3 key points to know the tolerances of stainless steel pipes

Nominal Pipe Sizes (NPS)

Steel pipe sizes around the world are standardized on historic dimensions that are now referred to as Nominal Pipe Sizes (NPS).

In addition to standardizing the nominal outside diameter (OD), a series of standard wall thicknesses was developed years ago and these were labeled as Schedules (SCH). The units for both of NPS and SCH are dimensionless. A larger number indicates a larger OD size or a heavier wall thickness. A typical description might look like 4 NPS SCH 40. In this case this description indicates a nominal OD of 4.500” (114.3 mm) and a nominal wall thickness of 0.237” (6.02 mm). Through 8 NPS, the nominal dimensions for both carbon alloy and stainless alloy pipe are the same. In larger sizes, there are differences in some nominal wall thicknesses.

Historically, the dimensions for smaller diameter pipe were such that the inside diameter (ID) of what are now known as SCH 40 thicknesses were very close to the OD size designator. In the example above, the nominal ID is actually 4.026” for 4 NPS. As it became possible to make lighter (and heavier) wall thicknesses, the schedule of sizes was expanded. The sizes are standardized in ANSI B36.10 for carbon pipe and in ANSI B36.19 for stainless and nickel alloys.

:: Quote form Rolled Alloys, Inc. :Pipe Sizes and Tolerances

Roundness tolerance of steel pipe & tube

Roundness

Pipes and tubes are usually measured by the outside diameter dimension – or OD. One might think that a 2.0 inch OD tube would measure 2.0 inches and a 6.5 inch OD tube would measure 6.5 inches. However, machines today cannot hold dimensions precisely to the nominal value, and because of that there must be acceptable degrees of variations. In other words, every dimension on a blueprint is subject to variations from the nominal value and so is “roundness”. But “out of roundess” measurement can be contained.

Tubes, pipes or solids will have a range of values that would be acceptable and considered as ‘x’ nominal OD. Roundness tolerance (+/- 0.01 inch) would mean the difference between major OD and the minor OD throughout the profile should not be greater or less than 0.01 inches.

Ovality

When bending round profiles to a certain radius, the greater the OD to bend radius, the higher ovality should be expected.

American Society of Mechanical Engineers ASME states the following, “on section B31.1-2001 104.2 Curved Segments of Pipe (B) For ferrous material, when the radius of a bend is 5 nominal pipe diameters or greater, and the nominal wall thickness of the pipe is schedule 40 or thicker, the difference between maximum and minimum diameters shall not exceed 8% of average measured outside diameter of the pipe before bending.”

As a guideline, we consider 5% ovality as an acceptable measurement. That said, the ovality should always be discussed and determined by the end user prior to rolling.

stainless steel tube & pipe tolerance

Pipe tolerances will depend on the alloy and the standard to which it was produced. The general requirement standard covering most stainless pipe is ASTM/ASME A/SA 999. The OD under tolerance on all sizes is -0.031”. The over tolerance increases with OD size but for the range of 1-1/2 to 4 NPS the plus tolerance is also 0.031”. This introduces the concept of ovality, where if the pipe exhibited both the maximum and minimum OD permitted, it would be out of round (oval) by 0.062”. An additional ovality allowance is permitted for thin wall pipe which is defined as a pipe where the nominal wall is 3% or less of the OD. In such a case, the OD tolerance range applies to the mean OD and the ovality is permitted to be 1.5% of the OD. (See Inset).
Only the minimum wall thickness is limited, which is -12.5% of the nominal wall. There is no maximum wall thickness limit. For seamless pipe, there is a limit of 10% by weight (mass) over the nominal weight. Note that this limits the average wall thickness of the seamless pipe to being no more than 10% heavy, but does not limit the specific maximum wall thickness at any given point. There is no maximum thickness or weight limit for welded pipe.

Tolerances are also provided for stainless pipe on straightness (1/8” in 10’) and for cut lengths (-0, +1/4”). Notice that there is no requirement or tolerance for the ID.
The tolerances for most nickel alloy welded pipe are identical or very similar to those for stainless. The general requirements for nickel alloy welded pipe are contained in ASTM/ASME B/SB 775. General requirements for seamless nickel alloy pipe are published in ASTM/ASME B/SB 729. Because nickel alloys are more difficult to hot work, tolerances for hot finished pipe are more generous than for stainless steel or welded and cold finished nickel alloy products.

