Gate valve simulation and parametric analysis using Ansys Fluent

Skills or knowledge: Knowledge of GATE valve and its applications

Software or programs used: CFD, Fluent, parametric analysis

• PROJECT: Gate Valve simulation and parametric analysis using Ansys fluent

• Objective:

• Gate valves are mechanical valves that are used in fluid flow systems for flow control. In this project, a simplified Gate valve model is simulated in Ansys fluent followed by a parametric study on the lifts created inside the valve by the disc. The lifts start from 10 mm up to 65 mm. The mass flow rate at the outlet of the valve is monitored to study the effect of lift on the mass flow rate through the valve. The velocity and pressure variations due to the lift were also studied. The flow coefficient & flow factor were calculated for the valve and the results are compared and concluded.

• 1. Simulation of fluid flow through a gate valve using the flow solver fluent.

• 2. Perform parametric study with various lifts and their effects on the flow variables.

• 3. Calculate mass flow rate at outlet, Cv & Kv for each valve lift design point.

• Geometry :

• Geometry consists of a handle on top, spindle, bonnet (cap), Disc inside it on the spindle and bottom part where the pipe fits with bolted joints.

• The geometry was created in Space claim with inlets, outlets for the fluid and a valve disc for providing boundary conditions for the simulations.

• Meshing the fluid domain: An unstructured tetrahedral mesh was generated for the fluid domain under study. Initially, a grid independency study was carried out using grids of sizes 1 million,2 million and 3 million cells. The 2 million size grid was chosen from the study. The grid was made finer near the valve stem and disc region to capture the flow features accurately. The quality of the cells was maintained to assist in the quick convergence of the solver.

• Solver and boundary conditions (Common):

• The solver properties and the boundary conditions used for the simulation are shown

• Solver: Finite volume Pressure based solver was used for the incompressible flow simulation. The fluent solver is a Reynolds averaged Navier strokes solver. The closing equations for the Reynolds stress term are the turbulence equations.

• Turbulence Mode: The K epsilon, a realizable turbulence model with scalable Wall functions (for capturing or modeling the near-wall velocity gradients) was used for the viscous simulations. The K epsilon is a two-equation eddy viscosity-based turbulence model where these k and epsilon transport equations are used as the closing equations for resolving turbulence (eddy viscosity) and using eddy viscosity to determine the Reynolds stress term.

• Time stepping: Steady-state simulation was carried out till convergence of the residuals and mass flow rate at the valve exit.

• Material: water

• Boundary conditions: The pressure inlet boundary condition with an inlet static pressure of 10 Pascal was given for the inlet fluid flow boundary and the outlet fluid flow boundary was given a pressure outlet boundary condition with an outlet gauge pressure of 0 Pascal. This maintains a pressure differential of 10 Pascal between the inlets and the outlet of the gate valve.

• The mass flow at the valve outlet and the flow variables residuals were monitored for the convergence of the simulation.

• Simulation and parametric analysis for valve lift :

• The initial simulation with the valve lift value of zero (closed condition ) was performed then the parametric studies of valve lift were performed with lift values of 10 mm, 20 mm, 40 mm & 65 mm.

• The velocity of the fluid through the gate valve for various lift values was observed from the velocity contours

• From the velocity contours, we can see that the flow velocity is almost uniform when it reaches the valve due to the small area of the passage created by the valve the flow accelerates and again attains the uniform velocity after the valve. The increase of lift causes an increase in the passage area, which causes an increase in the flow velocity and hence the mass flow rate of the fluid through the valve, which is evident from the velocity contours at the various valve lift values.

• Mass flow rate for all Lifts :

• Mass flow rate is studied as a part of a parametric study to understand the effect of the valve lift on the mass flow rate through the valve.

• The mass flow at the valve exit is monitored for all the parametric study simulations and the convergence of the mass flow rate is ensured in each simulation The negative value of mass flow rate denotes the flow getting out from the outlet boundary.

• The mass flow rate through the valve for different valve lift design points in the parametric study was plotted. The plot shows an increase in the mass flow rate with the increase in the valve lift.

• Flow Coefficient (Cv) & Flow factor (Kv) in Gate valve :

• When flow goes through a valve or any other restricting device it loses some energy. The flow coefficient is a designing factor that relates head drop (Δh) or pressure drop (ΔP) across the valve with the flow rate (Q).

• Each valve has its own flow coefficient. This depends on how the valve has been designed. Therefore, the main differences between different flow coefficients come from the type of valve and the opening position of the valve.

• Flow coefficient is important in order to select the best valve for a specific application. The valve with low head loss is generally preferred for reducing pressure drop and energy loss across the valve.

• Flow factor is the metric equivalent of Cv.

• `Kv = Q*sqrt((Sg)/(DeltaP)`

• where,

• Kv is flow factor expressed in m3/h/bar

• Q is flow rate (m3/h)

• Sg - specific gravity (water =1 )

• Delta P = Pressure drop different between inlet & outlet .

• Flow coefficient (Cv) is simply found by relation : Kv = 0.865 x Cv

• Cv expressed in Gpm/psi.

• Mass flow rate vs Flow coefficient :

• As the mass flow rate increases i.e more fluid is let inside the valve the flow coefficient increases. It determines the capacity of flow for the given mass flow rate required.

• These coefficients and Factor values act as a performance curve to help select the proper valve size for desired flow rate, the % of lift required to increase plant efficiency. The flow coefficient or valve coefficient denoted by “Cv”, is used to determine the valve size that will best allow the valve to pass the required flow rate while providing stable control of the process fluid.

• If the Cv for a control valve is not calculated correctly or accurately, the resultant selected valve will experience diminished performance. If the Cv is too small for the process, the valve itself or the trim inside the valve will be undersized resulting in the system being “starved” of the process fluid. Undersized valves exhibit a higher pressure drop across the valve to maintain adequate flow and exhibit limited flow capacity. Furthermore, since the restriction in the valve can cause a build-up of upstream pressure, higher back pressures created before the valve can lead to damage in upstream pumps or other upstream equipment.

• Cavitation can occur in liquid systems when high velocity reduces the static pressure inside the valve to below the pressure level at which the liquid starts to boil and produce vapor bubbles. These vapor bubbles collapse whenever the downstream pressure is higher than the vapor pressure causing high-pressure waves. These implosions result in very high noise levels and can cause considerable damage to the valve body or trim parts under prolonged service

• Pressure drop across the valve :

• The pressure drop across the valve is monitored for each lift design point in the parametric study. The monitor points were created at the inlet and the exit of the valve to get the pressure drop. From the contours, it can be seen that at a small valve lift the flow is restricted which results in pressure increase or build-up upstream of the valve inlet. As the lift increases the pressure built-up redu

• Also with regard to pressure plots, it's noticeable that when the lift is less there is more pressure created at the inlet of the disc and high-pressure drops at the outlet side. Whereas in the last lift there is considerably less pressure rise in the inlet side as the flow can pass and also pressure drop towards the outlet is higher.