Accumulator circuits (Applications of accumulators)

 As discussed in Section 9.1.2, accumulators are used as:

ACCUMULATOR CIRCUITS (APPLICATIONS OF ACCUMULATORS)

As discussed in Section 9.1.2, accumulators are used as:

(i) Leakage compensator,

(ii) Auxiliary power source,

(iii) Emergency power source,

(iv) Hydraulic shock absorber,

(v) Fluid make-up device,

(vi) Holding device, and

(vii) Lubricant dispenser,

Now, we shall explain the operation of some accumulator circuits.

1. Accumulator as Leakage Compensator

• The stored energy of the accumulator can be used to compensate any possible loss of energy due to internal or external leakage in a system. (usually, in situations when the system is pressurized but not in operation).

• This application is extremely helpful in circuits, such as are used for hydraulic presses, which require high pressure for long periods.

1. Circuit

Fig.9.9 illustrateş a basic hydraulic circuit employing a bladder-type accumulator as a leakage compensator. This circuit consists of a power unit (pump), a check valve, 4/2 DC valve, pressure switch, power cylinder, and an accumulator.

2. Operation

First operator places workpiece on the press and shifts handle of the 4/2 DC valve. Now the oil flows to blind end of cylinder and piston extends contacting the work. The pressure builds up and oil fills the accumulator. When maximum pressure is reached, the pressure switch stops the pump motor.

In these applications, the cylinder and piston arrangement is required to press the workpiece for a longer period of time. During this period, the internal and external leakage may reduce the cylinder pressure. The leakage oil is replaced with the oil from the accumulator. This leakage replacement of oil is carried for a longer period of time. The maximum length of time is determined by the volume of the accumulator and the rate of leakage in the cylinder.

Finally, when pressing cycle has been completed, the operator shifts the handle of the 4/2 DC valve to original position. Now the oil flows to the rod side of the piston and the piston is returned to starting position. Thus a cycle is completed.

The purpose of the check valve, in this circuit, is to prevent any reversal oil flow when the electric motor is stopped. The pressure relief valve is provided to protect against any overloads experienced by the actuators in the system.

3. Advantages

It may be noted that the power unit (the pump motor) operates only for a short period of time. That is, the power unit operates only when system pressure drops to the minimum pressure setting of the pressure switch. Thus, this arrangement saves electrical power and reduces heat in the system.

2. Accumulator as Auxiliary Power Source

• One of the most common uses of accumulators is to act as an auxiliary (or secondary) source of power. As we know, the electric motor or pump motor is the primary power source.

• In this application, the accumulator stores the oil during one portion of the work cycle and releases the oil during the remaining cycle. Thus the accumulator serves as a secondary power source.

1. Circuit

Fig.9.10 illustrateș a basic hydraulic circuit employing a gas-loaded type accumulator for serving as a secondary power source.

2. Operation

First operator places workpiece on slide-table and shifts handle of 4/2 DC valve. Now oil flows from the accumulator to blind end of slide cylinder. This extends the piston until slide table reaches end of stroke.

When the cylinder is in the fully extended position, the accumulator is charged with the oil by the pump. Then the operator shifts the handle of 4/2 DC valve for the retraction of the cylinder. Now the oil flows from the pump as well as from the accumulator to retract the cylinder quickly.

3. Advantage

The above application makes it possible to use a very small pump in conjunction with a large accumulator for producing the required performance.

3. Accumulator as Emergency Power Source

• In some hydraulic applications, it is necessary to retract the pistons of cylinders to their starting position (for safety reasons), even there may be an electrical power failure. In such applications, the accumulator can be used as an emergency power source to retract the piston of the cylinder.

• For example, consider a situation where a hydraulic system is operating a flood gate in a dam, and the electrical power fails. In such situation, the accumulator supplies the working fluid which operates the flood gate.

1. Circuit

Fig.9.11 illustrates how gas-loaded accumulator can be used as emergency source of power. This circuit essentially has a solenoid-operated 3/2 DC valve.

2. Operation

When operator depresses push button energizing solenoid of the 3/2 DC valve, oil flows to blind end of cylinder. At the same time, the oil also unseats check valve. So the oil under pressure flows to rod end of cylinder and into the accumulator. Now the accumulator charges the oil as the piston of cylinder extends.

When there is a power failure, the solenoid will deenergize. In the absence of solenoid- energy, the spring pressure forces the valve to shift to its spring-offset mode. Now the oil stored under pressure is forced from the bladder-type accumulator to the rod end of the cylinder. Thus the piston of the cylinder retracts to the starting position.

4. Accumulator as Hydraulic Shock Absorber

In many high-pressure hydraulic systems, the sudden stoppage or deceleration of a hydraulic fluid flowing at high velocity in pipelines can cause considerable damage to the piping. This hydraulic shock, also known as water hammer, may snap heavy pipes, loosen fittings and cause leaks. By installing an accumulator, this high-pressure pulsations or hydraulic shocks can be suppressed/absorbed.

1. Circuit

Fig.9.12 illustrates a basic hydraulic circuit employing a dead-weight type accumulator for serving as a hydraulic shock suppressor.

2. Operation

As shown in Fig.9.12, the accumulator is installed near the shut-off point in order to be more effective in quickly absorbing the shock wave. When the system demands to shut-off the supply suddenly, a 2/2 shut-off valve is used for the purpose.

When operator shifts handle of the 2/2 emergency shut-off valve, the fluid flow is stopped suddenly. This results in high-pressure pulsations or hydraulic shock. The pressure pulsation is blocked by check valve. The surges between the check valve and the shut-off valve is used to store the oil in accumulator and thus the pressure pulsations of the oil in the pipe line is absorbed.