(Great wall machinery is one of the domestic first-class grinding system integration suppliers. With the advantages of technology, equipment and brand of ball mill, rotary kiln, vertical roller mill for nearly 6 years, Great Wall Machinery devotes itself to carry out business service in many fields and gradually expend the business mode including EPC project, financial leasing and spare parts supply as the increasing maturity of single equipment and turnkey project, successfully realizing the change from manufacture to manufacture + service.)

It’s hard to avoid irregular faults occurred on the cement plant equipment. What troubles has your cement grinding mill occurred? As an operator of the cement plant equipment, you must summarize and accumulate the experience from all aspects all the time and optimize the design for the specific details of production line to avoid equipment occurring the faults. Now let’s see the faults occurred on the cement grinding mill in a domestic cement plant and what methods they adopted to improve and enhance it to ensure the normal operation of the equipment.

Abstract: the type of the main motor of the cement grinding mill in a cement plant is YRKK800-8, power 2500kW with Babbitt metal bearing, material of which is composed of tin, lead, antimony, copper, etc. with the feature of soft texture and low temperature. The highest operation temperature of Babbitt metal bearing of control bearing bush is 100℃. In order to protect bearing bush, the upper limit value of temperature protection is set to 63℃. Once exceeding the setting value, DCS will control the main motor of the mill and stop it. Lubricating oil station of main motor of the mill is equipped with two pressure switches and the pressure are respectively set to 0.12MPa and 0.2MPa. The normal value of oil supply pressure when the oil station operates is about 0.25MPa. If oil supply pressure is lower than 0.2MPa, emergency bearing oil pump will be started to allow the oil pressure to meet the lubrication demands of bearing of main motor; if oil supply pressure is lower than 0.12MPa, there are troubles in lubrication system. Oil pressure signal interlocks main motor of the mill and stops it to avoid the high temperature burning the bearing bush.

1. Fault phenomenon

On January 7, 2015, No. 2 control room of cement grinding mill of a cement plant trips because of the fault of DCS control power supply, which leads to the missing of DO signal of cement grinding mill and causes the tripping of all low-voltage equipment of No. 2 cement grinding mil. Staff finds that main motor of the mill is still operating as routing inspection and presses the emergency stop button on the spot. After inspected by electrical personnel, it finds that rotor journal of main motor wears seriously and front and rear bearing bushes are burned out due to high temperature. After the fault of DOS system leading to the tripping of lubrication oil station, oil pressure lower than 0.12MPa doesn’t stop the main motor and bearing bush with high temperature nor stop it, which is the root cause for serious wear of rotor journal and burn-out bearing bush.

2. Analysis for fault reason

Main motor of cement grinding mill is high-voltage electrical equipment, whose principle of start-stop circuit is shown in figure 1.

Figure 1: diagram of principle of start-stop circuit of main motor of cement grinding mill

As shown in figure 1, motor-driven relay K1 and K2 of DCS system respectively pulse trigger switch closing and switch opening to stop and start the main motor. After power-loosing of DCS system, switch opening commands that K2 cannot stop the main motor; on the other hand, both chain jump stop signal of low oil pressure of oil station and overheat of bearing bush control opening coil through DCS output. DCS fault cannot trigger switch opening of breaker of service truck, which lead to the failure of protection system and trouble parking of equipment.

3. Fault handling

Switch-opening circuit of main motor of the mill is controlled by soft chain of DCS system, while the influence of DCS fault on equipment is not fully considered at design. However, hard chain can solve the problem. For hard wire, low oil pressure signal of lubrication oil station is only need to be led to the switch-opening circuit of high-voltage board of main motor of the mill. While the lubrication oil station is set up on the spot, which is far from the high-voltage board and it is time-consuming and strenuous with high cost if directly install the cable. According to the site situations, electrical maintenance personnel decide to make use of the feature that the lubrication oil station is close to the emergency stop button box of main motor to parallel signals and then send them to high-voltage board. Temperature signal of bearing bush is no modification for gather the analog quantity. Control principle diagram after modification is shown in figure 2.

As shown in figure 2, realize the soft chain and hard chain control for main motor through adding intermediate relay in the control box ALC2 of lubrication oil station. Auxiliary contract of K3 is connected with DI of DCS to realize the soft control for switch-opening coil of equipment and auxiliary contract of K4 is paralleled to button box ALC1 near the main motor to directly act on the tripping circuit of the equipment and realize the hard control for switch-opening coil. After modification, it is successful to separate the switch. After that, the same modification is carried out for lubrication oil stations of grinding head, grinding tail and main reducer and low oil pressure signal of all oil stations is paralleled to switch-opening circuit in figure 2. In this way, fault of any oil station can stop the main motor timely and fault trip reliability of main motor of the mill is increased through soft chain and hard chain.

Figure 2: Diagram of control principle of main motor after modification

4. Conclusion

This accident exposes the hidden dangers in the design of equipment. Interlocking relationship among equipment with different voltage grades should be paid special attention in the process of design. Consider the influence of different faults on the equipment and timely find the control dead sector of equipment operation and parking to improve the design and maintain safe operation of equipment and safe production of enterprise.