There is enormous variety of pumps brands, pump types, and prices covered by this general description. This article will cover the most common variety. The usual domestic pump is a Centrifugal pump direct coupled to an electric motor.
MOTORS
The motor is usually 2-pole (2900 revs/minute), 240 Volt single phase, ranging generally from 300 Watts to 1500 Watts. The better one have stainless steel shafts, the lesser ones are of mild steel, perhaps a plastic sleeve covering them against corrosion. There can be open motors (not desirable) or TEFC (Totally Enclosed Fan Cooled) motors, which are weather-proof and therefore the preferred type. The motor needs to be placed to a location with easy access for air to the fan to keep the motor cool running. Avoid grass and rubbish accumulating around the fan end cover.
PUMPS
The centrifugal pump has one or more impellors of plastic or metal. The pump is mounted directly on the end of the motor, with a water seal onto the motor shaft. If this seal fails, water will be seen between the pump end and the motor.
The better pumps are of bronze or stainless steel construction. The very best are of cast metals, the cheaper ones are of fabricated sheet metal.
Smaller units generally have lower efficiencies, lower pressure and lower flows. This is because the diameter of the impellor is smaller, so the circumferential speed is lower, correspondingly reducing the efficiency, pressure and flow. To compensate for these limitations, most small pumps have a Jet & Venturi built into the pump casing to boost pressure. Regrettably, this further lowers the overall efficiency of the pump to about 15%, so it becomes relatively expensive to deliver a litre of water in comparison with a straight centrifugal pump with a larger diameter impellor and an operational efficiency of around 50% to 60%.
The small pump is cheap ($100-$400), generally low quality, more expensive to operate and normally has a shorter working life expectancy. It this often has a shirt Warranty period.
The better quality pressure pumps are of 750 Watts or more, multi-impellored, metal construction with TEFC motors. There price range is from $700-$2000 as at the time of writing this article.
CONTROLS
Like the pumps, there are various control methods to turn on with a fall of pressure, when you open a tap, or stop on a rise in pressure with a shut tap.
ELECTRONIC CONTROLLERS
They are popular because they are cheap. There primary disadvantages are:
- They are prone to damage by poor quality power supply or surges
- They start the pump every time water is used, even just enough to fill a kettle
- The pressure range is fixed, not adjustable to particular application
- Frequent stop-start operation is hard on the motor and harder on the power bill. An electric motor uses much more power to start than it does to run.
There is a variable speed controller available, which ‘soft-starts’ the pump, reducing power consumption on start-up; and will maintain a constant pressure in accordance with required flow. Generally used in larger and institutional applications because of the higher initial cost, as the motor has to be three-phase to be speed controlled. The sensitivity to power spikes etc remains as for other electronic systems. Not generally practical for normal domestic applications.
PRESSURE SWITCH & PRESSURE TANK CONTROLS
These are considered somewhat old-fashioned and they do take up more space. They are more costly initially but provide long service with very moderate maintenance. They are pressure adjustable and efficient because the number of starts is considerably reduced.
The pressure tanks stores water under pressure. This is drawn off first, until the pressure drops to the point where the pressure switch cuts into maintain supply mode. When the draw-off ceases the pumps runs on the re-charge the pressure with water stored ready for the next draw-off.
So when kettle is filled, or toilet flushed, the pump does not need to start. Quite obviously, the larger the pressure tank the fewer pump starts there will be. As a rule of thumb, the drawdown from tank with pressure switch set to 40/60 psi is about 27%, or a little more than a quarter. So, a 35 litre tank will supply about 9 litres of water for example. At higher pressures this is reduced slightly and conversely it is slightly more at lower pressure switch settings.
Tank pre-charge pressure should be about 10% below the cut-in settings of the pressure switch. This pressure should be checked about every six months, or in the event the pump is starting/stopping more frequently than normal. Rapid cycling is damaging to the pump, and to the power bill.
To check the pre-charge pressure, switch off the pump, close the supply valve, open a tap to drain all the pressure from the plumbing system, and leave it open until completed.
Use a tyre gauge to read the stored pressure in the pressure tank. A tyre pump or air compressor will bring the pressure to the required level, then re-start the system.
INSTALLATION
Mostly tanks are the water source but pressure pumps can be set up on dams or other surface water and on underground supplies with suitable pumps.
- The pump suction should be as short as is practical and must be level, or better sloping upwards to the pumps inlet. A high point in the pipeline anywhere else is inviting problems caused by air-locks.
- Install a check valve, preferably at the tank, otherwise at the pump inlet.
- Always keep suction pipelines at least as large as the pump inlet … bigger is better
- Minimise elbows etc. in the suction line
- Never put an outlet along the suction line – it will lose pump prime if opened.
- Prime the pump (fill it with water) prior to starting – a dry-running shaft seal can fail in a few seconds. Open the supply valve, and close the outlets until the pump runs up to the pressure and shuts down … if it fails to produce pressure almost immediately, re-prime, and ensure all air is out of the suction pipe.