Aerospace Tube Assemblies, Fluid Lines and Pressure System Hardware

What tube and fluid system work covers

It covers the hardware that carries and controls gases and liquids: bent and welded tube assemblies, fluid lines with flared or welded ends, manifolds, pressurization and purge panels, servicing carts and the plumbing on test stands. Buyers are launch providers, spacecraft makers, test facilities and the teams building ground support equipment.

Typical requests include:

• Tube assemblies bent to a bend table or a 3D model, with flared, flareless or orbital welded ends
• Pressurant, purge and propellant lines for vehicles, test articles and ground systems
• Machined manifolds and distribution blocks with straight thread ports
• Panels and skids that combine tubing, valves, regulators, gauges and relief devices
• Replacement lines built to an existing as-built drawing for a test stand or facility system

The work is judged on four things: the right tube and fitting specifications, joints made and inspected to a qualified procedure, a proof and leak test that matches the governing pressure standard, and internal cleanliness that survives until the system is assembled. The sections below follow that order.

Tube materials and the specifications buyers call out

Call out the tube by specification, not just by alloy. Two tubes in the same alloy can be bought to different specifications with different testing, tolerances and intended service, and the specification is what the mill cert is checked against at receiving.

Three buying notes:

• Check the revision. These specifications revise every few years. State the revision your program accepts or accept the current one, and make the mill cert show it.
• Specialty metals. Defense contracts that carry the specialty metals clause restrict where stainless and other covered alloys were melted. Flow that clause down before the tube is bought.
• Traceability to the heat. On assemblies with many lines, ask for heat numbers to be recorded against each line or tag, so a material problem can be traced to the affected assemblies. Our material certs page covers what a good cert shows, and the stainless steels page compares the alloys.

End connections: flares, ports and welded joints

Specify every end by its standard, and do not mix sealing systems without an adapter on the drawing. Most fluid system leaks at test come from mismatched or poorly made ends, not from the tube.

On flared lines, ask how the source checks flare geometry and surface condition. A scratched or off-angle flare will often pass a quick visual and leak at proof. On ports, ask for thread and port inspection on the manifold, since a port machined to the wrong profile will not seal no matter what fitting goes into it.

Tube bending: what to put on the drawing

Give the bend geometry in a form a bender can load directly, and put limits on the defects bending causes. Tube bending is fast when the data is clean and slow when the shop has to reverse engineer a model.

• Bend data. Provide XYZ centerline points or a length, rotation and angle table along with the model, and state which governs if they disagree.
• Centerline radius. Use radii that match standard tooling where the design allows. A one-off radius can mean new tooling and a longer lead time.
• Ovality, wall thinning and wrinkling. Bending flattens the tube, thins the outer wall and can wrinkle the inner wall. If pressure rating depends on wall thickness, put limits on the drawing and say how they are measured.
• Straight length between bends. Short straights between bends and near fitting ends can make a line impossible to bend or flare. A quick producibility review before release avoids redesigns.
• Fit check. For lines that connect fixed points on a vehicle or panel, ask whether the source will check against a fixture, a coordinate measuring machine or a tube measurement system.

Lines that go into an assembly with existing hardware often need a field fit. If that is the case, say so, and ask for lines to be supplied with extra length at one end for trimming and finishing on site.

Orbital welding of tube

Orbital welding is automated gas tungsten arc welding of tube butt joints, and for aerospace work it is normally done to AWS D17.1. Buyers should require a qualified weld schedule for each tube size and material combination, qualified operators and inspection of the inside of the joint, not just the outside.

AWS D17.1 is the fusion welding specification for aerospace applications. Its current fourth edition, 2024, has clauses covering joint design, procedure and welder qualification, fabrication, inspection, repair and non-flight hardware, with additional requirements for crewed spaceflight hardware. When a contract invokes it, all of its provisions apply except those the engineering authority or contract exempts.

