RFQ emails live or die on terminology. One wrong term — a dash where an NPS belongs, “class 150” read as 150 psi, “316” where the weld needs 316L — costs three rounds of clarification and possibly a mis-supply. The twenty-five terms below are the ones that actually decide whether a fittings quote comes back right, defined once, in the plain sense the factory will read them, grouped the way a spec sheet reads: sizes, threads, pressure, materials and certificates.
Key takeaways
- Sizes: NPS and DN are names, not measurements; schedule moves the wall, dash numbers count sixteenths of tube OD.
- Threads: four families cover almost everything — NPT, BSPP, BSPT, and the 55° Whitworth form behind the BSP pair.
- Pressure: Cv is flow, not pressure; cracking pressure is a differential; WOG, PN and Class are three different rating systems that do not convert into each other by arithmetic.
- Materials: PREN screens pitting, sensitization explains weld-zone corrosion, and the certificate names — C of O, EN 10204 3.1, PMI — are the audit trail.
- Write the medium, the temperature and the weld plan with the terms; the right material answer follows from those three facts.
Sizes
NPS — Nominal Pipe Size: a legacy name, not a measurement. Below NPS 14 the number matches no actual diameter; the OD is fixed per size by the pipe standard, whatever the wall. Pipe schedule numbers.
DN — diamètre nominal: the ISO metric pipe-size name in millimeters, cross-walked to NPS by convention (1 in ↔ DN 25). A naming convention, not exact arithmetic.
Schedule — the wall-thickness series for pipe: schedule sets the wall for a given NPS, never the OD. Pipe vs tube.
Dash number — the SAE tube code: dash ÷ 16 = inches, and on tube fittings the dash is the tube outside diameter. Dash numbers on drawings.
OD / ID / wall — outside diameter, inside diameter, wall thickness. Tube is ordered OD × wall; pipe is ordered NPS + schedule. Mixing the two ordering systems is the classic first-email error.
Threads
NPT — ASME B1.20.1 tapered pipe thread, 60° form, 1:16 taper, seals on the thread flanks with sealant. The American default.
BSPP / G — ISO 228-1 parallel thread, 55° Whitworth form; does not seal on the thread — seals at the face with a washer or O-ring. Why BSPP needs a washer.
BSPT / R — ISO 7-1 tapered 55° thread; seals on the thread like NPT but with the Whitworth form. The two are not interchangeable despite matching tapers. NPT vs G (BSP).
Whitworth — the 55° British thread-form family with rounded crests and roots, parent of both BSP variants and the obsolete BSW/BSF fastener threads.
Pressure and flow
Cv — the valve flow coefficient: US gallons per minute of 60 °F water through the fully open valve at a 1 psi drop. A flow number, not a pressure rating. What is Cv and valve sizing gone wrong.
Cracking pressure — the minimum inlet-to-outlet differential at which a check valve first passes flow: a sizing number, not a quality number, distinct from reseal pressure. Cracking pressure.
Working / proof / burst — working: maximum continuous operating pressure. Proof: hydrostatic test pressure held without deformation or leak. Burst: the pressure at which the part fails. Proof vs burst vs working.
WOG — Water-Oil-Gas: the cold, non-shock working-pressure class marking in psig for those benign services. Steam needs its own WSP rating.
PN — Nominal Pressure: the metric rating class — the number is a pressure in bar at a 20 °C reference temperature, per the EN 1092 flange family.
Class — the ASME pressure-temperature rating class (150, 300, 600, 900…): a dimensionless index, not psi. Class 150 flanges rate roughly 285 psig at ambient for standard material groups — the number and the pressure relate through rating tables, not by identity.
Full port vs reduced port — full port: the bore matches the line ID, so nothing restricts flow; reduced port: a smaller bore acts as a permanent contraction. Pick by duty and by the valve sizing consequences.
Materials, certificates and failure modes
Seat — the sealing surface the ball, disc or needle closes against; usually a polymer (PTFE, PEEK) in instrument valves, and the first thing the medium attacks.
Trim — the valve’s internal working set — stem, ball or disc, seats — specified as materials per part. “316 valve” says nothing about trim. Valve trim materials.
