Question 1

What formula is used for pipe wall thickness calculation?

Accepted Answer

Two main standards apply: 1. Barlow's Formula (simpler): t = P×D / (2×S×E) — where P = internal pressure (MPa), D = outside diameter (mm), S = allowable stress (MPa), E = weld efficiency factor. 2. ASME B31.3 (process piping): t_m = PD / (2(SE + PY)) + c — where Y = temperature derating coefficient (0.4 for most metals at ≤450°C), c = sum of mechanical allowances including corrosion. The minimum thickness must then be increased by a mill tolerance (typically 12.5% for seamless, or as specified). South African process piping design uses ASME B31.1 (power piping) and ASME B31.3 (process piping), while distribution systems follow SANS standards.

Question 2

What is weld joint efficiency and how does it affect wall thickness?

Accepted Answer

Weld joint efficiency (E) accounts for the reduced strength of a welded seam compared to the base metal. Values per ASME B31.3 Appendix A: Seamless pipe (SMLS): E = 1.0. Electric Resistance Welded (ERW) with 100% radiography: E = 1.0 down to 0.9. ERW spot radiography: E = 0.85. ERW no radiography: E = 0.8. Double submerged arc welded (DSAW): E = 0.9–1.0. A lower E value means more wall thickness is required to achieve the same pressure rating. For high-pressure systems (>50 bar) and hazardous services, seamless or 100% X-ray ERW pipe is specified.

Question 3

What is a corrosion allowance and how do I choose it?

Accepted Answer

Corrosion allowance (c) is the extra wall thickness added to the structural minimum to compensate for material loss over the service life. Typical values: General carbon steel in water service: 1.5–3.0 mm. Carbon steel in mild corrosive process: 3.0–5.0 mm. Carbon steel in severe corrosive / sulphide service: 5.0–10.0 mm. Stainless steel (non-corrosive service): 0 mm. Lined or coated pipe (internal): 0 mm (rely on lining). Calculation: c = corrosion rate (mm/year) × design life (years). A carbon steel pipe handling acidic mine water in South Africa would typically use 3 mm corrosion allowance with a 20-year design life, implying a corrosion rate of 0.15 mm/year.

Question 4

When does thin-wall (membrane) theory apply vs thick-wall (Lamé) theory?

Accepted Answer

Thin-wall theory (Barlow's formula) is valid when t/D_i < 0.1, i.e. wall thickness is less than 10% of the inside diameter. It assumes uniform stress through the wall. For thicker walls (t/D_i > 0.1), Lamé's thick-wall equations are used: σ_hoop = P(r_i² + r_o²) / (r_o² - r_i²). ASME B31.3 inherently uses thin-wall for most process piping. For high-pressure applications (HP steam drums, hydraulic cylinders, autoclaves), engineering codes with Lamé equations (ASME VIII Div 2, PD 5500) apply. In South African engineering, ASME B31.3 is the primary reference for process piping at refineries, petrochemical, mining, and power plants.

Question 5

What pipe schedules are standard in South Africa?

Accepted Answer

South Africa uses both ASME/ANSI (imported) and SABS/SANS local standards. Common schedules: Sch 10 / 10S — light wall, stainless systems, vacuum. Sch 40 (standard weight, STD) — most common general purpose. Sch 80 (extra strong, XS) — high pressure / corrosive. Sch 160 — very high pressure systems. XXS (double extra strong) — extreme service. Local SANS standards: SANS 62 — galvanised steel pipes (plumbing). SANS 719 — steel pipes (mining, industrial). SANS 966 — uPVC pipes (water supply). SANS 4427 — HDPE polyethylene pipes (water and sewage). For importing: schedule numbers match ASME B36.10M/B36.19M which define wall thickness per DN size globally.

Question 6

How do I calculate the MAWP of an existing pipe?

Accepted Answer

Maximum Allowable Working Pressure (MAWP) is back-calculated from the minimum measured wall thickness: MAWP = 2SE(t - c) / (D - 2Y(t - c)) per ASME B31.3. Steps: 1. Measure or obtain actual minimum wall thickness (t). 2. Subtract corrosion allowance (c) to get structural thickness. 3. Determine S (allowable stress from material tables at design temperature). 4. Determine E (weld efficiency). 5. Apply the formula. In South African plant integrity programs, pressure vessel inspections use this calculation for re-rating aging equipment. MAWP is typically 10–25% lower than design pressure to allow for uncertainties and aging.

Question 7

What safety factor applies to pipe burst pressure?

Accepted Answer

The commonly used safety factor for pipe design: ASME B31.3 uses allowable stress = SMTS / 3 for carbon steel at room temperature, effectively giving a safety factor of ~3 on burst. Hydrostatic test pressure is typically 1.5 × design pressure (ASME B31.3 clause 345). Pneumatic test: 1.1 × design pressure. Theoretical burst pressure from Barlow's formula: P_burst = 2S_UTS × t / OD. So, design pressure ≈ P_burst / 3. South African pressure equipment follows SANS 347 (pressure vessels) and NRCS (National Regulator for Compulsory Specifications) for boilers, which mandates ASME/EN compliance for locally manufactured equipment.

Question 8

What material should I specify for mining slurry pipelines in South Africa?

Accepted Answer

South African mine slurry pipe selection depends on: 1. Abrasion resistance — HDPE PE100 (SANS 4427) is excellent for slurry. Rubber-lined steel is used for severe abrasive. 2. Corrosion — acid mine drainage (AMD) requires HDPE, rubber-lined, or stainless. 3. Pressure rating — HDPE PE100 available up to 25 bar (PN25). For high-pressure, use steel with HDPE liner. 4. Operating temperature — HDPE rated to 60°C. Above 60°C use steel. 5. Fatigue — slurry pulsations from centrifugal or diaphragm pumps. Common SA mine practice: Gold tailings: HDPE PE100 SANS 4427. Platinum concentrate (pressurised): SANS 719 steel with rubber liner. Chrome slurry: 316 stainless steel or HDPE. Coal: HDPE or mild steel.

Material	S (MPa)	UTS
Carbon Steel A106 Gr B / SANS 719	138	415
Carbon Steel A53 Gr B	138	415
Stainless Steel 304	138	515
Stainless Steel 316	138	515
Stainless Steel 316L	115	485
Alloy Steel P11	138	415
Duplex Stainless 2205	172	620
Copper	55	220

DN	OD (mm)	Sch40 t
DN50	60.3	3.91
DN80	88.9	5.49
DN100	114.3	6.02
DN150	168.3	7.11
DN200	219.1	8.18
DN250	273.1	9.27
DN300	323.9	10.31

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