What Has to Be Measured on a Stainless Steel Fitting
A stainless steel pipe fitting is not inspected by eye. Whether the item is a butt-weld elbow, a tee, a reducer, a socket-weld coupling or a threaded nipple, five quantities decide whether it will fit and hold: outside diameter, wall thickness, center-to-end or overall length, angle or bevel, and the surface condition of the weld preparation. Each of these is defined and toleranced in a specific dimensional standard, and the correct method is to measure against that standard rather than against another fitting.
The pressure grade of a fitting is a separate matter from its dimensions. A fitting can be dimensionally perfect and still fail a pressure acceptance test, and it can pass a pressure test while being out of tolerance on center-to-end. Both checks are needed.
The Governing Dimensional Standards
| Product | Reference standard | What it controls |
|---|---|---|
| Butt-weld fittings, NPS 1/2 to 48 | ASME B16.9 | Outside diameter, wall, center-to-end, overall length, angle, tolerances |
| Short-radius elbows and returns | ASME B16.28 | Reduced center-to-end dimensions |
| Forged socket-weld and threaded fittings | ASME B16.11 | Socket bore, depth, thread form, body dimensions |
| Corrosion-resistant butt-weld fittings, Sch 5S and 10S | MSS SP-43 | Light-wall dimensions and tolerances |
| Swaged nipples and bull plugs | MSS SP-95 | End-to-end length and end thickness |
| Flanges | ASME B16.5 | Flange OD, bolt circle, thickness, facing |
| European stainless fittings | EN 10253-3 / EN 10253-4 | Dimensions and tolerances for wrought and welded fittings |
Measuring Outside Diameter and Wall Thickness
Outside diameter is measured with a calibrated vernier caliper or a large-diameter micrometer at three positions spaced roughly 120 degrees apart around the circumference, and it is normally measured at the weld end, not on the body of the fitting. On a butt-weld fitting the outside diameter must match the pipe outside diameter, so an out-of-round condition shows up as a variation between the three readings rather than as a simple size error. Record the maximum, minimum and the difference; the difference is what the dimensional standard tolerances.
Wall thickness on a formed fitting is not uniform. The extrados of an elbow thins during forming and the intrados thickens, so a single reading means nothing. The accepted method is ultrasonic thickness measurement with a dual-element transducer, taking at least eight readings around the circumference of the extrados, the intrados and both flanks at each end and at the mid-length. The thinnest reading is compared with the nominal wall of the ordered schedule. Where the wall thickness is critical, a wall-thickness report with the measurement grid attached is the appropriate documentation; a single spot reading on the body should be treated as unrepresentative.
Measuring Center-to-End, Overall Length and Angle
Center-to-end distance is the fundamental length dimension of an elbow or a tee. It is the distance from the centerline of one end face to the centerline of the other end, so it must be measured either with a purpose-built center-to-end gauge or on a surface plate from a scribed centerline, never from the outside of the fitting to the other end. A 90 degree long-radius elbow in NPS 2, for example, has a nominal center-to-end dimension of about 76 mm in the ASME B16.9 tables; the value scales with nominal pipe size and is tabulated for every size and both radii. Angled fittings are checked with a bevel protractor or a digital angle finder across the two end faces, and the measured angle is compared with the nominal 45 or 90 degrees.
For tees and crosses, the controlled dimension is the overall length from end face to end face through the run, plus the center-to-end of the branch. For reducers the controlled dimensions are the overall length and the two end diameters. For socket-weld and threaded fittings the governing dimensions are the socket bore diameter, the socket depth and the body length. In every case the reading is compared with the tabulated value for the nominal size, not with a value derived by measuring the mating pipe.
Bevel geometry deserves specific attention on heavy-wall stainless fittings. The end preparation is normally 37.5 degrees plus or minus 2.5 degrees with a root face of about 1.6 mm, checked with a bevel gauge. A shallow or irregular bevel is a welding defect waiting to happen and it is not corrected by the dimensional tolerance of the fitting.
Pressure Grades and the 150 Pound Class
The pressure grade of a fitting is described by its class number, and the class number is a conventional designation rather than a direct pressure value. The widely quoted 150 pound class corresponds to 150 psi, which converts as follows.
| Unit | Equivalent of 150 psi |
|---|---|
| Megapascal (MPa) | approximately 1.03 MPa |
| Kilogram-force per square centimetre (kgf/cm2) | approximately 10.5 kgf/cm2 |
| Bar | approximately 10.3 bar |
These conversions are useful for comparing a class rating with a metric datasheet, but they must not be used as an allowable working pressure. The actual pressure-temperature rating of a fitting depends on the material group, the wall schedule, the design temperature and the applicable code, so the rating tables of the governing code, not the arithmetic conversion, decide the permissible pressure. Above ambient temperature the allowable stress falls, and a fitting rated at 150 psi at room temperature will have a substantially lower allowable pressure at 400 C.
A Practical Measurement Procedure
Identify the standard, the nominal size, the schedule and the material from the marking before measuring anything. Verify the heat number and the material grade by positive material identification if the specification requires it. Record the outside diameter at three points per end. Record the wall thickness on the ultrasonic grid and keep the thinnest value. Measure center-to-end or overall length on a surface plate from scribed centerlines. Check the angle with a protractor and the bevel with a bevel gauge. Check the internal bore for ovality and for weld flash, particularly on welded fittings. Compare every value with the tabulated value for the nominal size, and record the deviation rather than a pass or fail judgement. Surface roughness and pickled or polished finish are assessed separately, as is hardness when the specification calls for it.
Frequently Asked Questions
Q: What does 150 pound class actually mean for a stainless steel fitting?
A: The class number is a conventional designation. 150 psi converts to about 1.03 MPa, 10.5 kgf/cm2 or 10.3 bar, but the allowable working pressure at temperature must be read from the rating tables of the governing code.
Q: Where should the wall thickness of an elbow be measured?
A: On a grid that covers the extrados, the intrados, the flanks and both ends. Forming thins the extrados and thickens the intrados, so the thinnest value on the grid is the value that matters.
Q: How is center-to-end distance measured correctly?
A: From the centerline of one end face to the centerline of the other end face, using a purpose-built gauge or a surface plate with scribed centerlines. Measuring from the outside of the fitting gives a wrong result.
Q: Which standard applies to ASME B16.9 fittings?
A: ASME B16.9 covers wrought butt-welding fittings from NPS 1/2 to NPS 48 and controls outside diameter, wall, center-to-end, overall length, angle and tolerances. Short-radius elbows fall under ASME B16.28 and socket-weld fittings under ASME B16.11.
Q: Is the outside diameter of a fitting the same as that of the matching pipe?
A: Yes for butt-weld fittings, which is precisely why the outside diameter is measured at the weld end and checked for out-of-round. Socket-weld and threaded fittings are instead matched on the socket bore and thread form.
Q: What bevel angle should be expected on a heavy-wall stainless fitting?
A: Approximately 37.5 degrees with a tolerance of plus or minus 2.5 degrees and a root face of about 1.6 mm, verified with a bevel gauge.
