At present, solid and hazardous waste sludge produced by various industries are generally divided into three categories. Most of them have a moisture content of 80%, a small part of the sludge that has been deeply treated has a moisture content of 50-60%, and a very small part of the sludge that has been dried has a moisture content of less than 40%. The following is an introduction to the common sludge conveying equipment currently available for reference.
1. Belt conveyor
Belt conveyor is a friction-driven machine that transports materials in a continuous manner. It can transport loose materials or pieced items. It is used for horizontal or inclined transportation and is very convenient to use. It has the advantages of large conveying capacity, simple structure, easy maintenance, low cost, and strong versatility. However, when conveying sludge, due to the high moisture content of the sludge, high viscosity, and easy to stick to the belt, it is easy to cause the conveyor belt to deviate and slip during the conveying process, seriously affecting the service life of the belt. Moreover, the belt conveyor is greatly affected by the site. It generally requires the construction of a longer belt corridor in its layout, and the lifting height is relatively limited, and the general inclination angle does not exceed 20 degrees. In terms of sludge transportation, belt conveyors are suitable for conveying dried sludge with a moisture content of less than 40%. They are suitable for conveying sludge over short distances (less than 50m) and with low lifts (less than 20m), and are often used to convey thermally dried sludge to a designated location.
Belt conveyors can achieve changes in the conveying inclination angle by setting a steering device; but when the conveying direction on the plane turns, an additional conveying device is generally required.
2. Screw conveyors
The screw conveyor pushes the sludge forward through rotating spiral blades. The force that keeps the sludge from rotating with the spiral blades is the dead weight of the sludge and the friction between the sludge and the casing. Screw conveyors can be divided into shafted screw conveyors and shaftless screw conveyors according to whether there is a screw shaft; they can be divided into single screw conveyors and double screw conveyors according to the number of screws. It can convey materials in horizontal, inclined or vertical directions, and can complete operations such as mixing, stirring and cooling while conveying materials.
For sludge, the screw conveyor is suitable for conveying sludge with a relatively loose structure and medium viscosity in the range of 60%~85%; it is suitable for conveying sludge over short distances (less than 25m) and low lifts (less than 8m). If the sludge contains flexible fibrous substances, a shaftless screw conveyor can be selected.
A single screw conveyor can only realize horizontal or inclined conveying. When the conveying direction and conveying angle change, an additional conveying device is required. When conveying sludge with a moisture content of about 80%, the conveying angle of the screw should not be greater than 25 degrees. Its conveying length is generally 5-30m. When it exceeds 40m, the energy consumption is particularly large, which is very uneconomical. The screw conveyor belongs to non-pressure conveying and is one of the commonly used sludge conveying methods.
3. Screw pump
Eccentric screw pumps are usually used in sludge pumping systems. Its main working parts are eccentric screws (rotors) and screw sleeves (stators). When the motor drives the pump shaft to rotate, the screw rotates around its own axis on the one hand, and rolls along the inner surface of the bushing on the other hand, thus forming a sealed chamber of the pump. Every time the screw rotates one circle, the sludge in the sealed chamber advances one pitch. With the continuous rotation of the screw and the rod, the sludge being transported is pressed from one sealed chamber to another sealed chamber in a spiral form, and finally squeezed out of the pump body. The stator and rotor of the pump are usually replaced every 2 months to half a year.
Eccentric screw pumps are suitable for occasions with short distances, small flows, low conveying pressures, and continuous sludge conveying. The screw pump does not produce turbulent pulsation when conveying sludge, and has basically no shear force on the medium.
The screw pump has a simple structure and low one-time investment. Under the condition of ensuring the cleanliness and singleness of the material, there is no problem in stable operation. In engineering applications, the working pressure of the screw pump should be controlled at 1/2-1/3 of the rated pressure; the speed of the pump should be controlled at a relative sliding speed of the stator and rotor of less than 0.5m/s; the horizontal sludge conveying distance of the screw pump generally does not exceed 100m; the vertical critical height is 50m, and the theoretical maximum pressure can reach 4.8 MPa. The biggest defect of the screw pump itself is that the stator and rotor rely on sliding friction to form a mobile closed cavity to convey the medium, so the stator and rotor are very easy to wear. The sand content of the sludge in my country's sewage plants is relatively large, which will aggravate the wear rate of the stator and rotor, which will not only increase maintenance costs, but also directly affect the normal operation of the sludge conveying system. The sand content in the sludge directly affects the service life of the screw pump.
The screw pump has a small adaptability to the moisture content of the sludge. The moisture content of the sludge in many sewage plants can be as low as about 75%. Such a low moisture content will reduce the pumping capacity by at least 20-40%. As the pumping capacity decreases, the delivery volume of the screw pump will also decrease, and it cannot be guaranteed to work stably at the designed flow rate.
If the sludge contains flexible fibrous materials (such as hair, plant stems, rods and plastic bags, etc.), after passing through the crushing device at the front end of the pump body, it will still entangle the rotor, causing the sealing chamber to leak, and the system working pressure can generally drop by 50-90%. Sludge will inevitably contain a certain amount of flexible fibrous materials, and the structural characteristics of the screw pump itself determine that it cannot guarantee continuous and stable transportation.
4. Plunger pump
The plunger pump relies on the reciprocating motion of the plunger in the cylinder to change the volume of the sealed working chamber to achieve sludge transportation. The plunger pump has the advantages of high rated pressure, compact structure, high efficiency and convenient flow adjustment. It is widely used in high pressure, large flow, high solid content and high debris material transportation occasions.
For sludge with a water content of 80%, a hydraulic double-cylinder plunger pump is used. It is designed for viscous materials with high solid content. The system has high conveying pressure (0-24Mpa), large conveying flow (0-80m3), long conveying distance (0-1000m), large tolerance for debris (it can convey debris with an outer diameter not exceeding 1/2 of the pipe diameter), low operating cost (few wearing parts, long life and low price), and has incomparable superior characteristics over traditional conveying methods.
The instantaneous flow of the plunger pump is pulsating, because in the plunger pump, the suction and discharge processes of the sludge are carried out alternately, and the speed of the piston is constantly changing during the displacement process. Since the double-cylinder plunger pump has two cylinders running alternately, the average flow rate is constant regardless of whether it is continuous flow in the pipeline. In theory, the flow rate of the pump depends only on the main structural parameters of the pump n (number of reciprocating times per minute), S (piston stroke), and D (piston diameter), and has nothing to do with the discharge pressure, and has nothing to do with the physical and chemical properties of the conveying medium such as temperature and viscosity. So the flow rate of the pump is constant.
In principle, plunger pumps can transport any medium, and are almost not limited by the physical and chemical properties of the medium. Through the configuration of the pump hydraulic system and the optimization of the pump body structure, as well as the selection of manufacturing materials and the strengthening of sealing technology, there is no technical problem in using plunger pumps to transport sludge.
The selection of sludge conveying equipment is a comprehensive consideration process, and the best choice needs to be made based on the specific properties of the sludge, the conveying distance and height, the processing capacity and other practical factors.
