Learn more about SimHydraulics

Pressure Compensator - Simulate hydraulic pressure compensating valve

Library

Pressure Control Valves

Description

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 p_xy.

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.

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 computed according to the following equations:

where

`q`	Flow rate
`p`	Pressure differential across the valve
p_xy	Pressure differential across valve control terminals
`p_A,p_B`	Gauge pressures at the valve main terminals
p_x,p_y	Gauge pressures at the valve control terminals
`p_set`	Valve preset pressure
`p_max`	Pressure needed to fully close the orifice
`p_reg`	Regulation range
`A(h)`	Instantaneous orifice passage area
`A_max`	Orifice maximum area
`C_D`	Flow discharge coefficient
ρ	Fluid density
`D_H`	Instantaneous orifice hydraulic diameter
ν	Fluid kinematic viscosity
`A_leak`	Closed orifice leakage area

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.

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.
Flow consumption associated with the spool motion is neglected.

Dialog Box and Parameters

Maximum passage area: Valve passage maximum cross-sectional area. The default value is 1e-4 m^2.
Valve pressure setting: 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 3e6 Pa.
Valve regulation range: 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 1.5e5 Pa.
Flow discharge coefficient: 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 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.
X: Hydraulic conserving port associated with the pressure control terminal that opens the orifice.
Y: Hydraulic conserving port associated with the pressure control terminal that closes the orifice.