Week 9 - Parametric study on Gate valve.

Objective:

To perform parametric study on flow in gate valve to visualize the velocity contour, velocity vector and calculate the flow co-efficient for each case. We shall perform parametric study from 10 mm valve open position to gradually increase the opening in units of 10 mm up to 80 mm valve open position.

Theory:

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).

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.

Construction: Common gate valves are actuated by a threaded stem that connects the actuator (e.g., hand wheel or motor) to the gate. They are characterized as having either a rising or a non-rising stem, depending on which end of the stem is threaded. Rising stems are fixed to the gate and rise and lower together as the valve is operated, providing a visual indication of valve position. The actuator is attached to a nut that is rotated around the threaded stem to move it. Non rising stem valves are fixed to, and rotate with, the actuator, and are threaded into the gate. They may have a pointer threaded onto the stem to indicate valve position, since the gate's motion is concealed inside the valve. Non rising stems are used where vertical space is limited.

Gate valves may have flanged ends drilled according to pipeline-compatible flange dimensional standards.

Gate valves are typically constructed from cast iron, cast carbon steel, ductile iron, gunmetal, stainless steel, alloy steels, and forged steels.

All-metal gate valves are used in ultra-high vacuum chambers to isolate regions of the chamber.

Functioning Principle:

Main Components of a Gate Valve are: body, seat, stem, bonnet and actuator. The main 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 fully open/close the valve. 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.

Applications of Gate Valve:

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.

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
• 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 whether to make changes.

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 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).

10pa=0.00145038psi

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:

Kv can be calculated from Cv using the equation as given below:

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.

Modelling Procedure:

1: Pre-Processing.

• Open Ansys Workbench→under Analysis system select Fluid flow(fluent) drag and drop it in Project schematic → Geometry → New spaceclaim geometry.
• File select the appropriate file and then Open the file.
• This project is an internal flow simulation project, so we have to extract the fluid volume by using Volume extract with edge selection option in spaceclaim.
• After fluid volume creation, we can use the Move tool to select the disc and move it in positive z direction and input 10 mm. Now in Groups we can select Create Parameter option to make it an input parameter. After this step, we shall select the volume and right click to select Update volume by context option to update the volume as per the new disc position.

• We don’t need the solid volume for solving, so we can select the suppress for physics option to see only fluid volume in physics. We are good to go now. Once the geometry is ready, we can close space claim and check that the fluid flow (fluent) geometry is updated indicated by a green tick.

Given Geometry:

For studying the fluid flow:

• Here we can see the green color arrow in upward direction, which shows the movement of "Gate Disc" in positive Z-Direction.
• Also we can see the symbol of "P".
• This symbol "P" is used to parametize the lift of the "Gate Disc".
• After selecting the symbol "P", it turns in Grey color, and message shows that a Group has been vreaated for lift.
• Created group for 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 parametrs from there directly.

Extracted Fluid Volume:

Lift adjustments:

Case-1: Lift adjustment=10mm in positive z direction.

Case-2: Lift adjustment=30mm in positive z direction.

Case-3: Lift adjustment=50mm in positive z direction.

Case-4: Lift adjustment=60mm in positive z direction.

Case-5: Lift adjustment=80mm in positive z direction.

2: Meshing.

• In this case we are using the mesh lenght as 7mm with element type as tetrahedron.
• Once the mesh is created we can name the inlets and outlet faces.

Generated Mesh:

No of elements: 315705

Element Quality:

3: Simulation.

• Setting up for Physics. Choose pressure-based solver and steady state condition with gravity enabled in Z axis as –9.81 m/s^2.

• The turbulence model we are using is K-epsilon turbulence model with Realizable and Scalable Wall Function.

• The material Liquid-Water is used.

• Boundary Conditions:

1. Inlet= Pressure Inlet with 10pa

2. Outlet= Standard Pressure Outlet.

• Solution Method is defined as coupled with second order upward pseudo Time method.

Results:

Case 1: 10 mm disc lift.

Scaled Residual:

Outlet Pressure:

Mass Flow Rate:

Velocity Contour:

Velocity Vector:

Pressure Contour:

Case 2: 30 mm dic lift.

Scaled Residual:

Inlet Pressure:

Outlet Pressure:

Mass Flow Rate:

Velocity Contour:

Velocity Vector:

Pressure Contour:

Case 3: 50 mm dic lift.

Scaled Residual:

Outlet Pressure:

Mass Flow Rate:

Velocity Contour:

Velocity Vector:

Pressure Contour:

Case 4: 60 mm dic lift.

Scaled Residual:

Outlet Pressure:

Mass Flow Rate:

Velocity Contour:

Velocity Vector:

Pressure Contour:

Case 5: 80 mm dic lift.

Scaled Residual:

Outlet Pressure:

Mass Flow Rate:

Velocity Contour:

Velocity Vector:

Pressure Contour:

Case	Gate Disc Lift ( in mm)	Flow coefficient (Cv)	Flow factor (Kv)	Flow Rate (Q) Kg/s (Leaving)	Pressure Drop ( Δ P) in psi
1	10	64.55	56.71	0.206	0.00142
2	30	153.61	130.47	0.504	0.00129
3	50	273.04	236.18	0.784	0.00107
4	60	648.54	560.9	0.901	0.00045
5	80	470.51	405.92	1.115	0.0007

Relation between Flow Coefficient (Cv) and Flow Factor (Kv)

Conclusion:

• From the observation of result table we can say that the mass flow rate is increases when we increase the gate disc lift of gate valve.
• The pressure drop is decreases when we Lifted the gate disc of gate valve
• The flow coefficient is also increases when we increase the gate disc lift of gate valve.
• The flow factor is also increases when we increase the gate disc lift of gate valve.

Interference:

• We are Performed a parametric study on the gate valve simulation by setting the opening from 10 % to 80% as we can see the above Table and Graph Data.
• From the graph, it can be seen that mass flow increases rather slowly than flow factor & flow coefficient which show their exponential nature.
• Hence much larger flow coefficient & flow factor each time there is an increment in mass flow rate.
• The mass flow rate is increases if we increase the gate valve opening (gate disc lift) .
• The velocity of the flow is also increases when we increase the gate disc lift of gate valve.
• The pressure drop is decreases when we increase the gate disc lift of gate valve.
• The graph of flow factor and flow coefficient with respect to lift of the gate valve is plotted.
• The design point 70mm and 80 mm is not Showing correct results because the reason behind that is the bug in ansys student version.