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Shuttle Valve

Hydraulic valve that allows flow in one direction only

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Directional Valves

Description

The Shuttle Valve block represents a hydraulic shuttle valve as a data-sheet-based model. The valve has two inlet ports (A and A1) and one outlet port (B). The valve is controlled by pressure differential . The valve permits flow either between ports A and B or between ports A1 and B, depending on the pressure differential pc. Initially, path A-B is assumed to be opened. To open path A1-B (and close A-B at the same time), pressure differential must be less than the valve cracking pressure (pcr <=0).

When cracking pressure is reached, the value control member (spool, ball, poppet, etc.) is forced off its seat and moves to the opposite seat, thus opening one passage and closing the other. If the flow rate is high enough and pressure continues to change, the control member continues to move until it reaches its extreme position. At this moment, one of the valve passage areas is at its maximum. The valve maximum area and the cracking and maximum pressures are generally provided in the catalogs and are the three key parameters of the block.

The relationship between the A-B, A1–B path openings and control pressure pc is shown in the following illustration.

In addition to the maximum area, the leakage area is also required to characterize the valve. The main purpose of the parameter is not to account for possible leakage, even though this is also important, but to maintain numerical integrity of the circuit by preventing a portion of the system from getting isolated after the valve is completely closed. An isolated or "hanging" part of the system could affect computational efficiency and even cause failure of computation. Theoretically, the parameter can be set to zero, but it is not recommended.

The model accounts for the laminar and turbulent flow regimes by monitoring the Reynolds number for each orifice (ReAB,ReA1B) and comparing its value with the critical Reynolds number (Recr). The flow rate through each of the orifices is determined according to the following equations:

where

qAB, qA1BFlow rates through the AB and A1B orifices
pAB, pA1BPressure differentials across the AB and A1B orifices
pA, pA1, pBGauge pressures at the block terminals
CDFlow discharge coefficient
AAB, AA1BInstantaneous orifice AB and A1B passage areas
AmaxFully open orifice passage area
AleakClosed valve leakage area
pcValve control pressure differential
pcrackValve cracking pressure differential
popPressure differential needed to fully shift the valve
pcrAB, pcrA1BMinimum pressures for turbulent flow across the AB and A1B orifices
RecrCritical Reynolds number
DHAB, DHA1BInstantaneous orifice hydraulic diameters
ρFluid density
νFluid kinematic viscosity

The block positive direction is from port A to port B and from port A1 to port B. Control pressure is determined as .

Basic Assumptions and Limitations

  • Valve opening is linearly proportional to the pressure differential.

  • No loading on the valve, such as inertia, friction, spring, and so on, is considered.

Dialog Box and Parameters

Maximum passage area

Valve passage maximum cross-sectional area. The default value is 1e-4 m^2.

Cracking pressure

Pressure differential level at which the orifice of the valve starts to open. The default value is -1e4 Pa.

Opening pressure

Pressure differential across the valve needed to shift the valve from one extreme position to another. The default value is 1e4 Pa.

Flow discharge coefficient

Semi-empirical parameter for valve capacity characterization. Its value depends on the geometrical properties of the orifice, and usually is provided in textbooks or manufacturer data sheets. The default value is 0.7.

Critical Reynolds number

The maximum Reynolds number for laminar flow. The transition from laminar to turbulent regime is supposed to take place when the Reynolds number reaches this value. The value of the parameter depends on orifice geometrical profile, and the recommendations on the parameter value can be found in hydraulic textbooks. The default value is 12.

Leakage area

The total area of possible leaks in the completely closed valve. The main purpose of the parameter is to maintain numerical integrity of the circuit by preventing a portion of the system from getting isolated after the valve is completely closed. An isolated or "hanging" part of the system could affect computational efficiency and even cause simulation to fail. Therefore, MathWorks recommends that you do not set this parameter to 0. The default value is 1e-12 m^2.

Global Parameters

Parameters determined by the type of working fluid:

  • Fluid density

  • Fluid kinematic viscosity

Use the Hydraulic Fluid block or the Custom Hydraulic Fluid block to specify the fluid properties.

Ports

The block has the following ports:

A

Hydraulic conserving port associated with the valve inlet.

A1

Hydraulic conserving port associated with the valve inlet.

B

Hydraulic conserving port associated with the valve outlet.

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