Lay flat hose pressure ratings explained: working, burst and why bigger bores rate lower

Lay flat hose is sold on a pressure rating, and that one figure is the most misread number on the datasheet. It changes with the bore, it means something different from the burst figure beside it, and on a real line it is often not the hose that sets the limit at all. Getting it right is the difference between a dewatering line that runs for a season and one that splits on the first morning.

/01Working pressure and burst pressure.

The ratio between the two is the safety factor. Our TPU dewatering hose and NBR discharge hose are rated at 3:1 — 21 bar working against 63 bar burst, and 10 bar against 30 bar. The TPU well riser hose works at 2.1 to 2.5:1, reflecting a steady, continuous duty. When comparing hoses from different sources, compare working pressures at the same bore; a higher burst figure with a lower safety factor is not a stronger hose.

/02Why bigger bores rate lower.

Pressure inside a hose tries to split it along its length. The force on the wall grows with the diameter, so for the same jacket a larger bore can hold less pressure — the relation engineers know from Barlow’s formula, where allowable pressure is proportional to wall strength divided by diameter. Makers offset this with heavier jackets and thicker walls in the larger sizes, which is why the rating steps down unevenly rather than in a straight line.

At the top of the range sits concrete placement: the high-pressure class of our TPU concrete hose is rated 85 bar at 3" to 4" and 75 bar at 5", with burst at 150 to 170 bar. That is a different product built for a different job, not a stronger version of a dewatering hose.

/03What pressure your line actually sees.

The hose has to carry the highest pressure anywhere on the line, and that is almost always at the pump discharge. It is the sum of three parts:

Worked example: 200 m³/h through 1 km of 6" TPU hose, discharging 20 m above the pump into an open tank. Friction is about 4.6 bar, lift about 2 bar, delivery nil — roughly 6.6 bar at the pump, comfortably inside the 14-bar rating of the 6" dewatering hose. The same flow through 4" hose would need over 30 bar for friction alone — above the 17-bar rating — so the bore, not the pump, is the problem.

/04Surge, heat and couplings.

Surge. When moving water is stopped suddenly — a valve snapped shut, a pump tripping — its momentum turns into a pressure spike that travels along the line. A flexible hose absorbs some of it, but on a long, full line the spike can still exceed the running pressure by a wide margin. Open and close valves slowly, start and stop pumps against a partly closed valve where possible, and leave headroom between the running pressure and the rating.

Heat. Ratings are given for normal ambient service. Our TPU hoses are rated for use from -50 °C to +80 °C and NBR from -20 °C to +80 °C; in the Gulf, a black hose lying empty in the sun gets far hotter than the air. Keep hoses full or shaded where you can, and ask for confirmation of the rating if the fluid itself is hot.

Couplings. A line is only as strong as its weakest joint. Banded small-bore couplings, large-bore lay flat couplings and pump or valve connections all have their own limits, and the lowest of them is the line’s limit. For high-pressure transfer lines use a coupling built for the duty — such as our high-pressure male and female coupling, each one leak-tested.

/05Testing a line before use.

Before a new line goes into service, fill it slowly, bleed the air out at the high points, and bring it up to pressure in steps while walking the line for leaks at couplings. Never test with air — compressed air stores energy that water does not, and a failure under air is violent. Test against the maker’s test pressure for the assembly, never towards the burst figure.