The flow resistance chart provides an easy way of determining how much flow in litres per second will occur in a pipe with a known pressure drop between two locations or alternatively it can be used to find the pressure drop between two locations for a known flow.
A domestic plumbing pipe installation is 50 metres long and the required discharge is 12.0 litres per minute. What is the pressure drop over this distance ignoring any differences in height between the start and finish point?
This flow corresponds to 0.2 litres per second. Draw a vertical line from the bottom axis of the chart at 0.2 l/s and where it crosses the pipeline being considered read horizontally to the vertical axis. For a DN 25 PEX pipe the reading can be estimated as 55 m loss per 1000 m (note this is a logarithmic scale!) or (55 x 50 / 1000 =) 2.75 m per 50 m of pipeline. This converts to 27 kPa pressure drop (i.e. 2.75 x 9.8).
For the same flow in a DN 20 B copper pipe the loss becomes 70m per 1000 m which is 3.5 m per 50 m or 34 kPa pressure drop.
If there is a height difference multiply the vertical rise in metres over the pipe length by 9.8 to convert to kPa and add to the chart calculation. (A fall should be subtracted).
A pipe is to be installed to transfer 144 litre per hour between two tanks with a 3.0 m height difference over a distance of 100 metes. What pipe is suitable?
This flow converts to 0.04 litres per second. The head loss can be expressed as 3.0 x 1000/100 = 30m per 1000m. Interpolate to find the 30m point on the vertical axis remembering this is a logarithmic scale. Draw a line horizontally to the point where it intersects the 0.04 l/s line. All pipes with flow lines to the right of this point would be suitable. That is DN 16 PEX would provide a flows slightly greater than required whereas DN 15 B copper would be less than required.
Iplex K2® PE-X pipes and fittings system must be installed in accordance with the manufacturer’s installation requirements, AS / NZS 3500 parts 1, 4 & 5 and any local by-laws with particular reference to the pressure and temperature relationship as described in AS 2492, pressure derating of pipes according to pipe material temperature:
|Working Pressure According to Pipe Material Temperature (PMT)|
|Pressure||2000 kPa||1870 kPa||1500 kPa||1330 kPa||1020kPa|
The thermal conductivity of plastics is generally lower than that of metals. It is the poor thermal conductivity of cross-linked polyethylene that restricts the heat loss through hot water pipes and reduces the need for lagging. In addition, the rate of heat flow through a body is not just directly proportional to the thermal conductivity but also inversely proportional to the thickness. The wall thickness of the cross-linked polyethylene pipe further restricts heat loss.
Lagging of cross-linked polyethylene water pipes is required where the pipe is installed in chases or where it penetrates a concrete slab or to meet the energy efficiency requirements of the Plumbing and Drainage Standard AS3500 Part 4 and Building Code of Australia.
Also in particularly cold climates lagging is recommended where freezing can occur, for example, where pipe is exposed above ground. Although cross linked polyethylene pipes have been shown to withstand freezing of water to a greater extent than many other materials, the pipe obviously is not useable if the water inside is frozen.
|Thermal Conductivity – watt per metre Kelvin (W/m.K.)|
Plastics pipes such as Iplex K2® PE-X inherently have a much lower thermal conductivity than metal pipes, for example copper, and therefore the R values are higher and heat losses are lower.
|Inherent R Values – K2™|
|DN16||R = 0.006|
|DN20||R = 0.007|
|DN25||R = 0.009|
However, in those applications where AS3500 requires copper hot water pipes to be insulated, it is also necessary to insulate plastics hot water pipes using a similar thickness of insulation. Note that the total R value of an insulated plastics pipe will be higher than an equivalent copper insulated with the same material. Heat losses will again be less for the plastics pipe.