Week 9 - Parametric study on Gate valve.

AIM

            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.

 

OBJECTIVE

1. The main objective of this challenge is to set up steady-state simulations of water flow through a gate valve..
2. To perform simulation for various gate valve openings from 10% to 80%
3. Perform a parametric study
4. Compare Mass flow rate, Flow coefficient and flow factor.

 

INTRODUCTION

            In this project simulation of a steady state flow of water inside a gate valve is performed for a various opening of valve from 10% to 80%. Only one case is simulated and remaining rest of the simulation is performed through parametric study. Opening distance is set a parameter. Velocity, pressure vector distributions are plotted to study the inside flow of the water. Mass flow rate, flow coefficient and 

flow factor are computed and compared to understand how the flow behaves for the different openings of the gate valve.

 

THEORY

 

Gate Valve

         A gate valve can be defined as a type of valve that uses a gate or wedge-type disk, and the disk moves perpendicular to flow to start or stop the fluid flow in piping.

         A gate valve is the most common type of valve used in any process plant. It is a linear motion valve used to start or stop fluid flow. In service, these valves are either fully open or fully closed. When the gate valve is fully open, the disk of a gate valve is completely removed from the flow. Therefore virtually no resistance to flow. Due to this, very little pressure drops when fluid passes through a gate valve. 360° surface contact is required between the disk and seats when the valve is fully closed to achieve proper sealing. 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.

   Types of Gate Valves

There are three ways to classify the gate valve.

1. Types of Disk
   1. Solid taper wedge
   2. Flexible wedge
   3. Split wedge or Parallel disks Valve
2. Types of Body Bonnet Joint
   1. Screwed Bonnet
   2. Bolted-Bonnet
   3. Welded-Bonnet
   4. Pressure-Seal Bonnet
3. Types of Stem movement
   1. Rising Stem or OS & Y Type (Outside Stem and Screw Type)
   2. Non-rising Stem type 

           A gate valve operates similar to other valves. To open the valve, turn the handwheel, which moves the gate up or down on the stem via the threads. A gate valve requires more than one 360° turn to open or close the valve fully. When the gate is lifted up, it opens the inlet to the outlet allowing an unobstructed passageway for the media to flow. When the gate is lowered, it closes and blocks the media flow.

Gate Valve Applications

• Gate valves are used in almost all fluid services such as air, fuel gas, feedwater, steam, lube oil, hydrocarbon, and almost any services.
• Some special gate valve is used in slurry and powder product also, such as knife gate valve.

Advantages of Gate Valve

• Gate valve provides good shutoff
• Pressure drop during operation is very less
• Most of the gate valves can be used as bi-directional
• They are suitable for high pressure and temperature application and require less maintenance

Disadvantages of Gate Valve

• It cannot be used to control the flow.
• A gate valve is slow in operation. Opening and closing take time which is good also as it reduces the chance of hammering.
• When partially open, it creates vibration and noise.
• Repairs, such as lapping and grinding of seats are more difficult due to limited access.

Parametric Study

         A parametric study that perturbs design variables in the product design model to explore design alternatives can effectively support product concept designs. A parametric study is simple and easy to perform as long as the mapping between CAD and simulation models has been established. The mapping supports fast simulation model generation for performance analyses. It also supports DSA using the finite difference method. The parametric study is possible for concept design because the number of design variables to perturb is usually small.

          Parameter optimisation is one of the biggest issues of engineers, particularly who use FE software. Sometimes, it needs to be investigated design parameters that are affecting to results as well as the mesh properties. However, it is so time-consuming to change the model every single time and run the analysis again. Fortunately, Ansys allows users to optimise their model without redesigning or running the model multiple times.  Parameter sets enable you to expose different parameters to the user. And, to return different information based on the parameters specified by the user.

 

FLOW COEFFICIENT

         The flow coefficient of a device is 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 or flow capacity rating Cv can be expressed as

Where             Q is the rate of flow (gallons per minute)

                        SG is the specific gravity of fluid (for water =1)

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

                        10 Pa = 0.00145038 psi

             In more practical terms, the flow coefficient Cv 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 are widely accepted by the industry.

 

FLOW FACTOR

            The K_v-factor for a given valve is a constant which in a simple way states the valve capacity. The K_v -factor is determined by the valve manufacturer by experiments. The K_v  factor specifies the water flow in m3 through the valve in one hour at a pressure drop across the valve of 1Bar

             The metric equivalent flow factor (K_v) is calculated using metric units. The metric equivalent factor Kv commonly used in europe and Aisa is calculated using metric units

where,

K_v is the flow factor (expressed in m³·h⁻¹).

