Week 9 - Parametric study on Gate valve.

AIM - For this challenge, we’ll have to perform a parametric study on the gate valve simulation by setting the opening from 10 % to 80%.

OBJECTIVE -

• Obtain the mass flow rates at the outlet for each design point.
• Calculate the flow coefficient and flow factor for each opening and plot the graph.
• Discuss the results of the mass flow rate and flow coefficient.

Gate Valve –

A gate valve, also known as a sluice valve, is a valve that 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).

Use –

Gate valves are used to shut off the flow of liquids rather than for flow regulation. When fully open, the typical gate valve has no obstruction in the flow path, resulting in very low flow resistance. The size of the open flow path generally varies in a nonlinear manner as the gate is moved. This means that the flow rate does not change evenly with stem travel. Depending on the construction, a partially open gate can vibrate from the fluid flow.

Gate valves are mostly used with larger pipe diameters (from 2" to the largest pipelines) since they are less complex to construct than other types of valves in large sizes.

Gate valves without an extra sealing ring on the gate or the seat are used in applications where minor leaking of the valve is not an issue, such as heating circuits or sewer pipes.

Flow Coefficient –

The flow coefficient of a device is a relative measure of its efficiency at allowing fluid flow. It describes the relationship between the pressure dop across an orifice valve or other assembly and the corresponding flow rate.

Mathematically the flow coefficient C_v is expressed as:

`C_v = Qsqrt((SG)/(ΔP)`

Where Q is the rate of flow in gallon/minute.

SG is the specific gravity of the fluid (for water=1).

ΔP is the pressure drop across the valve in psi.

Flow Factor –

Flow factor (K_v) is the flow coefficient in metric units that is defined as the flow rate in cubic meters per hour(m³/h) of water at a temperature of 16°C with a pressure drop across the valve or any other restricting device of 1 bar. The K_v is related to the head drop (Δh) or pressure drop (ΔP) across the valve with the flow rate (Q).

Mathematically, the Flow Factor is given as:

`K_v = 0.865*C_v`

Parametric Study –

One of the best ways to get value out of your simulation is to do parametric analysis. With very little marginal work a completed model can be parameterized to simulate scenarios limited only by your computational time and resources.

As you move forward in your design, you can assess the impact that changing certain parameters can have on the design. The parameters can include dimensional parameters. Parametric studies allow you to nominate parameters for evaluation, define the parameter range, specify the design constraints, and analyze the results of each parameter variation.

A parametric study requires the following:

• Design Objective is set to Parametric Dimensions
• Parameters nominated for use in the simulation
• Parameter ranges identified
• Design criteria specified

When you have the configurations generated you can then evaluate your simulation. You can further refine the parameters or design constraints until satisfied with the results.

Once you determine that a configuration satisfies your design needs, you are able to promote that configuration back to the model as a CAD edit. You are prompted whether to make changes.

ANALYSIS -

Geometry -

We import the gate valve model into the SpaceClaim and extract the required volume for the analysis as shown below.

Gate Valve model -

After importing Geometry, hide the "Bottom" Part, so we can clearly see the "Gate Disc".

Then Select "Gate Disc" component and move it upward direction by "10 mm" lift.

To analyze the flow and its effects, we must allow the flow, and for that we have given "10 mm" lift to the "Gate Disc".

Lift of the "Gate Disc" can be given as shown in below image:

Here we can see the green color arrow in upward direction shows the movement of Gate Disc in Positive Z-Direction.

We can also see the symbol "P", This symbol "P" is used to parameterize the lift of the Gate Disc.

After selecting the symbol "P", it turns Grey in color, and message shows that a Group has been created for lift, created group for the lift parameter shown in above image.

On the left side, in Tree structure, if we see the Group options, we can see created group and the amount of parameter, which has been set 10 mm lift, we can simply change the value of parameter from there directly.

After the parameterization of the lift, we can move on to generate the fluid volume required for the analysis.

To do that first we need to extend the inlet and outlet walls to 800mm as shown in the below given figure:

Edges selected for volume extraction -

Extracted volume -

Since we're done with the volume extraction, except the extracted volume we'll suppress all the other components for physics.

Mesh -

Given below are the mesh details and mesh images.

Element size - 4mm

Number of Elements - 9,63,692

Number of Nodes - 1,88,898

Named Selection -

Inlet -

Outlet -

In order to see if the mesh we have generated is accurate or not, we'll check a quantity known as orthogonal mesh quality which tells us about the quality of our mesh.

Element Quality -

SETUP -

Setting up the physics for analysis of Gate valve in ANSYS fluent.

• For this case we are using pressure-based steady state solver.
• Gravity is turned on in negative Z direction.

Model setup

• Viscous Model - k-epsilon (RNG) realizable with scalable wall function.

Material

• Water.

Gravity -

Material (Water) -

To let ANSYS know that we'll be using water instead of air inside the fluid volume, we'll have to change the material in cell volume as shown below:

Viscous Model -

Boundary Condition -

Inlet - Pressure inlet boundary condition

Every other boundary condition is same as already given by ANSYS.

Since we're giving the inlet pressure, to see the initial velocity of the fluid we can standard initialize the solution to see the initial velocity as shown below:

After initialization we can create the variable report, we want to see during our analysis. We'll create two such report that are, mass flow rate at outlet and the velocity contour on a plane passing through the pipe.

Mass flow rate -

Plane for velocity contour -

Velocity Contour -

Now we're done setting up all the required files for analysis and hence we can now start our calculation with 150 iterations.

Result -

Geometry -

Residuals -

Mass flow rate -

Velocity -

Preesure -

Now as we're done with analysis of the 10mm lift case, in order to get the result for the other cases in which the lift goes till 80mm, we can use the parametrization.

All we need to do is give the lift we want and just click on calculate and the ANSYS will do all the required work, from redoing the geometry to calculating and saving the result for us to see later as shown below:

To see a specific case all we need to de is right click on that case and set that case as the current case and after that we can see the modified geometry and the solution of that case as shown below:

DP1 - 20mm Lift

Geometry -

Residuals -

Mass flow rate -

Velocity -

Preesure -

DP2 - 30mm Lift

Geometry -

Residuals -

Mass flow rate -

Velocity -

Preesure -

DP3 - 40mm Lift

Geometry -

Residuals -

Mass flow rate -

Velocity -

Preesure -

DP4 - 50mm Lift

Geometry -

Residuals -

Mass flow rate -

Velocity -

Preesure -

DP5 - 60mm Lift

Geometry -

Residuals -

Mass flow rate -

Velocity -

Preesure -

DP6 - 70mm Lift

Geometry -

Residuals -

Mass flow rate -

Velocity -

Preesure -

DP7 - 80mm Lift

Geometry -

Residuals -

Mass flow rate -

Velocity -

Preesure -

Flow Coefficient and Flow Factor -

Conclusion –

• Flow-coefficient and flow-factor increases with increasing opening percentage.
  
  
• Differential pressure across the gate valve decreases with increasing opening percentage. This is because there is less blockage to the flow.
  
  
• Mass-flow rate and velocity increases with increasing opening percentage. This is because there is more pathway for the water to flow.
  
  
• Gate valve should operate in a fully-opened or fully-closed position. A partially-opened valve induces vibration which can damage the gate valve.
  
  
• Also the gate valve should be closed gradually rather than suddenly, as it causes a water hammer effect which can again damage the gate valve.