- Parameter definition
- Calculation example
- If the power is too high
- Pump installation and commissioning
- Downhole pump protection
- Glossary of terms
Unfortunately, no type of pump has as many failures and breakdowns as with household submersible borehole pumps. The reason is not in the quality of equipment, but in the fact that the project and selection is carried out not by professional designers, but by the owners of private houses themselves or insufficiently qualified personnel of installation organizations.
How, for example, does a non-specialist know that the engine of a pump selected with a large margin can burn out if the pump is not brought into the operating range during installation and setup of the system.
We hope that this article will help readers avoid such mistakes..
In all cases, for the correct choice of the pump, first of all, it is necessary to determine its operating parameters – flow (Q) and head (H).
The required water consumption is determined from the total productivity of all water points of the object, taking into account the likelihood of their simultaneous use.
In a simplified calculation, you can use the following water consumption rates for plumbing fixtures:
– washbasin – 60 l / h, – toilet cistern – 83 l / h, – kitchen sink – 500 l / h, – shower – 500 l / h, – bathroom – 300 l / h, – watering tap – 1080 l / h.
Watering lawns and flower beds requires 3-6 m3 of water per m2, while the consumption also depends on the method of irrigation and the intensity of irrigation. Sauna or bath will require about 1000 l / h.
To calculate the required head, the formula is used:
Htr = Hgeo + S + Hsvob, where
Ngeo – the height of the pipeline inlet in the building relative to the dynamic water level in the well (the numerical expression of the dynamic level must be present in the well certificate);
S is the sum of frictional head losses in the pipeline and local resistances (fittings, fittings, filters, etc.);
Nsvobod – the pressure that must be created at the entrance to the building, with the calculation of the provision at the most remote and highest located water point of pressure 0.5 atm.
Well parameters are fundamentally important to the user, as they are used to calculate the required head and performance of the selected pump. Drillers should indicate such parameters as static level, dynamic level, well flow rate in the well certificate. These data are determined experimentally by the organization that carried out the drilling work. Obviously, the calculation results will turn out to be incorrect if, when determining the dynamic level of the well, a pump of obviously lower power was used than is required to supply the facility in accordance with the consumer’s requests. And although it is difficult for the user to expect to quickly obtain an official passport for an artesian well (this is a state document that requires many permits and approvals), it is necessary to require the provision of detailed data on the well along with the act of work performed, including inquiring about the power of the pump that was pumped out water when determining the dynamic level. When concluding a contract for drilling work, you should pay attention to the availability of a license from the contractor. At the end of the work, only serious companies always give the client a guarantee and a detailed well passport, where all the mentioned characteristics are clearly stated, as well as the diameter of the casing string, the list of soils passed, information about the trial pumping of the well, etc. – up to the recommended pump brand and installation depth.
The parameters of the required Q and H for additional equipment (jacuzzi, washer, sprinklers, “sprinklers”, etc.) are specified by the manufacturers. When installing water treatment filters, head losses (usually about 2 atm) and water consumption for washing them are taken into account. For the pool, only the filling time is indicated.
It is required to provide water supply to a suburban area with a two-story cottage (a kitchen, two bathrooms and a shower with hydromassage require a flow rate of 1 m3 / h and a pressure of 4-5 atm.), A garage, a house for service personnel (contains a bathroom), a bathhouse, a 45 m3 pool, irrigation of the territory, water treatment system. A family of four and two of the staff permanently reside on the site. A well 80 m deep was drilled for water supply of the site; casing diameter – 150 mm; static level – 46 m; dynamic level – 50 m; flow rate measured during pumping – 3.5 m3 / h.
Taking into account the water consumption rates (see at the beginning of the article), we get the total consumption and pressure of consumers:
Qtot = 500 + 3 ґ (60 + 83 + 500) + 1000 + 1000 + 2 ґ 1060 = 6500 l / h = 6.5 m3 / h
Нтр = 50 + 8 + 20 + 2 + 30 = 110 m
Due to the impossibility and inexpediency of using all the water points at once, the required flow rate can be determined as 5 m3 / h.
The calculated data is satisfied by the pump with (Q = 5 m3 / h, H = 120 m); its characteristic is shown in fig. 1.
This ensures sufficient consumption for the kitchen, one bathroom and watering. (It is assumed that the owners will not use the bathhouse and take the bathroom, wash in the shower and fill the pool at the same time as watering the area from both taps.) It is cheaper to use separate pumps to maintain pressure on the irrigation systems and hydromassage – this will not keep the entire water supply system under high pressure and will make the operation of the submersible pump more stable, and the system flexible and independent (with the help of an additional pump, you can always get high pressure at any point of analysis). The pool will fill up at night. In this case, with the help of a valve on the head, the pump should be “throttled” (create additional resistance) so that when working on filling the pool, the flow does not exceed the permissible – 6.5 m3 / h.
