Consider Pipe Pressure Drop or Friction Losses In Choosing A Pond, Filter or
Waterfall Pump
In the list below we've provided you with a basic introduction to fluid
mechanics and the type of variables that will have an effect upon how well your
pond pump will work and therefore the factors you can take into consideration to
improve flow and ultimately reduce cost. The subject of fluid flow can become
very complicated (as it is in real streams and rivers) so we have used an
approximate and simplified but nevertheless accurate summary of the important
considerations.
In general under normal flow conditions in a pipe and in slow moving rivers
for example the speed of the water is slower at the wall than it is in the
middle of the pipe (in the river the speed at the bottom next to the river bed
is close to zero). This area is known as the boundary layer. In general it is
good to create a flow rate so that flow in a pipe is not too gentle (we call
this turbulent flow) because then no dead spots are created and this is good for
fish pond situations.
Turbulence in general is good for pond filters and UVC
systems also.
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When water is pumped through a pipe or tube
some of its energy is lost because the water slides along the walls of the
pipe creating friction losses. If a pipe is very rough on the inside then
the energy loss will be even greater. So always use smooth pipes. As you can
imagine these losses increase with the rate of flow (velocity) of the water.
The practical implication of this when specifying a pump for your pond is
that you must estimate how much friction losses are taking place and then
strive to reduce them.
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This amount of friction loss which can also be
described as "head loss" should be added to the height the pump is required
to pump to. For example if you have worked out you need to pump 3,000 lph of
water to a head of 2 metres then maybe you should allow for a head 10%
higher a 2.2 metres and this will compensate for some if not all of the
friction losses. This means choose a pump that will deliver 3,000 lph to a
head of 2.2 metres instead of 2 metres.
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This friction loss can be very significant in
long pipes and especially in narrow diameter pipes.
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If you use more than 3 metres of pipe length,
pump more than 2,000 litres per hour and especially if you use narrow tubing
you should estimate the friction losses or at least choose a pump one size
bigger and maybe 2 sizes bigger than you thought.
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In addition to the pipe friction loss, losses
also occur when valves and fittings are used in a pond filter and waterfall
pumping system.
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Losses also occur in restricted pipes, bends
and so on. These tend to be about 20% of the pipe friction loss.
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The calculation of friction loss is extremely
complex so be aware of these principles and adapt your system in a sensible
way.
What Kind Of Fittings Are Available? ... a few examples shown in this
picture
All these fittings restrict water flow (ie add friction to the system) so be
careful about using them unnecessarily. In particular avoid sharp bends like "T"
pieces ... a "Y" piece is always better than a "T" piece. Use smooth and long
curves in non-kinking flexible pipe to change water flow direction instead of
bends and right angle fittings.
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Poly or plastic linx Join two pieces of hose
together
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Poly or plastic "T" linx Join three
pieces of hose in a T shape to divert water flow
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Poly or plastic clips (pack of 3). Plastic
clips don't corrode. Ideal for securing hose to pump or on any underwater
joints, double Wire clips (pack of 3)
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Fits between the ribs of the hose for a really
secure joint
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Hose elbows tidies up those 90 deg. bends by
joining two pieces of hose
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Poly plastic hose or tubing 5m ... Wire
reinforced hose that will not kink or flatten. UPVC will not rot. 5m lengths
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Poly hose 30m ... The same high quality hose in 30
m lengths
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Clear hose or tubing ... 5m lengths of
transparent hose that let you see what's going on inside!
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In line tap ... Fits between 2 hoses to allow
control of water flow. The 32 and 40mm taps are metal and plastic the
smaller taps are all plastic
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