A hydraulic or pneumatic line whines, overheats, or the actuator moves slower than expected. The pump and valve were sized correctly, but the piping between them is too small. Fluid velocity in a pipe is not a detail; it sets pressure drop, heat, and response time. Pipe sizing is where good pump sizing goes to waste.
Why velocity matters
Flow through a pipe equals velocity times cross-sectional area. For a given flow, a smaller pipe means higher velocity. High velocity causes friction, pressure drop, heat, erosion, and noise. In hydraulics, excessive return-line velocity also damages seals and causes aeration. In pneumatics, high velocity starves actuators under demand.
Recommended velocity ranges
Suction lines should be slow, often below 1 m/s, to avoid cavitation. Pressure lines run faster, around 2 to 5 m/s for hydraulics, but over 6 to 8 m/s generates heat and erosion. Return lines should stay below about 2 to 3 m/s. Pneumatic supply lines keep velocity low enough to limit pressure drop, often under 6 m/s at the point of use. These ranges are starting points; the system vendor’s tables refine them.
Pressure drop with length and bends
Friction grows with pipe length and with every elbow, valve, and fitting. A long, thin line with several 90-degree bends drops pressure more than a straight short one. When an actuator is slow, measure pressure at the actuator, not at the pump outlet. The difference is line loss. Undersized piping turns pump pressure into heat before it reaches the work.
Size for the flow, not the port
A common mistake is to match the pipe to the valve port size rather than to the required flow and velocity. The port is a minimum, not a target. If the run is long or has many bends, go one size larger. The cost of a larger pipe is small against the heat and energy lost in a small one. Oversize pressure lines when in doubt.
Suction line sizing
The suction line is the most critical. Keep it short, straight, and as large as practical. A restricted suction line causes cavitation, noisy pumps, and shortened pump life. Use flexible hose with adequate bend radius, and never put a small filter on the suction without checking its pressure drop. Cavitation often looks like a pump problem but is a suction problem.
Return lines and reservoir
Return lines should enter the reservoir below the oil level to avoid aeration, and be sized for low velocity. A fast return line sprays and mixes air into the oil. Slope or size the return so oil flows gently back. Aerated oil causes spongy actuators and heat. The return line is not a dump line.
Pneumatic line sizing
Compressed air is compressible, so line sizing for pneumatics focuses on pressure drop at peak flow. A small or long air line drops pressure when several valves fire together, making actuators slow. Size the supply header and drops for worst-case simultaneous flow. Add a regulator and FRL close to the machine. Air that arrives late is air that does no work.
A worked line sizing
A hydraulic cylinder moves slowly despite a pump rated for the flow. Checking pressure at the cylinder shows 10 bar less than at the pump. The 6 m run uses 12 mm tubing with four bends and a small adapter. Upsizing to 16 mm tubing and using sweep elbows drops the pressure loss to near zero, and the cylinder speeds up. The pump was never weak; the line was throttling it. This is a common, cheap fix.
Pipe material and roughness
Friction depends on internal roughness. Smooth drawn tubing has less loss than rusty or scaled pipe. Use clean, deburred tubing. A corroded pipe inside adds friction year by year. For long runs, account for roughness. Hose has more loss than rigid pipe; minimize length and use smooth-bore hose where pressure matters.
Heat from line loss
Every pressure drop across a restriction becomes heat. A system running hot often has no cooler problem; it has a throttling problem in the lines or a bad filter. Check the temperature rise across suspected restrictions. Hot oil ages faster and seals fail. Sizing lines generously keeps the system cool without a larger cooler.
Fittings and valves as restrictions
Even the correct pipe can be undone by a small fitting, a partially closed valve, or a clogged filter. When calculating line loss, include the equivalent length of every fitting. A ball valve left half-closed throttles more than the whole pipe. Inspect restrictions before assuming the pipe size is wrong.
Transient and peak flow
Size for peak simultaneous flow, not average. If two cylinders move at once, the line must deliver both. A line sized for one actuator starves the second. Look at the machine cycle and sum the worst-case flow. Average sizing leaves the machine slow at peak moments.
Checking with gauges
Install pressure gauges at the pump outlet, at the valve, and at the cylinder. Read them during the cycle. A large drop between points localizes the restriction. This is faster and cheaper than guessing. Record the readings after a redesign to confirm the fix.
Common mistakes
Matching pipe to the smallest port, using long small suction lines, forgetting bends and filters, sizing for average flow, and dumping the return above oil level are recurring errors. Pipe sizing is not cosmetic. Treat lines as part of the power transmission path, and the system will deliver what the pump promised.
Flexible hose vs rigid pipe
Hose absorbs vibration and allows movement, but it has a smaller flow area and more internal friction than rigid pipe. Use the minimum hose length needed, and specify hose for the required flow and pressure. A long, tight-bend hose on a vibrating pump adds restriction. Keep bends generous. When a machine has flexible connections, account for them in the pressure loss budget.
Pipe supports and installation
Support pipe securely so vibration does not fatigue fittings. Long unsupported lines whip and wear. Leave slack at moving ends but avoid kinking. A pipe that rattles eventually leaks. Proper supports also keep the line aligned, reducing stress on pumps and valves. Installation quality affects reliability, not just flow.
Scaling an existing system
When adding an actuator to an existing circuit, check whether the supply header can feed it. Tapping a small line near the end of a long run starves both the new and old actuator. Add the new take-off near the supply, and upsize the header if needed. Retrofitting often fails because the original line was sized for fewer devices.
Noise as a clue
A whining or hissing line is usually high velocity or cavitation. Listen at suction lines: a gurgling, noisy pump often means the suction is restricted or the oil is aerated. A high-pitched pressure line means the velocity is too high. These sounds tell you what to measure before tearing into components. Fix the sound, and performance often follows.
Finally, when a system feels weak or hot, look at the lines before replacing pumps or valves. Measure pressure at the work, listen for restrictions, and account for fittings and filters. A properly sized system is quiet, cool, and responsive. The pipe is the power path; sizing it generously is one of the cheapest performance upgrades available.
When a system feels weak or hot, look at the lines before replacing pumps or valves. Measure pressure at the work, listen for restrictions, account for fittings and filters. A properly sized system is quiet, cool, and responsive. The pipe is the power path; generous sizing is a cheap performance upgrade.
When a system feels weak or hot, look at the lines before replacing pumps. Measure pressure at the work, listen for restrictions, account for fittings and filters. A well-sized system is quiet, cool, responsive. The pipe is the power path; generous sizing is a cheap upgrade.
When a system feels weak or hot, check the lines before replacing pumps. Measure pressure at the work, listen for restrictions. A well-sized system is quiet, cool, responsive. The pipe is the power path; generous sizing is a cheap upgrade.
When a system feels weak or hot, check the lines before replacing pumps. Measure pressure at the work, listen for restrictions. A well-sized system is quiet, cool, responsive. The pipe is the power path; generous sizing is a cheap upgrade.
When a system feels weak, check the lines before replacing pumps. Measure pressure at the work. A well-sized system is quiet, cool. The pipe is the power path; generous sizing is a cheap upgrade.
When a system feels weak, check the lines. Measure pressure at the work. A well-sized system is quiet, cool. The pipe is the power path.
Bottom line
Size pipe by velocity and pressure drop, not by matching ports. Keep suction lines large and slow, pressure lines moderate, and return lines gentle. Account for length, bends, and fittings. An undersized line wastes pump pressure as heat and slows the machine. When an actuator underperforms, check the line pressure before blaming the pump or valve. Good piping is cheap insurance against a system that feels weak.
