Week 9 - Parametric study on Gate valve.
Aim:
For this challenge, you will 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.
Theory:
Gate Valve
A gate valve is generally used to completely shut off fluid flow or, in the fully open position, provide full flow in a pipeline. Thus it is used either in fully closed or fully open positions. A gate valve consists of a valve body, seat and disc, a spindle, gland, and a wheel for operating the valve. The seat and the gate together perform the function of shutting off the flow of fluid. A typical gate valve is shown in Figure.
Gate valves are generally not suitable for regulating flow or pressure or operating in a partially open condition. For this service, a plug valve or a control valve should be used. It must be noted that because of the type of construction a gate valve requires many turns of the handwheel to completely open or close the valve. When fully opened, gate valves offer little resistance to flow, and their equivalent length-to-diameter ratio (L/D) is approximately 8.
The gate valves used in the mainlines carrying oil or gas must be of the full bore or through conduit design to enable smooth passage of scrapers or pigs used for cleaning or monitoring pipelines. Such gate valves are referred to as full bore or through conduit gate valves.
Functioning Principle:
The Main Components of a Gate Valve are the body, seat, stem, bonnet, and actuator. The primary operation mechanism is very simple. When the hand wheel is turned, it rotates the fluid, which is translated into the vertical movement of a Gate Disc via threads. They are considered multi-turn valves as it makes more than one 3600 turn to open/close the valve fully. When the Gate is lifted from the path of the flow, the valve opens and when it returns to its closed position, it seals the bore resulting in a full closure of the valve.
Uses:
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.
Types of Gate Valves:
Wedge Gate Valves
Plug-Disk Gate Valves
Subsea Gate Valve
Knife Gate Valve
Insider Screw Gate Valve
Rising Stem Gate Valve
Parametric Study:
One of the best ways to get value out of your simulation is to do a 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:
- The design Objective is set to Parametric Dimensions
- Parameters nominated for use in the simulation
- Parameter ranges identified
- Design criteria specified
- Various configurations generated
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 to make changes.
Simulation Procedure:
Now here we going to perform a simulation on the gate valve for which we have the STEP file. Here we going to lift the gate disc inside the valve which will vary from 10% to 80% of the total open conditions.
Geometry
After opening SpaceClaim in Fluent, import Geometry named “Gate Valve. STEP”.
We can see the simplified geometry of the Gate Valve as given below as well as the Structure of all parts mentioned as given below:
After importing Geometry, hide the “Bottom” Part, so we can clearly see the “Gate Disc”.
Then Select the “Gate Disc” component and move it in an upward direction by a “10 mm” lift.
To analyze the flow and its effects, we must allow the flow, and for that, we have given a “10 mm” lift to the “Gate Disc”.
The lift of the “Gate Disc” can be given as shown in below image:
Here we can see the Green color arrow in the upward direction shows the movement of the “Gate Disc” in Positive Z-Direction.
Also, we can see the symbol of “P”.
This symbol “P” is used to parameterize the lift of the “Gate Disc”.
After selecting the symbol “P”, it turns in Grey color, and the message shows that a Group has been created for lift.
Created group for the lift parameter shown in the above image.
On the left side, in a Tree structure, if we see the Group options, we can see the created group and the amount of parameter, which has been set to 10 mm lift.
We can simply change the value of the parameter from there directly.
After lifting the “Gate Disc”, move the face at the inlet and outlet of the “Bottom” part by an amount of 800 mm just to see the fluid flow and its effects as given below:
After extension, create Fluid volume by selecting the inlet, outlet, and upper edges of the “Bottom” part.
The creation of fluid volume will be as given below:
After creating Fluid Volume, Hide all rest parts of the Gate Valve and suppress it for Physics and save it for further process.
Then open Mesh settings.
Mesh:
Mesh for the Fluid Volume of the Gate valve, detailed as given below:
Details:
- Element Size: 10 mm
- Number of Elements: 192558
- Number of Nodes: 40234
After Meshing, update it for further process and go for the Fluent Setup.
In Fluent Setup, the necessary details for simulation are described below:
General Settings
Analysis type: Steady state
Solver: Pressure-based solver
Gravity: -9.81 m/s2 in Z-Direction
Material Specifications
Material: water-liquid (h2o)
Chemical Formula: H2O
Density: 998.2 Kg/m3
Viscosity: 0.001003 Kg/ (m-s)
Viscous Model
Model: K-Epsilon
K-Epsilon Model: Realizable
Near Wall Treatment: Scalable Wall Functions
Cell zone
Boundary Conditions
Inlet: >Type: Pressure-Inlet
>Gauge Total Pressure: 10 Pa
Outlet: >Type: Pressure-outlet
>Gauge Total Pressure: 0 Pa
Wall: >Type: Wall
>Wall Motion: Stationary Wall
>Shear Condition: No Slip
Interior: >Type: Interior
Here we have taken Pressure for inlet = 10 Pa. Typically for Pipe flow, a pressure of 10 Pa is acceptable.
In Viscous model, we have taken K-Epsilon model. The reason behind choosing the K-Epsilon Viscous model is the characteristic of the flow, which is turbulent.
