Flexible Metal Hose

What a Flexible Metal Hose Is and What Is Being Decided

Three subassemblies make the product, and each one is a separate engineering decision hiding inside a single part number. The corrugated tube is the pressure boundary and the flexible element: a thin wall formed into convolutions so the tube can bend at the roots instead of straining a straight wall. The braid is the strength member: wire, typically 304 or 316 stainless, braided over the tube at a set pitch and captured at both fittings, because pressure acting on the corrugations tries to stretch the tube like an accordion and the wire, not the thin wall, holds it back. The end fittings are the connection and the torque path, and they decide how the assembly behaves at installation, because a fitting that transmits torque into the hose has already begun to destroy it. A buyer who specifies a bore and ignores the other two decisions has bought a spring with unknown ends.

What is actually being decided in this purchase is whether flexibility is needed at all, and if so, how much and in which direction. The assembly is bought as a system against a duty statement: media, minimum and maximum temperature, working and test pressure, the type and amplitude of motion, the cycle count expected over the service interval, and the connections at both ends. A hose chosen on bore alone fits the pipe and fails the duty; the pressure class of the corrugation, the braid package behind it, and the length computed for the movement are what make the difference between an assembly that lasts twenty years and one that cracks at the convolution peak in its first winter. Those numbers, not the catalog photograph, are the specification.

Against its neighbors, the flexible metal hose occupies deliberate ground, and the full comparison belongs in the selection chapter. An expansion joint, the bellows type built to EJMA practice, is the engineered machine for large axial movement, and it demands anchors and guides that the piping designer must provide. The industrial hose families in rubber and composite are the commodity answer for water, air, and mild chemical duty at temperatures elastomers tolerate. On the rigid side, the pipe fitting and the tube fitting terminate the hard piping between which everything flexible must live, and a flange with its gasket makes the joint where the flexible meets the fixed. The metal hose is the multidirectional connector: moderate movement in more than one plane at once, vibration damping, and full vacuum to medium pressure capability in corrosion resistant metal.

Four references organize the datasheet. EN ISO 10380 is the governing standard for corrugated metal hose assemblies: design, manufacture, and the proof testing every assembly must pass. The NAHAD industrial hose assembly guidelines carry the fabrication and documentation practice the better shops work to. ASME B31.3 governs the process piping the assembly connects into, and its chapter on metallic bellows expansion joints is the boundary reference for the neighbor product. EJMA remains the design bible for that neighbor, not for hose. A datasheet that quotes none of these is a price tag, not a specification, and a shop that cannot produce pressure test evidence per assembly is a fabricator, not a supplier.

The numbers that frame every later chapter are worth stating up front. Working pressure falls steeply with bore: a DN 25 assembly with a single braid carries about 40 bar in catalog families while a DN 100 of the same construction is down around 16 bar, with the maker's table governing each build. Temperature runs from cryogenic duty at minus 196 deg C, liquid nitrogen and LNG service with oxygen clean documentation, to about 550 deg C continuous in stainless steel, with pressure derate practice above 200 deg C. And bend radius, the most abused number in the family, wants 10 to 12 times the bore for moving installations and 5 to 7 times for static bends, with anything tighter spending cycle life the specification never agreed to.

How the Corrugated Tube and Braid Carry Pressure

The corrugation is a hinge. A thin stainless wall, typically a few tenths of a millimeter at small bores, is formed into rings or a spiral so that bending the tube opens and closes the corrugation roots instead of straining a straight cylinder wall: the tube achieves in metal the flexibility that elastomer hose gets from its material. Pressure, however, does not only push outward. Acting on the corrugated profile, it produces a longitudinal force that tries to open the accordion, and an unbraided corrugated tube under pressure elongates visibly and fails early at the seam or the convolution root. The braid exists to cancel that force: braided over the tube at a helical pitch and anchored into the fitting collars at both ends, it carries the longitudinal load in wire tension, and the thin corrugated wall is left to do the only job it is good at, sealing while it flexes.

Convolution architecture decides how the flexibility is delivered. Annular corrugations are a stack of independent rings: each convolution bends locally, fatigue accumulates evenly, and the construction gives the best flexibility per unit length and the best cycle life, which is why it is the standard form for small and medium diameters and the default across catalogs. Helical corrugations run as one continuous spiral: the tube can be formed in longer continuous lengths, drainage behaves better in some orientations, and the construction suits larger bores and the highest flexibility demand. The choice is not aesthetic; it changes the fatigue map of the assembly, and the selection chapter returns to it when the motion statement is on the table.

