Flexible Metal Hoses: Braided and Corrugated Types, Ratings and Selection

Quick Answer

A flexible metal hose is a thin-walled corrugated stainless steel hose, usually covered with one or two layers of wire braid and fitted with flanges, threaded ends or unions. The corrugations give flexibility. The braid restrains the hose against the pressure thrust that would otherwise stretch it like a bellows, raises its pressure rating and protects the core. Most industrial hose has annular (ring) corrugations; helical hose drains more easily but tends to twist under pressure, and stripwound hose is not pressure-tight. Ratings are quoted at room temperature and multiplied by a temperature derating factor: 0.78 at 260 °C (500 °F) in one maker's table for stainless hose. Each hose has a static bend radius for a one-off installation bend and a larger dynamic bend radius for repeated flexing. Braided hose takes lateral offset, angular movement and vibration at right angles to its axis, but not axial movement or torsion; axial movement needs an expansion joint. For offset motion, the live length is L = √(6RY + Y²). Design standards include ISO 10380 and EN 14585. Flexible sprinkler hoses are approved to FM 1637 or listed to UL 2443, and FM 1637 limits them to 1.8 m (6 ft).

A typical hose enquiry reads: "DN 50 braided hose, 500 mm, flanged, stainless." It gives a size, a length and an end connection, but not the medium, the pressure at operating temperature or the movement. Those decide the braid, the length and whether a hose is the right component at all.

A common failure shows why: a braided hose at a pump cracks beside its end fitting within months. It was strong enough, but it was installed with a twist, in line with the vibration and bent hard at the fitting. This guide covers how metal hoses are built, rated, sized, installed and ordered.

How a Flexible Metal Hose Is Built

A hose assembly has three main parts:

• Corrugated hose: a thin-walled stainless steel tube formed into corrugations; it is the pressure boundary and gives the flexibility
• Braid: one or more layers of stainless steel wire woven over the hose
• End fittings: fixed or floating flanges, threaded nipples, unions or weld ends, attached to hose and braid together by welding, brazing or soldering

Options include an internal liner for high flow velocity and an outer guard against abrasion and over-bending.

Annular or Helical?

In an annular hose each corrugation is a closed ring, so internal pressure creates no twisting moment; that is why annular hose is the usual choice for industrial piping. In a helical hose the corrugation is a continuous spiral. It drains and cleans more easily, but it tends to twist under pressure.

What the Braid Does

Pressure acting on the corrugations tries to extend the hose like a bellows. One maker gives 1–2.5% elongation for unbraided hose at maximum working pressure, which is why unbraided hose must be free at one end or installed in a 180° loop. The braid:

• Carries the pressure thrust, so the hose does not stretch or push the piping apart the way an unrestrained bellows expansion joint does
• Raises the pressure rating: the unbraided version of a hose is rated at a small fraction of the braided one
• Protects the core from external damage

It has little effect on flexibility. A second braid helps only while braid strength governs; once the hoop strength of the corrugated hose governs, extra braid adds nothing.

One maker's standard-pitch annular stainless hose, rated at 21 °C (70 °F):

Ratings fall with size, since hoop stress rises with diameter at a given wall, and the second braid adds 50–60%, not 100%.

Pressure and Temperature Ratings

The catalogue figure is a maximum working pressure at room temperature; one maker sets it at 25% of the nominal design burst pressure, a 4:1 margin.

At higher temperatures, multiply the rating by the maker's derating factor. One maker's factors for stainless hose:

Factors differ between makers and alloys; use the table for the hose supplied, and check that the end-fitting attachment (welded, brazed or soldered) suits the temperature.

The same maker limits pulsating or shock pressure peaks to half the rated working pressure. It allows a proof test at up to 150% of rating with the hose straight, but limits field hydrotests of installed, bent or offset hoses to 120% of the room-temperature rating or 150% of the operating pressure, whichever is lower.

Bend Radius: Static and Dynamic

Bend radius is measured to the hose centreline, and every hose has two:

• Static (installation) bend radius: the tightest radius for a bend made once at installation, with only normal vibration afterwards
• Dynamic bend radius: for intermittent flexing, such as thermal movement, and the radius used in live-length formulas; for continuous lateral offset motion, one maker doubles it

In the table above, the dynamic radius is 2 to 2.3 times the static. Flexing tighter than the dynamic radius strains the corrugation crests on every cycle, and fatigue cracks follow.