If the power is too high
Fig. 1. Performance characteristics of a properly sized pump.
Self-selection of a pump by a customer with excessive requirements for flow and head often leads to the selection of a pump with too much power. As already mentioned, when installing an overpowered model, complications are possible..
Firstly, since with such a choice, the nominal flow significantly exceeds the average water demand, the pump will operate in the mode of frequent on / off switching. Manufacturers allow up to 30 pump starts per hour, but only for one hour per day, with a general limit of 60 cycles per day. In any case, frequent switching on negatively affects the service life of the electric motor and starting automatics. To avoid this, you will need to install a large diaphragm tank.
Secondly, with an overestimated pump power, as a result, the water pressure at the entrance to the house will be overestimated. At the moment of starting such a pump, strong hydraulic shocks will inevitably occur. Some fittings may simply not be designed for such pressure (dishwashers and washing machines, mixers), additional installation of pressure reducers will be required to reduce the pressure.
Fig. 2. Pump not matched correctly: duty point shifted to the right.
Thirdly, during the filling of the pool, the pump will operate on an “open pipe” without creating pressure. In such conditions, there is a large flow of water at minimum pressure. The operating point of the pump shifts on the characteristic curve to the right, in an area that does not correspond to the operating area of the pump (Fig. 2). The power on the shaft will be maximum, and with prolonged operation in this mode, the engine will fail..
The consequence of the use of a pump with an overestimated power will be a general increase in the cost of the entire system, caused by the use of more powerful electrical equipment, materials and fittings with a high permissible working pressure, an increase in the diameters of the pipeline and well, as well as an increase in the cost of water treatment.
If the nominal pump flow exceeds the well flow rate, it is necessary to install additional protection for “dry running”. Throttling and adjusting the pump will result in excessive energy consumption.
In other words, while ensuring the possibility of the simultaneous use of all water points by installing an overpowered pump, the cost of the water supply system increases. At the same time, the real water consumption will be much lower.
Therefore, although the final choice will always remain with the customer, it is cheaper and more correct to choose a pump taking into account real needs and with the help of specialists. It is possible to satisfy the user’s requirements for the water supply system, subject to the rules for its installation and operation, by choosing the pump with a flat operating characteristic that is optimal in this situation.
For a water supply system, a pump can be selected from the above example (Fig. 1). In the zone of possible flow rates (from 4 to 8 m3 / h), the curve of the pressure versus flow rate for this model has a flat shape, that is, at low water flow rates, there will not be too much pressure increase. At the same time, a certain allowable margin when calculating water consumption will exclude the possibility of water shortages.
Pump installation and commissioning
In any case, whichever pump is chosen, during installation it is necessary to adjust its operating point in all possible operating modes. During commissioning, the supplied flow rate should be measured (determined by the filling rate of any container of known volume, for example, a barrel), the pressure created (according to the pressure gauge on the head) and the current consumed (measured with current tongs).
The obtained characteristics are checked against the pump data sheet according to the catalog. If the operating parameters are exceeded (as a rule, some power reserve is provided, for example, for the subsequent installation of filters), it is necessary to close the valve at the exit from the well, create additional local resistance sufficient to establish the correct operating point – the middle of the Q (H) characteristic.
The installation, as well as the selection of the pump, must be carried out by trained specialists; the installation company must have a license to install this equipment.
Downhole pump protection
Before choosing and purchasing a pump, it is necessary to obtain accurate information about the power supply voltage at the facility. This is especially important when choosing an imported pump. All equipment supplied from abroad meets, first of all, the industrial standards of the country of origin. So for all German pumps, the permissible voltage deviation in the electrical network from the nominal is from +6 to -10%. Despite all the built-in protections, the pump is not designed to operate from a network with a voltage below 200 V, all possible drawdowns and voltage surges will negatively affect the service life of the electric motor. Here it is necessary to provide for adjustable voltage protection as part of the control cabinet, and for three-phase pumps also against non-phase modes of operation. It is not recommended to install powerful single-phase pumps. The starting current of a 2.2 kW motor can exceed the rated current by 4.4 times! To stabilize the voltage within the operating range with such surges, a stabilizer with a fivefold power reserve will be required (more precisely, the choice will be made by the manufacturers of stabilizers). Sometimes it is cheaper for the user to provide a 380 V power supply at the facility than the correct operation of a single-phase pump.
According to statistics, about 85% of failures occur precisely with the electrical part of the pump. The main reason is the turn-to-turn closure of the stator windings due to overheating due to hydraulic overload, or when operating at low or abruptly changing voltage. Both can be avoided with proper overcurrent protection. A conventional starter with overcurrent protection does this quite well, but some installers forget to adjust to the required current value. The result of such negligence is easy to calculate: you will have to pay for lifting the pump from the well, repairing it (the price of a new engine), for re-lowering the pump and putting it into operation. The amount may exceed the cost of a new pump.