The braid package is quantified, not described. Single braid, one layer of wire at the designed pitch and coverage, is the catalog standard and carries the published working pressure of the assembly. A second braid layer over the first raises the allowable working pressure of the same tube substantially, at the cost of stiffness, minimum bend radius, and price, and double braid is the standard answer when the pressure table says the single layer is not enough. Braid wire follows the chemistry of the tube, 304 for standard duty and 316 grades where chlorides are in play, because a stainless tube behind a corroding braid is a delayed failure either way. Braid also ends the torque question: it is the reason the installation chapter forbids twisting the assembly, since a twisted braid unloads half its wires and hands the load to the rest.

Pressure against bore is the family pattern worth memorizing. Catalog working pressure falls as the corrugated bore grows, because the same pressure acts on more area and the convolution profile cannot be scaled for free: about 40 bar and up at DN 25 in single braid families, mid twenties at DN 50, and about 16 bar class at DN 100, with double braid lifting each figure where the construction allows. Burst capability in this industry family runs to roughly four times working pressure, and the proof test that every assembly should carry is hydrostatic at 1.5 times working pressure. Those ratios are family patterns that the maker's table for the exact construction governs, and the RFQ should demand that table, not the pattern.

Temperature moves all the numbers. On the cold end, properly cleaned and documented assemblies carry liquid nitrogen and LNG at minus 196 deg C, and oxygen clean certification is its own paperwork for oxygen service. On the hot end, stainless constructions run to about 550 deg C continuous with short excursions above, and the derate practice across the family is roughly 10 percent of working pressure per 50 deg C above 200 deg C, stated as practice rather than as a table because the maker's derate curve governs. The honest specification statement is the working pressure at the operating temperature, never the catalog pressure at ambient with a silent hope attached.

Erosion is the failure mode that high velocity gas writes on the inside of the tube. Dry gas moving fast across the corrugation lands wears the convolution root, characteristically on the side opposite the seam weld where the flow field reattaches, and a smooth new assembly thins quietly until it weeps. The engineering answers are two: fit an interlock metal liner inside the bore to carry the velocity, or size the bore up until the velocity falls. The working threshold in practice is around 20 to 30 m/s of gas velocity, above which the liner stops being an option and becomes the specification, and abrasive solids duty asks for the same liner for the same reason. The velocity figure belongs in the RFQ, because no supplier can guess it.

Types, Construction, and Duty Boundaries

Tube materials sort by chemistry and temperature. Type 321 and 316L stainless steel are the catalog standards, with 316L the default wherever chlorides or cleaner service argue for the molybdenum grade. The special alloys exist for the services that eat stainless: Inconel 625 for high temperature strength and aggressive chemistry, and alloy C276 where the chloride and acid combination rules out everything cheaper. The honest material decision runs the whole environment, inside and out, because a coastal installation with a hot chloride bearing process can attack the tube from both sides and the 304 that saved money at purchase spends it at replacement, a failure the maintenance chapter returns to with numbers.

End fittings sort by connection and by how they are joined. The welded nipple end, a stub of pipe welded to the collar, becomes part of the permanent welding and suits the highest integrity joints. Threaded and swivel ends serve maintenance and mixed hardware, with the swivel preventing installation torque from reaching the corrugations. Flanged ends make the joint with a gasket, and on steel flanges the spiral wound gasket with inner ring is the same correct answer it is everywhere else in the plant; the flange at the hose end is torqued as a flange, with the wrenches on the hexes, never through the hose. Cam and groove ends serve loading and transfer duty, and sanitary clamp ends with polished bores serve food and pharma transfer where cleanability is the specification. Every welded collar is a pressure boundary weld, and porosity there is the leak the pressure test exists to catch.

Inside and outside, the accessories decide survival. The interlock metal liner, a spiral wound strip formed into a smooth bore, protects the corrugation roots from gas velocity and abrasive solids and reduces pressure loss at the same time, at some cost in minimum bend radius. External protection runs from a braid overbraid sleeve against abrasion, to guards where impact and foot traffic are real, to insulating wraps on hot service that must not block the movement the assembly was bought for. Chafing against structure is the most common external killer, and a sleeve at the rubbing point is cheaper than the outage that follows a worn braid.

Media duties carry their own rules. Steam service wants layout discipline: keep the assembly out of trapping dead legs where condensate can collect and hammer, let the steam trap stations do their work, and route any heat tracing cable clear of the braid so it neither binds the movement nor overheats the wall. Hot oil duty is kind to stainless but punishes under-documented welds. Cryogenic transfer demands the oxygen clean or cryo grade documentation and attention to icing loads on long vertical runs. Chemical duty is a material selection exercise through the whole wetted path, tube, liner, and gasket at the flanged end included. Water and air duty is where the family begins, and where the selection chapter keeps asking whether an industrial hose would not do the job for less.

The construction map below consolidates the build options into one table a specifier can scan before writing the RFQ.

