Pipe Material, Pipe Size, Pipe Schedule and Pipe Dimensions

[Source: From web]

Steel pipe sizes

“Nominal” refers to pipe in non-specific terms and identifies the diameter of the hole with a non-dimensional number (for example – 2-inch nominal steel pipe” consists of many varieties of steel pipe with the only criterion being a 2.375-inch (60.3 mm) outside diameter). Specific pipe is identified by pipe diameter and another non-dimensional number for wall thickness referred to as the Schedule (Sched. or Sch., for example – “2-inch diameter pipe, Schedule 40”).  Pipe size is quoted as a “Nominal Pipe Size” or NPS. The origin of the NPS numbers for smaller pipes (< NPS 12) is different to the origin for larger diameter pipes. However, all pipes of a specific NPS number have the same external or outer diameter (OD). The internal diameter will vary depending on the wall thickness of the metal. The reason for this is so that the same structural supports can be used for all piping of a specific NPS number regardless of the wall thickness.

Schedules

Steel pipe schedules are a way to describe the wall thickness of the pipe. This is a critical parameter as it is directly related to the strength of the pipe and the suitability for specific applications. A pipe schedule is a dimensionless number and is calculated based on the design formula for wall thickness, given the design pressure and allowable stress.

Examples of schedule numbers are as follows: 5, 5S, 10, 20, 30, 40, 50, 60, 80, 100, 120, 140, 160, STD, XS, and XXS—with the most common being schedules 40 and 80. As the schedule number increases, the wall thickness of the pipe increases. The schedule number of a pipe therefore defines the internal diameter, as the OD is fixed by the NPS number.

The schedule number is roughly calculated as: Schedule = 1000 x (P/S) where P is the internal service pressure of the pipe (psig) and S is the ultimate tensile strength of the pipe material (psi).

SCHEDULE = 1000(P/S)

P = internal service pressure of pipe (psig)

S = ultimate stress of pipe material (psi)

Piping Materials

Pipe Material

Piping material selection mainly depends on process conditions such as

• Fluid it transports
• at what temperature
• at what pressure it transports
• Cost of material
• Availability – whether the material is locally available or import is required.
• Weldability and Manufacturability of material also play a vital role in selection. Preference is always given to the material which does not require any special requirement during welding and manufacturing.

Material Characteristics

• density
• melting point
• thermal conductivity
• electrical conductivity (resistivity)
• thermal expansion
• corrosion resistance
• Ultimate Tensile Strength – that is the capacity of a material to withstand when subjected to tension.
• Yield Strength – that is the load at which plastic deformation or you can say permanent deformation start.
• Elasticity – is the ability of a material to resume its normal shape after the load is removed just like rubber.
• % Elongation – Percent elongation is a measure of ductility.
• Hardness – is ability of a material to resist plastic deformation.
• Toughness – ability of a material to absorb energy before fracture.
• Creep resistance – ability to resist any kind of distortion when under a load over an extended period.
• Fatigue Resistance – is ability of material to withstand cyclic load for a given number of cycles before it fails.

Frequently Used ASTM Grades for Various Piping Materials

ASTM Materials

Pipes

• A106 = This specification covers carbon steel pipe for high-temperature service.
• A335 = This specification covers seamless ferritic alloy-steel pipe for high-temperature service.
• A333 = This specification covers wall seamless and welded carbon and alloy steel pipe intended for use at low temperatures.
• A312 = Standard specification for seamless, straight-seam welded, and cold worked welded austenitic stainless steel pipe intended for high-temperature and general corrosive service.

Fittings

• A234 = This specification covers wrought carbon steel and alloy steel fittings of seamless and welded construction.
• A420 = Standard specification for piping fittings of wrought carbon steel and alloy steel for low-temperature service.
• A403 = Standard specification for wrought austenitic stainless steel piping fittings.

Flanges

• A105 = This specification covers standards for forged carbon steel piping components, that is, flanges, fittings, Valves, and similar parts, for use in pressure systems at ambient and higher-temperature service conditions.
• A182 = This specification covers forged or rolled alloy and stainless steel pipe flanges, forged fittings, and Valves and parts for high-temperature service.
• A350 = This specification covers several grades of carbon and low alloy steel forged or ring-rolled flanges, forged fittings and Valves for low-temperature service.

Valves

• A216 = This specification covers carbon steel castings for Valves, flanges, fittings, or other pressure-containing parts for high-temperature service and of quality suitable for assembly with other castings or wrought-steel parts by fusion welding.
• A217 = This specification covers steel castings, martensitic stainless steel and alloys steel castings for Valves, flanges, fittings, and other pressure-containing parts intended primarily for high-temperature and corrosive service.
• A352 = This specification covers steel castings for Valves, flanges, fittings, and other pressure-containing parts intended primarily for low-temperature service.
• A182 = This specification covers forged or rolled alloy and stainless steel pipe flanges, forged fittings, and Valves and parts for high-temperature service.

Bolts and Nuts

• A193 = This specification covers alloy and stainless steel bolting material for pressure vessels, Valves, flanges, and fittings for high temperature or high pressure service, or other special purpose applications.
• A320 = Standard Specification for Alloy-Steel and Stainless Steel Bolting Materials for Low-Temperature Service.
• A194 = Standard specification for nuts in many different material types.

What is Pipe NB (Nominal Bore)?

NPS is frequently referred as an NB (Nominal Bore). As such, there is no difference between NB and NPS. NB is also an American way to refer pipe dimensions.

What is DN (Diameter Nominal) Pipe Sizes?

DN or Diameter Nominal is International designation (SI or Matric Designator) and also a European equivalent of NPS to show pipe sizes. Here, you have to note that DN shows pipe sizes differently than NPS. For example, 2” pipe is simply mentioned as DN 50. You can get any NPS to DN by multiplying it with 25.

Standard Pipe Sizes and Dimensions

A Table of the Most Common Standard Pipe Sizes

Note: You can easily convert inch dimension to mm by multiplying it with 25.4 and rounding as follows:-

1. Outside diameter above 16 inches rounded to nearest 1 mm
2. Outside diameter 16 inches and below rounded to nearest 0.1 mm
3. Pipe wall thickness are rounded to nearest 0.01 mm

Dimension Tolerance for Carbon Steel and Stainless Steel Pipe Sizes and Schedule As per ASTM A530/A530M-12 and ASTM A999/A999M-15

Common dimension tolerances are listed in ASTM A530. However, each product has its own requirements and when given in that specification that will govern over the A530.

• Note-1: Pipe of NPS 4 (DN 100) and smaller may be weighed in lots; pipe in sizes larger than NPS 4 (DN 100) shall be weighed separately.
• Note-2: t = Nominal wall thickness. D = Outside diameter.
• Note-3: For welded pipe, the weld area shall not be limited by the over tolerance.
• Note-4: For thin-wall pipe, the ovality in any one cross-section shall not exceed 1.5 % of the specified outside diameter.

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