Design for Low Line Loss

As noted above, line diameter is the key variable. There are a number of other ways that line loss can be reduced. Table 8.1 below illustrates the equivalent line length in meters for swing check valves, 'T' junctions and long and short radius elbows.

Among the measures for reducing line loss in new installations are:

• Optimize line size based on running cost.

• Use VFD pump motors and full line size control valves for back-up if thought necessary.

• Use swept bends or long radius elbows.

• Consider expansion joints to avoid large expansion loops.

• Venturi meters offer less pressure drop than orifice plate flow meters. Where accuracy is not critical, use ultrasonic flowmeters.

• Select pipe with smooth inner surface.

• Use a 'Y' to split flow to the suction ofpaired pumps.

For existing installations, it is rare that any modification is worthwhile. Nevertheless, where plants have been de-bottlenecked, it is recommended to carry out a single gage pressure survey to identify where modification may be justified. Rerating orifice plate flowmeters (larger bore) and re-sizing control valve trim can help reduce loss.

For pumping applications with low head/high flow such as cooling water circuits, balanced check valves are preferred to swing check valves which often have significant pressure drop. Swing check valves in vertical lines should be avoided or fitted with external balance weights.

Table 8.1 Equivalent line lengths.

Line Size mm Check Valves 'T'Junctions Elbows r/d = 1.0 Elbows r/d = 0.5

100 150 200 250

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