Key Specification Parameters

The table below decodes the datasheet lines that decide whether a quoted assembly matches the duty. Each row is purchase defining; the paragraphs after the table explain what to demand and what a weak answer looks like.

The bend radius row deserves its own paragraph because it is where most assemblies are killed at installation, not at purchase. The family rule is 10 to 12 times the bore for a dynamic installation, one that moves or vibrates, and 5 to 7 times for a static bend that is set once and never cycles. A tighter radius does not merely look brave: it concentrates every cycle of movement into the outer corrugations, and fatigue life that the movement chart spread over the whole assembly is spent by the inch. The companion rule is the installation plane, because a hose installed straight and then cycled sideways in lateral offset loses cycle life at a steep rate, while the same assembly installed with a gentle 90 degree offset bend cycles in shear across many convolutions and lives its charted life. Bend radius and plane are specification inputs, and they belong in the RFQ movement statement.

The build options table compresses the pressure and convolution decision into the form a specification meeting can argue from.

Beyond the tables, the datasheet must state the evidence chain. The maximum allowable working pressure at the operating temperature, with the derate basis named, not the ambient catalog figure. The cycle life basis: how many cycles of what amplitude the rating assumes, because a movement chart with no duty cycle behind it is a drawing, not data. The weld procedure and examination at the collars, the material certificates to EN 10204 3.1 on tube and braid, the cleanliness statement or oxygen clean certificate where the service needs one, and the pressure test record per assembly, hydrostatic at 1.5 times working pressure or the low pressure pneumatic leak test where hydro is inappropriate, with helium testing available for high vacuum and gas service. A quotation that arrives without per assembly test evidence is asking the plant to do the shop's proof testing for free.

Selection Decision Factors and RFQ Language

The decision sequence is short enough to run as a checklist in a specification meeting, and each step eliminates a class of quotations before price comparison starts.

1. Write the media statement honestly: chemistry inside and atmosphere outside, gas or liquid, solids loading, and the actual velocity at the operating temperature.
2. Fix the temperature window with excursions, not just design points, because the derate practice above 200 deg C and the cryogenic documentation both hang off the extremes.
3. Fix working and test pressure at temperature, then let the pressure table choose the braid count instead of hoping a single layer will do.
4. Write the motion statement: type of movement, amplitude, plane, and cycles per year, because length and installation geometry are computed from it.
5. Check the bore against velocity before anything else, and write the liner into the specification when gas runs above roughly 20 to 30 m/s.
6. Choose convolution and braid against the pressure and cycle tables, annular as the default, helical where continuous length or drainage argues.
7. Choose the ends and their orientation, and compute length from the offset and travel allowances in the maker's practice, never by stretching the assembly to reach.
8. Fix the verification: per assembly pressure test at 1.5 times working, EN 10204 3.1 certificates, and helium or oxygen clean documentation where the service demands.

The boundary against the neighbors belongs here, stated once and clearly. For engineered large axial movement, the bellows expansion joint is the correct machine: it carries design axial travel with anchors, guides, and pressure thrust into the piping structure, per EJMA practice and the ASME B31.3 bellows chapter, and pretending a hose is one just moves the anchor design into a part that never had it. The flexible metal hose is the multidirectional vibration connector, the short run that absorbs pump shake and thermal growth and settlement across a few hundred millimeters of movement while remaining a pressure rated part. The industrial hose families in rubber and thermoplastic composite are the economical answer for commodity water, air, and mild chemical duty at temperatures elastomers tolerate, and specifying metal where a composite would serve is money spent on nothing. Small instrument connections make the same point from the other side: a needle valve and its gauge hang on rigid tubing and a tube fitting, because instrument signal lines want stiffness, and only the DN 6 metal assembly connects the piece that actually moves. Rigid piping flanks the assembly with a pipe fitting at each end, and a flange with a spiral wound gasket behind its inner ring closes the joint where the flexible meets the fixed.

Installation geometry is a selection output, and the pump discharge rule is the one to remember. The discharge of a centrifugal pump shakes in a mixture of planes, and the assembly that lives longest is installed with its length running at roughly 90 degrees to the pipe axis it serves, so the vibration is taken as lateral offset across the corrugations instead of as axial pumping along them. The same logic drives the length calculation: the maker's formulas take the required offset and travel and return the installed length with allowance, and an assembly stretched to reach its last centimeter, or compressed like a spring to take up slack, has consumed the movement margin before day one. On loading arms and transfer duty the geometry follows the reach envelope, and the guard and sleeve package follows the walkway reality around it. Station design completes the geometry: a check valve and an isolation ball valve sit at the pump side, a Y strainer shields the corrugations through commissioning, a gate valve or a butterfly valve carries utility bore isolation further out, and a solenoid valve gives the skid its remote shutoff, all placed so the connector can be broken into under a defined boundary. On monitored loops the pressure transmitter and the temperature transmitter read from the rigid runs flanking the connector, because the control system should treat the hose as pipe, and the hose was never designed as an instrument mount.

