Pipe vs Tube vs Tubing: Differences, Sizing, Standards

Quick Reference

PIPE: Round hollow section for fluid transport, sized by NPS (nominal) + Schedule (wall thickness). TUBE: Round/square/rectangular section for structural/mechanical use, sized by exact OD + WT (inches or mm). TUBING: Flexible/small-diameter tubes for instrumentation. Pipe sizes: ASME B36.10/B36.19. Tube sizes: BWG/SWG gauges.

Pipe vs Tube vs Tubing

These three terms get confused constantly, but in engineering and procurement they mean different things. Mixing them up on a purchase order leads to wrong materials showing up on site.

Pipe

• Shape: Cylindrical hollow section
• Purpose: Transporting fluids, gases, and/or solids
• Dimensions: Specified by nominal size (NPS) + schedule. ASME B36.10 and ASME B36.19 define standard sizes
• Customization: Circular cross-sections only
• Material: Steel, nickel alloys, non-ferrous (copper), PVC
• Application: Oil & gas, water distribution, plumbing, HVAC, process piping

Tube

• Shape: Round, square, rectangular, and oval hollow sections
• Purpose: Structural/mechanical applications, also fluid conveyance
• Dimensions: Exact OD and WT in inches or mm
• Customization: Multiple shapes and custom sizes
• Material: Metal, plastic, and glass
• Application: Construction, automotive, aerospace, instrumentation

Tubing

• Purpose: Flexible or small-diameter applications, instrumentation
• Measurement: OD and wall thickness, often associated with flexible materials
• Customization: Can be bent or fitted around structures as needed
• Material: Soft metals, plastics, rubber

Key Takeaway While pipe is defined by nominal sizes and schedules (NPS/SCH) for fluid transport, tube and tubing emphasize precise engineering dimensions and material flexibility (OD/WT). “Tube” pertains to structural and mechanical applications with exact size specifications, whereas “tubing” suggests a broader, sometimes more flexible, application.

Pipe, tube, and tubing comparison

Comparison Table

Pipe, tube, and tubing differ across every engineering parameter, from sizing method to cost:

Parameter	Pipe	Tube	Tubing
Sizing method	NPS (Nominal Pipe Size) or DN + Schedule	Exact OD + WT (inches or mm)	Exact OD + WT (fractional inches or mm)
Wall thickness designation	Schedule (SCH 5S, 10S, 40, 80, 160, XXS)	Inches/mm or BWG/SWG gauge number	Inches/mm or BWG gauge
Cross-section shapes	Round only	Round, square, rectangular, oval	Round only
Typical size range	NPS 1/8” to 80”+	OD 1/4” to 12”+ (round); larger for structural	OD 1/8” to 1” (instrumentation); up to 2” (hydraulic)
OD tolerance	Loose (per ASME B36.10/B36.19)	Tight (per ASTM product specification)	Very tight (per ASTM A269, A213)
WT tolerance	+/- 12.5% (standard)	+/- 10% or tighter	+/- 10% or tighter
Key material standards	ASTM A106, A53, A335, API 5L, A312	ASTM A179, A192, A213, A269, A519	ASTM A269, A213 (SS); A179 (CS)
Primary applications	Fluid/gas transport, process piping	Heat exchangers, boilers, mechanical/structural	Instrumentation, hydraulic, sampling lines
End connections	Beveled, plain, threaded (NPT/BSP)	Plain, expanded, flared	Compression fittings (ferrule type)
Typical cost (per kg)	Lower	Higher	Highest
Delivery lead time	Shorter (stock items)	Longer (often made to order)	Moderate (standard sizes stocked)

Inside Diameter vs Outside Diameter

Inside diameter (ID) and outside diameter (OD) measure different cross-sectional dimensions of a pipe or tube:

Dimension	Definition	Primary Use
Inside Diameter (ID)	The measurement of the diameter of the inner surface of a cylindrical object, representing the distance across the widest part of the inner circular cross-section.	Determines the flow capacity of a pipe or tube, as it directly affects the volume of fluid or material that can pass through the conduit.
Outside Diameter (OD)	The measurement of the diameter of the outer surface of a cylindrical object, representing the distance across the widest part of the outer circular cross-section.	Used to specify the size and dimensions of pipes or tubes, providing a reference for fitting, joining, or connecting the conduit with other components.

In practical terms: pipe ID determines how much fluid can flow through, while pipe OD determines which fittings and flanges will fit around it. For tubes, OD is the primary ordering dimension because it determines fit into tube sheets, compression fittings, and support clamps.

