Calculating the Pipe Flow
By Steve Stephenson October 2013
When you hear the phrase “Pipe Flow Calculator,” you need to ask, “What is it calculating?” There are a number of different parameters that need to be calculated concerning flow in piping systems. These include:
- Pipe sizing by pressure loss
- Pipe sizing by velocity
- Pressure drop through piping (pipe friction)
- Velocity through piping
- Flow rate through piping
- Economical Insulation thickness
In addition, based on how the piping is used, the pipe flow calculator may need to calculate other parameters such as:
- Pipe sizing for a steam vent
- Condensate load from piping at start-up
- Flow rate through individual valves
- Flow rate through a venturi tube
- Orifice flow
The first step in using a pipe flow calculator is to select what you are solving for. That will determine the information you'll need to input into the pipe flow calculator. For example, if you need to determine the flow rate, then you'll need to know the pipe diameter and velocity.
You also need to be sure you are using the same units of measurement for all data. If you have a pipe that is 8 inches in diameters, and a measured flow rate of 1,200 cubic centimeters per second, what is the velocity through the pipe? If the pipe flow calculator cannot accommodate mixed measurement systems, in this case English and Metric, you'll need to do the conversion before using the pipe flow calculator. This happened to the Mars Climate Orbiter which received control information in English units when it was expecting them in metric. The result was the disintegration of the spacecraft upon arrival at Mars because it entered the Martian atmosphere at an incorrect angle.
Also, be sure you know the units the pipe flow calculator is using for the results of the calculation.
Pipe Flow Calculator Example
As an example, here is the required input for a pipe flow calculator, and the calculated output:
- Pressure at pipe inlet
- Average fluid velocity in pipe
- Pipe diameter
- Pipe relative roughness (available from the manufacturer)
- Pipe length
- Elevation gain (or loss) along pipe length
- Fluid density
- Dynamic fluid viscosity
The outputs from the pipe flow calculator will be:
- Volume flow rate
- Mass flow rate
- Pressure at outlet
- Pressure drop
A pipe flow calculator may be used for a variety of applications. However, the characteristics of the material the pipe carries must be known as they affect the flow through the pipe. In the previous example a liquid was used in the calculation. Knowing the fluid density and viscosity were important. For other materials,, such as for natural gas, some of the calculations will require knowing the compressibility factor, specific gravity, and temperature of the gas. This means you must use a pipe flow calculator that has been designed to do the calculations for the material the pipe will be carrying.
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Definitions of Online Pipe Flow Calculators
Online pipe flow calculators can do the math and provided the needed calculation results. However, you must be sure to select the correct pipe flow calculator for the pipe and material being transported, and correctly enter the needed information. The following are some important definitions:
Friction coefficient - this depends on whether the flow is laminar, transient, or turbulent, and the roughness of the pipe. If the flow is transient, the friction cannot be determined. If the flow is transient, the friction can be ignored. If the flow is turbulent, the friction coefficient can be calculated based on the roughness of the pipe. The roughness will be supplied by the pipe manufacturer.
Kinematic viscosity - this is the viscosity we normally visualize when we hear the word “viscosity.” It is a measure of how “thick” a fluid is. Tables are available that give the kinematic viscosity for most materials.
Dynamic fluid viscosity - this is sometimes referred to as the absolute viscosity. The number is a characteristic of the material as it flows through a pipe, and it varies with the temperature of the material.
Reynolds number - this is a dimensionless number that is based on the fluid's density, viscosity, and velocity.
K factor - this is a function of the pipe length, pipe diameter, and the turbulent friction factor. It is a measurement of the comparatively minor losses in a pipe caused by fittings, bends and valves.