We often hear about open or closed circuits, open center, hydrostatic or pressure compensated hydraulic systems, just to name a few. It can be difficult to differentiate between them as the specifics are sometimes subtile. There are really two main types of hydraulic system categories: open and closed systems.  As shown in the diagram below, from these types of circuits, other types of circuits follow. We are going to explain their particularities in a simple and accessible way so that you will be able to understand their functioning and their differences.

Types of hydraulic systems - H.P. Hydraulique
Types of hydraulic systems

1.0 Open circuit hydraulic system

An open circuit is when the fluid from the pump passes through the directional valves before returning to the reservoir. Thus, the pump provides a continuous flow to the hydraulic system. The speed and direction of the system is achieved by means of directional controls and flow control valves in the circuit. An open circuit is a unidirectional circuit, which means that the fluid can only flow in one direction.

There are two subcategories of open circuit hydraulic systems:

  • Open center distributors
  • Closed center distributors.

Be careful not to confuse them with open and closed circuits.

1.1 Circuit with open center distributor

  • The oil in an open circuit is always renewed as it flows back and forth from the tank to the circuit. 
  • This type of open circuit uses a fixed displacement pump (often gear driven), which continuously feeds the hydraulic valves, called “body valves”. If the hydraulic distributor(s) is not used, the oil returns directly to the tank. It is possible to have a variable displacement pump to provide the necessary flow to the hydraulic system, but usually a fixed displacement pump is used.
  • This is an inexpensive technological investment, but it consumes a lot of energy since the pump works continuously, not to mention the fact that the oil circulating continuously in the circuit tends to heat up.
  • The role of the pressure relief valve is to control the maximum pressure of the circuit. This limiter allows the excess oil that is not used by the distributors to be sent to the tank. The relief valve causes energy losses in the system which results in unnecessary power consumption (in HP/CV) and generates heat in the system. This effect occurs when a fixed displacement pump is used.

The advantage of using a variable displacement pump in an open center circuit is that only the flow used is generated by the pump. This has the effect of limiting the amount of oil returned to the reservoir by the “relief”. This results in energy savings and less heat generation. Contrary to the fixed flow pump which always provides the same flow in the circuit and that the surplus returns by the “relief”.

 Open circuit diagrams with open center distributor

Schéma d’un système hydraulique à circuit ouvert avec distributeur à centre ouvert – Avec une pompe fixe sans fonction utilisée - H.P. Hydraulique
Fixed pump - No function used
Schéma d’un système hydraulique à circuit ouvert avec distributeur à centre ouvert – Avec une pompe fixe et une fonction utilisée - H.P. Hydraulique
Fixed pump - One function used
Schéma d’un système hydraulique à circuit ouvert avec distributeur à centre ouvert – Avec une pompe variable sans fonction utilisée - H.P. Hydraulique
Variable pump - No function used
Schéma d’un système hydraulique à circuit ouvert avec distributeur à centre ouvert – Avec une pompe variable et une fonction utilisée - H.P. Hydraulique
Variable pump - One function used
Schéma d’un système hydraulique à circuit ouvert avec distributeur à centre ouvert – Avec une pompe variable et deux fonctions utilisées - H.P. Hydraulique
Pompe variable - Deux fonctions utilisées

1.2 Circuit with closed center distributor

  • In an open hydraulic system with a closed center valve, the oil passes through the body valve and returns to the tank only when a function is used.
  • This type of system is found when the flow rates are much higher. 
  • A variable displacement pump is usually used. Unlike the open center circuit, the pump adapts its displacement according to demand and will supply the body valves only when they are used. The oil does not return to the tank.
  • The circuit remains at a constant pressure as long as no function is used
    The valve is controlled by the pressure of the circuit.
  • Le distributeur est commandé par la pression du circuit.
  • This type of circuit allows the pump flow rate to be regulated according to the circuit pressure.