Sizing Standards: NPS/DN for Pipes vs OD/WT for Tubes

One of the most critical distinctions between pipe and tube is how they are sized and ordered. Getting this wrong is the single most common procurement error in piping projects.

Pipe Sizing: NPS and DN

Pipes are specified by Nominal Pipe Size (NPS) in North America or Diametre Nominal (DN) in metric systems, combined with a schedule that defines wall thickness.

Key points about pipe sizing:

• NPS is not a physical measurement. For sizes below NPS 14, the NPS number does not correspond to either the inside or outside diameter. For example, a 4-inch NPS pipe has an OD of 4.500 inches, not 4.000 inches.
• NPS 14 and above: the NPS value equals the actual outside diameter in inches.
• The OD is constant for a given NPS, regardless of schedule. A 6-inch Schedule 40 pipe and a 6-inch Schedule 160 pipe both have an OD of 6.625 inches; only the wall thickness (and therefore ID) changes.
• Schedule determines wall thickness: higher schedule = thicker wall = smaller ID = higher pressure rating.

Tip

For a complete conversion table between NPS and DN values, along with pipe OD and wall thickness for every schedule, see the ASME B36.10 and B36.19 pipe size charts.

Tube Sizing: Exact OD and WT

Tubes are specified by their actual outside diameter and wall thickness, both expressed as exact measurements in inches or millimeters.

Key points about tube sizing:

• The OD is the true outside diameter. A 1-inch OD tube measures exactly 1.000 inches on the outside.
• Wall thickness is stated in decimal inches, millimeters, or gauge numbers (BWG or SWG).
• There is no “schedule” system for tubes. You simply specify the OD and WT you need.
• ID is calculated: ID = OD - (2 x WT).

Sizing Trap

A “2-inch pipe” (NPS 2) has an OD of 2.375 inches. A “2-inch tube” has an OD of 2.000 inches. These are completely different products. You cannot substitute one for the other because flanges, fittings, supports, and tube sheets are all designed around specific OD values.

What Is a Pipe?

The word “steel pipe” refers to round hollow sections to convey fluids and gases, including oil & gas, propane, steam, acids, and water.

The most important dimension for a steel pipe is the inside diameter (“pipe ID”), which indicates the rough (not the exact) fluid conveyance capacity of the tubular. The ID is expressed in “NPS” or “DN” (nominal pipe size, or bore size).

Note

The pipe outside diameter (OD) does not match the nominal size for pipes below NPS 14 inches. For example, a 2-inch pipe has an internal flow capacity of approximately 2 inches, but has an outside diameter of 2.375 inches.

For pipes of a given NPS, the pipe outside diameter is fixed, whereas the pipe inside diameter decreases by increasing schedule values (pipe wall thickness). The most important mechanical parameters for pipes are the pressure rating, the yield strength, and the ductility.

Standard combinations of pipe nominal diameter and wall thickness (schedule) are defined in the ASME B36.10 and ASME B36.19 specifications (for carbon/alloy pipes and stainless steel pipes, respectively).

Pipe Inside Diameter Calculator

Because the outside diameter of pipes of a specific NPS is constant, the inside diameter (ID) changes depending on the pipe schedule. To calculate the pipe ID, deduct the pipe wall thickness multiplied by 2 from the pipe OD (the WT can be taken from the schedule).

Example: for a 12 NPS pipe (DN 300 mm), schedule 40, the pipe outside diameter and the wall thickness are 12.75 inches (324 mm) and 0.406 inches (10.4 mm). Therefore, the pipe ID (internal diameter) is 12.75 inches - 2 x 0.406 inches = 11.94 inches, or Pipe ID = 324 mm - 2 x 10.4 mm = 303.2 mm.

This calculation is theoretical only, since pipes have a wall thickness tolerance of +/-12.5% per ASME standards. The actual ID of a given pipe may differ by +/- 12.5% from the calculated value.

What Is a Tube?

The word “tube” refers to round, square, rectangular, and oval hollow sections used for pressure equipment, for mechanical applications, and for instrumentation systems.

Tubes are designated by their outside diameter and wall thickness, which are exact measures in inches or millimeters. For tubes, the difference between the outside diameter and the wall thickness, multiplied by two, defines the inside diameter of the tube.

Key physical properties of steel tubes are hardness, tensile strength, and low manufacturing tolerances.

Manufacturing Methods: Pipe vs Tube

How pipes and tubes are manufactured is fundamentally different, and these differences directly affect cost, lead time, and available sizes.

