Parametric Study on Gate Valve Simulation

Introduction:

A gate valve can be defined as a type of valve that uses gate or wedge type disk and the disk moves perpendicular to the flow, to start or stop the fluid flow in piping in other words it is a linear motion valve used to start or stop fluid flow. In service, these valves are either in fully open or fully closed position. Operation of a gate valve is performed doing an either clockwise to close (CTC) or clockwise to open (CTO) rotating motion of the stem. When the gate valve is fully open, the disk of a gate valve is completely removed from the flow. Therefore virtually no resistance to the flow. Due to this very little pressure drops when fluid passes through a gate valve which is advantageous over other type of valves.

Gate valves are used in almost all fluid services such as air, fuel gas, feed water, steam, lube oil, hydrocarbon, and all most any services. They are suitable for high pressure and temperature application and required less maintenance. Gate valves should not be used for regulation or throttling of flow because accurate control is not possible. The high velocity of the flow in the partially open valve may cause erosion of the disc and seating surfaces and also creates vibration and noise and shortens service life of the valve.

Abstract:

A Parametric study is done on the gate valve simulation using Ansys Fluent by setting the opening from 10% to 80% to obtain the mass flow rates at the outlet at each design point, in addition to find the flow Coefficient and flow factor at each opening. After a thorough study, it is observed that mass flow rate and flow factor is lineraly increasing at each step by step opening which is seen by plotting graphs.

Importance of Parametric Study:

As you move forward in your design, you can assess the impact that changing certain parameters can have on the design. 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. 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. Once you determine that a configuration satisfies your design needs, you are able to promote that configuration back to the model as a CAD edit. You are prompted whether to make changes. The more design options you analyze in a parametric study, the more information you will have available to you to understand the impacts of your design decisions.

In this simulation Gate valve lift is set on Parametric feature to study the variation of mass flow rate, velocity, pressure drop and flow factor.

Flow Coefficient Or Flow Factor:

When flow goes through a valve or any other restricting device it loses some energy. The flow coefficient is a designing factor which relates head drop (Δh) or pressure drop (ΔP) across the valve with the flow rate (Q). It is also a relative measure of valve eficiency in allowing fluid flow.

Each valve has its own flow coefficient. This depends on how the valve has been designed to let the flow going through the valve. Therefore, the main differences between different flow coefficients come from the type of valve, and of course the opening position of the valve.

Flow coefficient is important in order to select the best valve for a specific application. If the valve is going to be most of the time opened, probably there should be selected a valve with low head loss in order to save energy. Or if it is needed a control valve, the range of coefficients for the different opening positions of the valve should fit the requirements of the application.

At same flow rate, higher flow coefficient means lower drop pressure across the valve.

Cv is the flow coefficient in imperial units. It is defined as the flow rate in US Gallons per minute [gpm] of water at a temperature of 60º fahrenheit with a pressure drop across the valve of 1 psi.

Kv(Flow Factor) is the flow coefficient in metric units. It is defined as the flow rate in cubic meters per hour [m3/h] of water at a temperature of 16º celsius with a pressure drop across the valve of 1 bar.

Kv = 0.865 *Cv
Cv = 1.156 * Kv

Processes:

Preprocessing: A geometry of Gate valve is created in space-claim or imported from some other sources like (Solidworks or Creo) as shown below, then \"move\' feature is used to lift the gate disc to required height ( say 20mm - means 10% of inner diameter of pipe is opened) then the parameric fearture is activated to analyze results of each paramter variation. Next, fluid volume extracted is alongated on either side of inlet and outlet to 800mm for simulation using pull tool . Then volume body is updated in context (current changes made to geometry) so that setting of volume extraction is always updated for every lift of the valve. The remaining parts except volume extraction is suppresed for physics and updated for discretizing the domain (meshing). The following figures shows the changes made to the created geometry for simulation.

Model:

Sectional View and Initial position of Gate Disc:

Measurement of ID of valve:

Measurement After 20mm lift (10% open position):

Extracted Volume:

Sectional View (Final Geometry setting for simulation):

Mesh:

Mesh Parmeters:

i) No. of elements =193057

ii) No. of Nodes =40271

iii) Size of element =0.01 m

Solving:

The Steady state simulation is run using Pressure based solver and Solver choosen here is K-epsilon model with Realizable and Scalable Wall Fuction Features. Gracity Accelaration is set to -9.81 m/s2 (on Z-Axis depending on Model plane).

Solving Prameters:

i) Material (fluid) = Water

ii) Boundary Condition

• Inlet ( Pressure-inlet) = 10 psi

Method for Initalizing used is standard computed from from inlet. A report is created on mass Flow rate for evaluation . The solution is run for minimum of 250 iterations.

• From the above figures residuals doesn\'t shows convergence of solution but mass low rate plot shows it as mass flow rate attained constant after almost 20 iterations.

Results:

Velocity Profile at each lift:

• 20mm lift(10% open)

• 35mm lift(25% open)

• 50 mm lift(40% open)

• 55mm lift(45% open)

• 65mm lift(55% open)

• 70mm lift(60% open)

• 85mm lift(75% open)

Parametric Table:

Note: Flow Factor(Kv) in above table is not real due to default setting of CFD post units and Minus sign of mass flow rate indicates the direction.

Flow Coefficient (Cv):

Cv = 1.156 * Kv

Plots:

Lift(Top and Bottom-mm) vs Mass flow rate (Left- kg/s) & Flow Factor(Right- m3/hr)

Lift(Top and Bottom-mm) vs Flow Coefficient(Us gallons per minute)

Discussions:

Parametric study is conducted successfully and from the above results it has been observed that upon increasing in the opening percentage of the valve there is an increase in the velocity (at inlet and at outlet as well after a convergence condition is reached), mass flowrate and flow factor or flow coefficient. In this way, parametric study helps a designer to analyze and select a valve with best flow coeffiect according to the requirements in very less time.