Operation Principle of Check Valve
The simplest type of directional control valve is the non-return or check valve which allows flow in one direction and prevents flow in the reverse direction. Such a valve, its symbols and characteristic curve is shown in Figure 1.
Check valves are available with different spring rates to give particular cracking pressures. The cracking pressure is that at which the check valve just opens. If a specific cracking pressure is essential to the functioning of a circuit, it is usual to show a spring on the check valve symbol. The pressure drop over the check valve depends upon the flow rate; the higher the flow rate, the further the ball or poppet has to move off its seat and so the higher the spring force.
Ball-type check valves have the least expensive form of construction, but as the ball is not guided there is a tendency for leakage to occur. Although manufacturers claim their check valves are leak-free in one direction of flow and allow free flow in the reverse direction, a tiny scratch, wear mark ,or imperfection on the poppet or scat will permit some leakage. Soft-seated check valves use Delrin or similar polymer material for the waling and 100% scaling is possible but at the expense of valve life. However, they are no generally suitable for pressures above 200 bar or temperatures above 35°C. Valves which seal satisfactorily at high pressure may leak at lower pressures. At high pressure, the poppet is forced onto the seat hydraulically giving a good seal; at low pressures, the sealing force is less and the valve may leak.
Figure 1: Poppet-type check valve with symbols and curves.
Pilot-operated check valves
These are normally closed check valves which may be opened by a pilot signal or less commonly held closed by a pilot signal. The pilot pressure needed to open the check valve against a load pressure depends upon the ratio of the areas of the pilot piston and check valve. A pilot-operated check valve is shown in Figure 2 (a). Most manufacturers offer I a range of pilot ratios, i.e. if the pilot ratio is 4:1, the pilot pressure required to open the valve is 25% of the load pressure. A typical application is shown in the circuit in Figure 2 (b) where a pilot-operated check valve is used to lock in pressure to prevent a load from falling. With a long stroke cylinder the lowering motion of the load may be jerky. If the load overruns, the pressure in the full bore end of the cylinder drops, the check valve closes and the cylinder jerks to a stop. The pressure at the full bore end increases, the check opens, the cylinder lowers the load, the load overruns and so on. This problem can be overcome by using:
1. A meter-out flow control valve to limit the cylinder speed;
2. A counterbalance valve to prevent overrun; or
3. An over-center valve.
Figure 2: Pilot-operated check valve. (a) Section. (b) Application.
With the directional control valve in the mid-position and the load raised, there will be a tendency for the pilot-operated check valve to leak at low loads, since the hydraulic sealing force on the check valve poppet is also reduced. Zero leakage is possible by using soft-seated versions of the valve.
Figure 3: Vented pilot-operated check valve.
Pressure on the pilot port X of the pilot-operated check valve shown in Figure 2 has not only to overcome the closing pressure which is present at cylinder port C but is also sensitive to any back-pressure at the valve port V. This can be overcome as shown in Figure 3 by incorporating a seal on the pilot stem and a separate vent or drain connection D for the spring chamber. Any back-pressure at port V will assist the pilot to open the valve.
Prefill valves are basically large pilot-operated check valves. They are used in hydraulic press circuits to prefill the main cylinder with fluid whilst the press dies are being closed. The valve shown diagrammatically in Figure 4 is similar to a large pilot-operated check valve both in construction and operation but incorporates a decompression feature.
Figure 4: Prefill valve with decompression feature.
Hydraulic fluids are compressible to varying degrees and the volume of free fluid compressed into a cylinder is greater than its internal capacity. For example, in a cylinder having an internal volume of 0.3 m3 , approximately 0.31 m3 of a typical mineral oil hydraulic fluid at atmospheric pressure will compress into the cylinder at 400 bar. (This quantity will be much greater if the oil is aerated). Special valves have to be employed to control the decompression of large cylinders because the additional fluid (10 liters in this case) will attempt to discharge instantaneously resulting in extremely high shock forces.
The decompression feature incorporated in the prefill valve (Figure 4) is composed of small poppet built within the main poppet. When the valve is piloted open by a pat port X, the main poppet is initially held firmly on its seat by pressure within the cylinder .The first part of the movement operates the pilot poppet opening up a small flow path facilitating a controlled decompression. Further movement opens up the main poppet and the valve functions as a normal pilot-operated check.
Figure 5 is a press circuit which utilizes a prefill valve. Operating directional control valve. A to the crossover condition initiates the closing of the dies. The side cylinder B drives down the main ram C and fluid from the reservoir which is mounted above the press,is sucked in through the prefill valve D to charge the full bore end of C. As the dies close onto the work piece, pressure builds up opening the sequence valve E and flow from the pump pressurizes the full bore end of the main cylinder. During the pressing operation the prefill valve isolates the cylinder from the reservoir. On the retract stroke, (valve A in tramline condition) the prefill valve is piloted open and as the side cylinder pulls back the main ram, fluid from the full bore end is pushed into the reservoir. Using a prefill valve in this manner enables rapid movement of a large bore cylinder from a small delivery circuit.
Figure 5: Press circuit utilizing a prefill valve.
Pilot to close check valves
In the valve shown in Figure 6(a), application of sufficient pilot pressure at port X prevents flow through the check in either direction. At other times the valve performs as a normal check, valve with free flow one way (B to A) and blocked flow in the opposite direction (A to B). A representative application could be as a safety valve. In Figure 6(b) if pressure is lost in circuit number 1, circuit number 2 exhausts immediately.
Figure 6: Pilot to close check valve. (a) Section. (b) Application.
Both check valves and pilot-operated check valves are manufactured as single or double units in sandwich plates for mounting in a valve stack between the directional control valve and base plate.
Check valve are available with small holes through or bypassing the poppet to give a controlled leakage rate in the normally blocked direction. Such a valve may be pilot as a safety feature in tome circuits, or to give u pilot supply through the closed check to the downstream circuit.
The shuttle valve is a single-ball check valve with two alternative inputs A and B and one output C. It is used for load-sensing and will accept a signal from the higher of two pressure inputs.
A typical application is in a reversible brake motor circuit (Figure 7) where used to release the brake when the motor is driven in either direction.
A double-ball shuttle valve or back-to-back check valve is able to sense signals from different inputs but prevents pressure feedback or interaction from other circuits . Care has to be taken in its use as it is possible to 'lock in' a pressure signal on the output side.
Figure 7: Shuttle valve in reversible brake motor circuir.
Check Valve Articles
- Review Of Some Of The Check Valve's Models
- Parts Of A Check Valve
- How to Maintain a Check Valve
- Guides To Help You Select The Right Check Valve