Pipe Manufacturing

Pipes are produced using high-volume, continuous processes optimized for output:

• Seamless pipe: A solid steel billet is heated and pierced using a rotary piercing mill (Mannesmann process), then elongated and sized through a series of rolling stands. See Types of Pipes: Seamless and Welded for details.
• Welded pipe (ERW): Flat steel coil (skelp) is formed into a cylinder and the longitudinal seam is welded using high-frequency electric resistance. Common for NPS 1/2” through 24”.
• Welded pipe (LSAW/DSAW): Steel plates are formed into a “U” then “O” shape and seam-welded with submerged arc welding. Used for large-diameter line pipe (24” and above).
• Production volume: Pipe mills run continuously, producing thousands of tons per day. Standard sizes are stocked by distributors worldwide.

Tube Manufacturing

Tubes require more controlled, lower-volume processes to meet tighter tolerances:

• Seamless tubes: Similar rotary piercing as pipe, but followed by cold drawing or cold pilgering to achieve precise OD, WT, and surface finish. Multiple drawing passes may be needed.
• Welded tubes: Formed from strip and welded (TIG, laser, or ERW), then often cold-drawn to final dimensions. The weld seam may be heat-treated and/or ground flush.
• Cold finishing: Most tubes undergo cold drawing or cold pilgering as a final step, which improves dimensional accuracy, surface finish, and mechanical properties.
• Inspection intensity: Every tube typically receives 100% non-destructive examination (eddy current testing, ultrasonic testing, or hydrostatic testing).

Note

Cold-drawn tubes can achieve OD tolerances of +/- 0.05 mm and surface finishes of Ra 0.8 micrometers or better, far tighter than what pipe mills deliver. This precision is essential for tube sheets in heat exchangers, where each tube must fit snugly into its hole.

Material Standards: Pipe vs Tube

ASTM and API standards draw a clear line between pipes and tubes. The specification printed on a material test certificate immediately tells you which product you are dealing with.

Common Pipe Standards

Standard	Description	Material
ASTM A106	Seamless carbon steel pipe for high-temperature service	Carbon steel (Gr. A, B, C)
ASTM A53	Seamless and welded carbon steel pipe	Carbon steel (Gr. A, B)
API 5L	Line pipe for oil & gas transmission	Carbon/HSLA steel (Gr. B through X80)
ASTM A335	Seamless alloy steel pipe for high-temperature service	Alloy steel (P5, P9, P11, P22, P91)
ASTM A312	Seamless and welded austenitic stainless steel pipe	Stainless steel (304/L, 316/L, 321, 347)
ASTM A790	Seamless and welded duplex stainless steel pipe	Duplex/super duplex (S31803, S32750)

Common Tube Standards

Standard	Description	Material
ASTM A179	Seamless cold-drawn carbon steel tubes for heat exchangers	Carbon steel
ASTM A192	Seamless carbon steel boiler tubes for high-pressure service	Carbon steel
ASTM A213	Seamless alloy and stainless steel tubes for boilers/superheaters	Alloy & stainless steel (T5, T9, T11, T22, TP304, TP316)
ASTM A269	Seamless and welded austenitic stainless steel tubing	Stainless steel (TP304/L, TP316/L)
ASTM A519	Seamless carbon and alloy steel mechanical tubing	Carbon & alloy steel
ASTM A789	Seamless and welded duplex stainless steel tubing	Duplex (S31803, S32205, S32750)

Tip

A simple rule of thumb: if the ASTM standard has “pipe” in its title, the product follows NPS/schedule sizing. If it says “tube” or “tubing,” the product is sized by exact OD and WT.

