The 2-way proportional flow control valve can control, independent o’ pressure and temperature, oil flow specified by the electrical signal. The most important components are the housing (t). the proportional solenoid with inductive positional transducer (2), the measuring orifice (3). the pressure compensator (4) as well as the optionally installed non-return valve (5).
The oil flow setting is determined by an electrical signal (command signal) set at a potentiometer. In conjunction with the electronic control (e.g. amplifier, type VT 5010). this set signal results in a corresponding current and therefore a proportional stroke of the proportional solenoid (stroke-controlled solenoid) Correspondingly, the measunng orifice (3) is shifted downwards, thereby releasing an opening to flow The position of the measuring orifice is fed back by the inductive positional transducer. Any deviations from the command signal are corrected by the closed loop control. The pressure compensator maintains the pressure drop at the measuring orifice at a constant value The oil How is therefore independent of load Good design of the measuring orifice ensures a low temperature drift
The measuring orifice is closed when the command signal is 0 % The measuring orifice closes in the case of power failure or cable breakage at the electrical positional transducer.
Starting without jump is possible from zero signal. The measuring orifice can be opened and closed with a delay via two ramps in the electrical amplifier.
Free return flow from B to A is possible via the non-return valve (5).
This is another type of flow control valve shown in Figure 6.31.
It is made up of a ball with a through hole which is rotated inside a machined seat. The manner in which flow control is exercised can be understood better with the help of Figures 6.32(a) and (b).
From Figure 6.32(a), it can be seen how flow assists opening and opposes closing of the valve. Conversely, from Figure 6.32(b), the flow is seen to assist closing and oppose opening of the valve.
Figure 6.33 shows the balanced version of a ball valve. This valve uses two plugs and two seats with opposite flows resulting in very little dynamic reaction onto the actuator shaft, although at the expense of higher leakage.
This is another type of flow control valve. It consists of a large disk which is rotated inside a pipe, the restriction in flow being determined by the angle. Figure 6.30 shows a simple design of a butterfly valve.
The advantage with this valve is that it can be constructed to almost any size. These valves are widely used for controlling gas flow. But a major problem associated with these valves is the high amount of leakage in the shut-off position.
A flow control valve is a device used for adjusting or manipulating the flow rate of a liquid or a gas in a pipeline. The valve contains a flow passage or a port whose flow area can be varied. The role of a flow control valve in a hydraulic circuit is very important and its very location is critical to optimum system performance.
The basic function of a flow control valve is to reduce the rate of flow in its leg of a hydraulic circuit. One of the most important applications of flow control valves in hydraulic systems is in controlling the flow rate to cylinders and motors to regulate their speeds. Any reduction in flow will in turn, result in a speed reduction at the actuator. There are many different designs of valves used for controlling flow. Many of these designs have been developed to meet specific needs.
Some factors, which should be considered during the design stage of a flow control valve are:
• The maximum and minimum flow rates and the fluid density, which affect the size of the valve
• The corrosive property of the fluid, which determines the material of construction of the valve
• The pressure drop required across the valve
• The allowable leakage limit across the valve in its closed position
• The maximum amount of noise from the valve that can be tolerated
• The means of connecting the valve to the process i.e. screwed, flanged or butt welded.
Flow control valves are classified as:
Fixed or non-adjustable flow control valves represented symbolically as in Figures 6.28(aHd).
Adjustable flow control valves represented in hydraulic circuits as
Additionally they may also be classified as:
and pressure-compensated flow control valves represented as: