Gate valve Simulation & its Parametric Study using ANSYS FLUENT

I. Introduction :

A gate valve opens by lifting a barrier (gate) out of the path of the fluid. Gate valves require very little space along the pipe axis and hardly restrict the flow of fluid when the gate is fully opened. The gate faces can be parallel but are most commonly wedge-shaped (in order to be able to apply pressure on the sealing surface).

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 upto 65 mm lift. Mass flow rate at the outlet of the valve is recorded showing how much fluid is flowing outside & velocity & pressure plots are captured. Various necessary data is extracted from the simulations of each lift to calculate flow coefficient & flow factor for every lift design point created within the parametric study and Results are compared and concluded. 

II. Objective : 

1. Simulate a flow of fluid through gate vavle

2. Perform parametric study with various lifts in it and retain all data.

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

4. Conclude all results observed in the study for every design point.

III. Geometry : 

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

 Geometry Preparation in Spaceclaim :

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

We then extract the fluid volume from under the bonnet, and the two pipe sides.

Since the Disc is fully closed in the bottom part inside the Gate valve, we create a lift of 10 mm as first lift and parametrize the lift using the solid component Disc. And set the Fluid Volume to adapt to changing Solid geometry by Setting it to change according to context. The option P below sets the parametric Study. Afte this all solid bodies are suppressed. 

IV. Meshing the Fluid Volume : 

The mesh is slightly kept coarse as the parametric study is time consuming. 

Element size 92.892 mm with 152061 Elements 32k nodes. Even so the element quality is quite high as can be seen from mesh metrics below.

V. Set up of the case (Common): 

1. Time : Steady state

2. Pressure Based solver

3. Gravity - 9.81 m/s^2 in -z dir

4. Viscous Model : K epsilon, Realizable with scalable Wall functions.

5. Material : water 

6. B.c : Pressure inlet - 10 pascal

          : Pressure outlet 0 guage pressure

7. Standard initialization - Computed from inlet

8. The report defination of mass flow rate was set to parametric study. 

VI. Solution & Post processing Results : 

•  Project Approach : 

All Cases are defined as Lifts from here on which are 10 mm, 20 mm, 40 mm & 65 mm. 

 Residuals : 

The residuals for all the studies are shown below, Theyre given their respective Lift names to differentiate which residual is from what study. 

From observing the equations the residuals for Lift 10 mm converge at about 160 iterations, Lift 20 mm are still converging slightly but converge around 130 iterations, Lift 40 mm converge at around 125 iterations however a very irregular convergence is seen suggesting it needs to be ran for longer time but since the study was already set for parameters it followed the same no. of iterations for all study. Lift 65 mm around 170 iterations 

 Velocity Plots for all Lifts : 

As Observed from the plots, we can see the velocity flow area increases with every lift, with increase in the area there will be an increase in the over all mass flow rate at the outlet of the valve. The velocity of the flow has seen a slight increase by a few points m/s as the lift has been increased. As the obstruction is less and less the fluid passes directly towards the outlet which is what the gate valve is operated for. The velocity flow near the outlet of the disc when the lift is small is kind of contained at a small area while the region about it shows 0 veocity, similarly as its increased the water flows through there too. 

 Mass flow rate for all Lifts : 

Mass flow rate is calculated as a part of parametric study to understand its increase as the lift is increased in the disc. As per our assumption that is observed in the simulation results as well. 

The above table shows the mass flow rate as calculated by the parametric study in ansys fluent and below are the individual plots created from the simulations.  The negative value denotes the mass flow rate getting out of the outlet, ansys notes it as negative while its to be considered a positive outflow. 

 Mass Flow rate Comparison at each lift :

This data is plotted from excel sheet. We can observe the linear increase in mass flow rate with increase in lift. If more numbers of lifts are to be created we shall observe a more accurate linear straight line increment.

VII. 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 which 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 to let the flow going through the valve. Therefore, the main differences between different flow coefficients come from the type of valve, and of course the opening position of the valve.

Flow coefficient is important in order to select the best valve for a specific application. If the valve is going to be most of the time opened, probably there should be selected a valve with low head loss in order to save energy. Or if it is needed a control valve, the range of coefficients for the different opening positions of the valve should fit the requirements of the application.

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. 

Cv & Kv for each Opening : 

Cv & Kv are calculated using the Formula given above & then plotted in excel sheet. 

Cv & Kv

VIII. Calculation of Pressure Drop : 

The calculation of pressure drop varies with every opening and also varies with different kind of valves used, hence to put in the formula, pressure drop is calculated using this method, creating points in left and right side of the pipe and calculated the pressure there and subtracted it for every opening. 

Also with regard to pressure plots, its noticable that when the lift is less theres more pressure created at the inlet of the disc and high pressure drops at the outlet side. Where as in the last lift theres considerably less pressure rise in the inlet side as the flow can pass and also pressure drop towards the outlet is higher than a sudden big drop showing a more continous flow. 

 IX. 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 acts as a performance curve to help select the proper valve size for desired flow rate, % 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 stats 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

Tabulated values : 

X. Animation : 

XI. Conclusions : 

1. Ansys fluent helps in Simulating lots of case studies through parametric case option which helps in initializing any design to make it more efficient and improve it by iterating further if changes seemed necessary.

2. Gate valve design requires extensive simulation and practical experimentation to be properly developed if required for custom cases. Luckily we have lots of standard sizes available for different kinds of uses right now which were again developed using the similar methods.

3. Cv & Kv are very important in designing Valves as their incorrect determination will damage the component and cause loss to the manufacturer. 

4. Good design will smoothen the flow of fluid velocity and create less pressure rise whilst controling the Cv to be optimum for the required lift. 

XII. References: 

1. http://www.valvias.com/flow-coefficient.php

2. https://en.wikipedia.org/wiki/Flow_coefficient

3. https://fluidflowinfo.com/control-valve-sizing/

4. https://www.womackmachine.com/engineering-toolbox/data-sheets/how-to-use-flow-coefficients-cv-for-hydraulic-fluids/

5. http://www.wfecn.com/newsdetail/what-is-flow-coefficient?-14.html

keywords - ANSYS-FLUENT, CFD, SIMULATION, GATE-VALVE , CAE

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