Process Engineer is responsible for specifying the diameter of all pipes in a processing unit. For pipe sizing, Pressure drop, maximum permissible velocity from the point of view of erosion and noise, and economics (when a fluid is compressed or pumped) must be taken into consideration.
The pipe must be sized to ensure that the total system pressure drop (including pressure drop due to equipment) does not exceed the available pressure drop.
Pressure Drop Criteria:
It is comparatively easy to size a line when a specified pressure or static head is accessible. It should be sized to eat at the rated flow conditions no more than the required pressure. The available frictional pressure loss is limited by pressure drops due to other system components.
A safety margin of 20 %, or a safety margin specified by the specific project or client, shall be added to all friction loss calculations, with the exception of steam systems, for which no margin is to be added.
To ensure that the resulting velocity is reasonable, each calculated line size must be verified. If a line size is not regulated by the accessibility of pressure drop or economics, then speed limits are generally the decisive factor. The velocity limits presented can be quite arbitrary and are intended as guidelines only.
In addition to these guidelines, other velocity limitations - erosional velocity, sonic velocity, entrainment velocity and noise must be considered when sizing lines.
Long lines that should be economically sized to transport pumped or compressed liquids. When significant incremental expenses are involved between two line dimensions, a thorough financial analysis is generally done. Usually these instances are categorized as follows:
Cooling water headers
Special pipe (e.g., refractory-lined pipe)
Acceptable Line Sizes:
The minimum acceptable line size is 3/4 “unless the client permits a lower size. Line sizes of 1¼”, 3½", 5", 7",and 9" are not to be used. Many customers want to have a minimum size of 2 “on tube shelves. Some customers also do not enable 2 1⁄2” or 14" piping to be used.
Design Criteria Considerations:
Customers, licensors, vendors of machinery make contributions to the design. Adequate details are created to be incorporated into the design to ensure stable operation of process units over the complete spectrum of expected working circumstances.
These circumstances include, but are not restricted to, variations in SOR, EOR, turn-down, feed and product structure, utilities and other emergency activities, conditioning/regeneration catalysts, cleaning of heat exchangers, etc.