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Ball Valve

Valve with a sliding ball control mechanism

  • Library:
  • Hydraulics (Isothermal) / Valves / Flow Control Valves

Description

The Ball Valve block models an orifice with a variable opening area controlled by a sliding ball mechanism. The opening area changes with the relative position of the ball—the valve lift. A displacement toward the orifice decreases flow while a displacement away increases flow. The interface between the orifice and the ball—the valve seat—can be Sharp-edged, shown left in the figure, or Conical, shown right.

Ball Valve Seat Types

Opening Area

The valve lift is a function of the displacement signal specified through port S. The two can, but generally do not, have the same value. The valve lift differs from the displacement whenever the Ball displacement offset parameter is nonzero:

h(x)=x0+s,

where:

  • h is the valve lift.

  • x0 is the ball displacement offset.

  • s is the ball displacement (relative to the specified offset).

The valve is fully closed when the valve lift is equal to zero or less. It is fully open when the valve lift reaches or exceeds a (geometry-dependent) value sufficient to completely clear the orifice. A fully closed valve has an opening area equal to the specified Leakage area parameter while a fully open valve has the maximum possible opening area. Adjusting for internal leakage:

AMax=πrO2+ALeak,

where:

  • AMax is the maximum opening area.

  • rO is the orifice radius.

  • ALeak is the internal leakage area between the ports.

At intermediate values of the valve lift, the opening area depends on the valve seat geometry. If the Valve seat specification parameter is set to Sharp-Edged, the opening area as a function of valve lift is:

A(h)=πrO(1[rBdOB]2)dOB,

where:

  • A is the opening area at a given valve lift value.

  • rB is the ball radius.

  • dOB is the clearance distance between the ball surface and the orifice edge.

If the Valve seat specification parameter is set to Conical, the opening area becomes:

A(h)=πhcos(θ)sin(θ)(dOB+hsin(θ)),

where θ is the angle between the conical surface and the orifice centerline. The geometrical parameters and variables used in the equations are shown in the figure.

Valve Geometries

Flow Rate

The volumetric flow rate through the valve is a function of the opening area, A(h), and of the pressure differential between the valve ports:

q=CDA(h)2ρΔp((Δp)2+pCr2)1/4,

where:

  • CD is the flow discharge coefficient.

  • ρ is the hydraulic fluid density.

  • Δp is the pressure differential between the valve ports, defined as:

    Δp=pApB,

    where pA is the pressure at port A and pB is the pressure at port B.

  • pCr is the minimum pressure required for turbulent flow.

The critical pressure pCr is computed from the critical Reynolds number as:

pCr=ρ2(ReCrνCDDH),

where:

  • ReCr is the critical Reynolds number.

  • ν (nu) is the hydraulic fluid dynamic viscosity.

  • DH is the orifice hydraulic diameter:

    DH=πrO2

Assumptions

Fluid inertia is ignored.

Ports

Input

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Physical signal input port for the ball displacement.

Conserving

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Hydraulic (isothermal liquid) conserving port associated with the valve inlet.

Hydraulic (isothermal liquid) conserving port associated with the valve outlet.

Parameters

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Parameters Tab

Valve seat geometry type. The seat geometry affects the opening area equations used during simulation.

Diameter of the ball used to control the valve opening area. This parameter must be greater than the Orifice diameter value.

Diameter of the valve orifice. This parameter must be smaller than the Ball diameter value.

Gate lift h corresponding to a zero displacement signal. This parameter must be greater than or equal to zero.

Ratio of the actual and theoretical flow rates through the valve. This parameter depends on the geometrical properties of the valve. Values are usually provided in textbooks and manufacturer data sheets.

Total area of internal leaks in the completely closed state. The purpose of this parameter is to maintain the numerical integrity of the fluid network by preventing a portion of that network from becoming isolated when the valve is completely closed.

Select the parameter to base the laminar-turbulent transition on. Options include:

  • Pressure ratio — Flow transitions between laminar and turbulent at the pressure ratio specified in the Laminar flow pressure ratio parameter. Use this option for the smoothest and most numerically robust flow transitions.

  • Reynolds number — The transition occurs at the Reynolds number specified in the Critical Reynolds number parameter. Flow transitions are more abrupt and can cause simulation issues at near-zero flow rates.

Pressure ratio at which the flow transitions between the laminar and turbulent regimes. The pressure ratio is the fraction of the outlet pressure over the inlet pressure.

Dependencies

This parameter is active when the Laminar transition specification parameter is set to Reynolds number.

Maximum Reynolds number for laminar flow. This parameter depends on the orifice geometrical profile. You can find recommendations on the parameter value in hydraulics textbooks. The default value, 12, corresponds to a round orifice in thin material with sharp edges.

Dependencies

This parameter is active when the Laminar transition specification parameter is set to Reynolds number.

Variables Tab

Volumetric flow rate through the valve at time zero. Simscape™ software uses this parameter to guide the initial configuration of the component and model. Initial variables that conflict with each other or are incompatible with the model may be ignored. Set the Priority column to High to prioritize this variable over other, low-priority, variables.

Pressure drop from port A to port B at time zero. The pressure drop is positive if pressure is greater at port A than at port B. Simscape software uses this parameter to guide the initial configuration of the component and model. Initial variables that conflict with each other or are incompatible with the model may be ignored. Set the Priority column to High to prioritize this variable over other, low-priority, variables.

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