Class 150 or 10K: matching ASME B16.5 and KS B 1503 flanges in Korea
Your general arrangement drawing says ASME B16.5 Class 150 RF. The tie-in detail that came back from the Korean site says 10K. Same nominal bore, two flange call-outs, and somebody has to decide which one leaves your works — usually while the skid is already painted and the booking is confirmed.
Settling it after the container clears customs is expensive in a specific way. A flange mismatch is normally found at hookup, inside a shutdown window, with the crane released and the erection crew already on the platform. What follows is a transition spool made locally, a new gasket and stud set, and a joint that was signed off as complete going back through inspection.
Two rating systems, not two dialects of one
ASME B16.5 is titled Pipe Flanges and Flanged Fittings: NPS 1/2 through NPS 24, Metric/Inch Standard. It carries rating classes 150, 300, 400, 600, 900 and 1500 across that size range, plus Class 2500 up to NPS 12, and it was reissued in 2025. Sizes are NPS, in inches.
The other family on a Korean site is the K series. In Korea the working document is KS B 1503, steel welded pipe flanges, which covers welded and blank flanges for general piping of steam, air, gas, water and oil at ratings from 5K to 30K. It does not carry the numbers alone. Its scope ties the basic dimensions to KS B 1511, and the pressure stages sit in KS B 1501, so a flange call-out that cites only KS B 1503 still needs those two documents behind it. Nominal sizes are given in A — 50A, 100A, 200A — not in inches. Japan's equivalent is JIS B 2220, and a Japanese-built package on a Korean site will normally arrive with it.
Two points are worth reading off the front matter of KS B 1503 itself. It was established in 1976 and its correspondence with ISO 7005-1 is declared NEQ — not equivalent. The standard is not claiming to line up with the international series, and nothing in it claims to line up with ASME. So 10K is a rating label inside one system and Class 150 is a rating label inside another. What a joint can hold is comparable across the two — both families rate pressure against temperature — but the dimensions do not carry across, and no official equivalence is declared between the families. An approximate table of the "10K is about Class 150" kind settles duty. It never settles fit, and it is not something to machine from.
What has to line up before anything bolts
The joint is decided by more than the rating on the drawing. Six items have to agree, and they are checked against the flange table, not against the pipe size:
- Bolt circle diameter, number of bolt holes, and bolt hole diameter. This is where the two families separate first, and it separates by size — a pair that is close at one diameter can be far apart at the next one up.
- Thread series. K-series bolting is metric. ASME B16.5 states its requirements in both metric and US customary units, but the diameters of bolts and bolt holes are given in inch. A flange dimensioned in millimeters on an ASME drawing is still an inch-bolted flange.
- Flange outside diameter and thickness. The OD decides clearance against supports, insulation and the next flange along; thickness decides stud length, and stud length is ordered long before anyone measures the joint.
- Facing. Raised face, flat face or ring joint, plus the raised face height and the face finish. Bolting a raised face onto a flat face on a cast body is a body-loading problem, not only a sealing one.
- Gasket dimensions. Inner and outer diameter have to suit the raised face and clear the bolt circle. Gasket tables belong to the flange family; they do not travel between families.
- Bolt hole orientation. Both families place holes straddling the flange centerlines rather than on them, so this is rarely where the mismatch is — it bites on a package nozzle or a flanged valve with a fixed top dead center, where the reference line is the equipment rather than the pipe. Confirm it on the GA.
Bore is a separate question. On weld-neck flanges the bore follows the pipe, and A-series pipe outside diameters are not identical to ASME B36.10M outside diameters at every size. That surfaces as a weld preparation problem at the fabricator, well before the tie-in, which is the one version of this that gets caught early.