Pipe vs Tube: 10 Basic Differences

#	PIPE VS TUBE	STEEL PIPE	STEEL TUBE
1	Key Dimensions (Pipe and Tube Size Chart)	The most important dimension for a pipe is the inside diameter (ID), expressed in NPS (nominal pipe size) or DN (nominal diameter), which defines its fluid conveyance capacity. The NPS does not match the true inside diameter, it is a rough indication	The most important dimensions for a steel tube are the outside diameter (OD) and the wall thickness (WT). These parameters are expressed in inches or millimeters and express the true dimensional value of the hollow section.
2	Wall Thickness	The thickness of a steel pipe is designated with a “Schedule” value (the most common are Sch. 40, Sch. STD., Sch. XS/XH, Sch. XXS). Two pipes of different NPS and same schedule have different wall thicknesses in inches or millimeters.	The wall thickness of a steel tube is expressed in inches or millimeters. For tubing, the wall thickness is measured also with a gage nomenclature (BWG, SWG).
3	Types of Pipes and Tubes (Shapes)	Round only	Round, rectangular, square, oval
4	Production range	Extensive (up to 80 inches and above)	A narrower range for tubing (up to 5 inches), larger for steel tubes for mechanical applications
5	Tolerances (straightness, dimensions, roundness, etc) and Pipe vs. Tube strength	Tolerances are set, but rather loose. Strength is not the major concern.	Steel tubes are produced to very strict tolerances. Tubulars undergo several dimensional quality checks, such as straightness, roundness, wall thickness, and surface, during the manufacturing process. Mechanical strength is a major concern for tubes.
6	Production Process	Pipes are generally made to stock with highly automated and efficient processes, i.e. pipe mills produce on a continuous basis and feed distributors stock around the world.	Tube manufacturing is more lengthy and laborious
7	Delivery time	Can be short	Generally longer
8	Market price	Relatively lower price per ton than steel tubes	Higher due to lower mill productivity per hour, and due to the stricter requirements in terms of tolerances and inspections
9	Materials	Various materials available	Tubing is available in carbon steel, low alloy, stainless steel, and nickel alloys; steel tubes for mechanical applications are mostly of carbon steel
10	End Connections	The most common are beveled and plain ends	Threaded and grooved ends are available for quicker connections on-site

When to Use Pipe vs Tube in Oil & Gas

In oil and gas projects, both pipes and tubes serve critical but distinct roles. Selecting the wrong product for a given service can result in safety incidents, failed inspections, or costly rework.

Use Pipe When:

• Transporting fluids in process piping systems: from wellhead to separator, through processing units, and to export. Governed by ASME B31.3 (process piping) or ASME B31.4/B31.8 (pipeline transport).
• Building pipeline infrastructure: cross-country and subsea pipelines use line pipe per API 5L with NPS/schedule sizing.
• Connecting equipment with flanged or butt-welded joints: all standard pipe fittings (flanges, elbows, tees, reducers) are manufactured to match NPS pipe dimensions.
• Fire protection and utility systems: firewater, potable water, drain, and vent systems use pipe.

Use Tube When:

• Heat exchangers: shell-and-tube heat exchangers use tubes expanded or welded into tube sheets. Tight OD tolerances are necessary for proper tube-to-tubesheet fit.
• Boilers and superheaters: boiler tubes per ASTM A192 or A213 must withstand high temperatures and pressures with precise wall thickness control.
• Condensers and coolers: condenser tubes require excellent heat transfer, corrosion resistance (often copper-nickel, titanium, or stainless steel), and consistent wall thickness.
• Structural and mechanical applications: support frames, handrails, and mechanical assemblies where strength-to-weight ratio and dimensional precision matter.

Use Tubing When:

• Instrumentation systems: impulse lines from process taps to transmitters, analyzer sample lines, and pneumatic signal tubing. Typically 1/4”, 3/8”, or 1/2” OD stainless steel per ASTM A269, connected with ferrule compression fittings.
• Hydraulic control systems: high-pressure hydraulic lines on BOP stacks, subsea trees, and control modules.
• Chemical injection: small-bore tubing for injecting corrosion inhibitors, methanol, and other chemicals into process streams.
• Sampling systems: sample transport lines from process taps to analyzer shelters.

Heat Exchanger Tubing

Heat exchangers are the single largest consumer of tubes in the oil and gas industry.

Why Tubes, Not Pipes?

Heat exchangers use tubes rather than pipes for several critical reasons:

1. Tight OD tolerances: tubes must fit precisely into drilled holes in the tube sheet. A typical tube sheet hole is drilled to the tube OD + 0.2 mm. Pipe OD tolerances are too loose for this.
2. Controlled wall thickness: heat transfer calculations depend on exact WT. The tube WT tolerance of +/- 10% (vs +/- 12.5% for pipe) allows more accurate thermal design.
3. Surface finish: smoother internal surfaces reduce fouling and improve heat transfer coefficient.
4. 100% NDE: every tube is tested (eddy current or hydrostatic) before shipment. Pipe inspection is typically sampling-based.

