Week 9 - Parametric study on Gate valve.

AIM:

To perform a parametric study on the gate valve simulation by setting the opening from 10 % to 80%, and calculate the following quantities:

1. Mass flow rates
2. Flow coefficient
3. Flow factor

Introduction:

Gate Valve:

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.

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.

At high pressures, friction can become a problem. As the gate is pushed against its guiding rail by the pressure of the medium, it becomes harder to operate the valve. Large gate valves are sometimes fitted with a bypass controlled by a smaller valve to be able to reduce the pressure before operating the gate valve itself.

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.

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 C_v (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 C_v 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 modelling the flow of liquids, gases and steam using the discharge coefficient.

FLOW FACTOR:

The metric equivalent flow factor (K_v; commonly used everywhere else in the world with the exception of the United States) is calculated using metric units :

where

K_v is the flow factor (expressed in

Q is the flow rate (expressed in cubic metres per hour

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

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

K_v can be calculated from C_v using the equation:

The k_v factor or value as it is also called is defined in VDI/VDE Richtlinien No. 2173. A simplified version of the definition is: The k_v 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. The complete definition also says that the flow medium must have a specific gravity of 1000 kg/m³ and a kinematic viscosity of 10⁻⁶ m²/s. e.g. water.

Modelling and Preparation:

1. The CAD .STEP model was imported onto Spaceclaim and the following processes were done:

2. The bottom faces were pulled by a distance of 800mm.

3. The GATE disk component is moved vertically in the Z direction by 10mm and is set as parameterized.

4. The fluid volume was extracted and the volume component is set to - "Update the volume in context"

5. The Gate Valve design is suppressed for physics as we only need to work on fluid volume.

MESH:

Named selections of Inlet and Outlet were creased and the following values for the mesh were set and the mesh was generated.

SETUP on FLUENT:

The solver is set to a steady-state, pressure-based and with a gravitational acceleration in the negative z-direction.

The inlet is set as a pressure-inlet. The cell zones are set to water-liquid as well.

OUTLET	Gauge pressure: 0
WALL	Static wall, no-slip, standard roughness

Applied inlet pressure: 10 Pa, and velocity of flow is 0.14158 m/s, and the characteristic length is 0.1 m (diameter of the pipe)

The fluid is water with a density of 998.2 kg/m^3, and dynamic viscosity of 0.001003 kg/ms.

Therefore, Reynold's number for the above case is 1.3x10^4 which is in the turbulent range.

Hence, the following viscous model has been selected.

Solved using standard initialisation with calculation from the inlet.

RESULTS:

CASE 1: 10mm lift

CASE 2: 20mm lift

CASE 3: 30mm lift

b

CASE 4: 40mm lift

CASE 5: 50mm lift

CASE 6: 60mm lift

CASE 7: 70mm lift

CASE 8: 80mm lift

MASS FLOW RATE:

-tive sign indicates the direction of flow.

FLOW COEFFICIENT AND FLOW FACTOR

CONCLUSION:

1. Mass flow rate increases with an increase in the lift of the gate disc.
2. As the lift increases, both the flow factor and flow coefficient increase.
3. The flow factor and flow coefficient are proportional to each other