In an industrial plant, depending on the process and the number of piping systems, the capital investment in piping could be up to 25 and even 40% of the total investment. Therefore, efficient sizing is vital to the plant economic feasibility. When an installation presents very long pipelines and/or it requires expensive materials (Cr-Mo, Stainless Steel), diameter selection must be scrutinized, and a detailed and comprehensive study shall be performed.
Flow of fluids in pipes is always accompanied by friction, produced between the particles themselves and between the fluid and the pipe wall, in other words, energy loss. It means that there is a pressure loss in direction of the flow. The energy used to overcome these resistances is called “head loss” or “pressure loss”, in turn resulting in a gradual decrease in the pressure of the fluid, falling from one point to another in the direction of the fluid flow (pressure drop).
The required diameter of a pipe depends on the fluid flow required, the elevation of the inlet and outlet of the system, the pressure level at these points, velocity and allowable pressure loss, fluid nature, piping material and pipe type. Except the elevation of the inlet and outlet of the system, all the other variables are related: the diameter depends on the flow, the flow depends on the velocity, the velocity depends of the diameter and the pressure loss depends on the square of the velocity.
Thus, to solve this system it is necessary to iterate. In other words, the designer needs to adopt acceptable values of those variables representing a compromise between the pipe diameter and the pressure loss of the system.
From all the values mentioned before, normally the parameter “fixed” or “adopted” is the velocity, since the diameter and pressure loss are a function of this. There are several publications and references that collect data from successful experiences, allowing designers develop different projects.
There is not a unique solution when it comes to pipe sizing. There are different solutions that bring different implications. Let’s analyse the following:
If the pressure loss of the system needs to be minimised, the velocity has to decrease and therefore the diameter of the pipes will increase.
If the size of the pipes in an installation need to be reduced, it will be necessary to increase the velocity of the systems and thus the pressure loss.
Summing up, reducing the pipe diameter means less capital investment and more pressure loss, with resulting increase of installed power and operative costs. On the contrary, reducing the pressure loss will require slower velocities and larger diameters, in turn more capital investment.
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