Parametric study on Gate valve with flow factor.

AIM:- Perform a parametric study on the gate valve simulation by setting the opening from 10 % to 80%.

• 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.

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INTRODUCTION

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 (to be able to apply pressure on the sealing surface).

Valve construction

Common gate valves are actuated by a threaded stem that connects the actuator (e.g. handwheel 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.

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.
• Generally in pipelines with high pressure, the gate valve opening can be quite difficult due to the high force exerted on the gate. So they are generally connected with a smaller bypass valve to release the pressure and then they are operated.

Different 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

Functioning Principle:

The Main Components of a Gate Valve are the body, seat, stem, bonnet and actuator. The main operation mechanism is very simple. When the handwheel 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 turns 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.

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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 can promote that configuration back to the model as a CAD edit. You are prompted to make changes.

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ANALYSIS

• Here we will perform a simulation for a Gate Valve and see the effect of lifting the Gate Disc at different amounts of percentages from 10 % to 80%.
• With the help of the Parametric option, for the lift of Gate Disc, we can parameterize the lift amount and see the different results or changes with different lift percentages.
• Once we study for 10% of the lift, we will parameterize the lift and perform the analysis.
• Then we will see the changes in contours and other variables like Mass flow rate, Flow coefficient and Flow factor for all results one by one.

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Modeling Approach:

Pre-Processing:

1. Importing geometry in SpaceClaim The following geometry was imported into SpaceClaim.

It has the following components:

• The bonnet is placed below the handwheel and prevents the splashing of fluid from the upper region.
• Handwheel and spindle that can be used to lift or lower the gate disc.
• The gate disc is attached to the spindle at the bottom.
• The bottom is a casing that covers the gate disc and the water flows through it. Two ports serve as an inlet and outlet for the fluid.

Geometry Preparation:

First, we extend the geometry of the inlet and outlet ports by 800 mm each. It will look as shown below.

After extension, create Fluid volume by selecting the inlet, outlet and upper edges of the “Bottom” part. The creation of fluid volume is shown below:

After generating fluid volume, hide all parts and suppress them for physics. Make sure to not hide gate-disc and spindle on behalf of which fluid volume must be created to involve necessary solid boundaries.

We then lift the gate valve by 10mm by using the move option and click on the icon P alongside the dimension to parameterize the gate valve opening.

Don't hide the bottom, gate disc, and spindle for parametrization, and set the solid part Volume inside the fluid volume to Update concerning context.

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Meshing :

Element Size = 5mm

Named Selections are given as:

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Setting Up Physics:

First of all the mesh is set up and the solver is defined as below:

Solver Type: Pressure Based Solver

Gravity: -9.81 (in negative z-direction)

Solver: Steady State

Model: k-epsilon model (realizable with scalable wall function)

Material: Water

Cell Zone: Fluid(water)

Inlet Boundary Condition: Pressure Inlet (10Pa)

Outlet Boundary Condition: Pressure Outlet

The model will be initialized with standard initialization and the data will be computed from the inlet.

Flux parameter is included for mass flow rate at outlet.

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PARAMETRIC STUDY

The lift parameter is set and mass flow rate is included as the output parameter.

DP 0) 10mm

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DP 1) 20mm

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DP 2) 30mm

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DP 3) 40mm

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DP 4) 50mm

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DP 5) 60mm

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DP 6) 70mm

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DP 7) 80mm

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CALCULATIONS

Mass flow rate:- It is defined as the mass of a substance flowing per unit time and it is given by the formula,

m = rho * Q

where m - mass flow rate ( kg/s )

rho - density of the fluid ( kg/m3 )

Q - flow rate ( m3/s )

FLOW COEFFICIENT AND FLOW FACTOR AT EACH OUTLET:-

Flow Coefficient:-The Flow Coefficient of a device is a relative measure of its efficiency at allowing fluid flow.

Q is Mass flow rate (expressed in US gallons per minute).

Sg is the specific gravity of water which is 1

△P is pressure drop (in PSI)

Flow factor:- The flow factor is the flow coefficient in metric units. It is defined as the flow rate in cubic meters per hour of water at a temperature of 16deg C with a pressure drop of 1 bar. It is given by the formula,

Where Kv - flow factor

Q - volume flow rate ( m3/hr )

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

`DeltaP` - pressure drop ( bar )

Converting between Flow Coefficient Cv and Flow Factor Kv

The relationship between Cv and Kv can be expressed as:

Cv = 1.16 Kv............ (1)

Kv = 0.862 Cv........... (2)

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.

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Conclusion:

1. Flow-coefficient and flow-factor increase with increasing opening percentage.

2. Differential pressure across the gate valve decreases with increasing opening percentage. This is because there is less blockage to the flow.

3. Mass-flow rate and velocity increase with increasing opening percentage. This is because there are more pathways for the water to flow.

4. Gate valve should operate in a fully-opened or fully-closed position. A partially-opened valve induces vibration which can damage the gate valve.

5. 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.