Superlok® USA · Tubing
Choosing Tube Wall Thickness for Pressure and Bending
Wall is the variable that decides how much pressure a run holds, how well a ferrule can swage it, and how tightly you can bend it.
Wall thickness is the quietest number on a tubing order and the one that decides the most. It sets how much pressure the run may carry, whether a ferrule can form a proper bite, how tight a bend the tube survives, and how much material is left after years of service. According to the Swagelok® tubing data sheet, a manufacturer publication, allowable working pressures for ASTM A269 tube are “calculated from an S value of 20 000 psi (138 MPa)” under ASME B31.3, are “based on maximum OD and minimum wall thickness,” and carry “no allowance” for corrosion, erosion, welding or bending. Each of those clauses is a design instruction, not a footnote.
How wall thickness relates to working pressure
Two relationships drive the whole table. At a fixed outside diameter, a thicker wall raises the allowable working pressure. At a fixed wall, a larger outside diameter lowers it, because the same thickness restrains a larger bore. So a heavier wall is often the cheapest way to keep a bigger line inside the pressure you need, and stepping up tube size without stepping up wall quietly costs margin.
The basis matters as much as the number. Because published values come from minimum wall and maximum outside diameter, they already assume the least favourable tube inside the mill tolerance. Because no allowance is made for corrosion, erosion, welding or bending, each of those is yours to add. A bend or a socket weld is not covered by the table value for the straight tube it came from. Published limits for Superlok® fittings by size are on our pressure rating pages, and the joint is limited by the lower of the tube and the fitting.
| Tube OD | Wall | Allowable working pressure | What the row shows |
|---|---|---|---|
| 1/4 in. | 0.035 in. | 7,500 psig | Baseline small bore instrument line |
| 1/2 in. | 0.035 in. | 5,100 psig | Same wall, larger OD, lower pressure |
| 1/2 in. | 0.049 in. | 6,700 psig | Heavier wall recovers much of the loss |
Figures as published in the Swagelok® tubing data sheet for annealed ASTM A269 316 stainless steel tube at ambient temperature, quoted to show the two relationships rather than to rate any product. Ratings for tube and for fittings are separate, and grade and rating for any Superlok® item are confirmed per order.
Too thin collapses, too thick resists
A ferrule works by deforming the tube wall, which gives wall thickness a lower and an upper bound that has nothing to do with pressure. Too thin, and the tube has no stiffness to resist the swaging load, so the bite can coin the wall inward and pull the tube out of round instead of gripping it. Too thick, and the tube pushes back harder than the ferrule was designed for. The Swagelok® tubing data sheet puts it in one line: “heavy-wall tube resists ferrule action more than thin-wall tube.”
Manufacturers therefore publish a wall range per size and are explicit about staying inside it. The same document says its fittings are “not recommended for tube wall thicknesses outside the ranges shown,” and for gas service advises choosing a wall “outside of the shaded area.” In practice, fitting selection and tubing selection are one decision, not two purchase orders. Check the wall against the range for the tube fittings you intend to use before the tube is cut.
Three wall thickness mistakes that show up at pull-up, not at the desk
Under the listed range
The bite deforms the tube instead of gripping it. The joint may pull up and gauge correctly and still fall short of the tube support the design assumed, which is the worst kind of defect because it looks finished.
Over the listed range
The wall resists the ferrule, the nut runs out of travel before the swage is done, and installers are tempted to keep turning. Extra turns load threads, not seals.
Right wall, wrong condition
Work hardened, cold drawn or scratched tube fails at a wall that annealed tube handles. Condition and surface are separate line items from thickness.
Hardness relative to the fitting
Thickness is only half the swaging story. The other half is that a ferrule cannot bite a tube as hard as itself. The Swagelok® tubing data sheet states that “the tubing must be softer than the fitting material,” with “hardness not to exceed 90 HRB or 200 HV” for 316 stainless steel tube. Parker® gives the same ceiling for fully annealed 316 stainless steel at 90 Rb, and adds that for gas service “a desirable hardness of 80 Rb is suitable for stainless steel.”
Both makers also call for high quality, fully annealed tube, seamless or welded and drawn, “free of scratches, suitable for bending and flaring.” That is why the mill certificate is worth reading rather than filing: a typical material test report shows what is documented for a heat, from grade and chemistry to mechanical results and condition. Superlok® fittings are listed in 316 stainless steel, a corrosion-resistant choice for process and instrument service, and are built with the finest interior surface finish of any tube and pipe fitting on the market, which helps on the wetted side but cannot rescue tube that arrived too hard.
