Parametric study on Gate valve using Ansys Fluent

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).

Applications of gate valve

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.

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.

Gate valves may have flanged ends drilled according to pipeline-compatible flange dimensional standards.

Gate valves are typically constructed from the cast iron, cast carbon steel, ductile iron, gunmetal, stainless steel, alloy steel, and forged steel.

All-metal gate valves are used in ultra-high-vacuum chambers to isolate regions of the chamber.

Bonnet: Bonnets provide leakproof closure for the valve body. Gate valves may have a screw-in, union, or bolted bonnet. A screw-in bonnet is the simplest, offering a durable, pressure-tight seal. A union bonnet is suitable for applications requiring frequent inspection and cleaning. It also gives the body added strength. A bolted bonnet is used for larger valves and higher pressure applications.

Pressure seal bonnet: Another type of bonnet construction in a gate valve is pressure seal bonnet. This construction is adopted for valves for high-pressure service, typically in excess of 2250 psi (15 MPa). The unique feature of the pressure seal bonnet is that the bonnet ends in a downward-facing cup that fits inside the body of the valve. As the internal pressure in the valve increases, the sides of the cup are forced outward. improving the body-bonnet seal. Other constructions where the seal is provided by external clamping pressure tend to create leaks in the body-bonnet joint.

Knife gate valve: For plastic solids and high-viscosity slurries such as paper pulp, a specialty valve known as a knife gate valve is used to cut through the material to stop the flow. A knife gate valve is usually not wedge-shaped and has a tapered knife-like edge on its lower surface.

Parametric study:

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.

Model:

Geometry preparation:

Pull the inlet and outlet of solid surface 

Extract the fluid volume using edge selection and suppress the unwanted components

Extracted volume

Set the lift of the valves parameter for the gate disc

Update the extracted volume and update it as per the context

Mesh

Mesh size 8mm

No of elements 256078

No of nodes 51219

The case set up:

Solver: Pressure based/steady-state/gravity enabled down the z-axis

Viscous model: Kepsilon/realizable/scalable wall function

Fluid: water

Boundary conditions: inlet/pressure inlet of 10 Pa

Standard initialization computing from the inlet

No of iterations 250

Case 1

10mm Rise

Case 2

25mm rise

Case 3

50mm rise

Case 4

80mm rise

Parametric table

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. 

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 (`m^3.h^(-1).Bar^0.5`), Q is the flow rate (`m^3/h`), SG is the specific gravity of the fluid (for water = 1),

∆P is the differential pressure across the device (bar).

K_v can be calculated from C_v using the equation:

Calculating Kv for each case

Q = 1kg/s = 3.6 m^3/h

∆P = 10-0 = 10 Pa = 10*`10^-5` = `10^-4`bar

For 10mm lift

`K_v = 3.6*0.1496sqrt(1/10^-4)=53.856 m^3/(hr.Bar^0.5)`

For 25mm lift

`K_v = 3.6*0.2981sqrt(1/10^-4)=107.316 m^3/(hr.Bar^0.5)`

For 50mm lift

`K_v = 3.6*0.5699sqrt(1/10^-4)=205.164 m^3/(hr.Bar^0.5)`

For 80mm lift

`K_v = 3.6*0.7755sqrt(1/10^-4)=279.18 m^3/(hr.Bar^0.5)`

`C_v` for each case

`C_v = K_v/0.856`

For 10mm Cv = 62.9158

For 25mm Cv = 125.3691

For 50mm Cv = 239.6775

For 80mm Cv = 326.1448

Tabulation:

Matlab Coding for plot

clear all
close all
clc
L = [10, 25, 50, 85];
Cv = [62.9158, 125.3691, 239.6775, 326.1448];
Kv = [53.856, 107.316, 205.164, 279.180];

plot(L,Cv,'color','r','marker','*')
hold on
plot(L,Kv,'color','b','marker','*')
xlabel('Gate disc lift in mm')
ylabel('Cv and Kv')
title('Lift vs Cv and Kv')
legend('Cv','Kv')

plot

Conclusions:

We can say that as the lift is increased the mass flow rate also increases

A parametric study is a time-reducing option to analyze the effect of certain changes in a parameter.

flow coefficient helps in computing the efficiency of the system and the effect of pressure on the system.

flow coefficient is calculated to determine the type and sizing of the value to be used for a particular system.