For Reynolds No. about 2000 and more, flow is turbulent. So we have taken the K-Epsilon Viscous model. But here, we have given Pressure Inlet, instead of Velocity Inlet. So knowing the velocity of the flow for correct Reynolds No. is required. So, to know the velocity of the flow, we have to go for initializing the solution. So, go to Solution, select Standard Initialization, and Initialize from Inlet.
After initialization, we can get the correct velocity and calculate the Reynolds No. and from that, we can decide on the appropriate viscous model.
Here, Reynolds No. is 14087.14, which is more than 2000. So the flow is turbulent. The viscous model will be K-Epsilon.
After initialization, create Report Definitions for the report of the Mass Flow Rate at the Outlet. Also, create Contours for velocity magnitudes and for Pressure.
We also can create animations for the flow of water inside the Gate Valve based on Contours.
After completing all procedures as described above, run the simulation for about 400 to 600 iterations.
Parametric Study:
On completion of the simulation, select “Parameter Set” and give input for different Design Points.
In the below image, we can see the Parameterization of the Lift Parameter before and after the Simulation. We can see the Parameter Set for the study at different Design Points of the Lift of the Gate Valve.
Before
Then Update all Design points and check all the results of different Design points.
After
In the above image, we can see Mass Flow Rate before and after Updating all Design Points. Before the Update, we have to give different inputs for the Lift of the Gate Disc as 20, 30, 40,50,60,70, and 80 mm and also select all checkboxes in Retain column for all Design Points. Then select “Update All Design Points”.
We can see in the above image that after updating all Design Points, the results of the Mass Flow Rate for all Design Points are generated automatically. These results are calculated by fluent based on the lift of the “Gate Disc”.
Now we can check all Design Points one by one for detailed results like Pressure and Velocity Contours, Residuals, etc. by selecting each Design Point and making it “Current”.
Results:
Now we will see all cases of lift of “Gate Disc” from 10% to 80% one by one.
We will discuss each case particularly, for all types of results, and calculate the Flow coefficient and Flow Factor accordingly. Here we will see Mass Flow Rate at the Outlet, Flow Coefficient Flow Factor, and required Plots for each opening.
Flow Coefficient
The flow coefficient of a device is a relative measure of its efficiency in allowing fluid flow. It describes the relationship between the pressure drop across an orifice valve or other assembly and the corresponding flow rate.
Mathematically the flow coefficient Cv (or flow-capacity rating of the valve) can be expressed as:
Where:
- Q is the rate of flow (expressed in US gallons per minute).
- SG is the specific gravity of the fluid (for water = 1).
- ΔP is the pressure drop across the valve (expressed in psi).
In more practical terms, the flow coefficient Cv is the volume (in US gallons) of water at 60 °F that will flow per minute through a valve with a pressure drop of 1 psi across the valve.
The use of the flow coefficient offers a standard method of comparing valve capacities and sizing valves for specific applications that are widely accepted by the industry. The general definition of the flow coefficient can be expanded into equations modeling the flow of liquids, gases, and steam using the discharge coefficient.
Flow Factor:
The metric equivalent flow factor (Kv; commonly used in Europe and Asia) is calculated using metric units.
Where:
Kv is the flow factor (expressed in). Q is the flow rate (expressed in cubic meters per hour.
SG is the specific gravity of the fluid (for water = 1).
∆P is the differential pressure across the device (expressed in [bar]).
Kv can be calculated from Cv using the equation given below:
kv = 0.865 * Cv
The kv factor of a valve indicates "The water flow in m3/h, at a pressure drop across the valve of 1 kgf /cm2 when the valve is completely open.
Case 1
Case 2
Case 3
Case 4
Case 5
Case 6
Case 7
Case 8
Observations:
The Table was prepared in Excel as shown below.
Graphs are obtained when plotting Cv and Kv with respect to valve opening.
From the above Tabular Data and Graphs, we can observe that
the opening of the Gate Valve or Lift of the Gate Disc increases the Mass Flow Rate increases.
With Increases in Mass Flow Rate, there is an increment in Flow Coefficient.
Also, Increases in Flow Factor are observed at each Design Point from 10 to 80 mm Lift.
So, the Graph of Mass Flow Rate, Flow Coefficient, and Flow Factor goes linearly with Gate Disc Opening.
Conclusion:
We can conclude from observations that velocity in the first case is very low. As the opening or lift of the Gate Disc increases, the velocity of the flow increases.
We can observe that at the front or inlet side of the valve, pressure is high. As flow passes through the valve opening, it decreases. And at the outlet, there are no any restrictions to the flow, so pressure becomes 0. It means there is a minimum Pressure Drop.
The 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.
There is a gradual increase in Mass Rate, Flow Coefficient, and Flow Factor as the Lift of the Gate Disc increases, or from 10% to 80%, or 10 mm 80 mm. When the valve is fully open, the Mass Flow rate, Flow Coefficient, and Flow Factor become maximum. Thus Flow-coefficient and flow factor increase with increasing opening percentage.
Thus, parametric study helps to analyze the different positions of valve opening and gives the benefit to get easy and quick results.