Flexible metallic hose is a thin-wall corrugated metal tube — usually 321 or 316L stainless — sheathed in one or two layers of wire braid, carrying steam, gas or cryogenic fluid where rigid pipe would crack. Unlike rubber or PTFE hose, it survives 600 °C and full-vacuum service without degrading. The corrugations flex axially, laterally and angularly to absorb thermal growth, pump vibration and misalignment between fixed equipment. A properly sized hose runs 20+ years on saturated steam at 150 psi where a rigid spool would fatigue out a flange in months.
Flexible Metallic Hose Interactive Calculator
Vary pressure, hose ID, braid angle, and braid layers to see pressure thrust and braid load in a corrugated metallic hose.
Equation Used
This calculator applies the pressure-thrust relationship described by the hose mechanism: internal pressure acts on the hose bore area, and the wire braid must carry that axial thrust. The braid-angle term resolves the axial load along the helical braid direction.
FIRGELLI Automations - Interactive Mechanism Calculators.
- Pressure thrust is based on the hose bore area.
- Braid angle is measured from the hose axis.
- Braid load is shared evenly by the selected number of braid layers.
- Use manufacturer ratings for final pressure, fatigue, and bend-radius approval.
How the Flexible Metallic Hose and Tubing Actually Works
The hose is two parts working together. The inner core is a thin-wall stainless tube, typically 0.20 to 0.40 mm wall, hydroformed into either annular (ring-shaped) corrugations or helical (continuous-spiral) corrugations. Each corrugation acts as a tiny accordion fold — when the line grows thermally or the pump shakes, the folds open and close instead of forcing the wall into bending stress. Around the core sits one or two layers of wire braid, usually 304 stainless, laid at a calibrated braid angle near 30 to 35° from the hose axis. The braid does not flex the hose. It restrains it from ballooning under pressure. Without that braid, a corrugated tube at 150 psi would extend like a Slinky and burst within seconds.
Why the corrugation geometry matters: the live length (the corrugated section between end fittings) sets how much movement the hose can absorb. A longer live length flexes more easily but droops under its own weight at temperature. The bend radius rule is firm — you must stay above the manufacturer's minimum static bend radius, and roughly 8x that figure for any dynamic flex application. Cross that line and the corrugation roots start work-hardening. You will see a single corrugation flatten out, then crack circumferentially within a few hundred cycles. We see this on poorly installed steam jacket hoses on jacketed reactors all the time — a hose routed in a tight S-bend instead of a relaxed U-bend fails in under a year.
Get the sizing wrong and the failure modes are predictable. Undersized ID throttles flow and drives velocity past the 200 ft/s wet-steam erosion limit, which thins the inner wall at the corrugation crests. Oversized ID gives sluggish drainage on horizontal runs and the hose pools condensate, hammering the braid every time a slug of water hits a bend. Wrong alloy — 304 instead of 321 on superheated steam over 425 °C — and you get carbide precipitation at the weld seam and intergranular cracking inside two years.
Key Components
- Corrugated Inner Core: The pressure-containing membrane, hydroformed from a longitudinally-welded thin-wall tube of 321, 316L or Inconel 625. Wall thickness sits between 0.20 and 0.40 mm depending on diameter. Corrugation pitch typically 4 to 8 mm and corrugation height around 15 to 25% of nominal ID.
- Wire Braid: One or two overlapping layers of 304 stainless wire — usually 0.30 to 0.45 mm strand — woven at a 30 to 35° helix angle. The braid carries the axial pressure thrust so the corrugations only have to flex, not contain hoop stress. Single braid handles roughly 70% of the rated pressure of double braid at the same diameter.
- End Fittings: Welded transitions from corrugated core into a rigid connection — flange, NPT, JIC, sanitary tri-clamp or weld-end. The braid is captured under a ferrule and TIG-welded simultaneously with the core. A poorly welded ferrule is the single most common origin of hose failure — listen for a hissing crack at the collar before any visible leak.
