What is Piping? | Basics of Piping and Pipelines Explained
1. Introduction
Behind nearly every industrial facility lies a hidden network of pipes. Globally, these piping systems and the long-distance pipelines they connect to stretch for millions of miles. Within a single large plant, however, the total length is more modest; typically 50 to 500 miles for a major refinery or petrochemical complex.
The term “piping” refers to the complete system of pipes, valves, flanges, fittings, and supports that convey fluids such as chemicals, fuels, steam, water, and compressed air throughout a manufacturing or power generation facility.
This page provides a detailed look at piping fundamentals, classifications, materials, sizing standards, and how piping systems differ from pipelines; making it an essential introduction for young engineers and technical professionals.
Piping systems are engineered to meet specific temperature, pressure, and corrosion conditions, governed by codes like ASME B31.3 (for process industries).
2. Types of Piping by Fluid Service
Piping systems can be categorized based on the type of fluid they transport, their application, and the materials used in their construction. Common types include:
| Fluid Type | Key Design Driver | Typical Material | Common Code |
|---|---|---|---|
| Water (cooling, potable, wastewater) | Corrosion, pressure | Carbon steel, ductile iron, HDPE | B31.3, B31.9 |
| Oil (crude, refined) | Flammability, viscosity | Carbon steel, stainless | B31.3, B31.4 |
| Gas (natural gas, propane, process gases) | Leak tightness, pressure | Carbon steel | B31.3, B31.8 |
| Steam | Temperature, thermal expansion | Carbon steel, alloy steel | B31.1, B31.3 |
| Slurry | Abrasion, erosion | Hardened steel, rubber-lined | B31.3 |
| Chemicals (corrosive, toxic) | Corrosion resistance, containment | Stainless steel, PTFE, PVC | B31.3 |
| Cryogenic (LNG, liquid nitrogen) | Brittle fracture at low temperature | Stainless steel, aluminum, special alloys | B31.3, B31.12 |
| Compressed air / vacuum | Pressure drop, leak tightness | Carbon steel, copper, aluminum | B31.3, B31.9 |
3. Core Components of a Piping System
A typical industrial piping system consists of several key components, each serving a specific function:
- Pipes: The main conduits for fluid flow, available in various materials and sizes to handle different pressures and substances.
- Pipe Fittings: Used to connect pipes, change direction, branch lines, or alter pipe diameters. Common fittings include elbows, tees, reducers, couplings, and unions.
- Valves: Control, stop, or divert fluid flow. Types include gate, globe, ball, butterfly, and control valves, chosen based on the application.
- Flanges and Gaskets: Provide leak-proof connections between pipes and equipment.
- Supports and Hangers: Maintain alignment and bear the weight of the piping, preventing sagging or vibration.
- Special Items: Such as strainers, steam traps, expansion bellows, and instrumentation for monitoring and control.
4. Piping Classification by ASME B31 Code:
5. Piping Materials
Selecting the right pipe material is one of the most consequential decisions in piping design. The wrong material can fail in weeks; the right material can last decades.
The decision flow below could be helpful to narrow down the right material for your service conditions.
1. Start with Carbon Steel (Unless You Have a Reason Not To)
Carbon steel (ASTM A106, A53) is the default for most oil, gas, steam, and water services between -20°F and 800°F. It is strong, weldable, and economical. The main limitation: it corrodes.
2. Move to Stainless Steel When Corrosion Is a Concern
Stainless steel (304, 316) resists rust and chemical attack. Use it for:
- Corrosive fluids (acids, salts, seawater)
- High-purity services (pharmaceutical, food, ultrapure water)
- Temperatures above 800°F (carbon steel weakens)
Downside: Much more expensive than carbon steel.
3. Use Alloy Steel for High-Temperature Strength
Alloy steels (chrome-moly, e.g., P11, P22, P91) maintain strength at high temperatures (800°F to 1200°F+). Found in power plant steam lines and refinery heaters.
4. Consider Plastics for Low-Pressure, Corrosive Services
PVC, CPVC, HDPE, PTFE are lightweight, immune to many corrosives, and inexpensive.
