Pipe Terminology & Characteristics: NPS, OD, Schedule

Quick Reference

Key pipe terms: NPS (Nominal Pipe Size) = approximate bore capacity, DN (European equivalent), OD = outside diameter, WT = wall thickness, ID = inside diameter, Sch = schedule (wall thickness designation), SRL/DRL = single/double random length, PE/BE/T&C = plain/beveled/threaded ends. Pipe sizes: ASME B36.10/B36.19.

Learn the basic terminology about steel pipes: dimensional concepts such as nominal pipe size (“NPS”), inside diameter (“ID”), wall thickness (“WT”), pipe schedule (“Sch.”), pipe length single/double random (“SRL”, “DRL”); finishing of pipe ends (“Plain”, “Bevelled”, “Threaded”); dimensional specifications (“Specifications”); material grades.

Steel Pipes: Key Terminology

Understanding pipe terminology is essential for accurate specifications and compatibility in piping systems.

Dimensions of Steel Pipes

Nominal Pipe Size (NPS) / Iron Pipe Size (IPS)

Definition: NPS (Nominal Pipe Size), also historically known as Iron Pipe Size (IPS), is a standardized system to designate pipe diameters. It does not represent actual dimensions - it’s a reference value for specifying pipe size.

How NPS relates to actual dimensions:

NPS Range	Relationship to Actual Dimensions
NPS 1/8 to 12	NPS roughly corresponds to inside diameter (ID), but not exactly. OD is the consistent reference.
NPS 14 and above	NPS equals the actual outside diameter in inches (e.g., NPS 14 = 14” OD)

Key points:

• NPS indicates the pipe’s approximate flow capacity (bore capacity)
• The European equivalent is DN (Diamètre nominal / Durchmesser Nach)
• Both NPS and schedule must be specified together - NPS alone is insufficient

Important: When specifying pipes, always provide both NPS and schedule number. The schedule determines wall thickness, which varies significantly for the same NPS.

Outside Diameter (OD)

Definition: The Outside Diameter (OD) is the total diameter measured from one outer edge to the opposite outer edge through the pipe’s centerline.

The OD is the most consistent reference dimension across all pipe types and materials. For a given NPS, the OD stays the same regardless of schedule or wall thickness.

Why OD matters:

Application	Why OD is Critical
Fittings & flanges	Components must match the pipe OD for proper fit and seal
Supports & clamps	Brackets and hangers are sized to the OD
Welding & fabrication	Cut lengths and bevel preparations depend on OD
Material interchangeability	Same OD across steel, PVC, copper for a given NPS

For NPS 14 and above, the NPS value equals the OD in inches. For smaller sizes, check the dimensional tables - a 2” NPS pipe, for example, has an OD of 2.375”.

Key pipe sizes (NPS, OD, ID, WT)

Inside Diameter (ID)

Definition: The Inside Diameter (ID) is the internal bore of the pipe - the space through which fluid actually flows.

Formula: ID = OD - (2 x Wall Thickness)

The ID directly determines flow capacity and pressure drop. Unlike OD (which stays constant for a given NPS), the ID varies depending on the pipe schedule.

What ID affects:

Factor	Impact
Flow capacity	Larger ID = more volume per unit time
Pressure loss	Smaller ID = higher velocity = more friction loss
Pump sizing	Must match system flow requirements
Pigging & instrumentation	Inserted devices must fit through the ID

Remember: Same NPS, different schedules = different IDs. A 4” NPS Schedule 40 pipe has a larger ID than a 4” NPS Schedule 80 pipe, even though both have the same OD (4.500”).

Pipe with “Controlled ID”

Standard pipes have controlled OD with variable wall thickness (and thus variable ID). “Controlled ID” pipes flip this - they maintain a precise internal diameter regardless of wall thickness variations.

When to specify controlled ID:

Application	Why Controlled ID Matters
Pharmaceutical/food processing	Precise flow rates for process control
Hydraulic/pneumatic systems	Consistent pressure and flow dynamics
Water supply systems	Regulatory compliance requires specific flow volumes
High-pressure applications	Thick walls needed, but ID must stay constant for flow calculations

Controlled ID pipes cost more due to tighter manufacturing tolerances and additional quality checks.

Wall Thickness (WT)

Definition: Wall thickness (WT) is the distance between the outer and inner surfaces of the pipe wall, measured in mm or inches.

WT determines how much pressure a pipe can handle and how long it will last in service.