Common Heat Exchanger Tube Specifications

Service	Standard	Typical Material	Common Sizes (OD)
General process	ASTM A179	Carbon steel	3/4”, 1”
High-temperature	ASTM A213	T11, T22 alloy steel	3/4”, 1”, 1-1/4”
Corrosive service	ASTM A213	TP304L, TP316L stainless	3/4”, 1”
Seawater coolers	ASTM B111	CuNi 90/10 (C70600)	3/4”, 1”
Highly corrosive	ASTM B338	Titanium Gr. 2	3/4”, 1”

Tube wall thickness for heat exchangers is commonly specified using the BWG (Birmingham Wire Gauge) system. Typical gauges are 14 BWG (0.083”, 2.11 mm) and 16 BWG (0.065”, 1.65 mm) for 3/4” OD tubes.

Note

When specifying heat exchanger tubes, always state: material standard (e.g., ASTM A213), grade (e.g., TP316L), OD (e.g., 3/4”), WT or BWG (e.g., 14 BWG or 0.083”), length, and condition (annealed, stress-relieved). Omitting any of these leads to inquiry delays.

Instrumentation Tubing

Instrumentation tubing is a specialized subset of tubing used throughout oil and gas facilities for connecting process instruments, transmitters, control valves, and analyzers.

Key Characteristics

• Small diameters: Most instrumentation tubing is 1/4” OD, 3/8” OD, or 1/2” OD. Larger sizes (3/4” and 1”) are used for some hydraulic and sampling applications.
• Material: 316/316L stainless steel is the standard choice (ASTM A269). For sour service or highly corrosive environments, alloy 625 (UNS N06625) or alloy C-276 is specified.
• Wall thickness: Common wall thicknesses are 0.035” (0.89 mm) and 0.049” (1.24 mm) for standard pressure ratings, and 0.065” (1.65 mm) for high-pressure service.
• Connections: Instrumentation tubing uses compression fittings (ferrule type), not pipe threads or flanges. The two most common brands are Swagelok and Parker A-LOK. See Ferrule Compression Fittings for a detailed guide.
• Seamless vs welded: Both are acceptable per ASTM A269, but seamless tubing is preferred for critical and high-pressure applications.

Instrumentation Tubing vs Small-Bore Pipe

A common question on construction sites: can you use 1/2” NPS pipe instead of 1/2” OD tubing for instrument connections? The answer is no. Here is why:

Parameter	1/2” OD Tubing	1/2” NPS Pipe (Sch 80)
Outside diameter	0.500” (12.70 mm)	0.840” (21.34 mm)
Wall thickness	0.049” (1.24 mm)	0.147” (3.73 mm)
Fitting type	Compression (ferrule)	Threaded (NPT) or socket weld
Bend radius	Tight bends possible with tube bender	Requires fittings for direction changes
Weight	Light, easy to route	Heavier, rigid

Do Not Interchange

Never substitute pipe for tubing (or vice versa) in instrumentation systems. Compression fittings are designed for exact OD tubing. Attempting to use them on pipe (which has a different and inconsistent OD) will result in leaks, particularly dangerous in hydrocarbon or toxic service.

Practical Selection Guide

Use this decision framework when specifying pipe or tube for a given application:

Step 1: What is the primary function?

• Fluid transport in process piping → Pipe
• Heat transfer in exchangers/boilers → Tube
• Instrument/hydraulic/sampling connections → Tubing
• Structural support → Tube (mechanical)

Step 2: What sizing system does the design use?

• NPS and schedule on the drawing → Pipe
• Exact OD and WT (or BWG gauge) → Tube/Tubing

Step 3: What fittings are specified?

• Flanges, butt-weld fittings, socket-weld fittings → Pipe
• Compression fittings (ferrule type) → Tubing
• Tube-to-tubesheet expansion → Tube

Step 4: What ASTM/API standard is called out?

• A106, A53, A335, A312, API 5L → Pipe
• A179, A192, A213, A269, A519 → Tube/Tubing

Pro Tip

When reviewing material requisitions, check that the sizing system matches the product standard. If someone calls out “ASTM A269, 1/2” Schedule 40”, that is an error. ASTM A269 is a tube standard; it uses OD and WT, not schedules. Catching these mismatches early saves weeks of back-and-forth with suppliers.

