Submersible sewage pump is a kind of pump product which is connected with motor and works under liquid. Compared with ordinary horizontal or vertical sewage pumps, submersible sewage pumps obviously have the following advantages:
1. Compact structure and small floor space. Because the submersible sewage pump works under liquid, it can be directly installed in the sewage pool, and there is no need to build a special pump room to install pumps and machines, which can save a lot of land and capital construction costs.
2. Easy installation and maintenance. Small submersible sewage pumps can be installed freely, and large submersible sewage pumps are generally equipped with automatic coupling devices for automatic installation, which is quite convenient for installation and maintenance.
3. Long continuous working hours. Because the submersible sewage pump is coaxial with the motor, the shaft is short and the rotating part is light, so the load (radial direction) on the bearing is relatively small and the service life is much longer than that of the general pump.
4. There are no cavitation damage and irrigation diversion problems. Especially the latter point brings great convenience to the operator.
5. Low vibration noise, low motor temperature rise and no pollution to the environment.
It is precisely because of the above advantages that people pay more and more attention to submersible sewage pump and its application scope is wider and wider. From the initial simple use of conveying clean water to the present, it can convey all kinds of domestic sewage, industrial wastewater, site drainage, liquid feeding and so on.
It plays a very important role in municipal engineering, industry, hospitals, buildings, hotels, water conservancy construction and other industries.
However, everything is divided into two parts, and the most critical issue for the submersible sewage pump is feasibility, because the submersible sewage pump is used under liquid; The conveying medium is some mixed liquids containing solid substances; The pump is close to the motor; The pump is arranged vertically, and the weight of the rotating parts is consistent with the direction of the water pressure borne by the impeller. All these problems make the requirements of submersible sewage pump in sealing, motor bearing capacity, bearing arrangement and selection higher than those of ordinary sewage pumps.
In order to improve the life of submersible sewage pump, most manufacturers at home and abroad are now trying to find a way to protect the pump, that is, when the pump fails, such as leakage, overload and overtemperature, it can automatically alarm and stop for maintenance. However, we think it is necessary to set up a protection system in the submersible sewage pump, which can effectively protect the safe operation of the electric pump.
But that's not the point. The protection system is only a remedial measure after the pump fails, and it is a relatively passive method. The key to the problem should be to start from the root cause and completely solve the problem of pump sealing and overload. This is a more proactive way. Therefore, we apply the hydrodynamic sealing technology of auxiliary impeller and the overload-free design technology of pump to submersible sewage pump, which greatly improves the sealing performance and bearing capacity of the pump and prolongs its service life.
First, the application of auxiliary impeller dynamic sealing technology
The so-called hydrodynamic seal of auxiliary impeller refers to the installation of an open impeller in the opposite direction on the same shaft near the back of the rear cover plate of the pump impeller. When the pump works, the auxiliary impeller rotates with the main shaft of the pump, and the liquid in the auxiliary impeller also rotates. The rotating liquid will produce outward centrifugal force, on the one hand, it will prevent the liquid from flowing to the mechanical seal and reduce 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 prolongs its service life.
The auxiliary impeller can not only seal, but also reduce the axial force. In submersible sewage pump, the axial force is mainly composed of the pressure difference force of liquid acting on the impeller and the gravity of the whole rotating part. Two forces act in the same direction, and the resultant force is the sum of the two forces. It can be seen that under the same performance parameters, the axial force of submersible sewage pump is greater than that of ordinary horizontal pump, and the balance difficulty is greater than that of vertical pump. Therefore, in the submersible sewage pump, the bearing is easy to be damaged, which is also closely related to the large axial force.
If the auxiliary impeller is installed, the direction of the pressure difference force acting on the auxiliary impeller is opposite to the direction of the resultant force of the above two forces, which can offset part of the axial force and prolong the service life of the bearing. However, the use of two-stage impeller sealing system also has a disadvantage, that is, a part of energy is consumed on the two-stage impeller, which is generally around 3%, but as long as the design is reasonable, this part of the loss can be completely reduced to a minimum.
Second, the application of non-overload design technology of pump
Generally, the power of centrifugal pump always increases with the increase of flow, that is to say, the power curve is a curve that rises with the increase of flow, which will bring a problem to the use of the pump: when the pump runs at the designed working point, generally speaking, the power of the pump is less than the rated power of the motor, so it is safe to use this pump; But when the pump head decreases, the flow will increase (as can be seen from the performance curve of the pump) and the power will also increase.
When the flow exceeds the design working 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, the protection system will stop the rotation of the pump; Either the protection system fails and the motor burns out.
In practice, the pump head is often lower than the design working point. In one case, the pump head is too high when it is selected, but it is lowered when it is actually used; On the other hand, the working point of the pump is not well determined in use, in other words, the flow of the pump needs to be adjusted frequently; Another situation is that the pump needs to be used frequently. These three conditions may overload the pump and affect its availability. It can be said that the application range of pumps (including submersible sewage pumps) without full lift characteristics will be greatly limited.
The so-called full lift characteristic (also known as no overload characteristic) means that the power curve rises very slowly with the increase of flow. Ideally, when the flow rate increases to a certain value, the power will not increase again, but will decrease, that is, the power curve is a hump curve. In this case, we only need to choose the rated power of the motor slightly exceeding the power value of the hump point, so in the whole range from 0 flow to maximum flow, no matter which working point you are running at, the power of the pump will not exceed the power of the motor, which will overload the pump. For a pump with such performance, it will be very convenient to choose and use. In addition, the motor power does not need to be too large, which can save considerable equipment cost.
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