Pipe Sizing in Piping Design – Practical Engineering Approach for Oil & Gas and Process Plants

Introduction

Pipe sizing is one of the most fundamental activities in piping design and piping engineering. In oil & gas facilities, refineries, chemical plants, and power plants, correct pipe sizing directly affects process performance, safety, operability, and lifecycle cost.

Pipe sizing is not just a mathematical exercise. It is a combination of process requirements, fluid behavior, material selection, industry experience, and applicable codes such as ASME B31. This article explains pipe sizing in a step-by-step, practical manner, suitable for beginners and intermediate engineers working with P&IDs, line lists, piping classes, and line numbering systems.

What Is Pipe Sizing in Piping Engineering?

Pipe sizing is the process of determining the appropriate nominal pipe size (NPS or DN) for a piping line so that the required flow rate can be transported safely and efficiently under specified design pressure and design temperature.

Pipe sizing is part of line design, not pipe thickness design.

Pipe sizing considers:

• Required flow rate
• Allowable fluid velocity
• Service type (liquid, gas, steam)
• Material specification
• Operating and design conditions
• Plant layout and operating philosophy

Pipe sizing does NOT decide:

• Pipe wall thickness
• Pipe schedule
• Stress flexibility

These are covered under mechanical design per ASME B31.3.

Pipe Sizing in the Overall Piping Design Workflow

In a typical oil and gas piping project, pipe sizing fits into the workflow as follows:

1. Process engineer prepares P&ID
2. Line list is developed (line number, fluid code, design pressure, design temperature)
3. Initial pipe sizing is performed
4. Piping class is selected
5. Pipe thickness is checked as per ASME B31.3
6. Layout and routing are developed
7. Stress analysis (if required)

Pipe sizing is therefore an early but critical activity that directly affects downstream design.

ASME B31.3 Code Perspective

ASME B31.3 (Process Piping) does not prescribe a direct pipe sizing method, but it sets important boundaries.

Relevant clauses include:

• ASME B31.3, para 300
  Defines the scope and responsibilities of the piping system designer.
• ASME B31.3, para 301 & 302
  Emphasizes that piping systems must be designed considering:
  • Pressure
  • Temperature
  • Fluid service
  • Material behavior
• ASME B31.3, para 304
  Covers pressure design of piping components, which assumes that pipe size has already been selected.

This means:

Pipe sizing is an engineering responsibility based on good practice, while ASME B31.3 verifies mechanical adequacy.

Basic Concepts Used in Pipe Sizing

Nominal Pipe Size vs Actual Diameter

Term	Meaning
NPS / DN	Nominal designation
OD	Fixed for a given NPS
ID	Depends on pipe schedule

Pipe sizing calculations always use inside diameter (ID), not OD.

Flow Rate and Velocity

• Volumetric flow rate (Q) – m³/hr or m³/s
• Velocity (V) – m/s

They are related by the continuity equation:$$Q = A \times V$$Q=A×V

Where:$$A = \frac{\pi D^2}{4}$$A=4πD2​

Velocity-Based Pipe Sizing – Industry Practice

Why velocity-based sizing is used:

• Simple and fast
• Proven by decades of plant operation
• Suitable for preliminary and detailed design

Typical velocity limits (Oil & Gas / Refinery)

Liquids

Service	Recommended Velocity
Pump suction	0.6 – 1.5 m/s
Cooling water	1.5 – 2.5 m/s
Process liquids	1.0 – 3.0 m/s
Corrosive / erosive	< 1.5 m/s

Gases

Service	Velocity Range
General process gas	10 – 25 m/s
Compressor suction	5 – 10 m/s
High-pressure gas	Limited by noise

Velocity limits are often defined in project design basis or company piping standards.