Standards Reference Table

Frequently referenced standards for pipes, tubes, and tubing:

Category	Standard	Title / Scope
Pipe sizing	ASME B36.10	Welded and seamless wrought steel pipe (carbon/alloy)
Pipe sizing	ASME B36.19	Stainless steel pipe
Tube sizing	TEMA (Tubular Exchanger Manufacturers Association)	Heat exchanger tube dimensions and tolerances
Carbon steel pipe	ASTM A106	Seamless CS pipe for high-temperature service
Carbon steel pipe	ASTM A53	Seamless and welded CS pipe (general service)
Line pipe	API 5L	Line pipe for oil & gas pipeline transport
Alloy pipe	ASTM A335	Seamless ferritic alloy steel pipe (P-grades)
Stainless pipe	ASTM A312	Seamless & welded austenitic SS pipe
Duplex pipe	ASTM A790	Seamless & welded duplex SS pipe
CS heat exchanger tube	ASTM A179	Seamless cold-drawn CS tubes
Boiler tube	ASTM A192	Seamless CS boiler tubes
Alloy/SS tube	ASTM A213	Seamless ferritic and austenitic alloy steel tubes
SS tubing	ASTM A269	Seamless & welded austenitic SS tubing
Mechanical tube	ASTM A519	Seamless CS & alloy mechanical tubing
Duplex tube	ASTM A789	Seamless & welded duplex SS tubing
Tube gauge	BWG / SWG	Birmingham Wire Gauge / Standard Wire Gauge
Pipe fittings	ASME B16.9	Factory-made wrought butt-welding fittings
Tube fittings	-	Proprietary (Swagelok, Parker, Hoke) per manufacturer specs

Bottom Line

The core distinction is simple: pipe is sized by NPS and schedule (nominal dimensions for fluid transport), while tube is sized by exact OD and wall thickness (precise dimensions for structural/mechanical use). When ordering, always be explicit about which system you are using. A “2-inch” pipe and a “2-inch” tube are physically different products that will not interchange.

Pro Tip

When ordering, be specific: “2-inch pipe” and “2-inch tube” are completely different products. A 2” NPS pipe has an OD of 2.375”, while a 2” OD tube has an actual OD of 2.000”. Confusing these on a purchase order means wrong materials, wasted time, and fittings that will not fit.

Common Mistake

Using pipe-based calculations (NPS/Schedule) when specifying instrumentation tubing. Tubing for instrument connections (per ASTM A269 or A213) is sized by actual OD and wall thickness. Applying pipe schedules to 1/4” or 3/8” tubing leads to ordering errors and incompatible compression fittings.

Frequently Asked Questions

What is the difference between pipe and tube?

Pipe is sized by nominal pipe size (NPS) and schedule for fluid transport; tube is sized by exact outside diameter (OD) and wall thickness (WT) for structural, mechanical, or heat-transfer applications. A 2-inch NPS pipe has an OD of 2.375 inches, while a 2-inch OD tube measures exactly 2.000 inches. Pipes use butt-weld and flanged connections; tubes use expansion joints, compression fittings, or flared ends.

What ASTM standards apply to pipes vs tubes?

Common pipe standards are ASTM A106 (seamless carbon steel), ASTM A53 (seamless and welded), and API 5L (line pipe). Common tube standards are ASTM A179 (heat exchanger), ASTM A192 (boiler), and ASTM A213 (seamless alloy and stainless steel tubes). For stainless steel, pipes use ASTM A312 while tubes use ASTM A269. The standard on the material test certificate is the quickest way to confirm whether a product is pipe or tube.

Can pipe and tube be used interchangeably?

No. Pipe and tube are not interchangeable because they follow different sizing systems, have different dimensional tolerances, and use different fittings. Pipe fittings (flanges, elbows, tees) are designed around NPS dimensions, while tube fittings (compression fittings, flare fittings) are designed around exact OD. Mixing them causes fit-up failures, potential leaks, and safety hazards, especially in hydrocarbon or toxic service.

What is tubing in piping?

Tubing refers to small-diameter tubes, typically 1/8" to 1" OD, used for instrumentation, hydraulic, and sampling systems in process plants. Tubing is connected with compression fittings (ferrule type, such as Swagelok or Parker) and is sized by actual outside diameter and wall thickness, not NPS. Common materials include 316 stainless steel (ASTM A269) and nickel alloys for corrosive service.

Why are tubes more expensive than pipes?

Tubes cost more per unit weight because they require tighter manufacturing tolerances (OD, WT, straightness, ovality, surface finish), additional cold-finishing steps (cold drawing or cold pilgering), and 100% non-destructive examination (eddy current or ultrasonic testing on every tube). Pipe mills produce at high volume with looser tolerances and sampling-based inspection, resulting in lower unit costs. The added tube cost is justified by the precision needed for heat exchanger tube sheets, instrument fittings, and mechanical assemblies.