Gate valve simulation and its parametric study using Ansys Fluent

I. Introduction

A gate valve is a barrier that acts as a obstruction to the path of the fluid.Passage to the fluid is given by lifting the gate.

A gate valve opens by lifting a barrier(gate) out of the path of the fluid. gate valve require very little space along the pipe axis and hardly restrict the flow of the fluid when the gate is fully opened.

Gate valves are widely used for all types of applications and are suitable for both above ground and underground installation.It is very important to choose right type of valve to avoid high replacement cost.

Here In this project,gate valve model is taken and the flow inside the gate valve is simulated by ansys fluent followed by a parametric study analysis for the mass flow rate in response to the 4 cases of lift created inside the valve.The lift was varied from 10 mm to 65 mm.Mass flow rate at the outlet of the valve is determined or evaluated from the parametric study. Velocity and pressure plots are plotted and compared with the end results.

II. Objective

To do the simulation of the flow of fluid through the gate valve and to perform parametric study with various lifts in it .To calculate mass flow rate at outlet and Cv and Kv for each lift cases are extracted out from the results of the simulation.

III. Geometry

Geometry consists of a handle on the top side ,spindle at the top, bonnet,disc inside of the spindle and the bottom part.

Geometry preparation in SpaceClaim

At first the sides of the bottom part are pulled to a distance of 800 mm on both sides, to increase the fluid volume to capture the fluid and flow properties.

We after that extract the fluid volume from under the bonnet and the two pipe sides.

We create a lift of 10 mm as first case lift and parametrize the lift using the solid component of the disc.

IV. Meshing the fluid volume

Element size was kept constant for all the cases which was 6mm.

For lift 10 mm: Elements- 417424;Nodes - 84419

For lift 20 mm: Elements - 415812; Nodes - 83820

For lift 40 mm: Elements - 411708; Nodes - 83170

For lift 65 mm: Elements - 432724:Nodes - 87689

V. Setup of the case

• Time : steady state simulation was performed.
• Pressure Based solver was used for this very simulation.
• Gravity was turned on by setting at : -9.81 m/s^2 in -z direction
• Viscous Model that was used was K-epsilon, Realizable with scalable wall functions
• Material of the fluid was taken as water(H2O-liq) and the air was deleted from the database so that no mixing of fluids takes place.
• B.C: Pressure inlet was kept automatic constant at 10 pascal in the settings.However we will see there is a varying pressure difference between the inlet and outket while reporting plots for the same at the end of the simulation.

Pressure outlet - 0 gauge Pressure

Standard initialization was done before running the simulation and the report plot and magnitude of mass flow rate was sent for parametric study so that other necessary datas can be extracted out of it to conclude with the final results of mass flow rate, lift coefficient and flow factor.

VI. Post Processing Results

All cases are observed for lifts of 10mm, 20mm, 40mm, 65mm.

Residuals:

The residuals for all the 4 cases are shown below.

We can see that the residuals for lift 10 mm converge at about 238 iterations.

For lift 20 mm the solution converged at (60-75) iterations.

Lift 40 mm converge at around 125 iterations but a very irregular convergence is seenwhich means that it needs to bne run for longer no. of iterations.

Lift 65 mm converges around 100 iterations and the same irregular convergence was observed.

Lift 10 mm

Lift 20 mm

Lift 40 mm

Lift 65 mm

Velocity plots for all lifts

From the plots we can see the velocity flow increases with every lift. The velocity of the flow has been increased by a few magnitude as the lift was increased case by case.

Lift 10 mm

Cut section view

Velocity contour

Streamline plot

Animation

Lift 20 mm

Cut section view

Velocity contour

Streamline plot

Animation

Lift 40 mm

Cut section view

Velocity contour

Streamline plot:

Animation

Lift 65 mm

Cut section view

Velocity contour

Streamline plot:

Animation

Mass Flow Rate for all lifts

Mass flow rate is calculated in terms of parametric study. It is performed to understand the increase in magnitude of mass flow rate as the lift is increased case by case in the disc.

Below here the tables and plots shows the mass flow rate as calculated by the parametric study in ansys fluent.

For lift 10 mm

For lift 20 mm

For lift 40 mm

For lift 65 mm

Mass Flow rate comparison that was made in excel by extracting out the datas from the fluent by the help of parametric study.

The mass flow rate datas are put in excel and plotted with respect to lift.We can observe a increase in mass flow rate with increase in lift.

VII. Flow coefficient (`C_v`) & Flow Factor(`K_v`)`in Gate valve

Every valve has its individual flow coefficients.This flow coefficient decides on how the valve is to be designed. So the types of valves signifies the different flow coefficients and the opening position of the valve.

Flow coefficient is important in order to select the best valve for a specific designed application.

Flow coefficient is measured from the following equation

`C_v = Q*root(2)(S_g/(DeltaP))`

and Kv is the flow factor expressed in m^3/bar/h.

Q is the volumetric flow rate(m^3/h)

`S_g`is the specific gravity(water =1)

`Delta P`= Pressure drop difference between inlet and outlet

`K_v`= `0.865*C_v`

`C_v`is expressed in Gpm/psi

`C_v & K_v` for each opening are calculated using the formula given above & then plotted in excel sheet

VIII. Calculation of pressure drop

The pressure drop varies with every opening and also varies with different kind of valves used.

For the pressure plots, its 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 of the valve for the fluid flow. But in the last case of lift there is comparatively less pressure rise in the inlet side.

For lift 10, pressure contour:

For lift 20, pressure contour:

For lift 40, pressure contour:

For lift 65, pressure contour:

IX. Mass flow rate vs Flow coefficient:

Flow coefficients and Flow Factor values behaves as a performance graph of greater significance to select the accurate size of valve.

Cavitation can occur in liquid systems when high velocity reduces its static pressure inside the valve to below the pressure level.This is the period at which the liquid starts to boil and produce vapour bubbles.Cavitation in any hydraulic machine means that the pressure in anywhere in the system should not fall below the vapour pressure of the liquid at the working temperature so bubble gets generated.

In general if bubble is formed, they collide each other and ultimately they come and hit the surface of the blade and the surface of the diffuser, resulting in erosion of the surface.This makes a detrimental effect and can damage the machine. so cavitation must be avoided. Infact even the pressure is little above the vapour pressure , cavitation formed due to dissolved air, they are liberatyed in the formation of bubbles which randomly moves in the hydraulic machine leading to erosion.This implosions result in very high noise levels and can cause considerable damage to the valve body or trim parts under prolonged service.

• As the mass flow increases with opening of gate valve disk, thus the fluid through the pipe will also increase.
• As the gate disc oprn, the velocity increases and pressure drop decreases and therefore mass flow rate and flow coefficient increases.
• The flow coefficient is used to determine the valve size that will best allow the valve to pass the required flow rate which provide stable control of process fluid.
• Due to high velocity and decrease in pressure drop there is formation of cavitation in the liquid system.
• Below pressure drop, the liquid start to boil and produce vapour bubble creating vapour pressure waves leading to erosion and implosions resulting in very high noise level.

X. Conclusions

From the above datas we can conclude that the lift of the gate disc increases(with the increased clearance between the gate disc and the walls) the mass flow rate also increases.Negative sign indicates that the mass flow rate is taking place in outward direction.Or the sign is negative because the fluent reports the flow leaving from the pipe or an enclosed geometry to make the free space as negative.

Parametric study is very useful and time saving where we can find the results at different design points.