design and performance of control valves are still insufficient to claim perfection in the agreement of theory and practice. The phenomenon of cavitation in control valves is the one in which some more progress can be achieved. Globe valves are widely used for throttling applications in the process industry for both liquid and gaseous applications. The main advantages are relatively low cost, linear characteristics and good controllability and wider range. To obtain the required flow and pressure drop characteristics for the valves, different types of internals have been evolved for globe type valves. Cage and plug internal is one among them. One of the major limitations associated with the use of globe valves in liquid application is cavitation. This limits the operating regime of valves. To combat cavitation in valves, valve manufactures have evolved different solutions including design improvement, use of harder materials to reduce erosion rate, limiting the valve operation to some critical values so that downstream pressure never goes below vapour pressure etc. Numerical models of Brennen [1], Wang & Brennen [2] and Davis & Stewart [3, 4] can be combined with two-phase flow starting after the convergent section of the nozzle. The starting of the flow is to be initiated by static pressure going below a threshold value. This value can be correlated to the local static pressure experienced in the valve. Ramamurthi & Nandakumar [5] studied characteristics of flow in the separated, attached and cavitated flow of small sharp edged cylindrical orifices. They have reported that the onset of cavitation observed is dependent on the diameter and aspect ratio of the orifices under study. They have studied on orifice plates with diameters varying from 0.3 to 2 mm. This study was extended by Ramamurthy and Patnaik [6] to investigate the effect of periodic disturbance present in the flow on the inception of cavitation. Cavitation characteristics of orifices were studied by Takehashi et al [7]. The spatial distribution of cavitation pressure downstream of the orifice along the pipe line was studied here. Cavitation of butterfly valve downstream of a multi holed orifice was also investigated in this study. Ishimoto & Kamiyama [8] described the numerical analysis of cavitating flow of a magnetic fluid in a vertical nozzle. Galson et al [9] modeled flow through venturi tube using bubble dynamics for S. Rammohan Fluid Control Research Institute, Palakkad, INDIA S. Saseendran Fluid Control Research Institute, Palakkad, INDIA S. Kumaraswamy Mechanical Engineering Dept., IIT Madras, INDIA