Pipe Pressure Rating Chart and Wall Calculation

People often inquire about a pipe pressure rating chart anticipating something akin to the flange tables, which include a class and a temperature resulting in a pressure value. However, pipe works differently. Flanges are classified according to a set standard while pipes have wall thickness, material grade, and temperature besides other variables from which their pressure ratings can be derived. That is why the same NPS 6 line can be good for very different pressures depending on schedule and material, and why a schedule alone never answers the question. Ranges are underpipes and tubes.

The Calculation

Under ASME B31.3 the pressure design thickness for straight pipe under internal pressure is:

t = P × D / (2 × (S × E × W + P × Y))

Symbol	Meaning
t	Pressure design thickness
P	Internal design gauge pressure
D	Outside diameter
S	Allowable stress for the material at the design temperature, from the code tables
E	Quality factor, which accounts for the weld seam. See theseamless vs welded guide
W	Weld joint strength reduction factor, which applies at high temperature
Y	Coefficient depending on material and temperature

The simpler Barlow form, t = P × D / (2 × S), is useful for a quick sense of magnitude but it omits E, W and Y, so it should not be used for design.

The required wall is then the pressure design thickness plus corrosion allowance, mill tolerance and any mechanical allowances. Mill tolerance is the one most often forgotten: pipe is supplied to a tolerance on wall, and the calculation has to allow for the thin end of it.

The Four Things That Change the Answer

Grade. Allowable stress differs between grades, so the same wall carries different pressure inA106 Gr B, inA312 TP316L and induplex. Duplex has roughly double the yield strength of austenitic stainless, which is why it allows a thinner wall for the same duty.

Temperature. Allowable stress falls as temperature rises, and it falls steeply once creep governs. A line adequate at ambient can be inadequate at operating temperature with no change to the pipe.

Seam. A welded seam is credited at less than parent metal through the quality factor E, and the credit depends on the weld type and the examination performed.

Corrosion allowance. This is a design decision rather than a material property, and on carbon steel in wet service it frequently exceeds the pressure design thickness itself.

Worked Example

For NPS 6 pipe, outside diameter 168.3 mm, in seamlessA106 Gr B at near ambient temperature, taking the basic allowable stress from the B31.3 table as 137.9 MPa, with E = 1.0 for seamless, W = 1.0 and Y = 0.4:

For a design pressure of 50 bar, that is 5.0 MPa:

t = 5.0 × 168.3 / (2 × (137.9 × 1.0 × 1.0 + 5.0 × 0.4)) t = 841.5 / (2 × 139.9) t = 3.0 mm

Add corrosion allowance, say 3.0 mm, giving 6.0 mm, then allow for mill tolerance on wall.Schedule 40 at NPS 6 is 7.11 mm, which accommodates it.

Two observations from that example. The pressure design thickness is the smaller part of the answer once corrosion allowance is included, which is typical of carbon steel in wet service. And the result lands on a standard schedule, which is why schedules remain a convenient ordering unit even though they are not the design input.

Allowable stress values must be taken from the current code tables for the actual design temperature. The figure used above is illustrative of the method, not a substitute for the table.

Why Pipe and Flange Ratings Are Different Questions

	Pipe	Flange
Rating basis	Calculated from wall, grade and temperature	Pressure class from a standard table
Standard	ASME B31.3 or the governing code	ASME B16.5 or B16.47
Varies with	Schedule, grade, temperature, corrosion allowance	Class, material group, temperature
Ordering unit	Schedule or wall	Class

A system is limited by its weakest component, so a Class 150 flange limits a line regardless of how heavy the pipe wall is. Flange pressure and temperature ratings are their own subject, covered in theflange pressure and temperature rating guide, and the flange types themselves are in theflange types guide.

Pipe Weight

Useful for freight, supports and lifting, and calculable without a chart. For steel pipe:

W = 0.02466 × (D − t) × t

where W is kilograms per metre, D is outside diameter in millimetres and t is wall thickness in millimetres.

For NPS 6 Sch 40: W = 0.02466 × (168.3 − 7.11) × 7.11 = 28.3 kg/m.

The constant reflects the density of steel, so for stainless, duplex, copper nickel, titanium and nickel alloys it changes, and the figure should be adjusted by the density ratio of the actual material.

Weight is where wall decisions become tonnage. On a long transmission line, an extra millimetre of wall at NPS 20 adds roughly 12.5 kg per metre, which is 625 tonnes over 50 km.

What to State on the Enquiry

Design pressure and design temperature, not operating values
Minimum design temperature, where it governs. See thematerial selection guide
Corrosion allowance assumed
The minimum wall required, alongside the schedule
Whether seamless is required or welded is permitted, with the quality factor assumed
The governing code, since the formula and the allowable stress tables come from it

Pipe Pressure Ratings: Buyer Questions

Does pipe have a pressure class like a flange?

No. A flange has a class because its geometry is standardised, while pipe capacity is calculated from wall thickness, grade and design temperature.

What is the wall thickness formula under ASME B31.3?

t = P × D divided by 2 × (S × E × W + P × Y), where S is the allowable stress at design temperature, E the quality factor, W the weld joint strength reduction factor and Y a material and temperature coefficient.

Is the Barlow formula adequate for design?

No. It gives a sense of magnitude but omits the quality factor, the weld joint strength reduction factor and the Y coefficient, so it should not be used for design.

What is most often forgotten in a wall calculation?

Mill tolerance on wall thickness, and on carbon steel in wet service the corrosion allowance, which frequently exceeds the pressure design thickness itself.

Does a heavy pipe wall raise the pressure capacity of a system?

Only up to the weakest component. A Class 150 flange limits the line regardless of the pipe wall, so flange ratings have to be checked separately.

How is pipe weight calculated?

For steel, W in kilograms per metre equals 0.02466 times (D minus t) times t, with D and t in millimetres, adjusted by density ratio for other alloys.

Applying This to an Enquiry

Send design pressure, design temperature and the corrosion allowance assumed, then state the minimum wall alongside the schedule. A schedule quoted without the wall it was derived from cannot be checked, and a substitution made later will not be caught.

Quotations are available in USD, OMR, AED, SAR, QAR, KWD and BHD, on EXW, FOB, CFR or CIF terms, with EN 10204 3.1 certificates as standard and 3.2 on request. Lead time is confirmed at quotation.

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Pipe Pressure Ratings: Calculated, Not Looked Up