Submersible Pumps

As well as the more common pressure pumps you will frequently see submersible pumps at your local supplier . . . . . So what’s the difference?

 

Characteristics of Submersible Pumps:

Most domestic submersible pumps come with an attached float switch which switches the pump on when the water level rises, and off before it runs dry.

  • They’re Quiet  The surrounding water absorbs any noise.
  • Don’t Need Priming  As the pump sits under the water it doesn’t need to be primed, which also improves efficiency.
  • Rarely Block These are centrifugal pumps, which can pass reasonably large solid particles.
  • Pump From Bottom of Tank/Well Useful  for emptying things, but bad if you need the best water quality (The worst water is typically at the bottom of a tank see: Understanding Tank Water Quality)

Uses

Best for transferring water in constant flow rate applications such as:

  • Emptying tanks
  • In a sump to keep basements dry
  • Pumping Storm Water
  • Pumping Grey Water
  • Irrigation systems (make sure there is a filter smaller than any nozzles)

 

Make sure you get the correct size pump, see: Pump Size

 

Rainwater – Pump Size

One of the reasons why most rainwater systems use too much power is because because the pump is too big. For other reasons see Rainwater – Pump Issues

Most pump suppliers and design charts will specify a larger pump than you really need (They sell a more expensive pump, It’s less likely that the pump will be returned because it isn’t big enough, and they aren’t paying the power bill)

If you want to pick the most economic pump here is how to go about it.

Instantaneous Flow

What is the most flow you need at any one time. Here are some figures for typical house fittings.

Fitting Flow Litres/min
Tap 10 to 15
Tap with Flow Restrictor 4 to 6
Low Flow Shower 7 to 9
Washing Machine* 4 to 10
Dishwasher 4 to 6
Toilet 3 to 5
Garden Sprinkler** 10 to 15

*To get to the lower figure you will need to close the supply valve this will add a few minutes to the wash but will help with issues like Water Hammer.

**It’s really better to irrigate the garden with a separate pump.

Add together the highest flow rate fittings that you think you will want to run together, which will give you a Total Flow Rate ‘Q’

Pressure

You need to aim for a pressure at the fitting of around 150kPa (15m of Head)

To get this pressure you need to:

  1. Measure the height of the furthest fitting above the lowest level in  the tank ‘Hs)
  2. Calculate the Pressure  Loss ‘Hf‘ due to Friction in the pipes See table below
Max Flow Rate
Litres/min
Hf  m head per 100m of pipe
20 25 32 40
12 10.9 3.7 1.2 0.4
24 13.4 3.9 1.3
36 8.3 2.8

The Required Pump Pressure is then calculated from:

Required Pump HeadP‘ m = 15 +Hs + Hf
Or
Required Pump Head ‘P‘ KPa = 150 +(Hs +Hf)) x 10

Ordering

To order a pump you just need to quote the Total Flow and Required Pump Head

 

When you are ordering a pump think about getting a Pressure Tank