1. Compact structure and small footprint. Because the submersible sewage pump works under submerged liquid, it can be installed directly in the sewage tank. There is no need to build a special pump room to install the pump and the machine, which can save a lot of land and infrastructure costs.
2. Easy to install and maintain. Small submersible sewage pumps can be installed freely. Large submersible sewage pumps are generally equipped with automatic coupling devices for automatic installation, and installation and maintenance are quite convenient.
3. Continuous operation time is long. Due to the coaxiality of the pump and the motor, the submersible sewage pump has a short shaft and a light weight. Therefore, the bearing (radial) is relatively small and the service life is much longer than that of a general pump.
4. There are no problems such as cavitation damage and irrigation water. Especially the latter point brings great convenience to the operator.
5. The vibration noise is small, the motor temperature rises low, and there is no pollution to the environment.
It is precisely because of the above advantages that the submersible sewage pump has been paid more and more attention by people, and the scope of use is becoming wider and wider. It can be used to transport all kinds of domestic sewage, industrial wastewater, and construction from the original purely used to transport clean water. Site drainage, liquid feed, etc. It plays a very important role in various industries such as municipal engineering, industry, hospitals, buildings, restaurants, and water conservancy construction.
But everything is divided into two. The most critical problem for the submersible sewage pump is the reliability problem, because the submersible sewage pump is used under the liquid; the medium to be transported is a mixed liquid containing solid materials; The pump is in close proximity to the motor; the pump is vertically arranged and the weight of the rotating part is in the same direction as the impeller is subjected to water pressure. These problems make the requirements of the submersible sewage pump in terms of sealing, motor bearing capacity, bearing arrangement and selection higher than that of the general sewage pump.
In order to improve the life of the submersible sewage pump, most manufacturers at home and abroad are looking for ways to protect the pump, that is, when the pump is leaking, overloading, over-temperature and other faults, it can automatically alarm and automatically stop the repair. However, we believe that it is necessary to install a protection system in the submersible sewage pump, which can effectively protect the safe operation of the electric pump.
First, the application of the sub-impeller fluid dynamic sealing technology
The so-called auxiliary impeller hydrodynamic seal means that an open impeller is mounted coaxially opposite to the back of the impeller rear cover of the pump. When the pump is working, the auxiliary impeller rotates with the pump main shaft, and the liquid in the auxiliary impeller rotates together. The rotating liquid generates an outward centrifugal force, which on the one hand bears against the liquid flowing to the mechanical seal and reduces the mechanical force. The pressure at the seal. On the other hand, the solid particles in the medium are prevented from entering the friction pair of the mechanical seal, which reduces the wear of the mechanical seal block and prolongs its service life.
In addition to the sealing function, the auxiliary impeller can also reduce the axial force. In the submersible sewage pump, the axial force is mainly composed of the differential pressure force of the liquid acting on the impeller and the gravity of the entire rotating part. The direction of action of the two forces is the same, and the resultant force is the sum of the two forces. It can be seen that in the case of the same performance parameters, the axial force of the submersible pump is larger than that of the general horizontal pump, and the difficulty of balancing is more difficult than that of the vertical pump. Therefore, in the submersible sewage pump, the reason that the bearing is easily damaged is also related to the large axial force.
If the auxiliary impeller is installed, the direction of the differential pressure of the liquid acting on the auxiliary impeller is opposite to the resultant force of the above two forces, so that a part of the axial force can be offset, which also plays a role in extending the life of the bearing. However, there is also a disadvantage in using the auxiliary impeller sealing system, that is, a part of the energy is consumed on the auxiliary impeller, which is generally about 3%, but as long as the design is reasonable, the loss can be minimized.
Second, the application of pump without overload design technology
In a typical centrifugal pump, the power always increases with the increase of the flow rate, that is, the power curve is a curve that rises with the increase of the flow rate, which brings a problem to the use of the pump: when the pump is When the design operating point is running, generally, the power of the pump is less than the rated power of the motor. The use of this pump is safe; but when the pump head is reduced, the flow will increase (as can be seen from the pump performance curve), The power also increases.
When the flow rate exceeds the design condition point flow and reaches a certain value, the input power of the pump may exceed the rated power of the motor and cause the motor to be overloaded and burned. When the motor is overloaded, either the protection system acts to stop the pump from rotating; or the protection system fails to burn the motor.
The lift of the pump is lower than that of the design working point. It is often encountered in practice. In one case, when the pump is selected, the lift of the pump is too high, and in actual use, the pump is lowered. In the other case, the operating point of the pump is not well determined in use, in other words the pump flow needs to be adjusted frequently; there is also a case where the pump needs to be changed frequently. These three situations may overload the pump and affect the reliability of the pump. It can be said that for pumps without full lift characteristics (including submersible sewage pumps), the scope of use is greatly limited.
The so-called full head characteristic (also known as no overload characteristic) means that the power curve rises very slowly with increasing flow rate. More ideally, when the flow rate increases to a certain value, the power will not rise any more, but will have some Falling, that is to say, the power curve is a curve with a hump. If so, we only need to select the power rating of the motor slightly more than the power of the hump point, then in the entire range of 0 flow to maximum flow, you are in that one When operating at the operating point, the pump power will not exceed the motor power and the pump will be overloaded. It is very convenient and reliable for the pump with this performance whether it is selected or used. In addition, the motor power does not need to be too large, which can save considerable equipment costs.