check valves (or two way valves)

CHECK VALVES (OR TWO WAY VALVES)

1. Introduction

• Check valves are the most commonly used and the simplest type of directional control valves.

• The check valve is a two-way valve because it contains two ports. Also a check valve is analogous to a diode in electric circuits.

• Functions: The check valves are used:

(i) to allow free flow in only one direction, and

(ii) to prevent any flow in the other direction.

• Since check valves block the reverse flow of the fluid, they are also known as non- return valves.

• The symbolic representation of a check valve, shown in Fig.6.4, illustrates its function clearly.

• 'Ports' or 'Ways' are nothing but openings present in a control valve. They refer flow path through the valve.

2. Types of Check Valves

Check valves are of several types. But the two important types of check valves are:

1. Poppet-type check valves, and

2. Pilot-operated type check valves.

3. Poppet-Type (or Simple) Check Valves

1. Construction and Operation

The construction and operation of a typical poppet type check valve is illustrated in Fig.6.5. Normally a spring holds the poppet in the closed position.

When flow is in the normal direction, the liquid pressure acts against the spring tension to hold the poppet offset the seat. When the liquid pressure overcomes the spring force, as shown in Fig.6.5(a), the valve allows the free flow. When flow stops, the spring seats the poppet and liquid cannot pass in the reverse direction.

Instead, if flow is attempted in the opposite direction as shown in Fig.6.5(b), the liquid pressure along with the spring force pushes the poppet in the closed position. Hence no flow is permitted in opposite direction.

2. Applications of Simple Check Valves

Usually poppet type check valves are used to provide the pilot pressure to operate larger valves.

4. Pilot-Operated Check Valves

The piloted operated check valve allows free fluid flow in one direction, but reversed flow. depends upon the pilot actuation. That means, this type check valve also allows the reverse flow, provided pilot pressure is applied at the pilot pressure port of the valve to overcome the spring force of the poppet.

1. Construction and Operation

The construction and operation of a typical pilot-operated check valve is illustrated in Fig.6.6.

First, the free flow in the normal direction from port A to port B is achieved in the usual manner. But the reverse flow is blocked as the fluid pressure pushes the poppet into the closed position.

In order to permit the fluid flow in the reverse direction i.e., from port B to port A, a pilot pressure is applied through the pilot pressure port. The pilot pressure pushes the pilot piston and the poppet down. Thus the fluid flow in the reverse direction is also obtained. The purpose of the drain port in the circuit is to prevent oil from creating a pressure buildup on the bottom of the pilot piston.

2. Graphical Symbol

Fig.6.7 shows the symbolic representation of a pilot-operated check valve. In Fig.6.7, the dashed line presents the pilot pressure line.

3. Applications of Pilot-Operated Check Valves

The pilot-operated check valves are widely used to hydraulically lock the cylinders such as in a hydraulic jack.