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Fluid Velocity in pipe, calculation and recommendation

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1. Introduction
2. Calculation
3. Pipe velocities recommendation

1. Introduction

Why is the fluid velocity in pipes important ?

The fluid velocity in a pipe is a fundamental data to calculate to be able to characterize the flow in a pipe, thanks to the Reynolds number, and size a pipe circuit calculating the pressure drop expected for a certain flow.


When designing the pipe, it is necessary to chose a fluid velocity to have good compromise in between pressure drop and economics, some recommendations are given below.

Calculation of fluid velocity in pipe
Figure 1 : Pipe fluid velocity calculation

2. Calculation of fluid velocity

How to calculate the fluid velocity in pipes ?

Fluid velocity in pipe calculation

With :
Q = Volumetric flowrate (m3/s)
D = Pipe diameter (m)
u = Fluid velocity in pipe (m/s)

Note that the velocity will be constant for a non compressible fluid but will vary with a compressible fluid such as a gas. For a gas it is therefore necessary to slice the pipe layout in several sections in order to recalculate the velocity along the pipe.

3. Pipe velocities recommendations

What are the recommended velocities for usual process fluids ?

Fluid Recommended velocity
Water
1.5-2.5 m/s [1]
1-2.5 m/s [2]
Air 10-30 m/s [1]
30 m/s [2]
Steam
20-40 m/s [1]
HP : 45 m/s [2]
LP : 30 m/s [2]
Gas Process flows 15-45 m/s
Dry gas 30 m/s [2]
Wet gas 18 m/s [2]

These values are given as an indication for pre-sizing. Each company should have its own guideline to be followed.

In addition to the sources given below, one may also wish to consult Applied Process Design for Chemicals and Petrochemicals plants, Ludwig, Gulf, page 85 which is giving values for specific applications like sodium hydroxide, ethylene, vinyl chloride...

Source

[1] Mecanique et Rheologies des fluides en Genie Chimique, Midoux, Tec et Docs, 1993
[2] Rules of Thumbs for Chemical Engineers, Brannan, Gulf, 2005