How to Choose a Well Pump
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1. What is a Borehole Pump and What Types are There
A borehole (submersible) pump is specialized equipment for extracting water from a depth of over 8 meters, which is typically the depth of boreholes. It appears as an elongated metal cylinder, installed inside the casing pipe, fully submerged in water, and operates in a vertical position. The construction includes a narrow body (usually 3-4 inches or 75-100 mm in diameter), an electric motor, impellers creating centrifugal force to pump water to the surface, a check valve keeping the water in the pipeline after the pump stops, and a filter protecting the impellers and other elements from large particles and damage.
5 – Compensating ring; 6 – Impeller bearing; 7 – Seal ring;
8 – Impeller; 9 – Suction chamber; 10 – Protective mesh;
11 – Coupling connecting motor and pump shaft; 12 – Motor shaft.
According to the operating principle, there are several types of borehole pumps:
- Centrifugal. The working element here is the gears (the number varies by model), fixed on the shaft and rotating thanks to the motor. Thus, through centrifugal force, water is swirled in the impeller and expelled under pressure. This is the most common type of pump as the devices are reliable, durable, have high performance and efficiency (about 95%), do not break with small solid particles in the water, and do not create much noise or vibrations. This equipment is compact (some models are suitable even for boreholes with a diameter of up to 8 cm) and can pump water from great depths—up to 100 meters or even more.
- Screw. The main element of such pumps is the screw, which rotates when the motor is switched on, thereby drawing water. The design is simple, minimizing the risk of breakdowns, and the devices are convenient to operate, do not make noise or vibrate. Moreover, this type of pump easily handles dirty water containing silt and clay, so they can supply water not only from boreholes but also from water bodies. Meanwhile, in terms of performance, screw pumps are inferior to centrifugal ones—at the same motor power, the efficiency will be 20-30% lower.
- Vortex. The construction includes impellers with blades that, by rotating, create a vortex which lifts the water to the surface. The main advantage of such pumps is the high pressure, though their efficiency is quite low—up to 45%. Therefore, they are chosen if water needs to be delivered to great heights, but the consumption volume is small.
2. Key Parameters
Usually, when drilling a borehole, specialists issue a certificate specifying all water intake parameters and sometimes even recommendations on choosing the right pump. However, users often have to make choices on their own, and numerous nuances must be taken into account.
Firstly, you need to learn the borehole parameters:
- Borehole Depth — the total length from the ground surface to the bottom
- Static Water Level — the distance to the water mirror in a still state, without the pump running. This parameter can be determined on your own using a weight, fishing line, and tape measure: tie the weight to the line and lower until you hear a splash — meaning it has reached the water surface.
- Dynamic Water Level — the water level with the pump running. It shows how deep the pump needs to be submerged to stay in the water at all times.
- Borehole Internal Diameter. The pump diameter should be 2-3 cm less than the casing pipe to allow it to be freely lowered into the borehole. Also, there must be enough space for water to effectively cool the internal components of the pump. Boreholes for private water supply are typically drilled with diameters from 10 to 12.5 cm, and pumps with a diameter of 10 cm are the most popular. For instance, a pump with a diameter of about 7.5-8 cm or even less (6.5 cm) would fit a borehole with a diameter of 11 cm.
You can learn the borehole parameters from the certificate of the object, from the company that drilled it, or calculate them independently. The borehole depth is determined in the same way as the static water level, but in this case, the weight is lowered to the very bottom until the line starts to sag.
Another important characteristic is the borehole's yield, or flow rate — the amount of water it can produce over a given time period. It is measured in liters per minute (l/min) or cubic meters per hour (m³/h), less commonly in liters per day. Specialists test the borehole's water yield when drilling and measure its performance. This can also be independently assessed by running the pump and measuring the volume of water in one minute, ensuring the water level does not drop.
Next, focus on the pump characteristics. The key parameter is capacity — the amount of water it can deliver per unit of time — liters per minute or cubic meters per hour. To choose a pump with optimum capacity, consider the water needs of all users. Here are average water consumption figures:
- sink and washbasin — 0.5 m³/hr;
- toilet — 0.5 m³/hr;
- bathtub and shower — 1 m³/hr;
- washing machine — 0.55 m³/hr;
- dishwasher — 0.25 m³/hr;
- drip irrigation, watering — more than 1 m³/hr.
