How Can Pressure Loss Be Reduced in a Hydraulic System?

Pressure loss in a hydraulic circuit is the pressure difference created as oil flows through pipes, hoses, filters, valves, fittings and other components. Some pressure drop is unavoidable because every restriction creates resistance to flow. The engineering objective is not to eliminate every pressure difference, but to prevent unnecessary pressure loss that produces heat, wastes energy or reduces machine performance.

One distinction is especially important: pressure drop across a component is not the same problem as a hydraulic system that cannot reach its required working pressure. A filter losing 6 bar between its inlet and outlet is a measurable pressure drop. A machine that should reach 200 bar but stops at 140 bar may instead have a pump, valve, internal-leakage or pressure-setting problem.

Start by measuring pressure at more than one point

Pressure loss becomes much easier to understand when measurements are taken on both sides of the suspected restriction. The basic relationship is:

ΔP = Inlet pressure − Outlet pressure

If a filter has 170 bar at its inlet and 164 bar at its outlet, the pressure drop across that part of the circuit is approximately 6 bar. If the same filter produced only 2 bar when clean, the change is useful diagnostic information.

For occasional testing, mechanical gauges may be sufficient. On machines where pressure needs to be monitored continuously, appropriately selected hydraulic pressure sensors and transmitters can provide pressure data to a PLC, indicator or monitoring system.

Undersized hydraulic lines create avoidable losses

When the same flow is forced through a smaller internal diameter, fluid velocity increases. Higher velocity normally means greater friction loss through the line and greater sensitivity to fittings, bends and restrictions.

Line size should therefore be selected from more than the thread size on the pump or valve. Flow rate, line length, fluid viscosity, operating temperature and the purpose of the line all matter.

Suction, pressure and return lines should not automatically be designed to the same criteria. A restrictive suction line can be especially problematic because excessive inlet vacuum can contribute to poor pump filling, noise and cavitation.

Fittings and bends add up

A single elbow or adapter may contribute only a small part of the total pressure loss. A long circuit containing many elbows, quick couplings, reducers and undersized fittings can behave very differently.

The objective is not to remove every bend. Hydraulic equipment still has to be serviceable and physically fit inside the machine. The better approach is to avoid unnecessary restrictions, abrupt changes in flow area and components with insufficient flow capacity.

A filter can protect the system and still create excessive ΔP when restricted

Hydraulic filtration is essential for component life, but the filter element becomes more restrictive as contamination is collected. This can gradually increase pressure drop across the filter.

A heavily restricted filter can increase pump load, generate additional heat and affect available flow. Some filter assemblies include a bypass function that opens at a defined differential pressure; the actual behaviour depends on the filter design.

Removing filtration to eliminate pressure loss is not a solution. The correct approach is to use a filter sized for the required flow and monitor its condition appropriately.

Fluid viscosity changes the pressure-drop picture

Cold hydraulic oil is more viscous and can produce much higher pressure losses through pipework, valves and filters than the same fluid at normal operating temperature.

Very low viscosity creates a different problem: internal leakage in pumps and valves can increase. This is why simply choosing a thinner fluid to reduce pressure drop can create new reliability and efficiency problems.

Hydraulic fluid should remain within the viscosity and temperature range required by the pump, valves and other components. For the wider fluid-selection subject, see hydraulic fluid types and their applications.

Valves must be evaluated at the real flow rate

Directional, check, flow-control and proportional valves all produce some pressure difference while oil passes through them. The relevant question is whether that pressure drop is reasonable at the machine's actual flow.

A valve that is acceptable at 20 L/min may produce a much larger pressure drop when the flow is increased substantially. Nominal connection size alone does not guarantee that two valves have the same flow capacity.

Pressure loss across a restriction is also an energy loss. For example, if 40 L/min passes through a component with an unnecessary 15 bar pressure drop:

15 × 40 ÷ 600 = approximately 1 kW

of hydraulic power is being dissipated rather than converted into useful actuator work. Much of that power eventually becomes heat. The broader energy effect is covered in hydraulic system energy efficiency.

Internal leakage is different from line pressure loss

Worn clearances inside pumps, valves and cylinders can allow oil to pass from a high-pressure region to a lower-pressure region without appearing outside the machine. This is internal leakage.

The symptoms can resemble ordinary pressure loss: the machine may become slower, fail to hold a load or require more pump operation to achieve the same result. But the corrective action is different because the problem is inside a component rather than resistance in the piping.

