Operation Principle of Flow Control Valve
It must be remembered that whenever a flow control valve is used in a system, there will always be some pressure drop and associated heat generation.
These are used to regulate the fluid rate to actuators and so give speed control. This is primarily achieved by varying the area of an orifice and flow characteristics of orifices play a major part in the design of hydraulic control devices. Flow through the control orifice is usually considered to be turbulent and the quantity of a fluid flowing can be given by
q = Kx ( δP)½
where q is the quantity flowing, x is the orifice area, δP is the pressure drop over the orifice, and K is a constant which may include such functions as the orifice characteristics, viscosity of the fluid and the Reynojds Number. An orifice is a sudden restriction in the flow path and may be fixed but is generally variable. Ideally it should be of zero length and sharp-edged in which case it will be insensitive to temperature (i.e. viscosity) changes in the fluid flowing. The flow through the orifice shown in Figure 1 will vary as the square root of the pressure drop and will be sensitive to viscosity changes. This type of orifice can be used to control flow rates if the pressure drop and fluid temperature is reasonably constant and minor variations in flow rate are acceptable.
Figure 1 Flow through a control orifice.
When precise speed control is required under varying load conditions it is necessary to maintain a constant pressure drop over the orifice. The relationship between flow and position of the adjusting device can be linear, logarithmic or specially contoured to follow a particular curve. The characteristics of a simple needle valve are shown in Figure 2. Generally, a non-return valve is incorporated enabling regulated flow in one direction with free flow in the reverse direction.
Figure 2 Characteristics of a simple needle valve
Three specialist forms of flow-control valves are now considered:
1. Deceleration valves
2. Viscosity or temperature-compensated valves
3. Pressure-compensated valves.
Deceleration valves
These are a throttle-type valve in which the throttle opening is controlled by a roller or roller lever. The valve may be either normally open or normally closed so that the flow and hence acceleration or deceleration may be controlled. A check valve and secondare throttle valve can be fitted. The former for free reverse flow; the latter to provide an adjustable minimum flow when the main throttle is closed. A sectional view through a deceleration valve is shown in Figure 3
Figure 3 Develeration valve.
In the circuit in Figure 4 a deceleration valve is used to retard a cylinder towards the end of its stroke. During the initial part of the stroke, speed is largely controlled by the restrictor A metering the flow leaving the cylinder with a small amount of flow through restrictor C. As the cam depresses the operating roller, the main spool B gradually closes the main flow path. Control over the final part of the stroke is by restrictor C. When the cylinder retracts, flow by-passes the deceleration valve through the check valve D.
Figure 4 Deceleration valve circuit.
Deceleration valves are most suitable for high-flow application and are generally not recommended for flows below 15 l/min.
Viscosity or temperature-compensated, flow-control valves
The viscosity of a hydraulic oil is dependent on the oil temperature; hence some valve manufacturers refer to temperature compensation and others to viscosity compensation. The simplest way to eliminate the effect of viscosity is to use a sharp-edged orifice, the flow through which is independent of viscosity.
In some designs of viscosity/temperature-compensated throttle valve the orifice aperture over which throttling of flow takes place consists of two adjacent flat plates – one fixed and one movable. A V-shaped notch in one of the plates is masked or unmasked as the movable plate is rotated relative to the fixed plate. The design of the throttle gives a sharp-edged orifice which makes the flow practically independent of viscosity and hence temperature, particularly at the higher flow rates. Problems can still occur at low flows (< 0.5 l/min) in which case a valve will function better with a low viscosity oil. Flow through these valves is load-dependent but this can be remedied by the addition of a pressure- compensating spool. A check valve is frequently incorporated to allow relatively unrestricted reverse flow.
An alternative method of temperature compensation favoured by some manufac-turers is to have part of the orifice adjusting mechanism made of a material with a high coefficient of thermal expansion. When the temperature of the fluid increases, a spindle in the mechanism lengthens thus reducing the control orifice opening.
Pressure-compensated flow-control valve
A pressure-compensating spool built into a flow control valve maintains a constant pressure drop across the metering orifice independent of changes in supply and load pressure.
Figure 5 Two-port pressure-compensated flow-control valve (with symbols), see text for explanation.
Figure 5 shows diagrammatically a two-port pressure-compensated flow-control valve together with its symbols. Flow rate is set by an adjustable metering orifice (1) which also be viscosity-compensated. In the unoperated condition, the compensating spool (2) is biassed fully open by the compensator spring (3). As soon as flows occurs, there will be a pressure drop across the valve and pressure upstream of the metering orifice tends to close the valve but this is opposed by the spring assisted by pressure from downstream of the metering orifice. The compensator spool adopts a balanced position with a consequen-tial pressure drop over the compensating orifice (4) formed by the partially closed spool. A rise in supply pressure tends to close the spool and the. increased pressure drop across the compensating orifice balances the increase in supply pressure. If the load pressure rises, the compensating orifice opens, again maintaining the pressure drop over the metering orifice at a set value. This pressure drop is usually 3-6 bar, dependent upon the size of the metering orifice. The total pressure drop across the valve is dependent upon the difference between supply and load pressure, but a minimum total pressure loss across the valve of 5-12 bar is normally required for the valve to function correctly. (Typical curves are shown in Figure 6) The damping orifice (5) stabilizes the compensator and prevents hunting as pressure fluctuates. A stroke limiter or anti-lunge device is sometimes fitted to the compensator spool to eliminate a flow surge which occurs when the circuit starts up. When there is no flow through the metering orifice, the pressure-compensating spool will be fully open and as soon as flow commences, there will be a pressure drop through the valve causing the compensator to lunge or jump. The stroke limiter is a movable end-stop which limits the travel of the compensating spool. This device, which has to be adjusted every time the setting of the flow control valve is changed, is used to position the compensating spool somewhere near its expected final location. However large variations in pressure can no longer be corrected.
Figure 6 Two-port pressure-compensated flow-control valve curves.
Pressure-compensated flow controls must be used when accurate speed control at varying supply or load pressures is required. The minimum regulated stable flow from a good-quality flow-control valve will be in the region of 0.1 l/min. In any precision flow-control valve application it is essential to have well-filtered fluid (better than 10 μm absolute) to promote efficient control and long life of the valve. The smaller the flow to be controlled, the finer the filtration necessary. Various types of valve adjusting mechanism are available - hand knob, lockable hand knob, lever, DC motor control etc. It must be remembered that whenever a flow-control valve is used in a system, there will always be some pressure drop and associated heat generation.