Reading the Korean tie-in drawing
| What the drawing says | What it usually points to | How to confirm | If you read it wrong |
|---|---|---|---|
| 10K, 16K, 20K, with sizes in A | The K series — KS B 1503 and KS B 1511 in Korea, JIS B 2220 on a Japanese-built package | Ask for the flange standard number and edition, plus the dimensioned flange detail at the tie-in | You ship a Class flange, the studs do not enter the holes, and the joint is re-made on site |
| 150#, Class 150, ANSI 150 | ASME B16.5 — ANSI is the legacy name for the same standard | Confirm the edition the project specification calls, not the current one | Usually a documentation query at inspection rather than a fit problem, but it holds the release |
| DN 100 PN 16 | EN 1092-1 — a third family, common on European-built packages already installed | Ask whether the counter-flange on site is EN or K series | PN and K are both metric labels for tables that do not match |
| RF with no height, or a finish with no value | Facing type given, facing detail left to whichever flange standard applies | Get raised face height and face finish from the flange GA | Gasket seats on the wrong land, or studs come up short by the thickness you assumed |
| Gasket drawn as a line with no spec | The gasket family has not been decided yet | Ask for gasket type, material and the standard it is dimensioned to | Gasket inner diameter intrudes into the bore, or the outer diameter fouls the studs |
| No standard cited at all, on an older utility line | Often a fabricated plate flange left from an earlier phase | Ask for the as-built isometric and the tie-in detail; where none exists, the site takes the dimensions during the planned shutdown survey | You manufacture to a standard that was never there |
The document that decides, and who holds it
On a Korean project the arbiter is the piping material specification — the line class list issued by the EPC contractor or the owner's engineering group. It fixes flange standard, rating, facing, gasket type and bolting material for each service. Your general arrangement drawing does not overrule it, and neither does the catalog your equipment is normally supplied against.
The second document is the terminal point or battery limit drawing, which is where the scope split lives. Your supply usually stops at a flange face. Whose flange, whose gasket and whose bolts sit at that face is a commercial question as much as a technical one, and it decides who pays for a transition spool if one turns out to be needed.
So three things are worth asking for in writing, and a project engineer can send all three the same day: the piping material specification extract for the line class at your tie-in, the dimensioned flange detail at that point, and the gasket and bolting call-out. Everything above resolves from those.
They travel with the same package as the material and examination records the client will ask for later, which is a separate exercise with its own traps — a Korean client asking for a B31.3 certificate is asking for something that does not exist under that name. And if the flanged item sits inside a scope that goes through Korean regulatory approval, a late flange change is worth checking against the submission rather than treated as a shop drawing revision; where KGS approval actually applies is not always where a foreign supplier expects it.
The near miss is worse than the obvious mismatch
An obvious mismatch is cheap. The outside diameters differ visibly, nobody attempts the joint, and it becomes a procurement item with a lead time.
The expensive case is the pair that nearly agrees. Bolt circles within a couple of millimeters of each other, studs that pass through, a joint that pulls up square and holds the hydrotest. Bolt shanks end up bearing on the edges of the holes instead of sitting clear, and the gasket sits off center on the raised face. It seals cold. Months later, after thermal cycling, it weeps, and the gasket gets blamed for a geometry problem.
Three field fixes get proposed and none of them should be accepted: drilling out the holes, slotting them, or running smaller bolts through oversized holes. A drilled flange is no longer the flange described on its certificate, so a mechanical shortcut becomes a documentation problem as well. The accepted answer is a transition spool or an adaptor flange carrying one face from each family, made to the project specification by a fabricator who can issue the material records for it.
One practical warning about where the numbers come from. A table headed 10K does not say which document it came from: the K series runs across KS B 1503 and JIS B 2220, both have been revised more than once, and a page copied before a revision can still be sitting on a supplier's website. Work from the standard edition your specification names, or from the manufacturer's certified GA drawing with its revision noted, and put that reference on the purchase document. Equipment bought with flanged ends is bought this way as a matter of course — a control valve such as the KOSO 501G takes its end connections from the line class it is going into, not from the valve catalog. It is also what the papers get read against at receipt on the Korean site.
The boxes that come in blank
Inquiries for flanged equipment into Korea arrive with size, rating and facing filled in, and stop there. Four boxes are almost always empty, and each one decides something.
The gasket is usually named by type and nothing else — no standard it is dimensioned to, and no statement of who supplies it. The bolting is worse: stud material grade, nut grade, thread series and length are rarely given at all, and on a joint that spans two flange families the thread series is exactly where the metric and inch question finally bites. Face finish comes through as the word "smooth" rather than a value. And the fourth is the one that matters most — the counter-flange. Senders describe their own flange in detail. The joint is decided by the flange already standing on the Korean site, and that is the drawing nobody attaches.
Send the tie-in detail and the line class for the connection you are shipping into, and we will come back with the flange, gasket and bolting call-out that joint takes on the Korean side — and, where the two families do not meet, what a transition spool would have to be made and certified to.