Q is the flowrate (expressed in cubic metres per hour m³·h⁻¹),

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,

 

SOLVING & MODELLING APPROACH

                             Gate Valve Model                                         

 

So the above challenge can be categorised into following cases

1. Valve Disc Lift
2. Case 1 – 10mm
3. Case 2 – 20mm
4. Case 3 – 30mm
5. Case 4 – 40mm
6. Case 5 – 50mm
7. Case 6 – 60mm
8. Case 7 – 70mm
9. Case 8 – 80mm

 

                                                                  Total Cases

 

The methodology which will be used in this simulation is to create the input parameter in space-claim and output parameter in FLUENT. The input parameter will be the valve lift height in assembly and it has to be update as new input parameter will enter for simulation. The output parameter is the mass flow rate at outlet which is defined in post-processing. The two main output required to calculate the flow coefficient and flow factor are mass flow rate and pressure drop between the inlet and outlet. The main advantage of parametric simulation is that; it will lower the pre-processing time for each case of the simulation. If the simulation cases are greater in number and number of variable to change for simulation are lower, then use of parametric simulation is one of the best option for engineers. The most industrial simulations are based on the parametric study which are used in numerous problem where product design and product optimization with respect to working conditions are main focus.

Parametric Setup

 

PRE PROCESSING AND SOLVER SETTING

In our challenge we will create a flow simulation of water through a gate valve and setup a parametric set to simulate and obtain the output parameter of all the opening positions of the gate valve.

 

Case 1

                        Gate valve opening : 10 mm

                        Inlet Pressure   : 10 Pa

            Step 1 : Load the gate valve in the geometry Space Claim. We are going to observe the flow pattern. So the inlet and outlet of the valve is extended using Pull command. Since we are analysing the inside volume mixing, we extract the volume using extract function in the Space Claim. To set the opening position of the gate valve, the disc of the gate valve is moved using move command for 10mm and set it to parameter. Only the volume is required for analysis and so the solid part is suppressed.

 

Once this process is done. We will close the space claim and open the mesh module.

 

            Step 2 : Open the mesh module and under the mesh details give CFD Fluent as preference and default Element.

                        After the meshing is done check for the quality criterion. Check whether the mesh quality is above 5%. Once this is done name the faces of the volume as Inlet, Outlet & Wall.

Step 3 : After the meshing and face naming are done move on to Fluent Solver. In the fluent launcher select double precision, display mesh after reading and give the appropriate solver processors and GPUs.

In the ANSYS CFD we need to give all the conditions, parameters and models. To start with go to physics menu and click general settings. In that select Pressure based type solver, Absolute velocity formulation and steady state flow. Next we need to give the model. For this case K Epsilon RNG model with swirl dominated flow is selected.

Next is the flow material selection. By default air is selected. Water is selected from fluent database and air is deleted.. In the cell zones, the volume fluid is assigned to water. After that boundary conditions are to be assigned. For that click on zones- boundaries.

Give all the boundary conditions such as pressure etc., to all the boundaries of the volume as required. Care should be taken while assigning the boundary type. In our case Inlet cold and hot initials values are to be assigned. Change the type of the inlet to pressure -Inlet and give the value as 10 pa.

After the physics part is done move to the solution part. To generate mass flow rate, click definitions-new and select the required one. In the popup window select the appropriate field variable, surfaces and other options.

Once the report definition part is completed click Initialize to initialize the boundary conditions. Hybrid method is selected before initialization. Click on Autosave to obtain a animation of the flow in the post results.

            Then run the calculation for a given number of iterations 600 till the convergence is obtained. Depending upon the number of elements and model selected the time required for convergence varies. In the CFD module itself we can compute, measure, plot, animate, etc., if needed.

 

Step 4 : After the solutions and calculations move to the result module to get different graphs, plots, contours, animations etc.,. Sectional views can be created if required.

 

Step 5 : Then in the parameter set the new design point is created for various opening positions and calculated the output parameter mass flow rate by updating it.

