Because they resist forces in opposite directions, and reinforcement designed for one does little for the other.

A pressure hose contains fluid pushing outward. Its braided or spiral wire carries that hoop stress in tension, which is what wire is good at.

A suction hose has a pump drawing fluid out, so pressure inside is below atmospheric and the atmosphere is pressing inward on the hose wall. Resisting that needs hoop stiffness in compression, which a braid does not provide - a braid can carry enormous tension and still let the hose flatten.

That is why suction hose carries a helical wire embedded in the wall, acting as a spring that holds the bore round. When specifying for a pump inlet, look for an explicit vacuum or suction rating rather than assuming a pressure rating covers it.

From flow rate against the permitted velocity for that line type, which is much lower than for a pressure line - so the bore comes out considerably larger.

The reason is that these lines have little pressure available to overcome friction. On a suction line the only driving force is atmospheric pressure pushing fluid into the pump, so any restriction directly starves it. On a return line, excessive velocity raises back pressure, which loads the system and adds heat.

Undersized suction lines cause cavitation - vapour bubbles forming at the pump inlet and collapsing violently inside the pump. That destroys the pump, and the hose that caused it usually looks fine.

Use the supplier's separate velocity figures for pressure, return and suction duty. Never size a suction line by matching the pump's port thread, and keep the run short with as few bends as possible.

It is usually a waste of money and sometimes does not solve the problem it was meant to.

A two-wire pressure hose on a return line costs several times as much per metre, is stiffer, has a larger bend radius, and is harder to route. None of that buys anything the return line needs, because the line does not see high pressure.

More importantly, if the line is a suction line the substitution may not help at all. Braid resists internal pressure; it does not stop the hose collapsing under vacuum. Unless the pressure hose happens to carry a helix, it can collapse just as the wrong return hose would.

The reverse substitution is genuinely dangerous. A low-pressure return hose fitted anywhere that sees system pressure will fail, and hydraulic fluid at pressure is an injection hazard. Match the hose to the line.

Because return fluid is the hottest in the circuit. Every inefficiency in the system - pressure drop across valves, work done against relief settings, friction in the pump and motors - ends up as heat in the fluid, and the return line carries all of it back to tank.

A hose selected on pressure alone can therefore be perfectly rated and still degrade quickly, because rubber ages faster the hotter it runs. Continuous operation near the top of a hose's temperature range shortens its life substantially rather than simply approaching a limit.

Check the return line's actual running temperature rather than the ambient, particularly on systems that work hard or run a high duty cycle, and select with margin.

It is also worth treating a persistently very hot return line as a symptom. It usually means the system is dumping more energy across a relief valve than it should be, and the hose is reporting a problem elsewhere.

Fittings suited to the hose construction and the bore, which for large low-pressure hose often means clamped or banded connections rather than the crimped fittings used on pressure hose.

On a large suction hose the connection frequently uses a barbed or shanked fitting secured with a heavy-duty clamp, sometimes two, which allows on-site assembly and lets the hose be removed and refitted during maintenance. Crimped fittings are also used and give the neatest, most reliable joint where the assembly does not need to be broken.

Whichever is used, the connection must seal against vacuum on a suction line, not just against pressure. A joint that holds fluid in will not necessarily keep air out, and air drawn in at a suction connection causes the same aeration and pump damage as a collapsed hose - while being much harder to find, because nothing leaks.

Check suction connections for air ingress when diagnosing a noisy pump.