Ask ten buyers what a Class 3000 fitting is rated for and eight will say 3,000 PSI. It is not. The class is a dimensional designation that ties the fitting to a pipe wall thickness — the actual working pressure depends on the grade you bought and the temperature it runs at. Get that wrong and you either pay double for steel you cannot use or install a weak link into a line that looks compliant on paper. This guide covers the five classes you will meet in the market, the sizes each comes in, the schedule each one matches, and how to choose in four questions.
What a “Class” Actually Means
A pressure class in forged fittings is a wall-thickness family. ASME B16.11 fixes the socket depth, bore, body thickness and centre-to-end dimensions for each class. The class number is a legacy label, not a rating.
Class vs Working Pressure
The working pressure comes from the pressure-temperature tables, and it moves with material grade. A Class 3000 A105 carbon steel fitting and a Class 3000 F316 stainless fitting have identical dimensions and different allowable pressures.
At ambient temperature, a Class 3000 A105 fitting sits well above 3,000 PSI. At 400°C it sits well below it. Same part, same stamp.
How Temperature Derates the Rating
Carbon steel loses roughly a third of its allowable stress between 200°C and 425°C. Chrome-moly grades hold on much further, which is the entire reason F11 and F22 exist. Stainless holds strength at temperature but starts weaker at ambient than carbon steel.
Read the derating table for your grade. Not the class number.
Class 800 — The Trade Class
Class 800 is not in ASME B16.11, but it trades heavily and you will be quoted it.
- Sizes: NPS 1/8 to 4, threaded
- Schedule match: roughly Sch 40 / Sch 80 depending on the maker
- Grades: mostly A105 and F304/F316
- Used in: general utility lines, compressed air, plant water, low-pressure instrument work, and as the body class for small forged valves
Because it is outside the standard, dimensions vary between manufacturers. If your spec says B16.11, Class 800 does not comply. Ask what the fitting is actually made to before you accept it.
Class 2000 — Threaded, Light Duty
- Sizes: NPS 1/8 to 4
- End type: threaded only — there is no Class 2000 socket weld fitting
- Schedule match: Sch 80
- Grades: A105, F304, F316
- Used in: utility air, cooling water, low-pressure drains and vents, non-critical dosing lines
Class 2000 is the lightest class the standard recognises. It is also the class most often substituted quietly when a supplier is short on Class 3000 stock. Measure the wall.
Class 3000 — The Industry Workhorse
If you buy forged fittings at all, most of what you buy is Class 3000.
- Sizes: NPS 1/8 to 4
- End type: socket weld and threaded
- Schedule match: Sch 160 threaded, Sch 80 socket weld
- Grades: A105, A350 LF2, F304/L, F316/L, F11, F22
- Used in: refinery process piping, steam and condensate, hydraulic power packs, chemical dosing skids, instrument impulse lines, fire water headers
Class 3000 covers the overwhelming majority of small-bore process piping in operating plants. It is the class to default to and then justify moving away from.
Class 6000 — High Pressure and Refinery Service
- Sizes: NPS 1/8 to 2 socket weld; up to NPS 4 threaded
- Schedule match: XXS threaded, Sch 160 socket weld
- Grades: A105, F316/L, F11, F22, F91, duplex F51
- Used in: high-pressure hydrocarbon lines, boiler feed water, hydrogen service, high-pressure hydraulics above 300 bar, offshore process modules
Class 6000 is where the price jump becomes noticeable — typically 40–60% over Class 3000 for the same size and grade. That gap is why over-specification hurts.
Class 9000 — Socket Weld Only, Critical Lines
- Sizes: NPS 1/8 to 2
- End type: socket weld only. There is no threaded Class 9000 in the standard
- Schedule match: XXS
- Grades: A105, F22, F91, F316L, duplex and super duplex
- Used in: high-pressure steam, urea and ammonia plants, hydrocracker units, supercritical boiler tapping points, subsea and high-integrity hydraulic circuits
Class 9000 fittings are the heaviest small-bore parts you can order. If someone quotes you a threaded Class 9000 fitting, they are quoting something that does not exist in B16.11.