Movement: What a Hose Can and Cannot Take

Live length L is the flexible length between the fittings, R the dynamic bend radius, Y the offset from the neutral position and θ the angle in degrees. If the movement goes both ways from neutral, use the total travel, 2Y. The largest offset a given live length can take is Y = √(9R² + L²) − 3R. Overall length adds the fittings; one maker also adds two nominal diameters of straight hose so that bending does not start at the weld.

Worked Example: DN 50 Hose at a Heat Exchanger Nozzle

A DN 50 (2 in) thermal oil line runs at 250 °C (482 °F) with a design pressure of 16 bar (232 psi). Each time the unit heats up, the exchanger nozzle moves 25 mm (about 1 in) sideways, at right angles to the hose. The hose is the DN 50 annular hose from the table above.

1. Pressure at temperature. The factor at 260 °C (500 °F) is 0.78, conservative for 250 °C.

• Single braid: 27.4 × 0.78 = 21.4 bar, above the 16 bar design pressure, so single braid will do
• Required room-temperature rating: 16 ÷ 0.78 = 20.5 bar
• At a 25 bar design pressure, single braid would fail and double braid would pass: 41.2 × 0.78 = 32.1 bar

2. Live length for intermittent offset. R = 351 mm (13.8 in), Y = 25 mm:

• L = √(6 × 351 × 25 + 25²) = √53,275 = 231 mm (9.1 in)
• Offset check: 25 mm is well below R/4 = 88 mm

3. What changes the answer

• Movement of 25 mm either side of neutral: use 2Y = 50 mm, so L = √(6 × 351 × 50 + 50²) = 328 mm (12.9 in)
• Continuous rather than intermittent motion: double R to 702 mm, so L = √(6 × 702 × 25 + 25²) = 325 mm
• The static radius used by mistake: L = √(6 × 175 × 25 + 25²) = 164 mm, about 29% too short, and the hose overflexes on every cycle

4. Overall length. Adding two nominal diameters (100 mm) gives 231 + 100 = 331 mm, plus the fittings. If the space allowed a 300 mm live length, the same hose could take up to √(9 × 351² + 300²) − 3 × 351 = 42 mm of offset.

Installation Rules

• No torsion. Bolt the fixed flange first, fit a floating flange or union at the other end, and hold threaded fittings with a second wrench. Makers warn that even a small twist sharply cuts hose life.
• One plane. Keep movement and vibration in the plane of the bend, at right angles to the hose axis.
• No bending at the fittings. Leave straight hose at each end and route the line with elbows; fit a guard where over-bending is likely.
• Support the pipe. Pipe weight must not hang on the hose or compress it, which bows the braid out and lets the hose buckle. Anchor at changes of direction and guide the pipe; see our guide to pipe supports and hangers.
• Planned offset only. Install any fixed offset so that no bend is tighter than the static bend radius; do not use a hose to pull misaligned flanges together.
• Pump connections. Fit the hose close to the pump with its axis at right angles to the main vibration, and use two hoses at 90° for vibration in more than one plane; hoses do not absorb axial vibration. See our guide to centrifugal pumps.
• Flow velocity. Fast flow over the corrugations can excite resonant vibration. One maker recommends a liner above about 46 m/s (150 ft/s) for gas or 23 m/s (75 ft/s) for liquid in braided hose, halving the limits for a 90° bend, and puts hose pressure drop at roughly three times that of comparable steel pipe.
• No quick-acting valves next to the hose, where they expose it to pressure surges.

Applications

Metal hose is used where rigid pipe cannot take movement, vibration or misalignment, or where temperature or the medium rules out rubber and plastic hose:

• Pump and compressor connections, to isolate vibration and take small misalignment
• Equipment nozzles that move with temperature: exchangers, vessels, turbines
• Steam, condensate, thermal oil, gas and vacuum lines, and burner connections
• Road and rail tanker loading and hygienic food and beverage plant
• Sprinkler drops in suspended ceilings

Flexible Sprinkler Hoses

A flexible sprinkler hose connects a sprinkler to the branch line in a suspended ceiling, clean room or duct. Each sprinkler can be placed to suit the ceiling layout without cutting rigid drops, and the hose is fixed to the ceiling with a bracket. Two documents apply:

