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Transformation of test control system for thin oil station in cement plant

- Jun 14, 2018 -

The working principle of a dilute oil station

The working principle of the dilute oil station is basically the same: the oil is sucked out from the oil tank by the gear pump, and through the one-way valve, the double tube net filter and the heat exchanger, it is directly sent to the lubricating point of the equipment. Each thin oil station has 2 oil pumps, one work and one standby. The output pressure of the oil pump is 0.4MPa and the flow rate is 2.5L / h in the thin oil station.

Lubricating oil flow: working oil pump, one way valve, filter, condenser (used in summer), pressure relay, main equipment lubrication point, return to the oil tank - oil pump.

Problems in the control of thin oil stations

1) The electrical control circuit is complex, with many intermediate relays (6 in total), high failure rate and extremely difficult to find and deal with problems

2) the control mode is backward. The main and auxiliary switchover of 2 oil pump motors must be carried out on the spot, and the control operation can not be realized; the site personnel are few and the equipment is many, often caused by problems found not in time, causing the damage of the host equipment and affecting the production.

3) thin oil stations measure the oil pressure on the pipeline through pressure relay to reflect the lubrication of the equipment. This method is less sensitive and accurate, and it is difficult to ensure reliable lubrication monitoring. The lubrication condition monitoring of thin oil station is pressure relay and pressure transmitter two. Both of them have some problems.

1. Under the condition of blockage of the oil pipeline, the tubing pressure is still very high, but the lubricating oil has no normal flow and the equipment can not be lubricated. The monitoring of the lubrication condition is to determine whether the pressure in the tubing is less than the set value, so the monitoring loop is still indicating normal. Before the transformation, the pressure relay of the thin oil station cannot accurately reflect the lubrication condition, causing the limestone crushing machine and the cement mill bearing bush to burn out.

2. The lubricating parts of the main equipment (axle bush or reducer) keep a certain drop from the oil tank of the oil station to ensure that the lubricating oil can return to the oil tank of the dilute oil station smoothly, which makes the port and oil tank of the drenching oil pipe also have a certain drop, and the outlet pipeline of the oil pump is equipped with a one-way valve to prevent the oil return. Therefore, even once the oil pump stops working, the lubricating oil in the pipeline still has a static pressure, even exceeding the setting value of the pressure relay.

3. The size of the tubing port at the oil drain line of the lubricating equipment also directly affects the oil pressure of the pipeline, resulting in the wrong judgement of the pressure size.

4. The pressure relay provides the switch quantity, and can not display the pressure value in the central control. It can only alarm or stop, and can not prefabricate emergency measures in advance.

 reform scheme

Through the investigation and demonstration, we change the pressure from the oil pipe to the monitoring flow, that is to add a flow transmitter in the oil road to monitor the flow of the lubricating oil. No matter whether the size of the outlet of the tubing, the blockage, the static pressure of the pipeline, or the motor turn instead of the oil pump, the flow condition can satisfy the lubrication condition.

 1. flow transmitter instead of primary contact pressure gauge

The differential capacitance flow transmitter is selected, which uses the differential capacitance as the monitoring element, and the whole transmission without mechanical transmission, adjustment device, and the measuring part is fully closed welding solid structure. Therefore, the instrument has simple structure, stable performance, reliability and high accuracy. The differential capacitance flow transmitter measured the change pressure, that is, the input differential pressure (pi.) only liquid oil can be acted on the sensing film film of the measuring component only when the liquid oil is flowing, so that it produces the displacement, thus making the capacitance of the differential capacitance of the sensing film sheet (the movable electrode) and the two fixed electrode change. The capacitance change is converted from a capacitor current conversion circuit to a DC current signal. The current signal is compared with the algebra of the zero signal and the model of the same feedback. The difference is sent to the amplifying circuit, and the output current I (standard 4~20mA) of the whole machine is obtained by amplification. It is used for the display and control of the screen in the control room.

Static pressure does not produce differential pressure, so it avoids the phenomenon that the pressure transmitter mistakenly considers the static pressure after the pipe blockage is normal lubrication.

 2. DCS program control instead of original complex intermediate relay

All the electric control of all the oil stations in the field is transformed into DCS program control. The centralized control and process pressure display of the control room of the main and auxiliary oil pump motor are fully realized, and the control circuit is simplified, the failure rate is greatly reduced, and the normal lubrication of the host equipment is ensured.