 

RESULT

 

Case 1

                        Gate valve opening : 10 mm

                        Inlet Pressure   : 10 Pa

                                                                 Residuals

 

                                                               Mass Flow Rate

 

                                                               Velocity

 

                                                                 Pressure

 

                                                                        Vector

 

                                                                 Stream Line

 

 

 

Case 2

                        Gate valve opening : 20 mm

                        Inlet Pressure   : 10 Pa

 

                                                            Residuals

 

                                                               Mass Flow Rate

 

                                                                   Velocity

 

                                                                     Pressure

 

                                                                     Vector

 

                                                                    Stream Line

 

 

 

Case 3

                        Gate valve opening : 30 mm

                        Inlet Pressure   : 10 Pa

 

                                                                   Residuals

 

                                                               Mass Flow Rate

 

                                                                    Velocity

 

                                                                      Pressure

 

                                                                     Vector

 

                                                                   Stream Line

 

 

Case 4

                        Gate valve opening : 40 mm

                        Inlet Pressure   : 10 Pa

 

                                                                Residuals

 

                                                           Mass Flow Rate

 

                                                                        Velocity

 

                                                                     Pressure

 

                                                                        Vector

 

                                                                      Stream Line

 

 

Case 5

                        Gate valve opening : 50 mm

                        Inlet Pressure   : 10 Pa

                                                                       Residuals

 

                                                                Mass Flow Rate

 

                                                                    Velocity

 

                                                                      Pressure

 

                                                                            Vector

 

                                                                     Stream Line

 

 

Case 6

                        Gate valve opening : 60 mm

                        Inlet Pressure   : 10 Pa

                                                                    Residuals

 

                                                                    Mass Flow Rate

 

                                                                          Velocity

 

                                                                        Pressure

 

                                                                          Vector

 

                                                                    Stream Line

 

Case 7

                        Gate valve opening : 70 mm

                        Inlet Pressure   : 10 Pa

 

                                                                     Residuals

 

                                                              Mass Flow Rate

 

                                                                         Velocity

 

                                                                          Pressure

 

                                                                          Vector

 

                                                                      Stream Line

 

 

Case 8

                        Gate valve opening : 80 mm

                        Inlet Pressure   : 10 Pa

                                                                       Residuals

 

                                                           Mass Flow Rate

 

 

                                                                           Velocity

 

                                                                           Pressure

 

                                                                     Stream Line

 

 

Flow Co-Efficient and Flow Factor :

As we know, flow co-efficient is mathematically expressed as,

 

In this particular case study,

Q= Mass Flow Rate = −0.1408kg/sec

                                     Q = −0.143*15.85

     =  2.267 gallons/min

SG = Specific Gravity of Water = 1

Pascal [Pa]	Psi [psi]
1 Pa	0.0001450377 psi

  

ΔP = Pressure Drop = 10 Pa

                                       = 0.001450  psi

   

Therefore,

                                                      

         Cv=2.267×26.261

Flow Co-efficient               Cv = 59.53

 

  

Similarly,

Flow Factor                      Kv=0.865⋅Cv

                                          Kv=0.865×59.53

∴∴                                    Kv = 51.497

 

Calculating the same for all the case we get the result as shown in the table

 

Graphical Representation :

We represent the above data in the form of line graphs and plots as follows.

 

1. Gate Disc Lift vs Mass Flow Rate

 

2. Variation of Flow Co-Efficient and Flow Factor with Gate Disc Lift

CONCLUSION

                 The simulations for all 8 case studies representing the different levels of lift of the gate disc has successfully been carried out and the parametric results, along with the plots and contours have been obtained, which signify the effects of lift on the mass flow rate of the fluid at the outlet. From the all simulations it is evident that increase in lift will increase in mass flow rate fluid at outlet.

                  The increase of lift also have increment in the flow coefficient and flow factor i.e. Valve lift increment is directly proportional to Flow coefficient and flow factor.

                   From the graph of Lift increment and flow coefficient and flow factor, the line Cv and Kv are not in linear direction and we have low Kv for respective Cv depend upon the unit system.

                  As the valve get lifted the pressure at outlet of the system also increases and pressure difference is decreasing with each valve disc height increment.

                  From the graphs and plots obtained, we can observe that with the increase in lift of the gate disc, the mass flow rate of the fluid across the outlet also increases.

Therefore, the gate disc lift is directly proportional to mass flow rate. Hence, larger the gate disc opening, higher the value of mass flow rate.
                   Also, from the pressure plots, we can observe that as the flow progresses, there is a sudden pressure drop and the pressure reduces as it crosses the gate disc.