Bonnet — the cover bolted to the valve body that carries the stem seal; its construction (integral, union, bellows) is a pressure-and-service decision. Integral vs union bonnet.
PREN — pitting resistance equivalent, %Cr + 3.3 × %Mo + 16 × %N: the desk-screen number for chloride pitting. Screens pitting only — it says nothing about stress corrosion cracking.
Sensitization — chromium-carbide precipitation at grain boundaries when austenitic stainless sits in the roughly 425–815 °C band, leaving chromium-depleted zones that corrode intergranularly: the weld-HAZ failure the L and stabilized grades exist to prevent. 316 vs 316L.
Passivation — nitric- or citric-acid treatment (ASTM A967) dissolving free iron and embedded contaminants so the protective chromium-oxide film re-forms. Not the same as electropolishing. Passivation of stainless steel fittings.
C of O — Certificate of Origin: the customs and trade document stating where the goods were manufactured. Asked for by trade compliance, not by engineering.
EN 10204 3.1 — the mill test certificate type: the supplied batch’s actual chemical and mechanical test results, validated by the manufacturer’s authorized inspection representative independent of production; 3.2 adds third-party witness. EN 10204 3.1 certificates.
PMI — Positive Material Identification: alloy verification of the received material against the specified grade, by X-ray fluorescence or optical emission spectroscopy: the receiving-dock check that catches the wrong-bar mistake.
CUI — Corrosion Under Insulation: hidden attack beneath insulation on insulated lines, driven by moisture and often by chloride-laden water.
SCC — Stress Corrosion Cracking: cracking under the combination of tensile stress and a specific environment — chlorides for austenitic stainless. Transgranular, fast, invisible until it is not. Chloride SCC of 316.
Pitting — localized chloride-driven attack that punches discrete pits through the passive film, self-propagating once started: the initiation site for many cracks. Pitting corrosion.
How to use this list in an RFQ
Terms do the work when they travel with facts. Write the medium and its chloride content, the temperature band, and where the welds go — three lines that convert a grade guess into a material answer — and put the sizes in the system they belong to: tube as OD × wall, pipe as NPS + schedule, threads by standard name. The quoting side of that discipline is how to write an RFQ for stainless fittings; the shipping-side vocabulary — FOB, CIF, EXW — is FOB vs CIF vs EXW. Get the terms right once and the quote comes back matching the drawing instead of matching the guesswork.
FAQ
What is the most commonly confused pair of terms?
Dash size and NPS. A “-6” is 3/8 inch tube outside diameter from the SAE system; “1/2 NPS” is a pipe whose actual OD is 0.840 inch. Both get called “the half-inch” in casual speech, they describe parts 68% apart in diameter, and only the right terminology sorts it out.
Is Class 150 the same as 150 psi?
No. Class is a dimensionless rating index whose pressure equivalence comes from material-and-temperature rating tables — a Class 150 flange rates roughly 285 psig at ambient for common material groups, and less as temperature rises. Reading the class number as psi under-specifies everything above ambient and mistakes an index for a measurement.
What is the difference between WOG and PN?
Two different rating systems that do not convert by arithmetic. WOG is a cold, non-shock working-pressure marking in psig for water, oil and gas service. PN is the metric nominal-pressure class — its number is a pressure in bar at 20 °C reference. A PN 16 valve and a 230 WOG valve are in the same neighborhood at room temperature, but the equivalence lives in the rating tables, not in the numbers.
Do I need EN 10204 3.1 or is a C of O enough?
They answer different questions. The C of O says where the goods were made — a trade document. The 3.1 certificate says what the batch actually is and passed — the engineering document, with the heat’s chemical and mechanical results. For pressure parts and any project with a material spec, the 3.1 is the floor; the C of O rides along for customs.
What is the difference between pitting and SCC?
Pitting is surface attack — discrete pits driven by chlorides against the passive film, visible with inspection. SCC is cracking — chlorides plus tensile stress plus temperature, running through the metal with little surface sign. Related: pits often act as the crack starter. PREN predicts the pitting side; nothing about the cracking side.
