Roots blower for conveying granular materials in polyethylene plant

Roots blower is the key unit for conveying granular materials in polyethylene plant. Once it breaks down, it will cause shutdown of the plant. The vibration speed of Roots blower in a polyethylene plant has been over 10 mm / s since it was operated. Because the vibration speed exceeds the standard, the faults such as the rupture of the fan shell, cover plate and the fracture of the rotor occur, which seriously affect the normal operation of the plant. In view of the above problems, the causes of large vibration are analyzed, and the treatment methods are put forward, thus prolonging the operation period of the equipment.

A Roots blower in a polyethylene plant is composed of motor, blower, inlet and outlet unit, etc. it is horizontal in shape, and the direction of inlet and outlet is up-down. Among them, the fan is a three blade roots fan, mainly composed of side cover, shell, gear, rotor, air seal assembly, radial bearing, tapered sleeve bearing assembly and other components.

The vibration value of Roots blower is over 10 mm / s all the time. Because of the high vibration speed value, the fan shell, cover plate and rotor are broken, which lead to the scrapping, and it has been overhauled for many times, which seriously affects the normal operation of the equipment.

The bearing installation accuracy is not up to standard and the fan speed is close to the critical speed. The specific analysis is as follows.

1. Installation accuracy of thrust bearing is not up to standard

It can be seen from the spot check of the last several overhauls that the overhauls are all installed according to the principle of consistent data before and after the overhauls, which does not reflect the clearance value of the thrust bearing. However, check that the clearance of the thrust bearing is 0.06mm, exceeding the reference value of 0.03-0.05mm. Randomly check 10 groups of clearance values of thrust bearing when the bearing and taper sleeve are installed to the same size as before disassembly by using the method of marking table.

After investigation, it is found that when the bearing clearance is lower than the standard value, the fan vibration speed increases rapidly with the decrease of the clearance value; when the bearing clearance is higher than the standard value, the fan vibration speed increases rapidly with the increase of the clearance value. It can be concluded that the installation accuracy of the thrust bearing is not up to standard, which is one of the reasons for the large vibration of the fan.

2. Speed close to critical speed

Through field measurement, the diameter of the motor pulley is 510mm, and that of the fan pulley is 410mm. According to the mechanical design manual, the reference diameters of the motor and the fan pulley are 500 and 500 mm respectively, and the motor speed is 1490 R / min. The rotating speed of the equipment should be controlled at 1430-1840r / min.

In view of the above problems, after analysis and research, it is proposed to check the clearance of the thrust bearing while installing, to ensure the installation quality of the bearing, and to increase the diameter of the fan pulley, so as to effectively control the speed of the fan.

Check the clearance of the thrust bearing while installing the watch as follows.

(1) Use a hook wrench to install the taper sleeve. Use hook spanner to install the taper sleeve to the original size difference of about 0.10 mm. At the same time, turn the main and driven rotors to make them rotate for 4-5 turns, so that the bearings are completely free.

(2) Make a table to check the clearance of the thrust bearing. Install the dial indicator base on the shell, and mount the dial indicator vertically on the driving and driven rotors respectively, and close to the bearing base. Use crowbar to lift up the driving and driven rotors respectively, and check the bearing clearance value. If the clearance value is not up to the standard, continue to use the hook wrench for installation, and use the same method to check the clearance value of the thrust bearing until it is within the standard range.

Process according to the calculated size, recheck the size of each part and check the dynamic balance, strictly control the quality, and reassemble on site after meeting the requirements to ensure that each step meets the quality standard.

Through the transformation of the pulley, the diameter of the pulley is increased, and the speed of Roots blower is effectively controlled; the clearance of the bearing is checked by the method of punch table, so that the installation quality of the bearing is guaranteed. After the modification of the self pulley, the vibration speed of the equipment is about 6mm / s, the vibration value is obviously reduced, the equipment is in good operation state, and the fault is obviously reduced. The successful transformation of the equipment provides a reference for the similar equipment problems of other similar devices.

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