Calgary, Alberta, Canada dzyanis@engineer.com

Fluid Power Pro

Open/Closed center circuits

Summary:

  • Open/Closed center circuits – what does it mean?
  • Which one I use?
  • What can happen if I connect my system to wrong center circuit?

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There are two types of valve circuits can be used to control your system: “Open center circuit” and “Close center circuit”.

Open center circuits use pumps which supply a continuous flow pumps.

Open center circuit

Open center circuit

The flow is returned to tank through the control valve’s open center; that is, when the control valve is centered, it provides an open return path to tank and the fluid is not pumped to a high pressure.

So, if you use control valves with open center, it is better if your equipment supplied from HPU with fixed displacement pumps.

At closed center circuits, the pumps vary their flow rate, pumping very little hydraulic fluid until the operator actuates a valve.

Close center circuit

Close center circuit

The valve’s spool therefore doesn’t need an open center return path to tank.

So, if you use control valves with close center, it is better if your equipment supplied from HPU with variable displacement pump.

Some valves, like HAWE valves, have an option to work with both close center circuits and open center circuits.

HAWE valve inlet section

HAWE valve inlet section

Additional external valve (marked by red arrow at the picture) is required:

  • to remote blocking of the flow trough HAWE valve sections when variable displacement pump used (close center circuits)
  • to connect pump line with return line when fixed displacement pump is used (open center circuits) until any valve’s spool is activated.

If you use this HAWE option you can connect your system at any HPU type, just double check the type of circuit  you selected by the external control valve has to match with HPU control system type.

What can happen if I connect my system to wrong center circuit?

If you connect HPU with variable displacement pump to your system with open center valves, the flow will always circulate trough these valves and heat the oil and hydraulic components.

If you connect HPU with fixed displacement pump to your system with closed center valves, the flow will always circulate trough pressure relief valve at the HPU and heat the oil and hydraulic components at HPU.

Both these cases are decrease the life time of oil and hydraulic components (pumps, valves, etc.).

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