- Low temperature and pressure limits
- Poor fire resistance
- Not for organics or solvents (may soften)
5. Other Materials for Special Cases
| Material | Best For | Limitation |
|---|---|---|
| Copper / brass | Plumbing, HVAC, refrigeration (thermal conductivity) | Low strength, not for high pressure |
| Ductile iron | Large-diameter water/sewer lines | Brittle, heavy |
| Nickel alloys (Inconel, Hastelloy) | Extreme corrosion (acids, sour gas) or high temperature | Very expensive, long lead time |
Quick Decision Flow:
1. Is the fluid corrosive or high-purity? → Stainless steel
2. Is temperature >800°F? → Alloy steel
3. Is pressure low (<150 psi) and fire risk low? → Consider plastic
4. Otherwise → Carbon steel
6. Pipe Sizing: Definition, Reading and Standards
Pipe size is most commonly defined using the Nominal Pipe Size (NPS) system, especially in North America and industries following imperial standards. NPS is a dimensionless designation. For NPS ⅛ through 12, the NPS number roughly approximates the inside diameter (ID) in inches for the original standard wall thickness (similar to modern Schedule 40). For NPS 14 and larger, the NPS number equals the outside diameter (OD) in inches exactly. Because pipe wall thickness varies with schedule, the actual ID changes while the OD remains fixed for a given NPS.
Fluid Flow Design: Establishes the minimum inside diameter required to achieve the desired flow rate while keeping pressure drops and velocities within acceptable limits.
Pressure-integrity Design: Determines the minimum wall thickness needed to withstand the system’s operating pressure and ensure mechanical integrity.
How Pipe Size is Defined
Pipe size is most commonly defined using the Nominal Pipe Size (NPS) system, especially in North America and industries following imperial standards. NPS is a dimensionless designation that refers to the approximate (not exact) inside diameter of a pipe. However, as pipe wall thicknesses changed over time (with the introduction of pipe schedules), the actual inside diameter may differ from the nominal size.
- NPS (Nominal Pipe Size): A standardized, dimensionless number (e.g., NPS 2, NPS 6). For NPS ⅛–12, the OD is larger than the NPS number (e.g., NPS 2 has OD = 2.375″). For NPS 14 and above, NPS equals OD in inches (e.g., NPS 14 has OD = 14.000″). The NPS alone does not define the ID; the schedule is also required.
- Pipe Schedule (SCH) / Wall Thickness: Indicates the wall thickness of the pipe. Common schedules include SCH 40, SCH 80, and SCH 160. A higher schedule number corresponds to a thicker wall capable of withstanding greater internal pressure. For a given NPS, the outside diameter (OD) remains constant, so increasing the schedule reduces the inside diameter (ID).
- Outside Diameter (OD): For a given NPS, the OD is constant, regardless of schedule or material (for carbon/alloy steel pipe under ASME B36.10). For NPS 14 and larger, OD in inches equals the NPS number. For smaller sizes, refer to standard tables (e.g., NPS 4 → OD = 4.500″).
- Inside Diameter (ID): The inside diameter represents the open passage within the pipe through which fluid flows. ID = OD − (2 × wall thickness). Even pipes with the same NPS may have different IDs depending on the schedule. For example, NPS 4 SCH 40 has ID ≈ 4.026″, while NPS 4 SCH 80 has ID ≈ 3.826″.
- Pipe Length: Pipes are manufactured in standard lengths, though cutting or threading can be done based on project needs and the material type. For steel pipe, typical lengths are 20 feet (6.1 m) or 40 feet (12.2 m), but actual dimensions vary by specification and application.
- End Finish: The end preparation of a pipe is called the end finish, which ensures proper connection to fittings or other piping. Typical end types include plain end (PE), beveled end for welding (BE), threaded end (TE), and grooved end for mechanical jointing. Beveled ends are common for butt-welded pipe systems in high-pressure applications.
Example:
An NPS 3 pipe has an OD of 3.5 inches, not 3 inches. If you select SCH 40, the wall thickness is 0.216 inches, so the actual internal diameter is about 3.068 inches.
