How to Build a Line List and Size Your Pipes
The line list is one of the engineering deliverables you produce. Here's a practical walkthrough of how to build one from scratch. In this article we will go through the steps you need to go through.
In this article — here are the steps to follow
- Pull every line off the drawing
- Collect stream data from the HMB
- Identify your sizing flow
- Find the required pipe ID
- Size to velocity and pressure drop criteria
- Build the line list
Steps at a glance
Pull every line off the drawing
Start with the PFD for preliminary sizing. Move to the P&ID for detailed work when you need every branch, drain, and instrument takeoff.
For each line, record:
- Line tag number and PFD/P&ID reference
- Stream name / service description
- Fluid phase (Liquid, Gas, Two-Phase)
- Service type — this directly controls which design criteria apply. Examples include pump suction line, pump discharge line, gravity drain line, compressor suction line, ...etc.
Don't skip the service type. It's not a label — it's the design limit.
Collect stream data from the HMB
Pull your flow conditions from the Heat and Mass Balance or process simulation. The minimum data you need depends on the fluid:
- Incompressible fluids (liquids): flow rate, density, and viscosity at operating temperature
- Compressible fluids (gases): flow rate, density, viscosity, operating temperature, and operating pressure — all five, because T and P define the thermodynamic state of the gas
For gas lines, you size on actual volumetric flow (Am³/h). If your HMB gives normal flow (Nm³/h), convert it to actual conditions using the operating T and P before sizing.
Identify your sizing flow
Size the pipe to the maximum operating flowrate — that's your primary case. Some companies or projects also require adding a design margin on top of that maximum; check your project design basis or company standard.
Don't ignore minimum flowrate entirely. In slurry or solid-bearing services, if velocity drops too low at minimum flow, you risk settling and plugging. Worth a quick check if the application calls for it.
Find the required pipe ID
With your sizing flow in hand, you need to find the pipe inside diameter (ID) that satisfies your velocity or pressure drop criteria. To do that, first establish the MOC and pipe schedule for the service — these define which pipe dimension table you pull from (ASME B36.10 for carbon steel, or equivalent standards for other materials). Then work through the available standard sizes until you land on one that meets your criteria.
Size to velocity and pressure drop criteria
This is the core of line sizing. The method is a two-check approach — velocity and pressure drop — but the order of priority differs between liquids and gases.
Liquid lines
Size primarily on velocity, then check that the resulting pressure drop is workable for your system.
The table below is based on Crane TP-410 and should be treated as guidelines, not hard limits. Going outside these ranges is acceptable as long as it's been reviewed and determined suitable for the project.
| Service | Velocity (m/s) | Reference |
|---|---|---|
| Pump suction | 1.2 – 2.1 | Crane TP-410 (4–7 ft/s) |
| Pump discharge / general service | 1.2 – 3.0 | Crane TP-410 (4–10 ft/s) |
| Boiler feed water | 2.4 – 4.6 | Crane TP-410 (8–15 ft/s) |
Suction lines get tighter velocity limits to protect available NPSH — especially on bubble-point liquids where any additional pressure drop risks flashing at the pump inlet.
Once you've selected a pipe size based on velocity, always check that the resulting pressure drop is something your pump can actually handle. If the pressure drop is too high for the available pump head, step up the pipe size and reduce velocity accordingly. Velocity and pressure drop are directly tied — a pipe sized to a reasonable velocity will generally also land in an economical pressure drop range. That said, other references and company standards do provide explicit pressure drop recommendations for liquid lines, and it's worth cross-checking both if your project has tight hydraulic constraints.
Gas lines
Gas velocity limits exist mainly to control noise and pipe vibration. For most gas services, pressure drop is the more governing sizing criterion — but both checks are required and either can end up controlling depending on the application.
| Service | Max velocity | Reference | Comments |
|---|---|---|---|
| General process gas (dry, no solids) | ≤ 30 m/s (Bechtel) ≤ 38 m/s (BP) |
Cheresources Ankur2061, 2013 — not published company standards |
Widely used onshore upper bound before noise and vibration become significant |
| General process gas — noise limit | ≤ 18 m/s (60 ft/s) | API RP 14E | Conservative; more appropriate for offshore where noise control is critical |
| General process gas — density-dependent noise limit | V = 175 × (1/ρ)0.43, max 60 m/s | NORSOK P-001 Ed. 5 | More flexible than API RP 14E; accounts for gas density — denser gas requires lower velocity |
| Saturated steam | 20 – 50 m/s | Crane TP-410 (4,000–10,000 fpm) |
— |
| Superheated steam | up to 50 – 70 m/s | Spirax Sarco | Dry gas — no moisture erosion risk; upper limit governed by pressure drop |
Why velocity limits exist — noise and vibration
High gas velocity causes flow-induced noise and pipe vibration. Two commonly referenced checks:
- API RP 14E — flags velocities above 60 ft/s (~18 m/s) as a potential noise concern. Conservative, doesn't account for gas density, and is more appropriate for offshore applications where noise control is critical.
- NORSOK P-001 — a Norwegian offshore process design standard applicable to single-phase gas and vapour lines. Uses a density-dependent formula that is more technically rigorous:
Vmax = 175 × (1/ρ)0.43, capped at 60 m/sA denser gas requires a lower velocity to stay within acceptable noise limits.ρ = gas density in kg/m³ | V in m/s
If no project-specific limit exists, the NORSOK formula is a good starting point.
Which operating point to use for gas velocity pipe sizing
- Short lines or small pressure drop: calculate velocity at the line inlet. Density doesn't change significantly so this is generally sufficient.
- Long lines or significant pressure drop: calculate velocity at the lowest pressure point, since gas density is lowest there and velocity highest. Sizing to the highest velocity avoids undersizing the pipe.
Build the line list
Once every line has a selected NPS and schedule, compile the list. Each row should capture:
- Line tag and drawing reference
- From / To (equipment tags)
- Service fluid and phase
- Operating T and P
- Flow rate (mass and volumetric), density, and viscosity
- Pipe MOC and pipe class
- Selected NPS and schedule
- Actual velocity and ΔP — confirmed against criteria
- Notes (insulation, tracing, slope, etc.)
Tie every line to the HMB revision it was sized from. When process conditions update, you'll know exactly which lines need to be revisited.
Final thoughts
Line sizing is straightforward once you know which criteria apply and why. Use these heuristics to build your preliminary line list, then validate against your project design basis and equipment requirements before locking in sizes.
Receive Free Discounts!Â
Join our mailing list to receive the latest engineering blogs, tools, resources and discounts on courses.
Don't worry, your information will not be shared.
Author