- Outer Cover (optional): Silicone-impregnated fibreglass sleeve or a stainless interlock armour for mechanical abrasion protection. On steam tracing lines run through walkways we specify interlock armour because operators routinely step on the hose.
- Liner (optional): A smooth-bore PTFE or stainless inner liner fitted inside the corrugations on high-velocity gas or sanitary service. The liner cuts the flow turbulence at the corrugation roots and stops product entrapment in pharma and food applications.
Industries That Rely on the Flexible Metallic Hose and Tubing
Flexible metallic hose shows up wherever rigid pipe cannot tolerate movement, vibration or extreme temperature. The use case is almost always the same: two pieces of fixed equipment must connect, but they grow, shake or settle relative to each other, and a rigid spool would fatigue and leak. Get the live length and bend radius right and the hose becomes the longest-lived component in the line.
- Steam Power: Boiler blowdown and sootblower supply lines on a Babcock & Wilcox FM-series industrial boiler — the hose absorbs the thermal growth between the drum and the blowdown manifold during cold startup.
- Cryogenics: Vacuum-jacketed transfer hose feeding liquid nitrogen from a Chart Industries bulk tank to a metallurgical Dewar at a heat-treat shop, holding -196 °C without frost on the outer jacket.
- Aerospace Ground Support: Hydraulic test stand jumpers between a Parker test cart and an aircraft wheel well at a Lockheed Martin C-130 depot, flexing as the gear is exercised under 3,000 psi.
- Pharmaceutical & Food: Clean-steam supply on a GEA jacketed kettle in a creamery — sanitary tri-clamp ends, smooth PTFE liner, surviving 12 SIP cycles per day at 134 °C.
- Chemical Process: Tank-truck loading arm jumpers on a Dow Chemical ethylene dichloride terminal, where the trailer settles 50 to 80 mm during loading and the hose must follow without tugging the loading arm.
- Heritage Steam: Auxiliary steam supply between the boiler manifold and the steam whistle on a preserved GWR Castle-class locomotive, isolating the whistle valve from boiler pulsation.
The Formula Behind the Flexible Metallic Hose and Tubing
The most useful sizing calculation is the live-length requirement for absorbing a known axial movement at a known cycle count. At the low end of typical service — small thermal growth on a short pipe run, maybe 5 mm total — almost any commercial hose handles it and the live length barely matters. At the nominal point — say 25 to 50 mm of axial movement on a long steam main — the live length must be sized properly or the hose fails by corrugation fatigue inside a year. At the high end — over 100 mm of movement, or any dynamic flexing application — you are usually outside what a single hose can do and need a pumping loop or a dedicated expansion joint. The sweet spot is between 10 and 60 mm absorbed movement on a hose 6 to 20 times the nominal ID in live length.
Variables
| Symbol | Meaning | Unit (SI) | Unit (Imperial) |
|---|---|---|---|
| Llive | Required live (corrugated) length of hose | mm | in |
| e | Total axial movement to be absorbed per cycle | mm | in |
| Ec | Corrugation stiffness modulus (manufacturer value, varies by corrugation pitch and wall thickness) | N/mm | lbf/in |
| fm | Movement factor (1.0 static, 0.25 to 0.5 for dynamic flex with cycle life > 10,000) | dimensionless | dimensionless |
| σallow | Allowable corrugation stress at operating temperature | N/mm | lbf/in |
Worked Example: Flexible Metallic Hose and Tubing in a brewery clean-steam jumper
You are sizing a flexible metallic hose jumper between a fixed clean-steam header and a movable jacketed mash tun at a craft brewery in Vermont. The header sits at 6 bar saturated (165 °C) and the tun shifts 35 mm axially during thermal cycling between cold start and full SIP. Nominal hose ID is DN50 (2 in), 321 stainless single-braid annular-corrugated. The hose sees roughly 600 SIP cycles per year and the brewery wants a 10-year service life — so 6,000 cycles minimum, which puts you in the dynamic-flex regime. You need to set the live length so the corrugation stress stays below the fatigue limit.