• FM 1637 (FM Approvals examination standard, October 2022 edition): minimum rated working pressure 175 psi (12 bar); nominal sizes 3/4 to 1-1/2 in; maximum length 1.8 m (6 ft) including end connections; tests include 20,000 pressure cycles with the hose bent 90°, vacuum, friction loss, and hydrostatic strength at four times the rated pressure (700 psi minimum)
• UL 2443: flexible sprinkler hose with fittings for direct connection to sprinklers, in systems installed to NFPA 13, 13R and 13D

Friction loss is published as an equivalent length of 1 in Schedule 40 steel pipe (Hazen–Williams C = 120), and that length goes into the hydraulic calculation. The approval or listing also fixes the number of bends and the minimum bend radius; FM 1637 tests from one bend on hoses up to 0.6 m (2 ft) long to four bends on hoses of 1.2–1.8 m (4–6 ft). See also our guides to sprinkler system valves and industrial fire protection systems.

Standards

Hose or Expansion Joint?

Both absorb movement, but not the same movements:

• Braided hose takes lateral offset, angular movement and vibration, but not axial movement, and carries its own pressure thrust through the braid.
• Metal bellows expansion joints take axial, lateral and angular movement; tied, hinged and gimbal designs carry the pressure thrust themselves and take only lateral or angular movement. An unrestrained bellows passes its pressure thrust to the pipe anchors, which must be designed for it.
• Rubber expansion joints and vibration absorbers isolate pump and machinery vibration where the medium and temperature allow.

For the axial growth of a straight run, use an expansion joint or a loop; for an offset or vibrating connection, a hose. See our guide to expansion joints.

Ordering Checklist

• Size (DN) and type: annular or helical; stripwound only for non-pressure duties
• Medium, including chlorides or other corrosive components
• Pressure: design, operating and test; any pulsation or shock
• Temperature: minimum and maximum
• Movement: type, amount, direction and frequency (static, intermittent or continuous)
• Length: live or overall, and installed shape (straight, offset, 90° or 180° loop)
• Materials: hose (for example 316L stainless), braid (for example 304), fittings (carbon or stainless steel)
• Braid: none, single or double
• End fittings: fixed or floating flanges with standard and rating (see pipe flanges), threaded ends, unions or weld ends
• Options: liner, guard
• Approvals and documents: test certificates; FM or UL for sprinkler hose; gas or drinking-water approvals where required
• Quantity and delivery

Common Mistakes

1. Twisting the hose at installation. Torsion sharply shortens hose life, and failures start beside the fitting.

   Prevention: Bolt the fixed end first, use a floating flange or union, and hold fittings with a second wrench.

2. Using braided hose for axial movement. Compression bows the braid and the hose buckles; extension overloads the braid.

   Prevention: Use an expansion joint for axial movement.

3. Sizing live length on the static radius. In the worked example the hose comes out 29% short.

   Prevention: Use the dynamic radius for moving hose, doubled for continuous lateral offset motion.

4. Ignoring temperature derating. A hose rated 27.4 bar at 21 °C is good for 21.4 bar at 260 °C in one maker's table.

   Prevention: Multiply the room-temperature rating by the factor for the design temperature.

5. Bending at the end fitting. The weld and the first corrugations take all the strain.

   Prevention: Leave straight hose at each end and route the line with elbows.

6. Fitting a pump connector in line with the vibration. A hose does not absorb axial vibration.

   Prevention: Set the hose at right angles to the main vibration, with two hoses at 90° for several planes.

7. Hydrotesting the system with bent hoses in place. The system test pressure can exceed an installed hose's limit.

   Prevention: Check the maker's field-test limit, or isolate the hose during the system test.

8. Long or over-bent sprinkler drops. More length, bends or a tighter radius than listed void the hydraulic calculation and the approval.

   Prevention: Install to the listing, with the listed bracket and equivalent length.

