Week 9 - Parametric study on Gate valve.

OBJECTIVE:

For this challenge, you will have 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.
• Discuss the results of the mass flow rate and flow coefficient

INTRODUCTON THEORY:

A gate valve also known as a sluice valve is a valve which opens by lifting a round or rectangular gate/wedge out of the path of the fluid. The distinct feature of a gate valve is the sealing surfaces between the gate and seats are planar, so gate valves are often used when a straight-line flow of fluid and minimum restriction is desired. The gate faces can be parallel, but are most commonly wedge-shaped. Gate valves are primarily used to permit or prevent the flow of liquids, but typical gate valves shouldn't be used for regulating flow, unless they are specifically designed for that purpose. Because of their ability to cut through liquids, gate valves are often used in the petroleum industry. For extremely thick fluids, a specialty valve often known as a knife gate valve is used to cut through the liquid. On opening the gate valve, the flow path is enlarged in a highly nonlinear manner with respect to percent of opening. This means that flow rate does not change evenly with stem travel. Also a partially open gate tends to vibrate from the fluid flow. Most of the flow change occurs near shutoff with a relatively high fluid velocity causing gate and seat wear and eventual leakage if used to regulate flow. Typical gate valves are designed to be fully opened or closed. When fully open, the typical gate valve has no obstruction in the flow path, resulting in very low friction loss.

Gate valves are actuated by a threaded stem which connects the actuator (e.g. hand wheel 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 takes the form of a nut which 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 upper end of the stem to indicate valve position, since the gate's motion is concealed inside the valve. Non rising stems are used underground
or where vertical space is limited. Gate valves are primarily designed to serve as isolation valves. In service, these valves generally are either fully open or fully closed. When fully open, the fluid or gas flows through the valve in a straight line with very little resistance. It should not be used in the regulation or throttling of flow because accurate control is notpossible. Furthermore, high-flow velocity in partially opened valves may cause erosion of the discs and seating surfaces. Vibration may also result in chattering of the partially opened valve disc. An exception to the above are specially designed gate valves that are used for lowvelocity
throttling; for example, guillotine gate valves for pulp stock.

Flow coefficient

The flow coefficient of a device is a relative measure of its efficiency at allowing flui flow. It describes the relationship between the pressure drop across an orificevalve or other assembly and the corresponding flow rate.

Mathematically the flow coefficient C_v (or flow-capacity rating of valve) can be expressed as :

where:

Q is the rate of flow (expressed in US gallons per minute),

SG is the specific gravityof 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 is widely accepted by industry. The general definition of the flow coefficient can be expanded into equations modeling 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 flowrate (expressed in cubic metres per hour]),

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

ΔP is the pressure drop across the valve (expressed in psi).

K_v can be calculated from C_v using the equation:

Disk lift 10mm :

Geometry :

Meshing :

solver conditions

SOLVER TYPE	PRESSURE BASED
SOLVER TIME	STEADY STATE
GRAVITY	ON( Z AXIS AS -9.81m/s^2)
VISCOUS MODEL	K-EPSILON+REALIZABLE+SWIRL FLOW +SCALABLE WALL FUNCTIONS
MATERIAL	WATER
METHOD	COUPLED
INLET(PRESSURE INLET)	10 Pa
OUTLET	PRESSURE OUTLET( 1 Pa) gauge pressure

Solving :

Residual :

mfr

pressure :

velocity contour :

20mm

pressure contour :

velocity contour :

30mm

pressure contour

velocity contour

40mm

pressure contour :

velocity contour :

50mm

pressure contour :

velocity contour :

60mm

velocity contour

pressure contour :

70mm

pressure contour :

velocity contour :

80mm

pressure contour :

Velocity contour :

Mass flow rate for all the cases :

parametric study :

1.The parametric study of gate valve indicated the increase in mass flow rate at the outlet section.

2.The graphical plot for mass flow rate support the simulation results.

3.The flow coefficient plays an important role in selection of valve and sizing of valve, it also determines the efficiency of the system and the pressure effect on the system.

4. Ansys fluent helps in Simulating lots of case studies through parametric case option which helps in initializing any design to make it more efficient and improve it by iterating further if changes seemed necessary.

5. Gate valve design requires extensive simulation and practical experimentation to be properly developed if required for custom cases. Luckily we have lots of standard sizes available for different kinds of uses right now which were again developed using the similar methods.