Installation and Commissioning of Decanter Centrifuge

In China, a combination of gravity dewatering and mechanical dewatering machines are mainly used in medium and large scale sewage treatment plants. Generally, the moisture content of the sludge to be treated is up to 97%. After dewatering, the solid content of the mud cake is 20% to 30%, and then the mud cake will be further treated.

The commonly used dewatering machines in sewage treatment plants are mainly belt filter press, plate and frame filter press and decanter centrifuge. Since the former two are open operations, sludge and mixed liquid are easy to form bacterial flocs with various pathogens in the air, which is harmful to the health of the operators. More and more sewage treatment plants are beginning to use decanter centrifuge.

1. Structural Principle

The decanter centrifuge is mainly composed of a rotary drum and a screw conveyor with a hollow shaft. After the sludge is sent into the drum through the hollow shaft, it is immediately thrown into the drum chamber under the centrifugal force generated by high-speed rotation. The specific gravity of the sludge particles is relatively high, so it will are thrown onto the inner wall of the drum, forming a solid layer. The water density is small, it will form a liquid layer inside the solid layer. The sludge will be transported to the cone end of the drum under the push of the screw shaft, and be continuously discharged through the outlet. The liquid will be discharge through the outlet at the column end of the drum.

The most important component of decanter centrifuge is the drum. The larger the diameter of the drum, the greater the processing capacity. The longer the length of the drum, the higher the solid content of the mud cake. However, if the drum is too large or too long, the performance-price ratio will decrease. Therefore, it is necessary to choose a suitable aspect ratio.

Since the operation of decanter centrifuge is carried out in a fully closed environment, there is no sludge or sewage around it, and there is no foul odor, which will greatly improve the working environment of the operators.

2. Issues Need to Be Payed Attention to in Mechanical and Electrical Installation Projects

2.1 Equipment Hoisting and Carrying

For medium and large sized decanter centrifuges, the overall weight is about 7 tons or more. During the construction process, the hoisting equipment in the production workshop is often unable to take into account all ranges, so corresponding measures must be formulated for the transfer of this large equipment. At the same time, in order to avoid floor load, during construction, sleepers, I-beams and rails should be used to build a transportation channel, and then rollers should be used to roll on the rails, and the equipment should be gradually moved to the equipment foundation.

2.2 Equipment Positioning

The allowable deviation of the plane position and elevation of the surface are: ±5mm for plane position and ±5mm for elevation. When the unit is leveled, the allowable deviation of the horizontal level is 0.1mm; the allowable deviation of the longitudinal level is 0.05‰. The leveling cannot be adjusted by tightening or loosening the anchor bolts. The allowable deviation of the verticality of the equipment is 1‰ of the total length, with a maximum of 30mm. When the outer diameter of the equipment is more than 2000mm, the maximum allowable deviation of the orientation is 15mm, and when the outer diameter is less than or equal to 2000mm, the maximum allowable deviation of the orientation is 10mm.

2.3 Protection of the Equipment Balance

The rotating parts of the centrifuge have a control accuracy of 1% - 1‰ millimeter during assembly, so the protection of rotating parts is very important, otherwise serious unbalanced vibration will occur during the operation of the drum. Although most decanter centrifuges are generally installed as a whole, it is still necessary to pay attention to the protection of the following parts during installation: bearing seat, bearing, drum shell, drum top, drum axis, coupling, screw conveyor, and it is strictly forbidden to apply external force to these parts. If disassembly and assembly are required, they must be strictly followed in order. Do not use a welding gun to heat the parts. Do not install them by knocking on the ball bearings, but only knock on the end surfaces of the bearings. The front and rear bearing seats are not allowed to be interchanged, otherwise the coaxiality cannot be adjusted on site.

3. Commissioning

It is mainly carried out in two stages: no-load and loaded.

3.1 Preparation Before Commissioning

(1) Make sure the safety cover is properly installed.

(2) Check whether the lubricating oil has been added as required, and remove the excess grease in the bearing seats.

(3) Check the tension of the belts to prevent them from slipping or overheating due to being too loose.

(4) Ensure that all wires and wiring do not touch driving and rotating parts. Ensure that all switches, contactors and relays have been safety tested.

(5) Check the motor direction and the maximum speed set in the frequency converter.

3.2 No-load Start

In this case, clean water is generally used as the feed material for test. First, the speed of the drum should be manually set to about 85% of the rated speed at the frequency converter. After starting the equipment, pay close attention to abnormal noise and abnormal vibration at this stage. Use vibration blocks for testing. Generally, the effective speed of each vibration block should be less than 7mm/s. Occasional vibrations can have an effective speed of 15mm/s, but if it occurs frequently, it is necessary to stop the machine immediately and find out the cause. Otherwise, the rotor, drum and bearings will be seriously damaged.

After the equipment reaches the rated operating speed (about 60 to 150s), gradually feed until the rated processing capacity is reached (this process also requires close monitoring of noise and vibration. If there is any abnormality, stop the machine immediately), and then start the flocculant dosing system. Before shutting down the decanter centrifuge, stop the feed pump and then run the cleaning program to check whether the relevant valves are opened and closed normally. At the same time, observe whether the washing time and the amount of washing liquid are appropriate, and make sure that the cleaning liquid enters the pipeline between the feed pump and the sludge input valve. After confirming that the feed pump, pipeline and centrifuge body are all effectively cleaned, shut down and enter the loaded commissioning stage.

3.3 Loaded Commissioning

The steps and precautions are basically the same as those for no-load startup. The difference is that in this stage, the focus tends to be on process commissioning. Gradually determine the liquid level height, the differential speed range between the screw and drum, the reasonable torque setting, and further determine whether the cleaning is in place.

Another important issue is that the consumption of flocculants should be reflected at this stage to determine whether the equipment meets the acceptance requirements.

It should be noted that this process will take a certain amount of time. In order to obtain the most economical combination, it is often necessary to conduct performance analysis through the entire trial operation process. Even in normal operation, performance analysis is often required to continuously find economical flocculant consumption while ensuring the treatment effect.