Gate Valve Parametric Study

1.Introduction:

Gate valve is a flow control device which is designed for fully open or fully closed operation. They are installed in pipelines as isolating valves, and should not be used as a control or regulating valves. The operation is done by Clockwise to close (CTC) or Clockwise to open (CTO) rotation of the stem. When operating the stem, the gate valve moves up or downwards on the threaded part of the stem.

Gate valves are often used when minimum pressure loss and a free bore is required to reduce the pump work. It is a multiturn valve meaning that the operation of the valve is done by the means of a threaded stem. As the valve has to turn multiple times to go from fully open to the closed position, the slow operation also prevents water hammer effects.

Suitable working conditions for a gate valve:

1. Potable water, wastewater, and neutral liquids; temperature -20 to +70 deg. celsius and maximum flow rate of 5 m/s and up to 16 bar differential pressure.
2. Gases; temperature between -20 to +60 deg. Celsius at a maximum flow rate of 20 m/s and up to 16 bar differential pressure.

Fig 1.1: A gate valve model.

2. The Study

In this study, a gate valve model was used to study the effect of lift on the flow at the outlet. The Ansys Fluent was used to simulate the case for this study. A CAD model of a gate valve was provided and it was used to set the lift of the gate valve as an input parameter and the mass flow rate at the outlet as an output parameter. Water was used as the working fluid and a comparison between the flow rate variation for the openings between 10 mm to 80 mm was made.

2.1 Geometry:

Fig 2.1.1: Cross-section view of the CAD model

The end flanges of the valve were pulled 800 mm to provide a bigger flow domain and then, the lift of the gate disc was set as an input parameter for the simulation. It was done by using the Parametrise option for the displacement provided to the gate disc in the SpaceClaim.

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Fig 2.1.1: Lift of the gate disc set as a parameter.

After that fluid volume was extracted from the CAD model and all other bodies except the extracted volume were suppressed for physics. Then the model was exported to mesher for meshing.

Fig 2.1.2: Pulled ends of the flange

Fig 2.1.3: The extracted fluid volume.

2.2 Meshing

Default mesh was used and a body sizing of 50 mm was used to refine the mesh. Named selections were made as inlet and outlet for the two ends of the fluid volume. The meshing resulted in 139001 elements of mix type i.e tetrahedral, hexagonal, wedge, and pyramid type.

Fig 2.2.1: The element quality for meshing.

Fig 2.2.2: meshed model.

As a control measure, the cross-section of the meshed model was checked if the meshing was proper. The gate was not intended to be meshed as we were only using the extracted fluid volume as our flow domain.

Fig 2.2.3: The cross-section of the meshed model.

2.3 Solver settings and simulation:

Then the meshed model was imported into fluent and Pressure based solver was used as the flow velocity was very low. The steady-state solver was used as we are interested in the end results of the simulation and gravity was enabled in the negative Y direction according to the model reference frame. water was used as the working fluid for the case.

Table 2.3.1: The boundary conditions used in the simulation

Setting the output parameters:

The mass flow rate at the outlet was created into an output parameter for the study. Two planes, one near inlet of valve and another near the outlet of valve were created for the measurement of pressure. Pressure at these two planes were made into output parametrs.

Table 2.3.2 :Coordinates of points used for creating valve inlet and outlet planes for pressure measurement.

Fig 2.3.1: View of valve inlet and valve outlet planes created for calculation of pressures in Fluent.

Solver settings:

The coupled scheme was used for pressure velocity coupling as it gives a good result for the pressure inlet cases and all other formulations were kept first-order upwind.

The standard initialization of the solution was done and compute from the inlet option was used.

After obtaining the solution for a 10 mm lift, in the parameter set window, other design points were created from 10 mm lift to 80 mm lift and the solution was obtained for each design point using the "Update all design points option".

3. Results and discussion:

Fig 3.1: Scaled residuals for the 10 mm lift simulation.

As clear from the plot solution has converged well as all the residuals have fallen below 1e-3 and are steady in nature.

Fig 3.2: Velocity contour plotted on midplane for a 10 mm lift.

Fig 3.3: Mass flow rate at the outlet

A value of 0.145 kg/s was obtained which is fine for the case.

3.1 Checking for the auto-update of geometry for each design point:

While performing such a parametric study it is vital to ensure that the geometry gets updated according to the value of the inlet parameter. In this case, the lift of the gate disc was checked for each design point by setting that design point as current and going to the project page and opening the geometry in the SpaceClaim.

Fig 3.1.1: Lift of 20 mm for the gate disc

Fig 3.1.2: Lift of 40 mm for the gate disc.

Fig 3.1.3: Lift of 70 mm for the gate disc.

It was updating correctly against all the design points as clear from the above figures.

3.2 Obtained Results

Fig 3.2.1: The parametric output from Workbench

Flow Factor and Flow Coefficient:

The flow coefficient of a device is a relative measure of its efficiency at allowing fluid flow. It describes the relations between the pressure drop across an orifice and the corresponding flow rate.

Mathematically:

`Cv= Q*sqrt((S.G.)/(Delta P))`

where Cv is the flow coefficient, Q is the volumetric flow rate, SG is the specific gravity of the fluid and delta P denotes the pressure drop.

Practically, Cv is used for comparison of valve capacities and sizing valves for specific applications.

Table 3.2.2: Calculation of flow factor and flow coefficient.

Note: Negative sign in mass flow rate indicates the fluid is leaving from flow domain.

Plot 3.2.1: Volumetric flow rate against lift of disc

The rise in flow rate was almost linear between 10 to 20 mm lift but afterwards it appears to follow a curved path.

Plot 3.2.2: Flow factor against lift of disc

The characteristic of the plot matches the curve for valves obtained from the website of Valvias-an enterprise that does design and production of valves.

Plot 3.2.3: Reference curve for Cv and Kv from Valvias.com (for reference)

Plot 3.2.4: Flow coefficient against lift of disc.

4. Conclusion:

• The Parametric study feature of Fluent is a very handy tool for iterative design process used in optimisation of designs.
• The case presented was a hypothetical one aimed at introduction to the Parametric study feature of the Fluent.
• The results obtained for the flow factor and flow coefficient are quite good as concluded by comparison of the plots with the reference plot 3.2.3 from Valvias.com.
• While conducting the parametric study simulation it is very important to ensure that the input parameters are updating correctly against all the design point values, otherwise very wrong results can be obtained.

5. References:

1. Flow coefficient definition, Available at: http://www.valvias.com/flow-coefficient.php
2. Flow coefficient definition, Available at: https://en.wikipedia.org/wiki/Flow_coefficient#:~:text=The%20flow%20coefficient%20of%20a,and%20the%20corresponding%20flow%20rate.