Hydraulic pressure and flow: what each one really does

In simple terms

Think of a garden hose. The tap decides how much water comes out each second: that is flow. Put your thumb over the end and the water pushes harder against your thumb: that is pressure. Your thumb didn’t make more water, it made the water work harder to get out.

In a hydraulic machine:

• Flow sets the speed. The pump pushes out a fixed amount of oil every turn. Send 30 litres a minute into a cylinder and it moves at a speed set by that flow and the cylinder’s size. Try the flow slider: the speed changes, the pressure barely does.
• The load sets the pressure. The oil pushes only as hard as it needs to lift what’s on the cylinder. Try the load slider: the pressure changes, the speed doesn’t.
• A restriction adds pressure before it. Close the needle valve and the gauge before the valve climbs while the gauge after it stays where the load put it. The pump now pushes harder than the load needs. That extra push is wasted and turns into heat in the valve.
• No flow, no difference. Press Stop. The oil stops moving and both gauges read the same. A restriction only creates a pressure difference while oil is flowing through it.

The relief valve is the limit. If the pump would need more pressure than the relief setting to push the oil through, the relief opens, some oil goes back to the tank and the machine slows. That’s what happens with the “partly blocked line” fault, or if the relief is set too low for the load.

For engineers

Three relationships cover almost every pressure and flow question on a real machine:

The pump pressure is the sum of everything downstream: p(pump) = p(load) + Δp(hoses) + Δp(valves) + Δp(anything else in the way). On a healthy machine the load is most of it. If the pump gauge reads well above what the load should need, there is a restriction you are paying for in heat.

The orifice equation is worth remembering for its shape: the pressure drop goes with the square of the flow. Double the flow through the same valve and the drop goes up four times. That’s why a machine that runs fine at slow speed can overheat when the flow is turned up, and why undersized hoses and fittings matter much more at high flow.

The pressure drop also goes with the inverse fourth power of the orifice diameter (area squared). Halve the opening and the drop rises sixteen times. In the animation, 6 mm costs about 3.5 bar at 30 L/min; 3 mm would cost about 56 bar.

Every bar of drop is power lost: Δp × Q / 600 kW. A 20 bar drop at 60 L/min is 2 kW of heat added to the oil around the clock.

Deep dive

Fixed and variable restrictions. A needle valve is a deliberate restriction, used to set a speed in meter-in or meter-out circuits. Partly blocked lines are accidental ones: a collapsed hose liner, a ball valve left half shut, a suction strainer or return filter full of debris, a hose too small for an upgraded pump. They all behave the same way: extra upstream pressure, heat at the restriction, and, once the relief is reached, lost speed.

Finding a restriction on site. Measure pressure at test points along the line while the machine works. The pressure falls a little along each hose and steps down across each restriction. A large step between two points that should be close together locates the problem. An infrared thermometer helps too: a restriction losing a few kilowatts is noticeably hotter than the hose either side.

Why pressure-compensated flow controls exist. In the simple needle valve, the flow depends on the pressure difference across it, so a heavier load (less margin to the relief) changes the speed. A pressure-compensated flow control valve keeps the drop across its orifice constant with an internal spool, so the speed holds regardless of load. Load-sensing pumps (lesson 16) apply the same idea to the whole machine.

Laminar and turbulent flow. The orifice equation describes a short, sharp restriction where the flow is turbulent. Along a long, smooth hose at low speed the flow is laminar and the drop is proportional to flow and to oil viscosity instead. In practice hydraulic lines run in between, which is why hose sizing uses velocity limits: about 0.5 to 1.5 m/s in suction lines, 2 to 4 m/s in returns and 4 to 6 m/s in pressure lines.

In UAE conditions. A restriction’s heat adds to whatever the sun and the ambient air are already putting into the oil. A machine that copes with a 10 bar restriction in winter may run its oil past 70 °C in August.