Gate Valve Parametric Study

Aim

To perform a parametric study on a gate valve assembly and to simulate the flow in the pipe for different valve openings

1. Introduction

A gate valve is a mechanical component assembly used to control the flow in a pipe having pressure difference across the inlet and outlet. The components of gate valve include a gate disc attached to a wheel through a spindle. The rotation of the wheel results in movement of the gate disc. When fully closed, the gate valve provides fluid flow resistance and can be used to shut off flow of liquids. The movement of the gate disc perpendicular and away from the pipe axis opens the fluid flow and at fully opened conditions, the gate valve offers negligible resistance to the fluid flow.

In the given study, the fluid flow is studied for different valve openings by running simulation on a CAD model of gate valve and the flux report is generated. A parametric study is conducted by setting distance of disc from pipe as the study parameter. The fluid coefficients are calculated and the trend for an increase in valve percentage is established.

2. Theory

2.1 Flow coefficient C_v

Flow coefficient describes the relationship between the pressure drop across the pipe and the flow rate. It is defined as follows:

`C_v= Q*sqrt((SG)/(triangleP)`

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 others words, flow coefficient is defined as the volume of the fluid that will flow per minute across a pipe with pressure difference of 1 psi. It can be used to assess the efficiency of flow across setups.

2.2 Flow factor K_v

It is the metric equivalent of the flow coefficient. Flow factor is used in Europe and Asia whereas flow coefficient is mainly used in USA. It is defined as:

`K_v= Q*sqrt((SG)/(triangleP)`

Q is the flow rate (expressed in cubic metres per hour{\displaystyle m^{3}/h}),

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

∆P is the differential pressure across the device (expressed in [bar]).

The relationship between flow factor and flow coefficient can be expressed as:

K_v = 0.865 C_v

3. Objective

The following are the objectives of the project:

• To perform a parametric study of gate valve simulation by setting the valve opening from 10 % to 90%.
• 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.

4. Solving & Modeling Approach

The following are the steps followed for solving the case:

1. The geometry is imported using Spaceclaim and the fluid volume is extracted
2. The gate disc is moved using Move command and the distance from the pipe axis is parameterized
3. The fluid volume is meshed using appropriate element size and the named selections are defined
4. The setup is done by selecting the turbulence model, boundary conditions and defining report plots and parameters. To conduct the parametric study, the report definitions are defined as output parameters
5. The case is initialized and solved for a certain set of iterations
6. Using parameters option in Workbench project, the case is simulated for different values of valve opening (parameterized in Spaceclaim) and the output parameters are obtained
7. For the obtained results, line plots are generated and the observations are discussed.

5. Pre-processing and Solver setting

5.1 Geometry

Fig 1- CAD of pipe fitted with gate valve

The figure shows the computer aided model of the pipe fitted with gate valve. It includes components- bottom pipe, disc, bonnet, spindle and wheel.

Fig 2- Components (top to bottom) a) Wheel b) Spindle c) Bonnet d) Disc

The fluid volume is extracted using the command under Prep and is shown in figure below:

Fig 3- Fluid volume extracted

The diameter of the pipe is measured to be 100 mm. Therefore, the range of motion for the disc is from 0 to 100 mm. For the simplicity of the case, the intervals for design point study are calculated as follows:

Table-1 Design points

For the initial case of 10% opening, the disc is moved in the positive z-direction by 10 mm and the dimension is parameterized as an input parameter P1- Group1.

Fig 4 – Section view of gate valve assembly a) With fluid volume b) Without fluid volume

5.2 Mesh

The model is meshed for an element size of 10 mm and the named selections are defined for inlet and outlet.

Fig 5 – Mesh generated

Fig 6- Mesh metrics

Fig 7- Named selections

5.3 Setup

The case is solved for a steady state, pressure based type using K-Ɛ Realizable with Scalable wall surface treatment. The next step involves defining the fluid as water and assigning the same to respective cell zones. The boundary conditions are defined as pressure-inlet with pressure of 10 Pa and pressure-outlet with zero gauge pressure. Using these settings, the case is run for 500 iterations.

6. Results

6.1 Convergence

In order to ensure the convergence of the solution, report plot definitions are generated for mass flow rate and pressure-inlet and out- in addition to the residuals

Fig 8- Residuals plot

Fig 9- Mass flow rate plot

Fig 10- Plot of Inlet pressure against iterations

Fig 11- Plot of Outlet pressure against iterations

From the plots, it can be observed that the solution has converged at iteration 450.

6.2 Mass flow rate and flow coefficients

The results obtained are tabulated below:

Table-2 Results for case- 10% valve opening

7. Parametric study

Using parameters in Workbench study, the different valve opening distance from pipe axis and different design points are created with the parameter inputs.

The next step is to update all design points and the observed outputs parameters are shown below:

Table- 3 Design Points and parameters

Note that for higher valve openings, the convergence is obtained at iteration 1500 and the table shown reports results for convergent solutions.

7.1 Geometry and results

To verify the nature of the parametric study, the sectional view of the gate valve is checked along with the velocity characteristics at a plane along the pipe for three different cases- 10%, 30%, 50% and 70%.

Fig -12 Case- 10% valve opening

Fig -13 Case- 20% valve opening

Fig -14 Case- 30% valve opening

Fig -15 Case- 50% valve opening

Fig 16- Case- 70% valve opening

7.2 Flow coefficients and flow factors

Using the output parameters obtained in the study, the flow coefficient and its metric equivalent, flow factors are calculated and tabulated below:

Table- 4 Results

7.3 Effect of valve opening on mass flow rate and flow coefficient

Fig 17- Plot of mass flow rate with valve opening

From the figure, it can be observed that with an increase in valve opening, the flux parameter- mass flow rate increases. That is, fluid resistance by the gate disc is reduced and the volume of fluid flow across the pipe increases.

Fig 18- Plot of flow coefficient/ flow factor with valve opening

A plot of the flow coefficient and flow factor against the different valve openings shows an increase in the coefficient with opening of the valve. This can be explained by increase in mass flow rate and a decrease in the pressure difference across the pipes (due to increase in pressure at outlet)

8. Conclusions

The study successfully simulates flow in a pipe fitted with gate valve. Using parametric study, the effect of opening the gate valve on the fluid characteristics- mass flow rate and flow coefficients are established.