What wall thickness affects:

Factor	Thicker Wall =
Pressure rating	Higher allowable pressure
Mechanical strength	More resistant to impacts, bending, ground loads
Corrosion allowance	Longer life in corrosive environments
Weight & cost	Heavier pipe, higher material cost
Flow capacity	Reduced ID for same OD

Wall thickness can be specified two ways:

1. By schedule number (SCH) - standardized thicknesses (SCH 20, 40, 80, 160…)
2. By actual dimension - in mm or inches for custom applications

Design balance: Thicker isn’t always better. Over-specifying wall thickness adds unnecessary weight, cost, and reduces flow capacity. Match the WT to actual service conditions.

Schedule Number (SCH)

Definition: Schedule (SCH) is a standardized designation for pipe wall thickness. Higher schedule = thicker wall = higher pressure rating.

Common schedule designations:

Schedule	Typical Use
SCH 5, 10	Light-duty, low pressure
SCH 40 (STD)	Most common, general service
SCH 80 (XS)	Higher pressure applications
SCH 160, XXS	High pressure, severe service

Key relationships:

• Same NPS + higher schedule = thicker wall, smaller ID, same OD
• Same schedule + different NPS = different actual wall thickness in mm/inches
• Same schedule + different material = different pressure rating

Selection factors:

Factor	Higher Schedule When…
Operating pressure	System pressure is high
Temperature	Elevated temps reduce material strength
Corrosion	Aggressive environments need allowance
Cost/weight	Only if pressure demands it - don’t over-specify

Remember: Schedule is not wall thickness - it’s a designation. SCH 40 for a 2” pipe is 0.154” thick, but SCH 40 for a 6” pipe is 0.280” thick. Always verify actual dimensions from the size charts.

Pipe Length

Pipes come in standardized length ranges. The term “random” means the mill delivers within a range - not a precise cut length.

Pipe length and pipe ends PE BE threaded

Length Type	Range	Typical Use
SRL (Single Random Length)	16-22 ft (4.9-6.7 m)	General applications, easier handling
DRL (Double Random Length)	35-45 ft (10.7-13.7 m)	Large projects, fewer welds needed
CTL (Cut-to-Length)	As specified	Custom projects, reduces field welding

SRL vs DRL decision factors:

Factor	SRL	DRL
Handling	Easier, standard equipment	Requires longer trucks, cranes
Welds needed	More joints	Fewer joints
Leak potential	More potential leak points	Fewer potential leak points
Cost per meter	Higher (more welds)	Lower (less fabrication)
Transport	Standard logistics	May need special permits

Practical tip: DRL saves money on large pipeline projects (fewer welds = less labor + fewer leak points). But for plant piping with many direction changes, SRL may be more economical due to easier handling and less waste.

NPS vs. IPS Pipe System

You’ll encounter both terms - here’s how they relate:

System	Origin	Basis	Current Status
IPS (Iron Pipe Size)	Historical, 19th century	Based on ID of wrought iron pipes	Largely obsolete, sometimes used interchangeably with NPS
NPS (Nominal Pipe Size)	Modern standard	Based on OD for sizes ≥14”, nominal for smaller	Current industry standard

In practice: NPS has replaced IPS as the universal standard. If you encounter “IPS” in specifications, treat it as equivalent to NPS unless you’re working with legacy systems where the original iron pipe dimensions matter.

Bottom line: Specify using NPS. It’s material-agnostic and the industry standard for steel, stainless, plastic, copper, and all other pipe materials.

Steel Pipes Finishing

Pipe Material Grade

Material grade defines the pipe’s chemical composition and mechanical properties. It determines what conditions the pipe can handle.

Why material grade matters:

Factor	What the Grade Determines
Mechanical strength	Yield, tensile strength, ductility
Corrosion resistance	Compatibility with process fluids
Temperature limits	Min/max operating temperature
Weldability	Pre-heat requirements, PWHT

Common pipe material grades at a glance:

Material	Common Grades	Typical Application
Carbon steel	ASTM A106 Gr.B, A53, API 5L	General service, moderate temp
Alloy steel	ASTM A335 P11, P22, P91	High temp, high pressure (refineries, power plants)
Stainless steel	304/304L, 316/316L	Corrosive service, hygiene applications
Duplex	UNS S31803, S32205	High strength + corrosion resistance
Nickel alloys	Alloy 625, 825	Aggressive environments, high temp

A more complete taxonomy of pipe materials:

Classification of materials grades (steel and non-ferrous)The specification of a pipe material grade involves considering the service environment, the physical and chemical properties required, regulatory standards, and cost. Proper selection based on these grades ensures the safety, reliability, and efficiency of the piping system across its intended lifespan.