This is why pressure should be checked at several points before parts are replaced. A systematic approach is described in hydraulic fault detection.

A low pressure reading does not automatically prove the pump is faulty

Hydraulic pumps primarily create flow. Working pressure develops as that flow encounters the machine load and circuit resistance.

A worn pump can certainly lose useful flow through internal leakage, particularly at higher pressure. But low system pressure can also result from a relief valve opening too early, internal valve leakage, cylinder leakage or another hydraulic fault.

Before condemning the pump, compare pressure and flow under known operating conditions. For the pump side of the circuit, see how a hydraulic pump works or visit the hydraulic pumps section.

Check the relief valve before increasing the pressure setting

If a relief or pressure-limiting valve begins opening below the intended setting, the machine may never reach its required working pressure. Continuous flow across the valve can also generate considerable heat.

Increasing the relief setting without diagnosis is not a safe troubleshooting method. Hoses, cylinders, pumps, manifolds and other components have maximum allowable pressures that must not be exceeded.

For the relationship between load, relief settings and control devices, see how hydraulic pressure is controlled.

Do not ignore return-line back pressure

The return line is often described casually as a “zero-pressure” line, but real systems can develop noticeable back pressure as oil passes through return filters, coolers, valves and pipework on the way to the reservoir.

Excessive return pressure can affect actuator performance and increase power loss. It may also place unwanted pressure on components that were not intended to operate with high return-line pressure.

Return filters, coolers and hose sizes should therefore be selected for the actual return flow rather than simply matching a convenient port size.

Pressure measurement can turn guessing into diagnosis

Suppose the pump outlet measures 200 bar, the valve inlet measures 198 bar and the valve outlet falls to 165 bar while the actuator is loaded. That pattern immediately provides more useful information than a single statement that “the machine has low pressure.”

The same principle applies to filters and coolers. Measuring upstream and downstream pressure can identify where significant resistance appears in the circuit.

For critical equipment, several pressure points can be monitored continuously. A transmitter does not repair the hydraulic fault; it makes the behaviour of the circuit visible.

A practical order for investigating hydraulic pressure loss

Check the symptom first: Is the problem low pressure everywhere, low pressure only at one actuator, slow movement or excessive heat?

Measure pump outlet pressure: Confirm what the power unit actually delivers under load.

Compare pressure before and after restrictions: Filters, valves, coolers and long hose runs are useful measurement points.

Check operating temperature: A cold system can have very different pressure drop from a machine at normal working temperature.

Verify relief and pressure-control settings: Do not assume the setting has remained unchanged.

Consider internal leakage: Pumps, valves and cylinders can lose flow internally without an external oil leak.

Review the history: Was the machine always like this, or has pressure performance gradually changed? A developing problem often points toward wear or contamination rather than an original design issue.

Maintenance can reduce pressure loss, but it cannot correct every design problem

Replacing a restricted filter, correcting fluid condition and repairing a leaking valve can restore lost performance. Regular maintenance therefore plays an important role in keeping pressure losses under control.

However, a circuit built with undersized lines or poorly matched components will not become efficient simply because the oil and filters are new. Design loss and maintenance-related deterioration should be treated as separate problems.

Routine inspection topics are covered under hydraulic system maintenance.

Pressure loss becomes an operating-cost problem on machines that run for long hours

The pump must supply the pressure required to overcome both the useful machine load and the unwanted resistance in the circuit. The second part produces no useful production work.

Reducing excessive pressure drop can therefore lower electrical demand and oil temperature, particularly on machines operating continuously or across several shifts.

If the aim is to determine whether the machine justifies a wider efficiency upgrade, continue with energy-saving potential in hydraulic systems.

Find the location of the loss before choosing the repair

When a hydraulic machine appears weak, the first reaction should not be to install a larger pump or increase the relief-valve setting. The more useful question is:

At which point in the circuit does the pressure stop behaving as expected?

Once that point is identified, pipe sizing, filtration, valve capacity, fluid condition, pump performance and internal leakage can be evaluated in a logical order.

Good hydraulic troubleshooting starts by locating the pressure change, not by guessing which component is expensive enough to be responsible.

Related hydraulic topics

Diagnostics and maintenance: Hydraulic fault detection · Hydraulic maintenance

Pumps and pressure control: Hydraulic pumps · Hydraulic pressure control

Measurement and efficiency: Hydraulic pressure sensors · Hydraulic energy efficiency