Understanding "Pressure" and "Pressure Head"

Applies To
Product(s): WaterCAD, WaterGEMS, HAMMER
Version(s): 08.XX.XX.XX and higher
Area: Output and Reporting

 

Problem

In WaterCAD, WaterGEMS and HAMMER, what does the "pressure" result represent and what is the difference between pressure head and pressure?

Solution

Pressure head is the hydraulic grade minus the physical elevation at a junction or the weight of a given height of water column at its bottom end. It is independent of the amount of water: the pressure head of a 10-foot high or deep 2-inch diameter pipe is the same as that of the 2-foot diameter pipe of the same height or depth (business dictionary.com, 2014).

The pressure reported is simply the hydraulic grade line (HGL) minus the physical elevation of the node, converted into your desired pressure units. It also takes into account the specific gravity of water, which defaults to 0.998. For example, if you have a pressure head of 56.70ft your pressure would be 56.79ft * 0.998 = 56.67 ft of H20.

In water distribution modeling, the "dynamic pressure" is usually negligible compare with the "static" and "total" pressure. Node elements, where pressure is calculated, are usually located at tees and crosses, so the velocity head would be ambiguous.

See Also

When the user changes the fluid, why doesn't the calculated pump head change?

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