Submersible sewage pump is a type of pump product that is connected to a motor and works underwater at the same time. Compared with general horizontal pumps or vertical sewage pumps, submersible sewage pumps have the following advantages:
1. Compact structure and small footprint. Submersible sewage pumps can be directly installed in sewage tanks due to their underwater operation, without the need to build specialized pump rooms for installing pumps and machines, which can save a lot of land and infrastructure costs.
2. Easy installation and maintenance. Small submersible sewage pumps can be freely installed, while large submersible sewage pumps are generally equipped with automatic coupling devices for automatic installation, making installation and maintenance quite convenient.
3. Long continuous operation time. Submersible sewage pumps have a shorter shaft and lighter weight of rotating components due to the coaxial design of the pump and motor. Therefore, the radial load on the bearings is relatively smaller, and the service life is much longer than that of ordinary pumps.
4. There are no issues such as cavitation damage or water injection. Especially the latter point has brought great convenience to the operators.
5. Low vibration noise, low motor temperature rise, and no pollution to the environment.
It is precisely because of the above advantages that submersible sewage pumps have been increasingly valued and used in a wider range, from simply transporting clean water to now being able to transport various types of domestic sewage, industrial wastewater, construction site drainage, liquid feed, and so on.
It plays a very important role in various industries such as municipal engineering, industry, hospitals, construction, restaurants, and water conservancy construction.
But everything is divided into two parts, and the key issue for submersible sewage pumps is reliability, because the use of submersible sewage pumps is underwater; The medium being transported is a mixture of liquids containing solid materials; The pump is very close to the motor; The pump is arranged vertically, and the weight of the rotating components is in the same direction as the water pressure borne by the impeller. These issues make the requirements for sealing, motor bearing capacity, bearing arrangement, and selection of submersible sewage pumps higher than those of general sewage pumps.
In order to improve the service life of submersible sewage pumps, most manufacturers are now trying to find ways in the pump protection system, which can automatically alarm and shut down for maintenance in case of pump leakage, overload, overheating and other faults. But we believe that it is necessary to install a protection system in submersible sewage pumps, which can effectively protect the safe operation of the electric pump. But this is not the key issue, the protection system is just a remedial measure after a pump failure, which is a relatively passive approach. The key to the problem should be to start from the root and thoroughly solve the problems of pump sealing, overload, etc. This is a more proactive approach. To this end, we have applied the hydrodynamic sealing technology of the secondary impeller and the overload free design technology of the pump to the submersible sewage pump, greatly improving the reliability and load-bearing capacity of the pump seal and extending the service life of the pump.
1, Application of hydrodynamic sealing technology for secondary impeller
The so-called secondary impeller fluid dynamic seal refers to the installation of an open impeller in the opposite direction of the same axis near the back cover plate of the pump impeller. When the pump is working, the secondary impeller rotates together with the pump spindle, and the liquid in the secondary impeller also rotates. The rotating liquid generates an outward centrifugal force, which on the one hand resists the liquid flowing towards the mechanical seal and reduces the pressure at the mechanical seal. On the other hand, it prevents solid particles in the medium from entering the friction pair of the mechanical seal, reduces the wear of the mechanical seal grinding block, and extends its service life.
In addition to sealing, the secondary impeller can also reduce axial force. In submersible sewage pumps, axial force is mainly composed of the pressure difference force of the liquid acting on the impeller and the gravity of the entire rotating part. The direction of these two forces is the same, and the resultant force is the sum of the two forces. It can be seen that under identical performance parameters, the axial force of a submersible sewage pump is greater than that of a typical horizontal pump, and the balancing difficulty is more difficult than that of a vertical pump. So in submersible sewage pumps, the reason why bearings are easily damaged is also closely related to the high axial force.
If a secondary impeller is installed, the direction of the pressure difference force exerted by the liquid on the secondary impeller is opposite to the combined force of the two forces, which can offset some of the axial force and prolong the bearing life. However, using a secondary impeller sealing system also has a disadvantage, which is that a portion of energy is consumed on the secondary impeller, usually around 3%. However, as long as the design is reasonable, this loss can be completely reduced to a low limit.
2, Application of overload free design technology for pumps
In a typical centrifugal pump, the power always increases with the increase of flow rate, that is to say, the power curve is a curve that rises with the increase of flow rate. This poses a problem for the use of the pump: when the pump operates at the design operating point, generally speaking, the power of the pump is less than the rated power of the motor, and the use of this pump is safe; But when the pump head decreases, the flow rate will increase. From the performance curve of the pump, it can be seen that the power also increases accordingly.
When the flow rate exceeds the design operating point and reaches a certain value, the input power of the pump may exceed the rated power of the motor, causing the motor to overload and burn out. When the motor is overloaded, either the protection system will activate to stop the pump from rotating; Either the protection system fails and the motor burns out.
The situation where the pump head is lower than the design operating point head is often encountered in practice. One situation is that when selecting the pump, the pump head is too high, but in actual use, the pump head is reduced; Another situation is that it is difficult to determine the operating point of the pump during use, in other words, the flow rate of the pump needs to be adjusted frequently; There is also a situation where the pump needs to be frequently relocated for use. These three situations may overload the pump and affect its reliability. It can be said that for pumps without full head characteristics, including submersible sewage pumps, their range of use will be greatly limited.
The so-called full head characteristic, also known as overload free characteristic, refers to the fact that the power curve rises very slowly with the increase of flow rate. Ideally, when the flow rate reaches a certain value, the power not only does not rise again, but also decreases. In other words, the power curve is a curve with a hump. If this is the case, as long as we choose a power value slightly higher than the hump point of the motor rated power, then in the entire range from 0 flow rate to high flow rate, no matter which operating point you operate at, the pump power will not exceed the motor power
