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Check Valve - Simulate hydraulic valve that allows flow in one direction only

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Directional Valves

Description

The Check Valve block represents a hydraulic check valve as a data-sheet-based model. The purpose of the check valve is to permit flow in one direction and block it in the opposite direction. The following figure shows the typical dependency between the valve passage area A and the pressure differential across the valve .

The valve remains closed while pressure differential across the valve is lower than the valve cracking pressure. When cracking pressure is reached, the value control member (spool, ball, poppet, etc.) is forced off its seat, thus creating a passage between the inlet and outlet. If the flow rate is high enough and pressure continues to rise, the area is further increased until the control member reaches its maximum. At this moment, the valve passage area 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.

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 (Re) and comparing its value with the critical Reynolds number (Re_cr). The flow rate is determined according to the following equations:

where

`q`	Flow rate through the valve
`p`	Pressure differential across the valve
`p_A,p_B`	Gauge pressures at the block terminals
`C_D`	Flow discharge coefficient
`A(p)`	Instantaneous orifice passage area
`A_max`	Fully open valve passage area
`A_leak`	Closed valve leakage area
`p_crack`	Valve cracking pressure
`p_max`	Pressure needed to fully open the valve
`D_H`	Instantaneous orifice hydraulic diameter
ρ	Fluid density
ν	Fluid kinematic viscosity

The block positive direction is from port A to port B. This means that the flow rate is positive if it flows from A to B, and the pressure differential is determined as .

Basic Assumptions and Limitations

The model is based on the following assumptions:

Valve opening is linearly proportional to the pressure differential.
No loading on the valve, such as inertia, friction, spring, and so on, is considered.
The transition between laminar and turbulent regimes is assumed to be sharp and taking place exactly at Re=Re_cr.

Dialog Box and Parameters

Maximum passage area: Valve passage maximum cross-sectional area. The default value is 1e-4 m^2.
Cracking pressure: Pressure level at which the orifice of the valve starts to open. The default value is 3e4 Pa.
Maximum opening pressure: Pressure differential across the valve needed to fully open the valve. Its value must be higher than the cracking pressure. The default value is 1.2e5 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 failure of computation. Extreme caution should be exercised if the parameter is set to 0. The default value is 1e-12 m^2.

Global Parameters

Fluid density: The parameter is determined by the type of working fluid selected for the system under design. Use the Hydraulic Fluid block or the Custom Hydraulic Fluid block to specify the fluid properties.
Fluid kinematic viscosity: The parameter is determined by the type of working fluid selected for the system under design. 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.
B: Hydraulic conserving port associated with the valve outlet.

Examples

The Graetz Flow Control Circuit demo (sh_Graetz_circuit) illustrates the use of check valves to build a rectifier that keeps the flow passing through a flow control valve always in the same direction, and to select an appropriate orifice depending on the flow direction.