Calculate approximate water consumption for each point in the house, add these figures together, and multiply by 0.5 - 0.7, as they will not all be working simultaneously. On average, a pump with a capacity of 2 to 4 m³/hr (or 35 - 70 l/min) is suitable for a family of 3-4 people with household needs and watering. It's wise to choose a pump with some reserve, but not excessively — excessive power leads to frequent starts and accelerates wear. Besides, the pump's capacity should not exceed the borehole's yield — otherwise, it will pump out all the water, leading to overheating and breakdowns. If the borehole yield is very small, an additional storage tank can be installed — this way, you'll have water at any time.
Apart from capacity, it's crucial to correctly select the head — the height to which the equipment can lift water. The required head is calculated by the formula:
H geo + H dyn + H dom + H pipe + H reserve, where:
- H geo — the pump's immersion depth;
- H dyn — the distance from the ground surface to the highest water intake point (e.g., shower on the second floor);
- H dom — the minimum pressure required at the water consumption point for the operation of household appliances (usually 1.5 - 2.5 atmospheres = 15 - 25 m water column);
- H pipe — head losses in the pipes due to friction, turns, valves (approximately 10 - 20% of the total pipeline length);
- H reserve — reserve for water level fluctuations, wear, pressure drops (recommended +10 - 20%);
As an example, consider the following calculations: dynamic water level 35 m, the highest consumption point 6 m (2nd floor), minimum system pressure 2 atmospheres = 20 m. Add 5 m (losses) and 10% as a reserve — resulting in a pump that must have at least a 72 - 75 meter head.
3. Other Considerations
3.1 Materials
Since the pump's body and working parts are constantly in contact with water, they should be made from quality materials:
- Stainless Steel — a robust and durable material not prone to corrosion, doesn't affect water taste, withstands sandy water and high pressure.
- Aluminum — affordable, lightweight, but less durable, can corrode.
- Plastic — used for making bodies and some internal parts, resistant to corrosion but less durable than metals. Commonly found in budget models.
- Brass — strong, wear-resistant, and costly, used for making parts in contact with water (mainly impellers), with good corrosion resistance.
3.2 Protection
Protection systems ensure the equipment's safe operation, prevent breakdowns, and extend service life. These can include:
- Overheat Protection — usually implemented with a thermal relay that cuts off the pump when reaching a critical temperature.
- Overload Protection — turns off the motor if it operates beyond its capabilities and risks overheating or breaking.
- Dry Run Protection — automatically stops the pump if there's nothing to pump, i.e., in the absence of water, as it may lead to overheating and damage.
- Pressure Relay — turns the pump on and off at specific pressure values in the system.
3.3 Power Supply
Standard household networks are rated for 230 V voltage, but there are few pump models powered by three-phase nets of 400 V. These are industrial-class devices intended for specialized facilities. Besides, do not forget the length of the power cable. Ideally, it should not be shorter than the maximum immersion depth, ensuring the cable connection to the network is above water or completely out of the borehole. This way, users won't spend time and effort on insulation.
3.4 Water Type in the Borehole
Water in boreholes is often clean, yet it can still contain silt, sand, and other mechanical impurities, which threaten damage to internal parts. Thus, if water contains impurities—it's better to have a filtration system on the pump.
3.5 Installation and Maintenance
A pump's design should allow easy access to the impeller, motor, and other crucial components for periodic cleaning and inspection. The pump must be suitable for the working temperature and pressure in accordance with the operating conditions. Additionally, a stainless steel cable or nylon rope is required to safely lower and raise the pump, capable of carrying the weight of the pump with reserve.
4. Common Mistakes When Choosing a Pump
- Ignoring borehole parameters, choosing a pump "by eye". It's crucial to carefully verify all information and perform necessary calculations before purchase.
- Overestimating needs. Inexperienced buyers choose a "more powerful" pump, thinking "it's better to choose with a reserve". In reality, if pressure is too great, it creates water hammer in the system, and too high capacity leads to frequent on/off cycles, causing engine and automation wear. The pump should match the actual borehole depth, water delivery height, and consumption volume.
- Incorrect installation. A pump shouldn't be installed at the borehole bottom, as silt, sand, and other contaminants are present there. The optimal height is 1-2 meters above the bottom.
- Lack of protection. Pumps running without water quickly overheat and fail, so it's better not to neglect dry run protection. Additionally, without automation, the pump will frequently turn on and off, destabilize pressure, and wear out faster.
- Improper operating conditions: water temperature above permissible levels, water with high sand content, continuous operation when not designed for it.
