Hydraulic valve that allows flow in one direction only
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 valve 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.
By default, the block does not include valve opening dynamics, and the valve sets its opening area directly as a function of pressure:
Adding valve opening dynamics provides continuous behavior that is more physically realistic, and is particularly helpful in situations with rapid valve opening and closing. The pressure-dependent orifice passage area A(p) in the block equations then becomes the steady-state area, and the instantaneous orifice passage area in the flow equation is determined as follows:
In either case, the flow rate through the valve is determined according to the following equations:
|pA, pB||Gauge pressures at the block terminals|
|CD||Flow discharge coefficient|
|A||Instantaneous orifice passage area|
|A(p)||Pressure-dependent orifice passage area|
|Ainit||Initial open area of the valve|
|Amax||Fully open valve passage area|
|Aleak||Closed valve leakage area|
|pcrack||Valve cracking pressure|
|pmax||Pressure needed to fully open the valve|
|pcr||Minimum pressure for turbulent flow|
|Recr||Critical Reynolds number|
|DH||Instantaneous orifice hydraulic diameter|
|ν||Fluid kinematic viscosity|
|τ||Time constant for the first order response of the valve opening|
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 .
Valve opening is linearly proportional to the pressure differential.
No loading on the valve, such as inertia, friction, spring, and so on, is considered.
Valve passage maximum cross-sectional area. The default value
Pressure level at which the orifice of the valve starts to open.
The default value is
Pressure differential across the valve needed to fully open
the valve. Its value must be higher than the cracking pressure. The
default value is
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
The maximum Reynolds number for laminar flow. The transition
from laminar to turbulent regime is assumed to take place when the
Reynolds number reaches this value. The value of the parameter depends
on the orifice geometrical profile. You can find recommendations on
the parameter value in hydraulics textbooks. The default value is
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
Select one of the following options:
Do not include valve opening dynamics —
The valve sets its orifice passage area directly as a function of
pressure. If the area changes instantaneously, so does the flow equation.
This is the default.
Include valve opening dynamics —
Provide continuous behavior that is more physically realistic, by
adding a first-order lag during valve opening and closing. Use this
option in hydraulic simulations with the local solver for real-time
simulation. This option is also helpful if you are interested in valve
opening dynamics in variable step simulations.
The time constant for the first order response of the valve
opening. This parameter is available only if Opening dynamics is
Include valve opening dynamics.
The default value is
The initial opening area of the valve. This parameter is available
only if Opening dynamics is set to
valve opening dynamics. The default value is
Parameters determined by the type of working fluid:
Fluid kinematic viscosity
The block has the following ports:
Hydraulic conserving port associated with the valve inlet.
Hydraulic conserving port associated with the valve outlet.
The Graetz Flow Control Circuit example 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.