1.2.1 Circuit types with closed center distributor

Load sensing circuit

A load sensing system is a hydraulic system consisting of a pump that automatically adjusts its pressure and flow rate in order to deliver a constant pressure maintained between the inlet of the variable area valve and the supply of the receiver(s). A load signal is sent to the pump so that it maintains its displacement to obtain a constant pressure. 

This type of circuit saves energy since the actual power produced by the pump is very similar to the actual power required from the receivers.

Pressure compensated circuit

This type of system is much more energy intensive. A pressure compensated system maintains a constant pressure variation through the flow function to ensure a constant flow rate regardless of pressure demand. Therefore, flow and pressure are independent. In short, if the pressure change increases, the flow rate decreases and conversely, if the pressure change decreases, the flow rate increases.

Open circuit diagrams with closed center valves

Schéma d’un système hydraulique à circuit ouvert avec distributeur à centre fermé – Avec une pompe fixe sans fonction utilisée - H.P. Hydraulique
Fixed pump - No function used
Schéma d’un système hydraulique à circuit ouvert avec distributeur à centre fermé – Avec une pompe fixe et une fonction utilisée - H.P. Hydraulique
Fixed pump - One function used
Schéma d’un système hydraulique à circuit ouvert avec distributeur à centre fermé – Avec une pompe fixe et deux fonctions utilisées - H.P. Hydraulique
Fixed pump - Two functions used
Schéma d’un système hydraulique à circuit ouvert avec distributeur à centre fermé – Avec une pompe variable sans fonction utilisée - H.P. Hydraulique
Pressure compensated pump - No function used
Schéma d’un système hydraulique à circuit ouvert avec distributeur à centre fermé – Avec une pompe variable à pression compensée et une fonction utilisée - H.P. Hydraulique
Pressure compensated pump - One function used
Schéma d’un système hydraulique à circuit ouvert avec distributeur à centre fermé – Avec une pompe variable à détection de charge (load sensing) sans fonction utilisée - H.P. Hydraulique
Variable pump (load sensing) - No function used
Schéma d’un système hydraulique à circuit ouvert avec distributeur à centre fermé – Avec une pompe variable à détection de charge (load sensing) et une fonction utilisée - H.P. Hydraulique
Variable pump (load sensing) - One function used
Schéma d’un système hydraulique à circuit ouvert avec distributeur à centre fermé – Avec une pompe variable à détection de charge (load sensing) et deux fonctions utilisées - HP Hydraulique
Variable pump (load sensing) -Two functions used

2.0 Closed Circuit Hydraulic System - Hydrostatic Type

In this type of hydraulic circuit, the fluid from the piston pump flows directly to a motor and back to the pump without passing through the tank. The particularity of this type of circuit is its bidirectional operation. In fact, when the control lever is in the neutral position, there is no flow. When the lever is pushed forward, the pump will supply fluid to the motor and the motor will begin to turn.  Gradually, the further the lever is pushed, the more fluid flows and the faster the motor will turn. Conversely, if the lever is pulled backwards, in the opposite direction, the fluid will move in the opposite direction, reversing the direction of rotation of the motor. This type of circuit makes it easier to control the speed and direction of the motor and has the advantage of being generally compact, light and efficient. The fluid that circulates in the circuit does not return to the tank but goes directly to the pump, unlike an open circuit. The oil that is pumped is 100% used.

Diagrams of hydrostatic closed circuit hydraulic systems

Schéma d’un système hydraulique à circuit fermé hydrostatique et fluide circulant dans un sens - HP Hydraulique
Fluid flowing in one direction
Schéma d’un système hydraulique à circuit fermé hydrostatique et fluide circulant en sens inverse - HP Hydraulique
Fluid flowing in the opposite direction

If you need advice on the choice of circuit types and equipment or assistance with the installation and maintenance of your hydraulic system, please do not hesitate to contact us. Our team of experts will be happy to answer your needs.