Three basic parameters

Outside diameter (OD), wall thickness and length are three basic parameters in the process of manufacture and distribution for stainless steel tube & pipe, and these parameters have been standardized.

Outer diameter and wall thickness are particularly important, they are not only the basic support for meeting the design requirements of various types of applications such as strength, stiffness and fluid delivery, but also have a decisive influence on the subsequent processing, therefore, standards for stainless steel tube & pipe diameter and wall thickness of the manufacturing accuracy (tolerance) should been standardized.

European Standard

European stainless steel tube & pipe standards specify 4 classes outer diameter and wall thickness in percentage or absolute value on the basis of nominal dimension.

Generally, larger diameter or heavy wall thickness stainless steel pipe use percentage, small diameter or thin wall thickness stainless steel tube use absolute value.

Standard	Manufacturing Process & Type	Outer Diameter Tolerance		Wall Thickness Tolerance	Others
		OD (Mm)	Class
EN10216-5	Hot Finished Seamless Tube	219.1～610	D1	+22.5%T，-15%T	T/D≤0.05
				T1	T/D≤0.09
				T2	T/D>0.09
		30～219.1	D2	T1
				T2
	Cold Finished Seamless Tubes	≤219.1	D3	T3
			D4	T4	Agreement
EN10297-2	Hot Finished Seamless Tube		D1	T1
			D2	T2	Agreement
	Cold Finished Seamless Tubes		D3	T3
			D4	T4	Agreement
EN10217-7	Welded Tubes	>168.3	D2	T3
		≤168.3	D3	T3
			D4	T3	Agreement
EN10296-2	Welded Tubes	>168.3	D2	T3
		≤168.3	D3	T3
		≤114.3	D4	T3	Agreement
EN10312	Welded Tubes	Series 1	D4	T3	T=0.6～2 Mm
		Series 2	D3 – D4	T3 – T4	T=1～3 Mm

From D1 to D4 class for outer diameter, and T1 to T4 for wall thickness tolerance comply to DIN EN ISO 1127 standard, stainless steel tubes – Dimensions, tolerances and conventional masses per unit length.

	Outside Diameter	Class	Wall Thickness
Class	Tolerance	Class	Tolerance
D1	±1.5%, Or ±0.75 Mm Min.	T1	±15%, Or ±0.6 Mm Min.
D2	±1.0%, Or ±0.5 Mm Min.	T2	±12.5%, Or ±0.4 Mm Min.
D3	±0.75%, Or. ±0.3 Mm Min.	T3	±10%, Or ±0.2 Mm Min.
D4	±0.50%, Or ±0.1 Mm Min.	T4	±7.5%, Or ±0.15 Mm Min.
		T5	+/- 5.0 %, Or +/- 0.10 Mm Min.

The tolerances on outside diameter and thickness from above table, greater value shall be selected.

EN 10312 has specific values in table series 1 and series 2.

America Standard

America standard has two standard rules to specify outside diameter and thickness, ASTM A999 / A999M and A1016 / A1016M.

ASTM A999	ASTM A1016
ASTM A312 / A312M ASTM A376 / A376M ASTM A409 / A409M ASTM A778 ASTM A790 / A790M ASTM A813 / A813M ASTM A814 / A814M ASTM A928 / A928M	ASTM A213 / A213M ASTM A249 / A249M ASTM A268 / A268M ASTM A269 ASTM A270 ASTM A688 / A688M ASTM A789 / A789M ASTM A803 / A803M

But A511 for seamless stainless steel mechanical tubing and A554 welded stainless steel mechanical tubing not conform to above standards, they have separate outside diameter and thickness tolerances.