What to ask for on orbital welded assemblies:

• The weld schedule used for each joint size and material, traced to a qualification record
• Operator qualification for the equipment and process
• Purge practice, because inside surface oxidation on stainless lines affects corrosion and cleanliness
• Internal inspection of the weld bead by borescope, which aerospace employers list as a routine part of orbital welding, and radiography where the class or drawing requires it
• Coupon or sample welds made at the start of a shift or schedule change if your procedure requires them
• A weld map tying each joint to the schedule, operator and inspection result

Weld classes and inspection methods are covered in more depth on our nondestructive testing page.

Which pressure system standard applies

The standard depends on where the system will operate and who owns it. Ground systems at NASA sites fall under NASA-STD-8719.17, which in turn points new process piping to ASME B31.3. Commercial sites usually invoke B31.3 or the owner's own pressure system rules directly.

NASA-STD-8719.17 at NASA facilities

NASA-STD-8719.17, the NASA standard for ground-based pressure vessels and pressurized systems, is at Revision D. It applies to ground-based equipment designed for or operating at positive or negative gauge pressure unless specifically excluded, including contractor or tenant systems operated on NASA property when the center's Pressure Systems Manager determines they pose a risk to NASA personnel, facilities or equipment. It also covers ground processing of flight pressure systems. Points that matter when buying hardware for those sites:

• New ground-based systems are to be designed, fabricated, inspected, examined and tested to the applicable consensus codes and regulations.
• Process piping is to meet ASME B31.3, and other piping the most applicable B31 code.
• New pressure vessels are to be ASME Section VIII code stamped and registered with the National Board.
• Flexible hoses are not to be used in place of rigid tubing unless rigid tubing is impractical, need restraint where a failure could whip, and those whose rupture would be unacceptable are retested at hose maximum allowable working pressure at least every five years.

The standard requires every covered system to be certified, recertified and documented in accordance with it, and the center's Pressure Systems Manager decides applicability and exclusions. If your hardware will operate at a NASA center, put that in the request so the fabricator builds the documentation the certification needs.

ASME B31.3 process piping

B31.3 governs design, materials, fabrication, examination and testing of process piping, and it defines fluid service categories that change the examination and testing required. If your system carries toxic or otherwise hazardous fluids, state the fluid and its service category in the request, because that determines how much weld examination is required.

Proof and leak testing

State the test type, the test pressure or the rule that sets it, the hold time, the test medium and the acceptance criterion. A request that says only "leak test" will be quoted at whatever the shop normally does.

Two notes from the buyer side. First, ASTM E1003 itself says hydrostatic leak testing is not sensitive enough for systems that will hold toxic or explosive gas under pressure, which is why gas systems usually get a helium leak check after the proof test. Second, sequence matters: welding, then NDE, then proof and leak test, then precision cleaning, then packaging. Testing with water after a part has been precision cleaned usually means cleaning it again.

Ask for a test record with the gauge or transducer identification and calibration date, the test pressure and hold time actually achieved, the medium, the leak rate measured and the witness. If you need a lab rather than the fabricator to run high pressure or helium testing, our test lab time page covers that route.

Cleanliness for oxygen and propellant service

Put the required cleanliness level and the governing specification on the drawing, and let the cleaning source choose a process that meets it. A note like "clean for oxygen service" without a level cannot be verified at receiving.

For fluid systems the practical questions are about design and sequence:

• Design lines and manifolds to be cleanable: no blind passages, no crevices at press fits and flow paths that can be flushed.
• Clean after the last operation that can introduce contamination, which usually means after welding, proof and leak testing.
• Cap ports with clean closures and double bag, with the cleanliness level and date on the outer bag label.
• For NASA work, center cleanliness specifications usually govern GSE and vehicle fluid systems, and program requirements take precedence.

Cleanliness levels, NASA center specifications and what a valid cleaning certificate shows are explained on our precision and oxygen cleaning page, and how to keep a clean package intact through shipping on the crating and packaging page.

Tagging and the documentation package

Ask for each line or assembly to be identified so its paperwork can be found from the part, and for one package that covers material, joints, testing and cleanliness. Fluid systems get rebuilt and modified, and records that cannot be tied to a specific line are of little use later.

