Pressure & Flow

Objectives

Many people think pressure is created by the pump. But this is not actually the case.

In this lesson we will differentiate the differences between pressure and flow, as well as examine how they are related to each other.

We will determine what is the actual cause of pressure, as well as what causes it to increase, and what causes it to decrease.
And we will break apart the various aspects of flow and examine the effects they have on a hydraulic actuator.
This isn't just theory! Effective hydraulic troubleshooters not only understand the difference between pressure and flow, but are able to use their understanding to diagnose problems faster.

Resistance to Flow

If we take a pump that moves one gallon per minute (1 gpm) and connect a one inch (1") diameter hose that is one foot (1') in length from its outlet, and we install a pressure gauge at the end of the hose, should we expect to see any noticeable pressure reading on the gauge?

1 Foot
1 gpm
Pump
1 Inch
Diameter
No, there is no pressure on the gauge. This is not because of a problem with the pump. If the pump is filling one gallon per minute, then the pump is working fine.

There was no pressure because there was no resistance to flow.
Key Concept:
The pump, or more accurately, the action of pumping, does not create nor determine the pressure value in the system.
1 Foot
1 gpm
Pump
1 Inch
Diameter
There is practically no pressure at the gauge.

Pressure is Caused By Resistance to Flow

Let's say that again, because it's so important: pressure is caused by resistance to flow.

So what resists flow? For starters, hoses or pipes that are too small resist flow.

Key Concept:
In a circuit where the fluid is moving, the pressure value indicated by a gauge is only ever the result of downstream resistance.
Pump
Motor
250 psi
125 psi
0 psi
1 gpm
1/4"
100'
Check out the pressure values on the gauges above to see how pressure is affected by downstream resistance.
Key Concept:
The gauge can only indicate back-pressure. This means the highest normal working pressure in a hydraulic system would be at the pump outlet.

Elbows in the pipe will also cause more resistance, creating more pressure near the pump.

Pump
Motor
3 psi
2 psi
1 psi
0 psi
1"
1"
12"
1 gpm
Elbow

A far the greater source of resistance to flow occurs as hydraulic fluid tries to flow through heavily loaded actuators.

Motor
A heavily loaded motor.
An example of a cylinder lifting a boom.

A heavily loaded actuator could be a hydraulic motor trying to move a heavy material along a conveyor belt in a mine, or a cylinder trying to lift the boom on a crane.

In very basic terms then, pressure is an indication of a force resisting the flow.

Determining Pressure Values

10,000 lb Brick
Assuming the only pressure is from the brick on the cylinder.

The exact system pressure that shows on a gauge is determined by the resistance of the entire system.

This includes fittings, hoses, valves, cylinders, motors, filters, etc. In this example, lets assume that the only resistance in the system is from the load to be lifted by the cylinder.

If the pump pushes fluid into a cylinder that is required to lift a 10,000 pound brick, how much pressure will there be?

In order to determine the pressure, we will also need to know the surface area of the piston.

If the piston is 10 square inches, then the system pressure gauge will read 1000 pounds per square inch (psi).

Pressure is developed in the system by resistance to flow, which is easy to calculate when weight and actuator surface areas are known.

P
F
A
Pressure = Force / Area
Weight of Load
Surface
Area
psi
Assuming that the only pressure in this system is coming from the 10,000 lb brick on top of the cylinder.
Whether it is intentional or unintentional, a restriction causes a rise in pressure.

Pressure also builds in systems whenever there is a pinch or blockage in the lines. This can be caused from a malfunction or from the setting on a restrictive valve such as a flow control or throttle valve.

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