The Hydraulic Blueprint: Decoding the Different Types of Fluid Circuits

Hydraulic systems serve as the muscle of the industrial system powering everything from heavy construction excavators to accurate manufacturing presses. However, it is how fluid - the 'blood' of the hydraulic system - is managed and directed that determines the overall hydraulic performance of the system. This fluid management system is called a hydraulic circuit. 

Learning the basic types of hydraulic circuits is the first step to understanding how one machine might be fast and efficient while another is slow but makes a lot of force.

There are three standard solutions engineers have developed to design hydraulic power flow. 

1. Open-loop hydraulic circuits: The basic flow-through system.

Open-loop circuits are the most typical and common form of hydraulic system. Think of it like a river flows through a machine to do work then drains back into a very large lake (the reservoir). 

How it works:

In an open-loop system, the pump inlet is always connected to the reservoir (tank). The pump takes fluid from the reservoir. The fluid is pressurized and sent to the actuator (cylinder) through control valves. When the work is done, the fluid is sent back to the reservoir before it is pumped back into the pump for the next cycle.

Key features:

• Pump type: Fixed displacement pump (pumps a constant volume of fluid per rotation). 
• Control: Flow and direction is controlled by directional control valves. 
• Suitable: Simple, repetitive, and/or low pressure applications. 
• Disadvantage: If the actuator is idle (not moving), the pump is still working. It is still pushing the fluid out and ultimately it will be dumped back to the reservoir, often over a relief valve. This result in heat and wasted energy.

2. Closed-Loop Hydraulic Circuits: The Constant Power Loop

The Closed-Loop Circuit (more commonly referred to as Hydrostatic Transmission in mobile equipment) essentially functions as a complete recycle loop system, similar to a closed water-cooling system in an engine. This emphasizes excellent control and superior efficiency.

How It Functions:

In a closed-loop system, the actuator's return line is connected back to the intake of the pump.Fluid exits the pump and goes to the motor/actuator.Once work has occurred, the fluid is returned to the intake side of the pump.A small, secondary Charge Pump (or a feed pump) operates continuously in this system to replace minor fluid losses and supply cool, filtered oil to the low-pressure side of the loop.

Typical Characteristics: 

• Pump Type: Typically uses a variable displacement pump (which can vary the volume of fluid it pushes).
• Control: Provides very accurate control over the speed and torque of the fluid power system and is bi-directional; it does not require a separate directional valve.
• Most Useful For: High performance mobile applications, such as in forklifts, compact loaders, and hydrostatic drives, that require smooth, continuous, and high-pressure power in both directions.
• The Benefit: The pump is directly connected to the actuator allowing for an immediate response, as the pump is moving liquid with little going back to the main tank of oil so it is more efficient and wastes less power.

3. Open- or Closed-Center Control Circuits: Valve Designs

Beyond open-loop and closed-loop (which describes how the pump and reservoir connect), hydraulic systems also can be defined by the Center of their Directional Control Valve (DCV).

A. Open Center Circuit (Usually used with open-loop systems)

In an Open Center circuit when the DCV is in it’s neutral (or the center) position the path of fluid flow is opened - the fluid just passes through the valve directly back to the tank.

• Idle State: The pump is at low pressure as they do not restrict the flow of liquid.
• Advantage: It allows the pump not to work against high pressure when no machines are running so little extra heat is generated.
• Challenge: If multiple valves are plumbed in series and one is actuated by the operator then all flow is dedicated to the actuated valve and potentially starving the others.

B. Closed-Center Circuit (Typically used with Variable Pumps)

In a Closed-Center circuit, when the DCV is neutral, all ports (P, A, B, T) are blocked, with no place for the fluid to go.

• Idle Position: System pressure rises quickly. A special Pressure Compensated Variable Pump senses the pressure rise, and automatically reduces flow to near zero to maintain a constant stand-by pressure.
• Benefit: You can connect multiple valves in parallel and when turned on, each actuator receives full system pressure immediately, thus providing a faster response time.
• Best Use: Applications requiring immediate power across multiple functions, for example, large mobile cranes and logging systems who may derate the pump when multiple functions are in use.