Bend radius implications
Bending redistributes wall. The outside of a bend stretches and thins, the inside bunches and can wrinkle, and the section can go oval. None of that sits inside the published pressure figure, which makes “no allowance” for bending. A tighter radius takes more out of the wall, so heavier wall is what buys a tighter bend, and thin wall at a tight radius is the combination that flattens.
The second bending rule is about where the bend ends. Waverley Brownall, a distributor training publication, tells installers to “always bend tube prior to insertion in connectors” and to leave enough straight tube to fully enter the connector abutment, with a table running from 0.82 in. (20.8 mm) at 1/4 in. size up to 1.5 in. (39.3 mm) at 1 in. Parker® lists comparable figures for its own fittings: free length 0.80 in. and insertion depth 0.60 in. at 1/4 in., and 1.19 in. and 0.90 in. at 1/2 in. If a bend starts inside that straight length, the tube cannot bottom on the shoulder and the ferrules swage a tube that is already deformed. Our installation guide covers insertion and pull-up, and process piping shows where these runs live.
What to check on the tube before ordering fittings
Six things. The outside diameter and its tolerance, since the ferrule seals on the OD and the table value assumes the maximum. The wall, and whether it sits inside the range the fitting maker lists for that size. The specification and condition, typically ASTM A269 or A213 and fully annealed. The hardness on the certificate, under the published ceiling. The surface, free of scratches and scale where the ferrules will sit. And the straight length you will actually have between each bend and each fitting. Then pick size and series from the Superlok® catalog.
Buying questions
If I go to a heavier wall, do I get a higher rated joint?
Only up to the fitting. A joint is limited by the lower of the tube and the fitting, and a wall above the maker’s listed range is not an upgrade.
Why does the published pressure drop when I keep the same wall and go up a size?
Because the same thickness restrains a larger bore. In the Swagelok® data, 0.035 in. wall gives 7,500 psig at 1/4 in. OD and 5,100 psig at 1/2 in. OD for annealed 316 stainless steel tube.
Can I use hard drawn tube if the wall is right?
Not as a substitute for annealed tube. The published requirement is that the tube be softer than the fitting, with 90 HRB or 200 HV as the ceiling for 316 stainless steel and lower preferred for gas service.
How much straight tube do I need after a bend?
Enough to bottom fully in the fitting. Published figures start around 0.8 in. at 1/4 in. tube and rise with size, so confirm the number for your fitting.
Does the pressure figure already include a corrosion allowance?
No. The source states that no allowance is made for corrosion, erosion, welding or bending, so anything you expect to lose in service has to be added by you.
Have the tube OD and wall, and need the matching fittings?
Send the tube spec and we will confirm grade, rating and configuration per order.
Superlok® North America
Instrumentation fittings and valves, cross-referenced by form, fit, and function.
Sources
- Swagelok® Company, “Tubing Data,” MS-01-107, manufacturer publication, supports the 20 000 psi S value for ASTM A269 under ASME B31.3, the maximum OD and minimum wall basis, the no allowance statement, the 316 stainless steel hardness ceiling of 90 HRB and 200 HV, the tube softer than fitting requirement, the wall thickness range recommendation, the gas service shaded area note, the heavy wall resistance statement, and the 7,500 psig, 5,100 psig and 6,700 psig table values quoted. swagelok.com
- Swagelok® Company, “Tubing Data,” MS-01-107, mirrored copy used to confirm the quoted pressure table values independently. idealvac.com
- Parker® Hannifin, “Instrumentation Tube Fitting Installation Manual,” BUL-4200-B4, supports the 90 Rb maximum for fully annealed 316 stainless steel, the 80 Rb preference for gas service, and the free length and insertion depth figures at 1/4 in. and 1/2 in. sizes. parker.com
- Waverley Brownall, “Tube Bending Tips,” distributor training publication, 20 March 2017, supports bending before insertion and the straight length figures of 0.82 in. (20.8 mm) at 1/4 in. tube through 1.5 in. (39.3 mm) at 1 in. tube. waverleybrownall.co.uk
Technical note: the pressure, hardness, wall range and straight length figures quoted here are published by manufacturers and distributors for their own products and for tube made to ASTM A269 and A213. They prove what those documents state at ambient temperature and for the conditions named in them. They do not prove a rating for any particular Superlok® item, they do not include any corrosion, erosion, welding or bending allowance, and they are not a substitute for a pressure calculation for your service. Grade, pressure rating and configuration for any Superlok® item are confirmed per order.
Trademark disclaimer: Swagelok® is a registered trademark of Swagelok® Company. Parker® is a registered trademark of Parker® Hannifin Corporation. Superlok® is a registered trademark of its owner. All other marks belong to their respective owners. Use of these names is for identification only and does not imply any affiliation, endorsement or approval. Cross-references are made by form, fit, and function only.