LINE SIZING IN PROCESS INDUSTRY (2024)

WHAT IS LINE SIZING ?

Line sizing is one of the most important and critical activities done by a process engineer for any project. Piping is a major cost component in any process plant and it may vary between 5-15% of the total plant investment and optimum design may result in considerable cost reductions. Line size is determined with proper sizing calculations considering various parameters.

WHAT ARE THE MAIN PURPOSES OF LINE SIZING ?

• To determine pump head requirements.
• To meet design process parameters such as flow, velocity & pressure.
• To fill appropriate data on P&ID’s, data sheets, and line lists (for common, water-like liquids, gases etc).

CATEGORIES OF LINE SIZING

A.SINGLE PHASE LINE SIZING

1. Liquid Line Sizing
2. Vapour Line Sizing

B.TWO PHASE LINE SIZING

1. Two phase (Horizontal)

• Bubble Flow - Liquid occupies the bulk cross section and bubbles (gas) are dispersed throughout the liquid & moves along the upper part of the pipe due to their buoyancy.
• Plug Flow - As the vapor flow rate increases the vapor bubble coalesces and alternate plugs are formed of liquid and gas where the gas portion moves along the upper part.
• Stratified Flow - The plug flow becomes a continuous phase as the vapor rate increases. Interface between the two phases is relatively smooth and fraction occupied by each phase remains constant.
• Wavy Flow - As vapor rate increases vapor moves appreciably faster than liquids and resulting friction at the interface forms waxes in liquids.
• Slug Flow - When vapor rate reaches a critical value, the crests of the waves in the liquid touch the top of the pipe and forms forthy slugs.
• Annular Flow - The liquid flows as an annular film of varying thickness along the wall, while the gas flows as a high speed core down the middle resulting in a great deal of slip between phases.
• Dispersed Flow - When vapor velocity becomes great enough all of the liquid is torn away from the wall and is carried by vapor as entrained droplets.

2.Two phase (Vertical)

• Bubble Flow - Upward flowing fluid is in the continuous phase with dispersed bubble of vapor rising upwards
• Slug Flow - The dispersed bubbles coalesce into slugs which occupy bulk of cross sectional area.
• Annular Flow - The flow is similar as in horizontal pipes but slip between liquid film is slowed down by gravity, increasing in the difference in velocities between vapor and liquid.

Mist Flow - The flow is same as spray flow in horizontal pipes

VARIOUS CONSIDERATIONS DURING LINE SIZING -

Line sizing decisions are affected by:

1. Economic Considerations:

Line sizing decisions have economic impacts, including:

• Cost of pipe
• Cost of pipe supports

2.Velocity and Pressure drop Calculations

It becomes most important when fluid flows under gravity. Sometimes, it may not be possible to absorb entire pressure n pipe because of excessive velocity and additional restrictions to flow may be necessary.

3.Structural & Mechanical Requirements

If Material of Construction is very high then lower sizes are considered. But 1.5’’ is kept as the minimum pipe size on pipe racks and sleepers.

4.Unusual Flow Situations

• Vortexing
• Vibration
• Water Hammering

5.Velocity Requirement

• Velocity should be less than the erosion velocity otherwise other piping material will start eroding.
• If velocity exceeds certain limit, the protective layer gets eroded & corrosion inhibitors become ineffective.
• To avoid settling of solid particles a minimum velocity of about 0.9 m/s should be maintained in the pipes.

6.Mode of Operation

Higher velocity and more pressure drop can be allowed in those lines which operate intermittently and for short duration than compared to normal lines.

7.Equipment Limitations

Equipment specifications also govern line sizes.

8.Space Limitations

When new pipe is to be provided to a existing plant, the available space may dictate pipe size.