The Schedule Mapping Most Buyers Get Wrong
Here is the uncomfortable part. Search this topic and half of page one will hand you a single mapping table — Class 3000 equals Sch 40, Class 6000 equals Sch 80, and so on. That table is wrong, because it collapses two different mappings into one.
The standard treats threaded and socket weld separately, and it has to: a threaded fitting loses wall to the thread cut, so it needs more meat to reach the same rating.
Threaded Fittings
| Class | Matching pipe schedule |
| 2000 | Sch 80 |
| 3000 | Sch 160 |
| 6000 | XXS |
Socket Weld Fittings
| Class | Matching pipe schedule |
| 3000 | Sch 80 |
| 6000 | Sch 160 |
| 9000 | XXS |
Read those side by side and one thing jumps out. A threaded Class 3000 fitting and a socket weld Class 6000 fitting match the same pipe. Buyers who use a single table routinely order one class too light on socket weld lines and one class too heavy on threaded ones — and both errors pass inspection, because the stamp says what the purchase order said.
What Over-Specifying Costs You
Over-specification is the quiet money leak in fitting procurement.
- Class 6000 typically costs 40–60% more than Class 3000 in the same grade
- Class 9000 can run double
- Heavier fittings mean thicker weld preparations, more filler, more welding hours and more radiography
- Heavier sockets on lighter pipe create a stiffness mismatch — the pipe becomes the fatigue point right at the weld toe
That last one is the part that surprises people. Fitting a Class 9000 socket weld elbow to Sch 80 pipe does not make the joint stronger. It moves the failure into the pipe wall just outside the fillet, where thermal cycling and vibration concentrate.
Matching beats maximising.
How to Pick the Class in Four Questions
- What pipe schedule is the line? That single answer usually fixes the class, using the correct table above.
- Threaded or socket weld? This decides which of the two tables you read.
- What is the design temperature? Take the class you landed on, look up the derated allowable for your grade, and confirm it clears the design pressure with margin.
- Does the client spec override any of this? Many refinery and EPC specs mandate a minimum class regardless of calculation — commonly Class 3000 socket weld minimum for all hydrocarbon service, even where Class 2000 would pass.
Work in that order and the class chooses itself.
FAQs
Can I weld a Class 3000 fitting to Sch 160 pipe? Physically yes, but you have created a thickness mismatch at the weld and the joint is no longer covered by the matching-schedule logic in B16.11. Use Class 6000 socket weld for Sch 160.
Why is there no Class 2000 socket weld fitting? Because a socket weld fitting has no thread cut removing metal. At Class 2000 wall thickness it would be lighter than the pipe it joins, which defeats the point.
Do stainless and carbon steel fittings of the same class cost the same? No. Expect stainless to run roughly 2.5–4 times carbon steel for the same size and class, and duplex or F91 higher still. The class affects the weight; the grade affects the rate.
Is a higher class always safer? No. Beyond the matching schedule, extra thickness adds stiffness and moves the stress concentration into the pipe. It also adds cost and welding time for no rating benefit.
What documentation should confirm the class? The class must be permanently marked on the fitting body alongside the grade, size and heat number, and it must appear on the mill test certificate against that heat number.
Conclusion
Five classes, two mapping tables, four questions. Get the schedule right first, read the right table for your end type, derate for temperature, then check the client spec. Everything else in fitting selection is downstream of that.
If you have a line list sitting on your desk, send it across and we will return the class and grade breakup against it.
About Krishna Forge
Krishna Forge manufactures socket weld and threaded forged fittings to ASME B16.11 in Classes 2000, 3000, 6000 and 9000, across carbon steel, stainless, alloy, low-temperature and duplex grades, NPS 1/8 to 4. Wall thickness is verified against the class before dispatch, and every piece carries a heat number traceable to its mill certificate.
Send us your line list for a class-wise quotation.
