Hydraulic pressure compensating valve
Pressure Control Valves
The Pressure Compensator block represents a hydraulic pressure compensating valve, or pressure compensator. Pressure compensators are used to maintain preset pressure differential across a hydraulic component to minimize the influence of pressure variation on a flow rate passing through the component. The following illustration shows typical applications of a pressure compensator, where it is used in combination with the orifice installed downstream (left figure) or upstream (right figure). The compensator can be also used in combination with metering pumps, flow dividers, and so on.
The block is implemented as a data-sheet-based model, based on parameters usually provided in the manufacturer's catalogs or data sheets.
Pressure compensator is a normally open valve. Its opening is
proportional to pressure difference between ports X and Y and the
spring force. The following illustration shows typical relationship
between the valve passage area A and the pressure difference
The orifice remains fully open until the pressure difference is lower than valve preset pressure determined by the spring preload. When the preset pressure is reached, the valve control member is forced off its stop and starts closing the orifice, thus trying to maintain pressure differential at preset level. Any further increase in the pressure difference 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, or pressure compensator static error, and usually is provided in manufacturer's catalog or data sheets.
The main parameters of the block are the valve maximum area and regulation range. In addition, you need to specify the leakage area of the valve. Physically, it represents a possible clearance in the closed valve, but 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.
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:
|p||Pressure differential across the valve|
|pxy||Pressure differential across valve control terminals|
|pA, pB||Gauge pressures at the valve main terminals|
|px, py||Gauge pressures at the valve control terminals|
|pset||Valve preset pressure|
|pmax||Pressure needed to fully close the orifice|
|A||Instantaneous orifice passage area|
|A(p)||Pressure-dependent orifice passage area|
|Ainit||Initial open area of the valve|
|Amax||Orifice maximum area|
|Aleak||Closed orifice leakage area|
|CD||Flow discharge coefficient|
|τ||Time constant for the first order response of the valve opening|
|pcr||Minimum pressure for turbulent flow|
The minimum pressure for turbulent flow, pcr, is calculated according to the laminar transition specification method:
By pressure ratio — The transition from laminar to turbulent regime is defined by the following equations:
pcr = (pavg + patm)(1 – Blam)
pavg = (pA + pB)/2
|pavg||Average pressure between the block terminals|
|patm||Atmospheric pressure, 101325 Pa|
|Blam||Pressure ratio at the transition between laminar and turbulent regimes (Laminar flow pressure ratio parameter value)|
By Reynolds number — The transition from laminar to turbulent regime is defined by the following equations:
|DH||Instantaneous orifice hydraulic diameter|
|ν||Fluid kinematic viscosity|
|Recr||Critical Reynolds number (Critical Reynolds number parameter value)|
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 . The control pressure differential is measured as , and it creates a force acting against the spring preload.
Valve opening is linearly proportional to the pressure differential.
No loading on the valve, such as inertia, friction, spring, and so on, is considered.
Flow consumption associated with the spool motion is neglected.
Valve passage maximum cross-sectional area. The default value
Pressure difference that must be maintained across an element
connected to ports X and Y. At this pressure the valve orifice 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 orifice 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
Select how the block transitions between the laminar and turbulent regimes:
Pressure ratio —
The transition from laminar to turbulent regime is smooth and depends
on the value of the Laminar flow pressure ratio parameter.
This method provides better simulation robustness.
Reynolds number —
The transition from laminar to turbulent regime is assumed to take
place when the Reynolds number reaches the value specified by the Critical
Reynolds number parameter.
Pressure ratio at which the flow transitions between laminar
and turbulent regimes. The default value is
This parameter is visible only if the Laminar transition
specification parameter is set to
The maximum Reynolds number for laminar flow. 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
12. This parameter is visible
only if the Laminar transition specification parameter
is set to
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. The parameter value
must be greater than 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.
Hydraulic conserving port associated with the pressure control terminal that opens the orifice.
Hydraulic conserving port associated with the pressure control terminal that closes the orifice.