MANAWATU FARMING LIFESTYLES
Irrigation Matters with Clem Powell
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January 2017
Save Lives
The mystery of pipelines Pipelines are those expensive things you have to have to convey water or other fluids (or gases) and best bought at cheapest possible price — correct? Wrong! If you consider it, they are often difficult to alter if the system is incorrect. All liquids and gases moving along a containment vessel have pressure loss due to friction from the walls and turbulence from various changes in the shape or direction of the vessel. Temperature of the liquid also has a bearing, but let’s keep it to the most basic issue of pressure loss. Also some things which can be done to cut down on the turbulence including long radius bends instead of tight bends, welded fittings in place of threaded sockets, etc. One of the biggest mysteries for amateurs is the relationship between pipe sizes and friction loss/flow rate. In a nutshell, the friction loss multiplies as the pipe size reduces — they are not proportional! Often a bargain price for next-sizesmaller pipe has a major negative impact on the efficiency of the system. As a scenario: Consider a small block irrigation scheme of 4ha. The well and shared pump are 200m into neighbouring property and the most distant hydrant on the block is a further 300m, making maximum delivery length of 500m. The flow required is 300L/min. At this
point it doesn’t matter what type of irrigator is used, seeing as we’re only considering the pressure loss in the main pipeline. Sizes discussed will all be internal to avoid confusion between PVC, LDPE and MDOD pipe options (never mind aluminium). If 3" pipe were used the friction/pressure loss could be 9psi for the full distance, if 2½" used could be 22psi loss, 2¼" could be 50psi loss, and good ole 2" alkathene could be 87psi loss! This is without the pressure required to run the irrigator or allowing for rising ground, etc. If 2" alkathene were selected for this job then the pump would quite possibly need to be 7.5hp larger than if 3" pipe were used. Rather a sober consideration for a small system and a great energy waste which
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DANNEVIRKE
FARM SHEDS
No job too big, No job too small
could easily be avoided. Next time — The mystery of pumps! If you wish to have further discussion regarding this article you can contact Clem Powell on 027 599 9273.
T: Nigel Walshe 06 374 6045 155 – 163 High St, Dannevirke OPEN 7 DAYS HOURS: Mon–Fri 7:30am–5pm Sat 9am–4pm, Sun 10am–4pm Timber Yard Sat 9am–1pm, Sunday Closed
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