Pressure control valve maintaining reduced pressure in portion of system
Pressure Control Valves
The Pressure Reducing Valve block represents
a hydraulic pressure-reducing valve as a data-sheet-based model. Pressure-reducing
valves are used to maintain reduced pressure in a portion of a system.
The following figure shows the typical dependency between the valve
A and the pressure
from the valve.
The pressure-reducing valve is a normally open valve and it remains fully open while outlet pressure is lower than the valve preset pressure. When the preset pressure is reached, the valve control member (spool, ball, poppet, etc.) is forced off its stop and starts closing the orifice, thus trying to maintain outlet pressure at preset level. Any further increase in the outlet pressure causes the control member to close the orifice even more until the point when the orifice if fully closed. The pressure increase that is necessary to close the valve is referred to as regulation range, and is generally provided in the catalogs, along with the valve maximum area. The valve maximum area and regulation range are the 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 block is built as a structural model based on the Pressure Compensator block, as shown in the following schematic.
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 passage maximum cross-sectional area. The default value
Preset pressure level, at which the orifice of the valve starts
to close. The default value is
Pressure increase over the preset level needed to fully close
the valve. Must be less than 0.2 of the Valve pressure setting parameter
value. 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 Power Unit with Pressure Reducing Valve example illustrates the use of the Pressure Reducing Valve block in hydraulic systems. The pressure reducing valve is set to 20e5 Pa and maintains this pressure downstream, as long as the upstream pressure is higher than this setting.