Ham-Let® vs Swagelok®: Reading a Pressure Rating and Its Temperature Derating

A printed pressure rating looks like a fact you can compare across a catalog page. According to Swagelok®’s own tubing data sheet, MS-01-107, a manufacturer publication, allowable working pressures rest on “an S value of 20 000 psi (138 MPa) for ASTM A269 tubing,” and the same document instructs that “to determine allowable working pressure at elevated temperatures, multiply allowable working pressures from Tables 1 through 25 by a factor shown in Table 26.” Ham-Let®, whose compression tube fittings are sold under the LET-LOK® name, publishes a derating table of its own, with its own base pressures and its own worked example. Two tables, two sets of inputs. This post is about what feeds a printed rating, how each company derates that rating for heat in its own literature, and why lifting a factor out of one catalog and dropping it into the other produces a number that means nothing.

What actually goes into a printed number

A working pressure rating is the output of a calculation, not a property of the alloy on its own. Swagelok®’s tubing data sheet ties its figures to one named tube standard, ASTM A269, and to one stated stress value for the alloy, and it adds a limit in plain words: “Swagelok tube fittings are not recommended for tube wall thicknesses outside the ranges shown in the accompanying tables for each size.” Change the wall, change the tube specification, or change the alloy, and the figure on the page has stopped describing your joint. The spread inside a single tube size makes the point on its own. For 1/2 in. outside diameter in 316 stainless steel, the same document publishes 2,600 psig at a 0.035 in. wall and 5,100 psig at a 0.156 in. wall. That is roughly double, with no change of brand, alloy or diameter. Ham-Let® organises its own tables the same way, by outside diameter and wall, with separate tables for inch tubing in psi and metric tubing in bar. So two catalogs can print a figure against what reads as the same tube size and still be describing different walls, different governing specifications, or different reference temperatures.

Temperature derating: the same idea, two different tables

Steel gets weaker as it gets hotter, and both companies publish a multiplier to account for it. Swagelok®’s Table 26 gives 316 stainless steel a factor of 0.96 at 400°F, 0.80 at 800°F and 0.76 at 1000°F, each applied to the base pressure taken from that document’s own size-by-wall tables. Ham-Let®’s catalog shows the arithmetic in full for one part: the allowable pressure for “Type 316 stainless steel, size 1/2″ OD x .049″ wall at 800°F (427°C) would be equivalent to 3750 psi x 0.79 = 2962.5 psi.” Set the two 800°F factors beside each other, 0.80 and 0.79, and the temptation is to call them the same. They are not answering the same question. One is a generic multiplier for an alloy, published separately from the pressure tables it applies to. The other is worked against one specific base pressure for one specific wall. Neither factor was calculated for the other company’s base number, and neither company suggests it was.

Why these figures will not sit on one chart

Plot the Ham-Let® example and the Swagelok® factors on a single line and you have drawn something that does not exist. The Ham-Let® worked example runs on a 0.049 in. wall. Swagelok®’s published 1/2 in. table starts at 0.035 in. and reaches 0.156 in., so the wall behind a Swagelok® figure depends entirely on which row you were reading. Wall thickness moves the base pressure before heat is considered at all, which is why the factor is the last step of the sum and never the first. None of this is a fault in either company’s literature. Both are doing the arithmetic correctly and stating their inputs. The mistake only happens downstream, when somebody copies a multiplier out of one book and applies it to a number from another.

Superlok® describes its own machining as the finest interior surface finish of any tube and pipe fitting on the market, wording we pass on as Superlok®’s rather than as a ranking against anyone. The range itself is listed in 316 stainless steel, an austenitic grade that is corrosion-resistant and specified by instrument shops out of long habit. What we will not do is read your rating off somebody else’s chart: working pressure, grade and certification are all settled per order, against the service data you send. Start with the tube fittings range, look at the relief valves that protect a system above its design point, the gauge valves that sit at the reading end of it, our certifications record, and a companion piece on high-pressure hydrogen service, where derating bites hardest.

Questions about pressure ratings and derating

Technical note: the sources above establish how each named manufacturer states its own published working pressures and temperature derating factors, for the tube sizes, walls and reference points each document gives. They do not establish that a figure or a factor from one manufacturer’s table applies to another manufacturer’s fitting, and they do not certify a rating for your tubing or your service. Superlok® working pressure, grade and documentation are confirmed per order.

Trademark disclaimer: Swagelok® is a registered trademark of Swagelok Company. Ham-Let® and LET-LOK® are trademarks associated with Ham-Let Group, a Watts Water Technologies company. Superlok® is a registered trademark of BMT Co., Ltd. Superior Fluid Solutions is not affiliated with, authorized by, endorsed by or sponsored by any of these companies, and the marks are used here only to identify the products they name. Cross-references are made by form, fit, and function only.