• Part number, revision and serial or line number on a tag or by marking method approved on the drawing
• Mill certs for tube, fittings and bar used for machined manifolds, with heat numbers
• Weld procedure, schedule and operator records, and NDE reports
• Bend and dimensional inspection results, and first article inspection if required
• Proof and leak test records with calibrated instrument identification
• Cleanliness certificate with the specification, level achieved and verification data
• As-built drawing or schematic for panels and skids
• Certificate of conformance to the purchase order

For hardware headed to a NASA center, the Pressure Systems Manager will expect records that support certification, so it is cheaper to collect them during fabrication than to reconstruct them afterward. Our first article inspection guide covers the FAI side.

What to put in a tube or fluid system RFQ

Lead with the fluid, the pressure, the governing standard and the cleanliness level. Those four facts decide which suppliers can bid, and leaving them out is the most common reason fluid system quotes come back qualified or late.

1. Drawings, models and bend data at the released revision
2. Fluid, design pressure, temperature range and the pressure standard that governs (NASA-STD-8719.17, ASME B31.3, program specification)
3. Tube and fitting specifications, with revisions if your program restricts them
4. End connection standards for every end
5. Weld code and class, and required NDE
6. Proof and leak test type, pressure, hold, medium and acceptance
7. Cleanliness specification and level, and packaging requirement
8. Tagging and documentation requirements
9. Quantity, need date and delivery location
10. Export control status of the drawings and the system data

Propulsion and pressurization data is often more sensitive than the hardware drawing. Read our confidentiality page and the controlled programs process before sending controlled files. If you are buying for a vehicle program, our launch provider page and the launch vehicle industry page cover the related work we source.

How Aerospace Sourcing handles a fluid system job

We are a sourcing service, not a fabrication shop or test lab. We match your job to suppliers whose bending, welding, testing and cleaning capability fits, and manage it to delivery.

• One quote, one lead time, one point of contact and one purchase order, even when a fabricator, a machinist, a test provider and a cleaning house are involved
• Tube specs, weld code, test and cleanliness requirements flowed down to each supplier
• Every supplier signs a flow-down NDA before seeing your data and does not learn the end customer unless you approve
• Records checked before shipment, with a certificate of conformance on every order and the other certs your request requires

We do not promise a supplier for every job, and we will tell you if a pressure, test or cleanliness requirement is beyond what we can match.

Request a tube or fluid system quote

Use the fluid systems quote form. It is preset for tube assemblies, manifolds and pressure system work, so it asks for the fluid, pressure, governing standard and cleanliness level first, which is the information a supplier needs before it can quote with confidence. A person replies within one business day, and rush requests are prioritized.

Questions

Related

• Ground Support Equipment Fabrication for Launch and Spacecraft Programs
• Sourcing GSE, Pad Hardware and Fluid System Parts for Launch Providers
• Booking Environmental Test Lab Time for Space and Defense Hardware

Sources

• NASA-STD-8719.17 Revision D, Ground-Based Pressure Vessels and Pressurized Systems
• ASTM A269/A269M: Seamless and Welded Austenitic Stainless Steel Tubing for General Service
• SAE AMS 5557K: 321 hydraulic tubing
• SAE AMS 5561H: high pressure hydraulic tubing 9.0Mn-20Cr-6.5Ni-0.28N
• SAE AMS 4083N: 6061-T6 hydraulic tubing
• SAE AS4395B: Fitting End, Flared Tube Connection, Design Standard
• SAE AS33583: Tubing End, Double Flare, Standard Dimensions
• SAE AS5202: Port or Fitting End, Internal Straight Thread, Design Standard
• ASTM E499/E499M: Leaks Using the Mass Spectrometer Leak Detector in the Detector Probe Mode
• ASTM E1003: Standard Practice for Hydrostatic Leak Testing
• What Is Piping: ASME B31.3 pressure leak test summary
• ANSI Blog: AWS D17.1:2024 Fusion Welding for Aerospace Applications