PRESSURE LOSSES 09
As a result of this characteristic, the pressure losses on the pipeline sections present low values, facilitating the design calculations and rendering installations more economical by the use of pipes with smaller dimensions and pumps of lower power for the same requirements of water supply. The installation operates quietly and smoothly, allowing the possibility to use, where needed, faster water velocities, without having to worry about the noise levels or the vibrations of the pipes during operation. The diagram illustrates the curves calculated for water temperature at 80°C. For ø 16x2.0 pipe and a flow rate of Q=140l/h, the flow velocity is V = 0.37m/sec and the loss of head is equal to R = 17mmH20/m. However, since we have calculated the diagram for a water temperature of 80°C (central heating), we have to divide the R= 17mm value by a correction factor (Fc), if we want to calculate the loss at a water temperature of 25°C (plumbing installation). The correction factor is provided in the following diagram and in our example equals to 0.935.
Cross-linked PE pipes present an exceptionally smooth inner surface, resulting in a very low roughness factor (0.006mm), which compared to other pipe types, even copper (κ=0.014 mm), is much reduced. The surface of every solid material, no matter how smooth it may look, still presents some level of roughness. The level of this roughness is quantitatively expressed by a characteristic length, e, which is called absolute roughness of the surface. The length e is equal to the statistical mean of the heights of the recesses and the projections of the surface on a large (compared to their size) area. The value of e depends on the construction material and the processing of the inner walls of the pipe. Corrosion and depositions increase roughness. Since the inner diameter also plays a role on the influence of the pressure drop, in order to decide whether the roughness is negligible or not, we use the ratio e/d that is called relative roughness.
The roughness coefficient, n, is used in hydraulic calculations and is calculated by Manning’s formula:
n = [R2/3xS1/2] / V
Therefore, R = 17/0,935 = 18,18mm H2O/m, which is the true value (loss of head) at 25°C.
where
With the help of the equivalent Pa/m (1000 Pa = 0,01 Bar), the press loss converted to Bar in the specific example is 18,18/10000 = 0,001818 Bar/m.
R = hydraulic radius, S = the slope of the pipeline, V = flow velocity
The roughness coefficient depends from the material of the pipe, but also from other factors such as bad connections, bad lining, etc.
In this diagram, it is quite simple to determine the loss of head when all other parameters are known, e.g.:
R = loss of head value in mm Η 2 Ο V = water velocity in m/s Q = flow rate value in l/h L = pipe length to be calculated in m.
0.935
25
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