Supply from Kasko Makine

Kasko Makine supplies flexible metal hoses and related products:

• Flexible metal hoses: metallic flexible line elements for steam, water, oil and gas systems with high pressure, temperature, vibration and mechanical stress; stripwound and corrugated types; hose in AISI 316L stainless steel, braid in AISI 304, fittings in carbon or stainless steel; DN 6 (1/4 in) to DN 300 (12 in); for heating and air-conditioning, chemical and petrochemical plants and food processing
• Braided metal hoses: metal hose cores with single- or multi-layer stainless steel wire braid for high-pressure, high-temperature and corrosive duties, including industrial gases, hydraulic and pneumatic systems, vacuum, steam and thermal fluids, and food and beverage plants; available with country-specific approvals, including for gas installations and drinking-water systems
• Flexible sprinkler hoses: stainless steel braided drops connecting sprinklers to branch lines in suspended and gypsum board ceilings, with mounting bracket, branch line nipple and sprinkler reducing nipple; DN 25 (1 in) hose in 316L with 304 braid; 1/2 in or 3/4 in sprinkler connections; lengths 500–2000 mm; FM approval listed for the braided models (FM 1637 covers assemblies up to 1.8 m including end connections; ask which models and lengths are approved)
• Vibration absorbers: rubber and elastomer absorbers and tuned dynamic absorbers for motors, pumps, compressors, HVAC equipment and machinery bases

Engineering support: Send the medium, design pressure and temperature, the movement (type, amount and frequency), the space available and the end connections. We will check braid, live length and installation against your data and quote the assemblies.

Send the hose size, medium, pressure, temperature, movement and end connections for a quotation — we reply within 24 hours. Reach us at info@kaskomakine.com or WhatsApp +90 (537) 521 1399.

Continue Reading

• Expansion Joints: Types, Materials and Applications
• Centrifugal Pumps: Types, Specifications and Selection
• Pipe Supports and Hangers: Types, Spacing and Selection
• Sprinkler System Valves: Alarm, Dry Pipe, Deluge and Pre-action
• Industrial Fire Protection Systems
• Stainless Steel Pipe: 304 vs 316 to ASTM A312

Frequently Asked Questions

Q: What is a flexible metal hose?
A: A flexible metal hose is a thin-walled corrugated stainless steel hose, usually covered with one or more layers of wire braid and fitted with flanges, threaded ends or unions. The corrugations provide flexibility and the braid carries the pressure thrust. Metal hoses absorb vibration, offset, angular movement and misalignment where rubber hose would not suit the temperature or the medium.

Q: What does the braid on a metal hose do?
A: Internal pressure tries to stretch a corrugated hose like a bellows. The wire braid restrains it, carrying the pressure thrust so the hose neither elongates nor pushes on the piping, and it also raises the pressure rating and protects the core. A second braid raises the rating further only while braid strength governs. The braid does little for flexibility and must not be compressed axially.

Q: How does temperature affect the pressure rating of a metal hose?
A: Metal hose ratings are given at room temperature and must be multiplied by a derating factor for the operating temperature. In one maker's table for stainless hose, the factor is 0.88 at 149 °C, 0.78 at 260 °C, 0.60 at 538 °C and 0.40 at 816 °C. Factors vary by maker and alloy, so use the table for the hose supplied.

Q: What is the difference between static and dynamic bend radius?
A: The static bend radius is the tightest centreline radius for a bend made once during installation, with only normal vibration afterwards. The dynamic bend radius is larger and applies when the hose flexes repeatedly, for example with thermal movement; it is the radius used in live-length formulas. For continuous lateral offset motion, one maker doubles it. In one catalogue, a DN 50 hose has radii of 175 mm and 351 mm.

Q: How do you calculate metal hose live length for offset motion?
A: For lateral offset at right angles to the hose axis, live length L is the square root of 6RY plus Y squared, where R is the dynamic bend radius and Y the offset. Use the total travel if the movement goes both ways from neutral. For a DN 50 hose with a 351 mm radius and 25 mm of offset, L is about 231 mm, plus straight hose and fittings.

Q: Can a braided metal hose take axial movement?
A: No. Axial compression bows the braid away from the hose so that it can buckle or squirm, and extension overloads the braid. Makers restrict axial movement to short lengths of unbraided annular hose. Install braided hose so that movement and vibration are at right angles to its axis, and use a bellows expansion joint where a pipe run grows axially with temperature.

Q: What standards apply to flexible sprinkler hoses?
A: Flexible sprinkler hoses are approved to FM Approvals Class 1637 or listed to UL 2443, and installed in sprinkler systems designed to NFPA 13, 13R or 13D. FM 1637 requires a minimum rated working pressure of 175 psi and limits assemblies to 6 ft, or 1.8 m, including end connections. Friction loss is published as an equivalent length of 1 inch Schedule 40 pipe for the hydraulic calculation.