Given
- e = 35 mm
- Ec = 180 N/mm
- fm = 0.4 dimensionless (dynamic, 6,000+ cycles)
- σallow = 12 N/mm at 165 °C for 321 SS
Solution
Step 1 — at the nominal 35 mm movement, plug straight into the live-length equation:
Round up to a stocked length, so call it 1,400 mm of live corrugated section between fittings. That puts the hose at roughly 28× the nominal ID — comfortably inside the recommended 6 to 20× range when you account for the dynamic factor, and well above the static minimum of around 12× ID for this service.
Step 2 — at the low end of expected movement, only the cold-start swing of 15 mm:
That is a short, stiff jumper — about 11× ID. It would work, but it leaves no margin if the tun ever shifts more than designed (a foundation settle, a gasket replacement that changes the standoff). We do not specify the minimum unless the geometry forces it.
Step 3 — at the high end, if the brewery later upgrades to a larger tun with 60 mm of expected travel:
Now you are at 45× ID. The hose flexes easily, but at 165 °C and 6 bar a 2.25 m unsupported corrugated run will sag visibly under its own weight plus condensate, and the sag itself becomes a fatigue source at the ferrule. Above roughly 1,800 mm of live length on DN50 you must add an intermediate support saddle or switch to a pumping loop with two shorter hoses.
Result
Specify a 1,400 mm live-length DN50 321 stainless single-braid hose with sanitary tri-clamp ends. At nominal 35 mm movement that hose sees roughly 4.8 N/mm of corrugation stress per cycle, well below the 12 N/mm allowable, and should hit the 10-year target. The 15 mm low-end case would let you shrink to a 600 mm jumper but leaves no headroom for plant changes; the 60 mm high-end case forces you past 2 m of live length and into supported-hose territory. If your installed hose fails earlier than predicted, check three things in order: (1) a kink at installation — look for a single flattened corrugation near the inlet ferrule, that is a torsion install error not a fatigue failure; (2) condensate hammer — a hissing crack at the 6 o'clock position of the corrugation root means slugs of water are pounding the hose because the line lacks a steam trap upstream; (3) braid fretting — fine red-brown dust under the braid means the braid angle has migrated and the wires are sawing through the corrugation crests, almost always caused by torsion rather than pure axial movement.
When to Use a Flexible Metallic Hose and Tubing and When Not To
Flexible metallic hose is one of three common ways to handle movement in a piping system. The other two — rubber/composite hose and metallic expansion joints (bellows) — each win on specific dimensions. The right pick depends on temperature, cycle count, the type of motion, and how much axial thrust your anchors can take.
| Property | Flexible Metallic Hose | Rubber/Composite Hose | Metallic Bellows Expansion Joint |
|---|---|---|---|
| Maximum continuous temperature | 600 °C (321 SS), 800 °C (Inconel) | 120 °C (EPDM), 230 °C (PTFE) | 650 °C (321 SS bellows) |
| Pressure rating at DN50 | 20 to 40 bar (single/double braid) | 10 to 16 bar typical | 16 to 100 bar depending on ply count |
| Movement type best suited | Combined axial, lateral, angular | Lateral and angular, light axial | Pure axial (with tie rods for lateral) |
| Cycle life at rated movement | 10,000 to 100,000 cycles | 50,000+ cycles (limited by ageing) | 1,000 to 10,000 cycles |
| Anchor thrust load on piping | Low — braid carries pressure thrust | Low | Very high — full pressure × cross-section |
| Cost (DN50, 1 m) | USD 200 to 400 | USD 80 to 150 | USD 350 to 800 |
| Typical service life on saturated steam | 15 to 25 years | 3 to 7 years | 10 to 20 years |
| Failure mode | Corrugation fatigue cracking | Cover blistering, cover swelling | Bellows squirm or convolution cracking |
Frequently Asked Questions About Flexible Metallic Hose and Tubing
Yes — and it is the single most common installation error we see. A corrugated metal hose has effectively zero torsional strength. If the two end fittings are not rotationally aligned before you torque the flanges, pressurising the hose locks that twist into the corrugations permanently. You will see the braid pattern spiral instead of running parallel to the hose axis.
