Parametric study on the gate valve

Aim:

To perform a parametric study on a gate valve at different valve opening settings.

Theory:

Gate valves work by inserting a rectangular gate or wedge into the path of a flowing fluid. They are operated by a threaded stem which connects the actuator (generally a hand wheel or motor) to the stem of the gate. If the valve has a rising stem its position can be seen just by looking at the position of the stem. A gate valve is used to control the fluid flow through lifting and descending the gate. A gate distinct feature is to provide an unobstructed passageway, which includes minimal pressure loss over the valve.

When the gate is fully opened there is no restriction to the flow hence the pressure drop is low comparatively. Generally, the gate valve are fully closed or fully open as partially opened valve may experience vibration which may lead to excessive wear. A gate valve's main component is body, seat, gate, stem, bonnet, and actuator. the primary mechanism is straight forward where the handwheel is rotated to move the gate up and down via threads. they require more than a 360-degree turn to fully open the valve.

Gate valves tend to be slightly cheaper than ball valves of the same size and quality. They are slower in actuation than quarter-turn valves and are for applications where valve operation is infrequent, such as isolating valves. Gate valves should be used either fully open or fully closed, not to regulate flow. Automated gate valves exist with either an electric or pneumatic actuator, but a manual gate valve is cost-effective since they have infrequent usage. Gate valves are widely used for all types of applications and are suitable for both above-ground and underground installation. They are suitable for most fluids including steam, water, oil, and gas, etc.

Part-1:

The solid geometry consists of bottom housing, handwheel, spindle, bonnet, and gate valve. The fluid moves from the inlet to the outlet, the gate valve is used to restrict this motion, and this is actuated manually by turning the handwheel which is supported by the bonnet and is connected via the spindle. When a lesser mass flow is required at the outlet, the gate valve is lowered, which restrict the mass flow of the fluid at the outlet. The mass flow rate of the fluid at the outlet varies with the lift of the gate valve stopper, and that is to be simulated.

To simulate the effect of the stopper on the mass flow rate of the fluid at the outlet the stopper is moved 10mm in the positive Z-direction so that the flow path is partially open. This is parametrized. Next, the fluid is extracted from the solid geometry. The parameterization helps us change its value and compute the required output parameters without having to repeat the same processes again. Since we are not interested in the analysis of solid geometry, it is suppressed.

Once the geometry is defined, the meshing is done. The mesh element size taken was 10mm. Once the required mesh was generated, a total of 40524 nodes and 193912 elements were generated.

The resultant mesh quality is as follows

The fluid medium is water. Acceleration due to gravity is taken into consideration (along the negative Z-direction). The problem was solved using the steady-state equations, and the K-epsilon realizable turbulence model with scalable wall functions was used.

The boundary conditions at the inlet and outlet were pressure inlet and pressure outlet. The gauge pressure at the inlet and outlet is 10 Pascals and 0 Pascals respectively. When standard initialization was done from the inlet, the velocity computed was 0.1415488 m/s.

A flux report of mass flow rate is created and it is defined as an output parameter so as to calculate this property at different positions of the stopper. The plot below gives the mass flow rate of the fluid at the outlet at 10mm lift of the stopper.

The solution was run for 250 iterations. and the residual plot is as follows

Next, we move on to the parameters tab to calculate the mass flow rates for the different stopper heights, and the following results were obtained

The negative signs for the mass flow rates imply that the fluid is exiting the domain. The mass flow rate is calculated for the stopper lifts from 10mm to 70mm. The mass flow rate increases with the increase in the lit of the stopper. When the stopper is lifted gradually, the space available for the passage of the fluid increases hence the mass flow increases. When we perform the parametric study, Ansys automatically changes the mesh as per new geometry using the same mesh specifications given earlier, and solves and calculates the output parameters, thereby saving time comparatively if it was done manually.

Part-2:

The flow coefficient(Cv) 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.

`C_v=Q*sqrt((SG)/(DeltaP))`

where:

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

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

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

The metric equivalent flow factor(Kv) is calculated using metric units

`K_v=Q*sqrt((SG)/(DeltaP))`

where:

Q is the rate of flow (expressed in m^3/h)

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

∆P is the pressure drop across the valve (expressed in bar).

Kv can be calculated from Cv using the equation:

`K_v=0.865*C_v`

The following table gives the flow factor and flow coefficient of the fluid at different valve displacement magnitudes. Before the parameters are substituted to the formula, the units must be converted to the specific units defined above.

The magnitude of the valve displacement	Mass flow rate (Kg/s)	Flow factor (Kv)	Flow coefficient (Cv)
10	0.14821	53.3556	61.6855
20	0.2376	85.536	98.8855
30	0.36416	131.0976	151.5579
40	0.47315	170.334	196.9179
50	0.61058	219.8089	254.1143
60	0.64818	233.3448	269.7627
70	0.72901	262.4436	303.403

From the above trend plots, we can say that with the increase in the opening of the gate valve the mass flow rate also increases. Along with the mass flow rate, the flow coefficient and flow factor also increase.

Conclusions:

1. 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. Using the parametric study method saves a lot of time in the simulations which require to perform the same operations for different settings multiple times.

2. 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
3. In the parametric study of the gate valve, it was found that as the valve stopping is lifted gradually, the mass flow rate of the fluid, and hence the flow factor and flow coefficient increases.