Week 9 - Parametric study on Gate valve.

AIM

To perform a parametric study on the gate valve

OBJECTIVES

• To 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.

THEORY

Gate valve

A gate valve, also known as a sluice valve, is a valve that opens by lifting a barrier 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.

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 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.

Flow factor

The metric equivalent flow factor (K_v) is calculated using metric units :

where,

K_v is the flow factor (expressed in `m^3/h`).

Q is the flowrate (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:

SIMULATION PROCEDURE

Open Ansys Workbench, select fluid flow(fluent) and open spaceclaim. Import the file containing gate valve into spaceclaim.

Extract the fluid volume using volume extract option.

The next step is to parametrise the lift of the gate

After creating geometry, close spacelaim then the next step is to create the mesh. Before meshing name the boundaries.

Meshing is complete. Double click on setup to open CFD solver.

Element size = 5.5mm

CFD Case Setup

• Pressure based solver
• Steady state simulation is performed
• Gravity is enabled in -ve Z axis.
• Viscous model - K epsilon Realizable with scalable wall functions
• Set the boundary conditions: inlet type - pressure inlet
• Initialize the solution using standard initialization
• Run the calculation

RESULTS

For 10 mm lift of gate valve:

Residual plot

Mass flow rate

Velocity plot

For 20 mm lift of gate valve:

Residual plot

Mass flow rate

Velocity plot

For 30 mm lift of gate valve:

Residual plot

Mass flow rate

Velocity plot

For 40 mm lift of gate valve:

Residual plot

Mass flow rate

Velocity plot

For 50 mm lift of gate valve:

Residual plot

Mass flow rate

Velocity plot

For 60 mm lift of gate valve:

Residual plot

Mass flow rate

Velocity plot

For 70 mm lift of gate valve:

Residual plot

Mass flow rate

Velocity plot

For 80 mm lift of gate valve:

Residual plot

Mass flow rate

Velocity plot

Parametric study table

CALCULATIONS

For 10mm of lift,

Rate of flow, Q = `(mass flow rate)/(density)`

= `0.14884/998.2`

= `1.491*10^-4 m^3/s`

= `(1.491*10^-4 )*3600 m^3/h`

= 0.53676 `m^3/h`

Pressure = 10 pascal

= `10*10^-5`

= 10^-4 bar

SG = 1 for water

Flow coeffecient, `C_v` = `0.53676*sqrt(1/10^-4)`

= 53.676

Flow factor, `K_v` = `0.865*C_v`

= 46.429

Lift (mm)	Mass flow rate (kg/s)	Q (`m^3/h`)	Flow coeffecient	Flow factor
10	0.148	0.536	53.6	46.4
20	0.239	0.862	86.2	74.6
30	0.361	1.305	130.5	112.9
40	0.472	1.703	170.3	147.3
50	0.566	2.044	204.4	176.8
60	0.640	2.309	230.9	199.7
70	0.713	2.573	257.3	222.5
80	0.786	2.836	283.6	245.3

Flow coefficient and flow factor for each opening

Mass flow rate v/s lift

CONCLUSION

• A parametric study was conducted on a gate valve by setting the opening from 10 % to 80%.
• From the results, it is clear that as the lift of the gate disc increases the velocity of the flow increases.
• As the lift of the gate valve increases from 10mm to 80mm the mass flow rate, flow coeffecient, flow factor increases gradually.