Common Pipe Mistakes: 12 Errors to Avoid

Pipe selection and specification errors lead to burst lines, corrosion failures, and costly reorders. These 12 mistakes cover the most frequent technical and commercial errors in pipe engineering for oil and gas.

Technical Mistakes

1. Confusing NPS with Outside Diameter

Error: Assuming that NPS 6 pipe has a 6-inch outside diameter (it’s actually 6.625”). Risk: Wrong calculations for supports, clamps, insulation thickness, and orifice plates. Fit-up problems at site. Fix: NPS equals OD only for NPS 14 and above. For NPS 12 and below, check ASME B36.10 dimensional tables.

2. Confusing Schedule with Wall Thickness

Error: Assuming that Sch 40 always means the same wall thickness regardless of pipe size. Risk: Sch 40 for NPS 2 = 3.91 mm, for NPS 8 = 8.18 mm, for NPS 24 = 14.27 mm. Wrong wall thickness = wrong pressure rating and weight calculations. Fix: Always look up the actual wall thickness in ASME B36.10/B36.19 for the specific NPS and schedule combination.

3. Using Welded Pipe Where Seamless Is Required

Error: Specifying ERW or LSAW pipe for high-pressure, sour service, or critical process lines. Risk: The longitudinal weld seam is a potential weakness under high pressure, cyclic loading, or corrosive (H₂S) environments. Weld seam cracking and failure. Fix: Use seamless pipe for Class 600+, NACE MR0175 sour service, lethal service, and high-cycle fatigue applications.

4. Using A106 Gr. B for Low-Temperature Service

Error: Specifying ASTM A106 Grade B pipe for design temperatures below -29°C (-20°F). Risk: A106 is not impact tested at low temperatures. The material becomes brittle and can crack under pressure at cold temperatures. Fix: Use ASTM A333 Grade 6 for low-temperature service (impact tested at -46°C).

5. Wrong Hydrostatic Test Pressure

Error: Testing at 1.5× design pressure without checking the actual requirement per ASME B31.3 or the pipe mill certificate. Risk: Over-testing can yield the pipe (especially thin-wall). Under-testing misses defects. The test factor depends on the code edition and temperature. Fix: Calculate test pressure per the applicable code (ASME B31.3 §345.4) and verify against the pipe manufacturer’s test certificate.

6. Ignoring Supplementary Requirements for Sour Service

Error: Ordering A106 Gr. B or A333 Gr. 6 without specifying NACE MR0175/ISO 15156 hardness requirements. Risk: Standard A106/A333 can have hardness above 22 HRC, which is susceptible to sulfide stress cracking (SSC) in H₂S service. Fix: Specify hardness limits (≤22 HRC per NACE MR0175) and add ASTM supplementary requirements for product analysis and hardness testing.

7. Pipe vs Tube Dimensional Confusion

Error: Using pipe dimensions (NPS/schedule) for tube calculations, or vice versa. Risk: Pipe is ID-controlled (nominal bore); tube is OD-controlled (exact OD). Mixing them up gives wrong flow areas, wrong fittings, and wrong support spacings. Fix: Verify whether the design calls for pipe (ASME B36.10) or tube (ASTM dimensional standards) and use the correct dimensional tables.

8. Not Accounting for Corrosion Allowance

Error: Selecting pipe wall thickness based only on pressure calculation, without adding corrosion allowance. Risk: Over the design life (typically 20-30 years), internal corrosion thins the wall below the minimum required thickness, leading to rupture. Fix: Add the specified corrosion allowance (typically 1.5-3.0 mm for carbon steel in hydrocarbon service) to the calculated minimum wall thickness.

Commercial Mistakes

9. Not Specifying Pipe End Preparation

Error: Ordering pipe without specifying bevel angle, land thickness, and root face for butt-weld ends. Risk: Non-standard bevels cause weld fit-up issues, poor root pass penetration, and WPS non-compliance at site. Fix: Specify end preparation per ASME B16.25 (standard 37.5° bevel, 1.6 mm land) in the PO, or call out project-specific requirements.

10. Forgetting to Specify Mill vs Shop Coating

Error: Not clarifying whether pipe requires bare delivery, mill-applied coating (e.g., FBE), or shop-applied coating. Risk: Bare pipe arrives and corrodes in the stockyard. Or coated pipe is ordered when the project plans to coat at a local shop; double cost. Fix: Specify coating type, standard (e.g., NACE SP0394 for FBE), and application stage in the PO.

11. Ordering by Weight Instead of Length

Error: Placing a PO in metric tons without specifying the lengths (random, double random, cut-to-length). Risk: Random-length pipe (5-7m) arrives when the project needs double random (10-12m), causing excessive field welds and wasted material. Fix: Specify pipe length type (random, double random, or exact cut-to-length) in addition to total quantity.

12. Missing Color Coding Specification

Error: Not specifying pipe color coding requirements for utility and process identification. Risk: Pipes installed without identification bands cause confusion during commissioning and operations, especially for safety-critical lines (fire water, instrument air, nitrogen). Fix: Specify color coding per ASME A13.1 or the project-specific color coding procedure in the PO.

Frequently Asked Questions

Is NPS the same as pipe outside diameter?

No. NPS (Nominal Pipe Size) is a dimensionless designator, not a measurement. For NPS 14 and above, the NPS number equals the actual OD in inches. For NPS 12 and below, the OD is larger than the NPS number; for example, NPS 4 has an OD of 4.500 inches. See pipe terminology.

What is the difference between pipe schedule and wall thickness?

Schedule is a standardized series (Sch 5S, 10, 40, 80, 160, XXS) that assigns a specific wall thickness for each NPS size per ASME B36.10. The same schedule number produces different wall thicknesses across different pipe sizes. Always look up the actual value in the dimensional table.

When should I use seamless pipe instead of welded pipe?

Use seamless pipe for high-pressure (Class 600+), high-temperature, sour service (NACE MR0175), lethal service, and cyclic-loading applications. Welded pipe (ERW, SAW) is acceptable for utility services, low-pressure process lines, and structural applications where the cost savings justify it.

Can I use A106 Grade B for low-temperature service?

No. ASTM A106 is not impact tested and is limited to a minimum design temperature of approximately -29°C. For lower temperatures, use ASTM A333 Grade 6, which is mandatory impact tested at -46°C and designed for cryogenic and cold-climate service.

What is the difference between pipe and tube?

Pipe is ordered by NPS and schedule, with the inside diameter being the nominal dimension. Tube is ordered by exact outside diameter and wall thickness (OD-controlled). They follow different ASTM standards, have different tolerances, and require different fittings. Never interchange them in calculations or specifications.