The application map below turns the rules into a table the project file can carry.

The RFQ block should be written once and reused. A complete block reads: media with chemistry and velocity; minimum and maximum temperature with excursions; working pressure and required test pressure at temperature; nominal bore; convolution type and braid layers; tube and braid materials with EN 10204 3.1 certificates; end fittings with type, standard, and orientation; installed length with the offset and travel basis stated; the motion and cycle specification; liner requirement where gas velocity or solids demand it; and the test regime, hydrostatic at 1.5 times working per assembly, pneumatic or helium where the service requires, oxygen clean where oxygen requires. A quotation returned without per assembly test evidence or the pressure table for the exact build is incomplete at any price.

Installation, Standards, Failure Modes, and Maintenance

Installation quality decides whether the assembly ever reaches its charted life, and the first rule is torque. The hose must never be twisted or used as the wrench reaction: fitting up flanged or threaded ends means two wrenches on the fitting hexes, one making the joint and one holding, so the torque stops at the collar and never travels down the braid. The second rule is geometry: do not stretch the assembly to reach, do not compress it to take up slack, and keep the installed bend radius inside the 10 to 12 times dynamic or 5 to 7 times static figure for the build. Support long vertical runs with pipe clamps that carry weight without pinning movement, guard the outside world from the assembly and the assembly from the outside world, and where an insulating wrap goes on hot service, make sure it never binds the corrugations it was bought to free.

Station practice protects the assembly from its neighbors and vice versa. Isolation sits at both ends, a ball valve at small and medium bore or a gate valve and butterfly valve at utility sizes, so the assembly can come off under a defined boundary, and a check valve at the pump discharge protects the machine upstream of the connection. Commissioning debris is real: the Y strainer upstream catches weld bead and scale before they can machine the corrugation roots from the inside, and the first month of operation is when that protection earns its price. Skid boundaries add remote shutoff, where a solenoid valve lets the control system close the line without a walk. Instrumentation stays on the hard pipe: the pressure transmitter and the temperature transmitter for the loop mount on rigid runs, with a needle valve isolating any gauge takeoff on the same hard pipe, never on the moving hose, because a transmitter on a flexing line adds mass, resonance, and a creep of error into exactly the measurement the loop depends on.

The standards set the verification frame. EN ISO 10380 is the governing document for corrugated metal hose assemblies, from design through manufacture to the proof testing behind every rating in this chapter. The NAHAD assembly guidelines carry the fabrication and documentation practice for shops. ASME B31.3 governs the process piping the assembly joins and carries the bellows chapter that draws the line toward the expansion joint neighbor, whose design world is EJMA. The test regime follows from the standard: every assembly takes a hydrostatic proof test at 1.5 times working pressure, a low pressure pneumatic leak test substitutes where hydrostatic testing is inappropriate, and helium leak testing serves high vacuum and gas service where the tiniest leak matters. Burst capability in the family runs to roughly four times working pressure as a class pattern, and the certificates, EN 10204 3.1 on the metals and oxygen clean paperwork where required, close the evidence chain.

Failure modes in service follow a short and predictable list. Braid wear from rubbing on structure, the most common external killer: wires abrade, the braid loses section, and the assembly should have been sleeved at the rubbing point when it was installed. Chloride stress corrosion cracking of 304 tube and braid in hot coastal or chloride bearing environments: the cracks live at the convolution roots, the answer is 316L or C276 construction, and the failure is found by inspection, not by schedule. Fatigue cracks at the convolution peak from over bending, twist at installation, or vibration resonance: the cure is geometry, a retuned length or plane, sometimes a stiffer braid package, and never a thicker hose alone. Leak at the weld collar from porosity in a collar weld: the reason every assembly carries its own pressure test record. And mechanical damage, foot traffic, impact, and lifted loads, all of which the guard package exists to prevent.

Maintenance is inspection discipline on a calendar set by duty severity. Walk down the installation looking for broken braid wires, especially near the collars where fatigue concentrates, and apply the acceptance limits the standard practice gives for broken wires per braid band rather than a vague opinion. Check the routing has not been altered by later work, that clamps still carry weight, that guards are in place, and that nothing is stored on top of the assembly. On steam service, the winter check writes itself: confirm the steam trap stations are working and that heat tracing cable along the run is intact and clear of the braid, because a frozen condensate pocket hammers the corrugations from the inside. Replace on condition, log the movement history on severe duty, and keep one spare assembly with its test certificate on the shelf for the connection the plant cannot afford to wait for, because a hose emergency is a procurement event with the clock running backward.