What is pressure intensification in hydraulic cylinders

Questions & Answers - Hydraulic Cylinders

Pressure intensification is an increase in pressure above the relief valve setting. When you use a hydraulic system, you think that the pressure is limited

by the relief valve; well? sometimes is not. If there are lines that are not connected to the relief valve in certain moments, they are not protected.

We are going to talk this time about the intensification of the pressure in hydraulic cylinders when the flow is controlled, meter-out.

The rod of the hydraulic cylinder is extending at a speed controlled by the meter-out flow control. In the graphic, the flow is coming from the pump to the back of the cylinder and the reading in the pressure gauge is the pressure relief valve setting. The flow control that is reducing the flow that exits the rod side of the cylinder creating a back pressure that is higher than the relief valve setting.

Looking at the forces acting in the cylinder piston, we can see that the force acting from the left side of the piston is equal to the force acting from the right side + the load. or:

A x P_in = A_r x P_out + Load

Where:

A= Area of the piston

Ar= Area annular (A - rod area)

P_in= Inlet pressure or relief valve pressure setting

P_out= Outlet pressure before the flow control.

The worst case scenario is when the load is insignificant or very low and the rod diameter is big compared to the piston diameter. Taking the load off the formula, we have that the force from the left is equal to the force from the right, or:

A x P_in = A_r x P_{out    so, Pout = Pin x A/Ar}

Knowing that A_r is always smaller than A, then P_out is always going to be higher than p_in. Now, the question is how much higher?

It is inversely proportional to A/Ar or in other words, the bigger the rod against the piston, the higher the pressure intensification.

Let's put an example:

A cylinder has:  4" piston diameter, 3" rod.

Then, A = 12.56 sq inch. A_r = 12.56 - 7.06 = 5.5 sq inch

If P_in = 2,000 psi, then Pout = 2000 x 12.56/5.5 = 4,567 psi a lot higher than 2,000 psi.

In the following video, the relief valve pressure setting is 85 psi. The intensified pressure goes up to 125 psi.

If the load is not neglected, then it lowers the intensification effect.

Whenever you have meter-out flow meters and low loads, you can expect pressure intensification.