- The actual inside diameter (ID) is calculated as: ID = OD − (2 × Wall Thickness) = 3.500 − (2 × 0.216) = 3.068 inches
- If you select a higher schedule, such as SCH 80 (wall thickness = 0.300 inches), the ID becomes smaller: 3.500 − (2 × 0.300) = 2.900 inches, even though the NPS and OD remain the same.
How Pipe Size is Read
When reading or specifying pipe size, it is typically presented as:
- NPS x Schedule (e.g., 6″ NPS SCH 40)
- For metric systems, the DN (Diameter Nominal) system is used, which is based on millimeters and closely matches the NPS system for reference.
Associated Pipe Sizing Systems
- IPS (Iron Pipe Size): An older system that preceded NPS. IPS used the same OD as modern NPS for most sizes, but the nominal size referred to the approximate ID. NPS replaced IPS, though the terms are sometimes still used interchangeably for legacy systems.
- DN (Diameter Nominal): The international metric equivalent of NPS, used mainly outside North America. DN values in millimeters roughly correspond to NPS in inches multiplied by 25 (e.g., NPS 2 ≈ DN 50). Key rule: For DN 350 and above (≈ NPS 14 and larger), DN equals the OD in millimeters. For smaller sizes, DN is an approximation of the ID in millimeters.
- Pipe Schedule: As noted above, the schedule defines the wall thickness and is essential for determining the inside diameter, pressure rating, and mechanical strength of the pipe.
Why Proper Pipe Sizing Mater
- Ensures energy efficiency by minimizing pressure losses and pump requirements.
- Maintains system performance by preventing excessive velocities, erosion, and noise.
- Balances cost by avoiding oversized (expensive) or undersized (inefficient, unsafe) pipes.
- Supports safety by ensuring pipes can handle the required pressure and flow without risk of failure.
7. Pipelines
What is a pipeline?
A pipeline is a system of connected pipes designed for the long-distance transportation of fluids-such as oil, natural gas, water, or refined products-typically from one facility to another, often spanning hundreds or thousands of kilometers. Pipelines are engineered to move large volumes of liquids or gases efficiently and reliably across regions, countries, or even continents, and can be installed underground, above ground, or underwater (as with subsea pipelines)
Key Characteristics of Pipelines
- Function: Pipelines are primarily used to transport bulk quantities of fluids over long distances, such as moving crude oil from offshore platforms to refineries or delivering natural gas from production fields to distribution networks.
- Size: They typically use large-diameter pipes to accommodate high flow rates and minimize pressure loss over extended distances.
- Components: A typical pipeline system includes injection (supply) stations, pump or compressor stations, block valve stations for isolation, regulator stations for pressure control, and delivery stations at the endpoint.
- Fittings: Pipelines use a minimal number of fittings-mostly long-radius bends and valves-to reduce flow resistance and maintenance needs.
- Construction: Most pipelines are constructed from welded steel, though materials like concrete or plastics may be used for specific applications.
- Codes and Standards: Design and operation are governed by standards such as ASME B31.4 (for liquid transportation) and ASME B31.8 (for gas transmission)9.
Typical Applications
- Oil and Gas: Transporting crude oil, natural gas, and refined products over long distances.
- Water and Sewage: Moving potable water, irrigation water, or wastewater between treatment plants and distribution points.
- Industrial Uses: Conveying chemicals, slurries, or other industrial fluids
Difference Between Piping and Pipeline
| Feature | Piping (Oil & Gas Facilities) | Pipeline (Long-Distance Transport) |
|---|---|---|
| Location | Within plant/facility boundaries | Across cities, regions, or countries |
| Function | Transfers fluids between equipment/process units | Transports bulk fluids over long distances |
| Size Range | Typically ½” to 80” (varied as per plant design) | Large diameter, optimized for high-volume flow |
| Fittings | Extensive use (elbows, tees, reducers, valves) | Limited, mostly long-radius bends and valves |
| Design Codes | ASME B31.3, B31.1 | ASME B31.4 (liquid), ASME B31.8 (gas) |
| Construction | Complex, with many branches and connections | Straight runs, minimal branches |
| Materials | Wide variety (steel, alloys, plastics) | Primarily steel, designed for durability |