Pipe Standards

Pipe standards (ASTM, API, EN, BS) specify manufacturing processes, material properties, dimensions, and tolerances. Always reference the applicable standard in your specifications.

Pipe End Finish

The pipe end preparation determines how it connects to fittings, other pipes, or equipment.

Plain end pipe PEBeveled end pipe BEThreaded End PipeGrooved end pipe

End Type	Abbreviation	Description	Typical Application
Plain End	PE	Square-cut, no preparation	Slip-on flanges, mechanical joints
Beveled End	BE	Angled cut (30-37.5°) for weld prep	Butt-weld connections (most common)
Threaded End	TE	External or internal threads	Small bore pipes, maintenance access
Threaded & Coupled	T&C	Threaded with coupling attached	No-weld assembly, easy disassembly
Socket Weld	SW	Fits into socket of fitting	Small bore, high-pressure
Grooved End	-	Groove for mechanical coupling	Fire protection, HVAC, quick assembly
Bell End	-	Enlarged end for insertion joint	PVC/plastic, water & sewer

Selection considerations:

Factor	Best End Type
Permanent high-pressure joint	BE (butt-weld)
Needs disassembly for maintenance	TE or T&C
Quick field assembly	Grooved
Small bore (<2”) high pressure	SW

(Video source: FTPipelineSystems)

Pipe End Types: Beveled Ends

Mechanical Properties

Mechanical properties determine how a pipe behaves under load, pressure, and stress. Understanding these is essential for material selection.

Key mechanical properties summary:

Property	What It Measures	Why It Matters	Test Method
Tensile Strength	Max stress before breaking	Pressure containment capacity	Tensile test
Yield Strength	Stress where permanent deformation begins	Design limit for allowable stress	Tensile test
Elongation	% stretch before fracture	Ductility, ability to deform without cracking	Tensile test
Impact Resistance	Energy absorbed before fracture	Resistance to shocks, low-temp brittleness	Charpy V-notch
Hardness	Resistance to indentation	Wear resistance	Brinell, Rockwell
Toughness	Energy absorbed with plastic deformation	Combined strength + ductility	Impact tests

Strength Properties

Yield Strength vs. Tensile Strength: Yield is where permanent deformation starts. Tensile (UTS) is the maximum load before failure. Design calculations use yield strength with a safety factor.

Term	Definition
Yield Strength	Stress where elastic behavior ends (permanent deformation begins)
Ultimate Tensile Strength (UTS)	Maximum stress before necking and failure
Breaking Strength	Actual stress at fracture point

Ductility & Toughness

Elongation indicates ductility - how much the material can stretch before breaking:

• High elongation (>20%): Austenitic stainless steels (304, 316), aluminum
• Low elongation (<5%): Cast iron, hardened steels - more brittle

Toughness combines strength and ductility - the ability to absorb energy without fracturing. Critical for impact and shock loads.

Time-Dependent Properties

Property	Definition	Critical When
Creep Resistance	Resistance to slow deformation under sustained load at high temp	Operating above ~40% of melting point
Fatigue Resistance	Resistance to failure from cyclic loading	Vibrating lines, thermal cycling

Design principle: Match mechanical properties to service conditions. High-pressure needs high yield strength. Shock loading needs impact resistance. Cyclic service needs fatigue resistance. High temperature needs creep resistance.

Pro Tip

When ordering pipes, always specify both NPS and schedule (or actual wall thickness), never just NPS alone. “2-inch pipe” could mean anything from Schedule 5 (0.065” wall) to Schedule 160 (0.344” wall) - over 5x difference in wall thickness and pressure rating. Incomplete specifications lead to wrong deliveries and project delays.

Common Mistake

Assuming the pipe schedule gives the same wall thickness across all sizes. Schedule 40 for a 2” pipe is 0.154”, but Schedule 40 for a 12” pipe is 0.406”. Schedule is a designation, not a dimension - always verify the actual wall thickness from size charts for your specific NPS.

Field Note

When inspecting pipe deliveries, measure wall thickness at multiple points around the circumference and at both ends. Pipe wall thickness has a manufacturing tolerance of typically +/-12.5%. A pipe that meets spec at one point could be under minimum at another - especially important for pressure calculations that assume minimum wall.

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• ASTM Materials for Carbon Steel Pipes - Common carbon steel pipe specifications
• API 5L Line Pipes - Specifications for oil and gas pipeline materials
• ASME A13.1 Pipe Color Coding - Standard pipe marking and identification
• Piping Isometrics - How pipes are represented in engineering drawings
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