The fix is to align both fittings hand-tight, mark a chalk line down the braid, then torque both flanges while watching the chalk line stay straight. If your installed hose already shows a spiraled braid, replace it — it will fail at roughly 30% of the predicted cycle life because torsion concentrates strain at one corrugation root instead of distributing it across the live length.
Annular (ring) corrugation flexes more freely and drains better — pick it for steam, condensate, and any service where you want the hose to lie completely empty between cycles. Helical (spiral) corrugation handles higher pressure for the same wall thickness and gives smoother gas flow, but it acts like a shallow auger and traps a film of condensate on horizontal runs.
For a saturated-steam jumper between fixed equipment, always specify annular. We have replaced helical hoses on brewery clean-steam runs that pooled condensate at the low corrugation crests and corroded through inside 18 months despite being 316L.
No, and this is where most brewery and chemical-plant retrofits go wrong. Forcing the 800 mm hose to absorb the same 35 mm of movement triples the strain per corrugation. Your fatigue life drops from a calculated 10 years to roughly 14 months because corrugation fatigue scales with the cube of strain amplitude.
Two real options: route the hose in a U-loop or 90° offset to add live length within the available straight-line distance — a U-loop typically lets you fit 2.5× the straight-line length into the same envelope. Or move to a pumping loop with two shorter hoses sharing the movement. Do not undersize the live length and hope.
The decision hinges on operating pressure relative to the hose's burst rating, not on whether the application feels demanding. Single braid gives roughly 4:1 burst-to-working-pressure ratio. Double braid gives the same ratio but at roughly 50% higher working pressure for the same diameter.
Specify double braid when working pressure exceeds about 60% of the single-braid rating, when the hose sees pressure pulsation (reciprocating compressor discharge, for example), or when a hose burst would endanger personnel — steam over 8 bar in an occupied area is the classic case. For a static 6 bar clean-steam jumper at DN50, single braid is correct and the double braid is wasted money plus added stiffness that hurts cycle life.
That is fretting corrosion — the braid wires are micro-sliding against each other and against the corrugation crests, abrading the chromium oxide passive layer faster than it can re-form. The powder is fine iron oxide from the abraded steel.
Two causes worth checking: vibration excitation (a nearby pump running at a frequency that matches the hose's natural frequency, typically 15 to 60 Hz) or torsion in the install. The diagnostic is to bend the hose gently by hand — if you can hear a faint crunching from the braid, it is fretted and needs replacement. Add a mid-span support clamp or a vibration-dampening sleeve and the next hose will last full-life.
Only within strict limits. The corrugated core was not designed to carry weight along its axis — it was designed to flex. A vertical hose with the lower fitting unsupported drags the corrugations open, which extends them into the plastic range over time and you will measure the hose growing 5 to 10 mm longer per year until a corrugation root cracks.
Rule of thumb: a vertical static hose can support its own weight plus contained fluid, nothing more. If you need to support equipment, put a separate mechanical hanger on the equipment and let the hose carry only the fluid. On vertical clean-steam drops we always specify a sling under the lower flange that takes the equipment weight independently.
Static bend radius is the tightest you can install the hose and leave it there. Dynamic bend radius — usually 6 to 10 times larger — is the tightest the hose can be while it is actively flexing under load.
Thermal cycling is dynamic, even if it happens slowly. Each heat-up and cool-down is one full flex cycle, and over a 10-year service life you are looking at 3,000 to 6,000 cycles in process service. Use the dynamic bend radius. We see installers default to the static value because the hose is not visibly moving — and the failures show up at year three to four, exactly when the corrugation fatigue curve predicts.
References & Further Reading
- Wikipedia contributors. Hose. Wikipedia
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