Hi all. Ok if you have an orfice that takes 400 psi to go through it and a cylinder just after the orfice with a relief valve setting of 500; will a there be an available 500 psi worth of work at the cylinder or just 100 psi before the oil goes over relief valve??

Thank you.

You would have 100 psi available to do work.

The Cylinder will MOVE at the rate of FLOW the Orifice (FLOW CONTROL) is set for at any pressure it needs, up to, but not exceeding 500 PSI.

Orifices control FLOW and cause PRESSURE due to the Restriction to FLOW of a given FLOW and VISCOSITY.

They are flow restrictors and since your scenarion has the restriction to flow at 400 PSI, between it and the Pump, and the Relief Valve is set at 500 PSI all the oil the pump is producing is going to the Cylinder at the 400 PSI Pressure Drop required by the Restricted Flow of the Orifice.

Pressure after the Orifice is whatever the Cylinder requires to overcome its work load. Theoreticlly the cylinder will move at almost a steady rate, set by the Orifice, until the load it is pushing requires 500 PSI or more. At that point the Relief valve (Pressure Control) opens and sends all pump flow to tank to protect the circuit from excessive pressure damage.

If the Orifice can be adjusted and was set at a flow that caused a Gauge in the line between the Pump and the Orifice to read 400 PSI as stated, restricting flow more would cause pressure to increase but flow would to the Cylinder would decrease until the Restriction was adjusted to cause Upstream Pressure to try to go above the 500 PSI set on the Relief Valve. At that point some oil would go to the Cylinder and some across the Relief Valve. Further closing of the Orifice would reduce Flow to the Cylinder until it finally stops all the Pump Flow to it and forces it to Tank across the Relief valve at 500 PSI.

In all the classes over the years it seems it is more difficult to get across the point that Flow moves an actuator, Pressure gives it Force.

Thanks for the replies! The orfice is a valve, flow control and po check gauge on manifold. Most of the pressure drop is occuring going into the cyl. So Bud T. are you saying there is 500 psi available to the cylinder before it starts to slow down and some oil go over relief valve not just 100 psi? Orfice or pump flow does not change. I do know that if i meter out of the cyl on ext that I have all the 500 psi acting on the rod area of the cylinder if some oil is going over relief valve.

quote:

The orfice is a valve, flow control and po check gauge on manifold."
That's a wierd setup if I ever heard one.

Most of the pressure drop is occuring going into the cyl. So Bud T. are you saying there is 500 psi available to the cylinder before it starts to slow down and some oil go over relief valve not just 100 psi?

Yes. You have adjusted the Flow Control to cause a Back Pressure of 400 PSI while it is still passing all Pump Flow. The reason it is passing all pump flow is because the only other path for flow, the Relief Valve, will remain closed until you restrict flow enough that pressure raises above 500 PSI.

The Flow Control is a Pseudo Pressure Control that would maintain that pressure until the heat the restricted flow is making heated the oil and the viscosity dropped so that the unchanged orifice would pass more oil due to the oil being thinned out.

If you replaced the Flow Control with a Reducing Valve set at 100 PSI the cylinder would operate at 100 PSI, Full Flow until it met a resistance that required more than 100 PSI. at that point the Relief valve would open and send all pump flow to tank at 500 PSI.

That is because a Reducing Valve is Normally Open and allows full flow until pressure at its Outlet reaches its set pressure. At that point it closes and pressure at its Inlet will increase to the Relief Valve setting and send all flow to tank.

It's in the Book:
http://www.hydraulicspneumatics.com/200/eBooks/