Standard	Manufacturing & delivery condition	Outside Diameter OD / mm	OD tolerance mm	Thickness T / mm	T tolerance mm
					Minimum	Average
ASTM A1016 A1016M	Hot drawn seamless tubes	<=100	+0.4, -0.8	<=2.4	+0.4t, 0
		100 – 200	+0.4, -1.2	2.4 – 3.8	+0.35t, 0
		200 – 225	+0.4, -1.6	3.8 – 4.6	+0.33t, 0
				>=4.6	+0.28t, 0
	Cold drawn seamless tubes	<25	+0.1, -0.11		+0.20t, 0	+/-0.10t
		25 – 40
		40 – 50	+/-0.2		+0.22t, 0	+/-0.10t
		50 – 65	+/-0.25
		65 – 75	+/-0.3
		75 – 100	+/-0.38
		100 – 200	+/-0.38, -0.04
		200 – 250	+/-0.38, -1.14
	Welded tubes	40 – 50	+/-0.2		+0.18t, 0	+/-0.10t
		50 – 65	+/-0.25
		65 – 75	+/-0.3
		75 – 100	+/-0.38
		100 – 200	+/-0.38, -0.04
		200 – 250	+/-0.38, -1.14
ASTM A999 / M	Seamless tubes and welded tubes	<48.3	+0.4, -0.8			-0.125t
ASTM A312 / M	Seamless tubes and welded tubes	48.3-114.3	+/-0.8	OD=10.3-73		+0.20 t, -125t
		168.3-219.1	+1.6, -0.8	t/OD<=5% OD=88.9-457.2		+0.225 t, -125t
		219.1-457.2	+2.4, -0.8	t/OD>5% OD=88.9-457.2		+0.15 t, -125t
	Welded tubes	508-660	+3.2, -0.8	OD>=508		+0.175 t，-0.125 t
		711-864	+4.0, -0.8	t/OD<=5%, OD>=508		+0.225 t，-0.125 t
	Seamless tubes	914-1209	+4.8, -0.8	t/OD>5%, OD>=508		+0.15 t, -125t
ASTM A409 / M	Welded tubes		+/-0.2%	T<4.8
			+/-0.4%	t>=4.8
ASTM A358 / M	Welded tubes		+/-0.50%
ASTM A511	Seamless tubing	<=12.7	+/-0.1			+/-0.15t
		12.7-38.1	+/-0.2			+/-0.10t
		38.1-88.9	+/-0.3
		88.9-139.7	+/-0.4
		139.7-203.1	+/-0.8
		203.1-220	+/-1.1
		220-325	+/-1.6

American standard outside diameter tolerance mainly used to represent the absolute value, over and below tolerances are often asymmetric, SS tube and pipe in American standard average tolerance can reach even higher than D4 class in EN ISO 1127 standard.

Wall thickness tolerance should be +/-10%t or better, and average thickness tolerance should be ( +/-20 – 22%t, 0 ), pipeline pipe has a larger tolerance, hot finished seamless pipe largest.

China Standards

China has more quantity standards compared to European standards, but less than american standards,
SS tube and pipe standards are similar with EU standards in system, and absorb favorable experience in USA specifications.

Specification	Manufacturing	O.D(Mm)	O.D Tolerance	W.T(Mm)	W.T Tolerance
GB13296	Cold Rolled Seamless Pipe	6~30	±0.15/-0.2	1~3	+20%	-0%
		＞30~50	±0.3	＞3	+22%	-0%
		＞50	±0.75%	Also Average Wall
GB/T14976	Cold Rolled Seamless Tube	6~10	±0.15	1~3	+12.5%	-12.5%
		10~30	±0.2	＞3	+12.5%	-10%
		30~50	±0.3	Average Wall, +22%

Standard Comparison

Stainless steel tube & pipe wall thickness tolerance are the same as the general accuracy of American ASTM / ASME and EU standards, but slightly lower than the high requirements of the EU standards.

Stainless steel tube & pipe outer diameter tolerances are mostly lower than American standards, and approaching the EU standards. it is more reasonable that the American standard specify the OD tolerance is tight, compared to thickness tolerance.

China GB standards and EU standard have no weight tolerance specified, seamless tube & pipe in GB standard delivery goods as actual weight or theoretical weight, welded tube & pipe as theoretical weight or actual weight.