Principles of Volumetric Machines and Hydraulic Accumulators

Volumetric Machines and Energy Exchange

Volumetric machines are mechanical devices designed to achieve an energy exchange between a fluid and an axis of rotation. This energy exchange occurs intermittently in this type of machine. They can be classified as:

• Generating: These increase the energy of the fluid (e.g., pumps).
• Receptor: These extract energy from the fluid (e.g., motors).

Both motor and pump operating characteristic curves are as follows.

Types of Losses in Volumetric Systems

Volumetric machines consist of fixed and mobile parts in contact with a fluid and other fixed parts of the circuit. This produces a series of losses, including:

• Volumetric losses: These are caused by the presence of clearances between the fixed and mobile parts, plus a lack of sealing joints. Their yields are expressed as: EXPRESSION OF VOLUME LOSS.
• Hydraulic losses: These are caused by direct contact with the fluid. The formula for calculation is similar to losses in pipes. Yield: EXPRESSION OF HYDRAULIC LOSSES.
• Mechanical losses: These are generated by the friction of the various fixed and moving parts of the machines. Returns: [Expression].

Flow, Power, and Torque Expressions

Within volumetric machines, we can define the following expressions:

• Flow expressions: Three types can be defined:
  • In generating machines: input flow, leakage flow, and outflow.
  • In receptor machines: the same.
• Power expressions: For both generating and receiving machines.
• Torque (Par) expressions: For receptor machines and generators.

Hydraulic Accumulators in Oil Circuits

Accumulators are used in oil hydraulic circuits as extra fluid storage to utilize energy throughout the work cycle of the circuit. In a hydraulic circuit, they perform the following functions:

• Pressure reserve: Maintaining the pressure in the circuit.
• Dampening: Smoothing sudden changes of flow in the circuit.
• Protection: Absorption of pressure spikes to protect the circuit from damage.
• Flexibility: Improving the operation of an existing circuit.
• Energy recovery: They provide an economical way to recover energy in braking circuits.
• Other features: Auxiliary power source, thermal expansion compensator, leak compensator, etc.

Classification of Hydraulic Accumulators

There are three main types of accumulators:

• Weight: These use a mass to create resistance against the filling of an auxiliary shell. The pressure of the fluid remains constant as the vessel volume increases.
• Spring: A spring opposes the filling of the tank. The tank pressure increases linearly with the increase of the fluid.
• Gas: The effort is achieved by compressing a specific gas. The fluid pressure increases with the volume of oil but also depends on the type of gas.

Gas accumulators can be separated into two groups:

• Non-separated: The gas is in direct contact with the fluid.
• Separated: The gas is not in contact with the fluid.

Isothermal Processes in Accumulator Calculation

For the calculation expression of an accumulator, it should be noted that if the compression occurs over more than 3 minutes, the process is considered isothermal because there is sufficient time for the system to reach room temperature.