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Hydraulically Operated Remote Control Valve

Normally closed or normally open hydraulically operated remote control valve

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Directional Valves

Description

The Hydraulically Operated Remote Control Valve block represents a hydraulically operated remote control valve as a data-sheet-based model, meaning that most of the model parameters are generally available in catalogs or manufacturer data sheets. Hydraulically operated remote control valves are widely used in hydraulic systems as hydraulic switches, unloading and sequencing valves. You can also use them as pressure-relief and pressure-reducing valves. The block covers both the normally closed and normally open configurations, shown in the following figure.

The valve opens (closes) by the pilot pressure. The valve control member remains in its initial position as long as the pilot pressure is lower than the cracking pressure. When cracking pressure is reached, the valve control member (spool, ball, poppet, and so on) is forced off its seat and starts opening the normally closed valve, or closing the normally open valve. The control member displacement is directly proportional to pilot pressure. The member reaches its maximum displacement after the pilot pressure becomes equal or greater than the preset maximum value. The valve maximum area, cracking pressure, and maximum pressure are the key parameters of the block. These three parameters are usually provided in catalogs or data sheets.

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.

Schematic fragments in the next illustration show some typical valve applications: remote control valve (a), pressure-relief valve (b), and pressure-reducing valve (c).

The flow rate through the orifice is proportional to the orifice opening and the pressure differential across the orifice. The flow rate is determined according to the following equations:

For the normally closed valve, the instantaneous orifice passage area A(p) is computed with the equations:

For the normally open valve, the equations are similar:

The rest of the equations apply to both valve configurations:

where

qFlow rate through the valve
pPressure differential across the valve
pA, pBGauge pressures at the block terminals
ppGage pressure at the pilot terminal
CDFlow discharge coefficient
A(p)Instantaneous orifice passage area
AmaxFully open valve passage area
AleakClosed valve leakage area
pcrackValve cracking pressure
pmaxPilot pressure to shift the control member to its maximum
pcrMinimum pressure for turbulent flow
RecrCritical Reynolds number
DHInstantaneous orifice hydraulic diameter
ρFluid density
νFluid kinematic viscosity

Connections A and B are hydraulic conserving ports associated with the inlet and the outlet of the valve. Connection X is the pilot port, which is a hydraulic conserving port that provides the pilot pressure. 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

  • Control member displacement is linearly proportional to pilot pressure.

  • No flow consumption is associated with the pilot chamber.

  • 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=Recr.

Dialog Box and Parameters

Valve type

Select the valve configuration: Normally closed valve or Normally open valve. The default is Normally closed valve.

Maximum passage area

Valve passage maximum cross-sectional area. The default value is 1e-4 m^2.

Cracking pressure

Pressure level at which the valve control member is forced off its seat and starts to either open or close the valve, depending on the valve type. The default value is 3e4 Pa.

Maximum control member displacement pressure

Pilot pressure at which the valve control member shifts to its maximum displacement and remains there until the pressure falls below this level. 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 simulation to fail. Therefore, MathWorks recommends that you do not set this parameter to 0. The default value is 1e-12 m^2.

Global Parameters

Parameters determined by the type of working fluid:

  • Fluid density

  • Fluid kinematic viscosity

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 that acts as the control port and provides the pilot pressure.

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