Type a line (114.3 x 6.02 mm S 120 MPa) or fill in the fields: the line and the fields stay in step.
Enter the outside diameter of the pipe and two of the three, pressure, wall thickness and allowable stress S, and the calculator works out the third by the Barlow formula P = 2 S t / D: the pressure for a wall, the wall for a pressure, or the hoop stress in the wall. A joint factor E and a corrosion allowance c can be added, and with the tensile strength it gives the Barlow burst estimate. It shows the working with your numbers, draws the pipe cross-section, and gives a line you can copy or add to a request.
No allowable stresses for materials here: you enter S from the material standard or the class rules, and the calculator never assumes one. No safety factors either. Class rules and the ASME B31 codes add their own terms and factors; this is the thin-wall formula itself, and with a ratio D / t below 20 the calculator says it is approximate.
P = 2 S t / D, with D the outside diameter, t the wall and S the stress entered. A pipe of 114.3 x 6.02 mm at S = 120 MPa: P = 2 x 120 x 6.02 / 114.3 = 12.64 MPa, or 126.4 bar. That is the pressure at which the hoop stress equals S, not the allowed pressure of the pipe.
t = P D / (2 S). At 16 bar (1.6 MPa), an OD of 60.3 mm and S = 100 MPa: t = 1.6 x 60.3 / (2 x 100) = 0.48 mm. With a joint factor and an allowance, in the codes' form t = P D / (2 S E) + c: at 25 bar, 88.9 mm, S = 110 MPa, E = 0.85 and c = 1 mm it comes to 1.19 + 1 = 2.19 mm.
S = P D / (2 t). A pipe of OD 168.3 mm with a 7.11 mm wall at 40 bar (4 MPa): S = 4 x 168.3 / (2 x 7.11) = 47.3 MPa. Compare it with the allowable stress from the material standard or the class rules.
| Quantity | Relation | Note |
|---|---|---|
| Pressure | P = 2 S t / D | D the outside diameter, t the wall, S the stress you enter |
| Wall thickness | t = P D / (2 S) | the wall at which the hoop stress equals S |
| Hoop stress | S = P D / (2 t) | the stress in the wall at the pressure P |
| With a joint factor and an allowance | t = P D / (2 S E) + c; P = 2 S E (t - c) / D | E and c as entered; the codes add their own factors |
| Barlow burst estimate | 2 x UTS x t / D | with the tensile strength; an estimate, not a rating |
| Thin wall | D / t of 20 or more | below 20 thin-wall theory is approximate |
| Units | 1 in = 25.4 mm; 1 bar = 0.1 MPa; 1 N/mm2 = 1 MPa | 1 psi = 0.006894757293168 MPa; 1 ksi = 1000 psi |
D is the outside diameter. The pressure is the gauge pressure, the difference across the wall. The joint factor E and the allowance c apply only when entered; the burst estimate leaves them out.
The formula P = 2 S t / D with the outside diameter, its forms for the wall, t = P D / (2 S), and for the hoop stress, and the burst estimate with the tensile strength are each given in at least two independent engineering references and pipe calculators. The form with a joint factor and an allowance, t = P D / (2 S E) + c, is the codes' form, built from the two terms: the joint factor E multiplying the stress and the allowance c added to the wall for the pressure each appear in two independent sources, summaries of the pipeline codes and reference wall calculations. The codes and the class rules add their own factors on top. The thin-wall limit of D / t = 20 is the one two sources agree on.
It relates the internal pressure P, the outside diameter D, the wall thickness t and the stress S in the wall: P = 2 S t / D. Any one of the three, pressure, wall or stress, follows from the other two: t = P D / (2 S) and S = P D / (2 t), the hoop stress. A pipe of OD 114.3 mm with a 6.02 mm wall at S = 120 MPa: P = 2 x 120 x 6.02 / 114.3 = 12.64 MPa, or 126.4 bar. The same pipe at 40 bar, 4 MPa: S = 4 x 114.3 / (2 x 6.02) = 38.0 MPa.
The allowable stress for the pipe material at the design temperature, as the material standard or the class rules give it, or another stress chosen for the check. The calculator holds no stresses for materials and never assumes one: without S it says what is missing. With the tensile strength of the material instead, the result is only the Barlow burst estimate.
The joint factor E multiplies the stress, and the allowance c is added to the wall worked out for the pressure; put together, that is the codes' form t = P D / (2 S E) + c, and the other way round P = 2 S E (t - c) / D. At 25 bar, OD 88.9 mm, S = 110 MPa, E = 0.85 and c = 1 mm: t = 2.5 x 88.9 / (2 x 110 x 0.85) + 1 = 1.19 + 1 = 2.19 mm. Design codes, class rules and ASME B31, add their own terms and factors, such as a design factor, a temperature factor and the mill tolerance, which the calculator does not apply.
The Barlow formula with the tensile strength of the material in place of S: 2 x UTS x t / D. It is an estimate on ideal conditions at room temperature, not a rating, and is to be reduced by a factor of safety; the factor comes from the design code or the class rules, and the calculator gives none. A pipe of OD 114.3 mm with a 6.02 mm wall and a tensile strength of 415 MPa: 2 x 415 x 6.02 / 114.3 = 43.7 MPa.
It treats the wall as thin, with the stress even through the wall. That holds while the ratio of the outside diameter to the wall, D / t, is 20 or more; below 20 thin-wall theory is approximate, and thick-wall relations (Lame) are used instead. The calculator shows D / t with each result and notes when it is below 20; a wall of half the OD or more leaves no space inside the pipe and is refused.
The OD and the wall in mm or inches, written as 114.3 x 6.02 mm or with the words OD and wall; a size without a unit is read in mm. The gauge pressure in bar, MPa, kPa or psi; the stress S in MPa, N/mm2, psi or ksi; the joint factor as E 0.85 or 85 %; the corrosion allowance as c 1 mm. Nominal sizes (DN, NPS, schedules) are not taken: the pipe size identifier gives the OD and the wall for them.
The calculator gives the pressure, the wall or the hoop stress by the Barlow formula. When a pipe or piping parts are needed, send the line from the calculator with the material, the standard and a photograph of the marking, and Vessel Core checks the details before an offer goes back.
