Plumbing Tubes: Hydraulic Characteristics of Smooth-wall Materials

The hydraulic character of a piping material, determined by the size and smoothness of its bore, enables it to deliver water at sufficient pressures to overcome static head and operate fixtures, appliances and equipment. When comparing two smooth-wall materials, such as copper and polypropylene, internal diameter becomes the critical factor in evaluating hydraulic performance.

Table 1:

Comparison between 460/0 Copper tube (Cu) and Polypropylene tube (PP) showing area of bore, percentage difference and flow rates.

Tubes:  Hydraulic Characteristics of Smooth-wall Materials

Table 2:

Comparison of flow rate using the same pressure drop values for the same nominal tube size. The pressure drop values used are the same as those for copper tube in Table 1.

Tubes:  Hydraulic Characteristics of Smooth-wall Materials

Today there is a tendency to use pipes made from plastic-type materials. These pipes usually have thicker walls than conventional copper tubes, so their internal diameters or bores are considerably smaller. It is after all the bore of the pipe that is important when considering volumes, flow rates and velocities and not the outside diameter of the pipe.

Just how much the bore does vary is clearly shown in the tables above. it is therefore very important to note that one type of pipe cannot necessarily be directly substituted for another even though they have the same outside diameter.

When using plastic pipes, it would be a good rule of thumb to install one nominal size larger than would normally be specified in steel or copper.

Copper offers substantially greater bore areas than plastic for the same outer diameter of pipe. Example as per illustration above: 22mm outer diameter copper pipe offers 60% greater bore area than 22mm outer diameter plastic pipe.

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