Step-by-Step Pipe Sizing with Numerical Example

Design Data (Typical Refinery Line)

Parameter	Value
Fluid	Process water
Flow rate	120 m³/hr
Design pressure	10 barg
Design temperature	50°C
Material	Carbon steel
Piping class	CS-150
Service	Continuous

This information typically comes from:

• P&ID
• Line list
• Piping class specification

Step 1 – Convert Flow Rate

$$Q = \frac{120}{3600} = 0.0333 \, m³/s$$Q=3600120​=0.0333m3/s

Step 2 – Select Design Velocity

For process water in carbon steel:

• Selected velocity = 2.5 m/s

This value is based on:

• Service type
• Material
• Operating experience

Step 3 – Calculate Required Internal Diameter

$$D = \sqrt{\frac{4Q}{\pi V}}$$D=πV4Q​​ $$D = \sqrt{\frac{4 \times 0.0333}{\pi \times 2.5}}$$D=π×2.54×0.0333​​ $$D = 0.130 \, m$$D=0.130m

➡ Required ID ≈ 130 mm

Step 4 – Select Nearest Standard Pipe Size

Pipe Size	Approx. ID
DN 125	122 mm
DN 150	154 mm

DN 125 is marginal.
DN 150 provides operational margin.

✔ Selected Pipe Size: DN 150

Step 5 – Velocity Check

$$A = \frac{\pi (0.154)^2}{4} = 0.0186 \, m²$$A=4π(0.154)2​=0.0186m2 $$V = \frac{0.0333}{0.0186} = 1.79 \, m/s$$V=0.01860.0333​=1.79m/s

✔ Velocity is acceptable and conservative.

Relationship with Line Numbering and Piping Class

Once the pipe size is finalized, it becomes part of the line designation and is reflected in the line numbering system and piping class.

• Line number is frozen
• Piping class is confirmed
• Thickness calculation begins

Example line number:

12”-PW-150-CS-001

Where:

• 12” → Nominal pipe size
• PW → Fluid code
• 150 → Pressure class
• CS → Material
• 001 → Sequence number

Pipe sizing therefore directly influences the line designation and line numbering system.

Common Pipe Sizing Mistakes in Plants

• Using OD instead of ID
• Ignoring future flow expansion
• Over-sizing leading to low velocity and fouling
• Under-sizing causing noise and erosion
• Not coordinating with pump suction requirements
• Not aligning with project design basis

Best Engineering Practice

Experienced piping engineers follow these rules:

• Start with velocity-based sizing
• Apply conservative judgment for critical services
• Coordinate with process and mechanical teams
• Follow project and company standards
• Use ASME B31.3 for verification, not sizing

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FAQ

1. What is the difference between pipe sizing and pipe thickness calculation?

Answer:
Pipe sizing and pipe thickness calculation are two different but closely related activities in piping engineering.

• Pipe sizing determines the nominal pipe size (NPS / DN) based on:
  • Flow rate
  • Allowable fluid velocity
  • Service type (liquid, gas, steam)
• Pipe thickness calculation determines the minimum required wall thickness based on:
  • Design pressure
  • Design temperature
  • Material specification
  • Corrosion allowance
  • Code requirements (ASME B31.3)

In practice:

Pipe sizing is performed first during line design, and thickness calculation is performed later during mechanical design as per ASME B31.

2. Does ASME B31.3 specify how to calculate pipe size?

Answer:
No. ASME B31.3 does not provide a direct pipe sizing formula.

ASME B31.3:

• Defines design responsibilities (para 300)
• Specifies pressure design requirements (para 304)
• Ensures mechanical integrity of piping systems

Pipe sizing is based on:

• Velocity limits
• Industry best practices
• Company standards
• Engineering judgment

ASME B31.3 is then used to verify that the selected pipe size and thickness are safe for the specified design pressure and design temperature.

3. Why is velocity-based pipe sizing commonly used in oil and gas plants?

Answer:
Velocity-based pipe sizing is widely used in oil & gas, refinery, and power plant piping design because it is:

• Simple and fast
• Proven by long-term operating experience
• Suitable for early and detailed design stages
• Effective for avoiding erosion, noise, and excessive pressure drop

Velocity limits are usually defined in:

• Project design basis
• Company piping standards
• Client specifications

After velocity-based sizing, the selected pipe size is verified by:

• Pressure drop calculations
• Pump and compressor checks
• ASME B31 mechanical design

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Conclusion

Pipe sizing is a core skill in piping engineering, especially in oil & gas, refinery, and power plant projects. While equations provide a starting point, correct pipe sizing depends heavily on engineering judgment, operating experience, and proper use of standards such as ASME B31.3.

A well-sized piping system ensures:

• Safe operation
• Stable process performance
• Lower maintenance cost
• Long service life

Pressure drop evaluation, pump matching, and stress analysis build upon this foundation and are best addressed as separate, focused engineering studies.