Week 9 - Parametric study on Gate valve.

Aim : 

        To perform a 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. 

INTRODUCTION

                    A gate valve controls the media's flow by lifting the gate (open) and lowering the gate (closed). A gate valve's distinct feature is the straight-through unobstructed passageway, which induces minimal pressure loss over the valve. The unobstructed bore of a gate valve also allows for a pig's passage in cleaning pipe procedures, unlike butterfly valves. Gate valves are available in many options, including various sizes, materials, temperature and pressure ratings, and gate and bonnet designs. A gate valve's main components are body, seat, gate, stem, bonnet, and actuator (Figure 2). The primary operation mechanism is straightforward. Turning the handwheel rotates the stem, which moves the gate up or down via the threads. They require more than one 360° turn to fully open/close the valve. Lifting the gate from the path of the flow, the valve opens. Lowering the gate to its closed position seals the bore resulting in a full closure of the valve.

             For a gate valve, the relationship between the gate's vertical travel and the flow rate is nonlinear, with the highest changes occurring near shutoff. When used to regulate flow, the relatively high velocity of the flow at partial opening results in gate and seat wear, which along with possible vibrations of the gate, shortens the valve's service life.

Flow Coefficient:

                          A device's flow coefficient is a measure of how well it allows fluid to flow. It explains the connection between the corresponding flow rate and the pressure drop via an orifice valve or other component.

           The flow coefficient Cv, often known as the flow-capacity rating of the valve, can be mathematically stated 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).

                The use of the flow coefficient provides an industry-accepted standard method for comparing valve capacity and sizing valves for particular applications. The discharge coefficient can be used to extend the generic definition of the flow coefficient into equations that simulate the flow of liquids, gases, and steam.

               The Cv for the same assembly can be combined with a more complicated equation to model gas flow in a pneumatic system. For gas, absolute pressures (psia) must be employed instead of only differential pressure.

Flow Factor:

               A flow factor (Kv), also known as a flow coefficient, is the amount of water flowing through a device in cubic metres per hour (m3/h) at a temperature of 16°C with a pressure drop of 1 bar across the valve or other restricting device. The head drop (h) or pressure drop (P) over the valve is related to the flow rate by the Kv (Q).

Kv = 0.865 * Cv

Critical Flow Factor:

            Cf, or critical flow factor, is a coefficient that describes how pressure will rise after falling to its lowest point inside the control valve. For the purpose of sizing valves, the flow coefficient (Cv) or flow rate must be calculated, and this factor is called Cf.

First, liquid or gas enters a control valve (seen above in light blue). As a result, the pressure decreases until it reaches the lowest point, or the vena contracta. After that, it flows downstream, and the pressure returns (or recovers).

Parametric analysis

                          You can define the parameter range, describe the design restrictions, and evaluate the effects of each parameter variation using parametric studies. To do a parametric study, you must: Parametric Dimensions is selected as the Design Objective. Nominated variables to be used in the simulation. The height of the gate valve is the parameter specified for this specific simulation.

 Geometry Setup :

                            3D model inlet & exhaust has been extended to accommodate fluid volume necessary to simulate gate valve.The valve has been opened 10mm to calculate mass flow rate of water.The fluid volume extracted using Volume extract option by selecting boundaries to make it closed volume.                  

                           The simulation has been carried out number of valve opening from 10mm to 80mm. The dimension of valve opening has been parameterize to simplify simulation process of similar type with different valve opening. The parameter has been created for valve opening by creating group shown in the below image. 

 

Meshing :

The meshing has been performed on the extracted fluid volume. The element size is considered as 70mm. The number of element generated during mesing process is 152485.Inlet & outlet of the fluid volume is assigned the name using named selection option.

Simulation Setup :

k-epsilon (2-eqn) with Realizable viscous model has been considered scalable near-wall treatment condition.

 

Create Material - Water

             Inlet boundary condition is assigned from the Boundaries options available in Physics tab of Ansys fluent. The boundary is considered as pressure-inlet type & 10Pa gauge total pressure is inserted in the inlet condition dialogue box.

 

        Standard initialization is performed before solving the physics of the model.After initializating the solution process, the output parameters & reauired contour images & animation file has been activated for post processing. 

        First of all, flow velocity contour graph has been created to visualize the flow condition in the fluid volume. The flow velocity has been projected onto the reference plane created in the YZ direction passing from the centre of the fluid volume.

Solution and Results :

Case 1 :  10 mm lift

 scaled residals :   

      

 

Case 2 :  20 mm lift

Case 3 :  30 mm lift

Case 4 :  40 mm lift

Case 4 :  50 mm lift

Case 5 :  60 mm lift

Case 7 :  70 mm lift

calculations for  flow factor and flow co-efficient

Flow factor for 10 mm lift

    =mass flow rate x3600/1000xsqrt(1/10x10^-5)

    = mass flow rate x360.

    =0.1657x360

    = 59.65

Flow coefficient = flow factor/0.865

Flow coefficient for 10 mm lift= 59.65/0.865 = 68.95

Similarly calculate for remaining lift values

S.no	Gate Valve Lift(mm)	mass flow rate(kg/s)	flow factor(in PSI drop per GPM)	flow co-efficient
1	10	-0.1657	59.65	68.95
2	20	-0.25766	92.75	107.23
3	30	-0.36685	132.06	152.67
4	40	-0.48348	174.05	201.21
5	50	-0.60118	216.42	250.20
6	60	-0.72252	260.10	300.70
7	70	-0.84575	304.47	351.98

conclusion :

•  With increased gate valve opening, mass flow rate rises.
• High pressure is present on the inflow side and decreases throughout the valve.
• The flow velocity rises as the valve opening widens.
•  The boundary of the outlet pressure is specified as zero.
•  The parametric study aids in the analysis of various valve opening positions and provides speedy simulation results.
•  The mass flow rate, flow coefficient, and flow factor are all at their maximum when the valve is fully open