Unexpected shutdown of the screw air compressor? What’s going on?

Unexpected downtime mainly refers to the temporary outage caused by equipment abnormality, which is not a normal planned downtime for maintenance. In order to improve production efficiency and safety, reducing the unexpected downtime of air compressors is the direction of equipment management workers’ efforts. Under normal circumstances, the reasons for unexpected downtime of air compressors are mainly in the following aspects.

1. The exhaust temperature is too high

High exhaust temperature is the most common failure of screw air compressors, and it is the most harmful. Usually the working temperature of the screw air compressor is between 65°C and 85°C, and the maximum exhaust temperature is set at 100°C. When the temperature exceeds this temperature, it will automatically stop and alarm. If the equipment is operated under the condition of high exhaust temperature for a long time, the excessively high temperature will change the physical coefficient values of the rotor and bearing materials, causing abnormal friction loss of the bearings, and even bearing loose ball accidents.

Excessively high oil temperature will also reduce the gas transmission coefficient and increase power consumption, and the viscosity of the lubricating oil will also decrease. If the temperature is too high, the lubricating oil will be thermally decomposed under the catalysis of the metal, and free carbon and acids harmful to work will be generated. If it is serious, it will cause the screw of the whole main engine to be stuck or stop jumping. Although the air compressor is equipped with a protection device to prevent high temperature, the existence of a fault will cause the device to operate frequently, affecting the normal production of the enterprise.

The high exhaust temperature of the screw air compressor is generally caused by the high temperature of the oil cooler and water cooler, and it may also be caused by sensor failure. Some alarms can be eliminated immediately through the operation of the controller page, but sometimes the alarm of high exhaust temperature appears after the elimination. At this time, in addition to checking the circulating water, the problem of lubricating oil must be considered. Common air compressor exhaust temperature detection and possibility are as follows:

(1) The actual exhaust temperature is not too high

The thermometer display is inaccurate; the temperature sensor is faulty; the controller is faulty; the temperature switch is faulty.

(2) The actual exhaust temperature is too high

The lubricating oil has been used for too long; the oil filter is clogged; the oil separator is clogged; the temperature control valve is faulty; the lubricating oil level is too low; the ambient temperature is too high; the fan motor is reversed; the fan motor does not turn; the outside of the air cooler is always too dirty; Serious fouling in the oil circuit of the air cooler; poor exhaust air passage of the cooler; too low cooling water pressure; too high cooling water temperature; blockage of the cooling water outlet pipeline; The check valve is stuck; the oil cut-off valve cannot be fully opened; the seal of the temperature control valve core is invalid.

Note that there are two types of high temperature: ①The air compressor itself generates too much heat; ②The normal heat generated cannot be taken away.

For the first problem, the cause should be found from the air path, such as: air backflow and eddy current caused by damage to the suction and exhaust valves at all levels, dirt, and failure of the seal under the valve, resulting in a large amount of heat. The air compressor has to go through the four processes of suction, compression, work, and exhaust to generate high-temperature and low-pressure gas. In order to ensure the normal suction of the next-stage cylinder or the gas used by the user, the high-temperature gas must be forcibly cooled. In this case, part of the normal heat is dissipated in the environment, and part is taken away by the cooling water (or cooling fan). Poor heat dissipation and poor cooling system circulation are the main causes of high exhaust temperature. The radiator is blocked, the cooling water is insufficient, the water pump is damaged, there may be too much scale, and the performance of the fan is poor.

2. The thermal relay tripped

The working principle of the thermal relay is that the current flowing into the heating element generates heat, which causes the bimetal sheets with different expansion coefficients to deform. When the deformation reaches a certain distance, the connecting rod is pushed to disconnect the control circuit, so that the contactor When the power is lost, the main circuit is disconnected to realize the overload protection of the motor. As the overload protection element of the motor, the relay has been widely used in production because of its small size, simple structure and low cost.

(1) Common fault causes of thermal relays

The current flowing through the thermal relay is seriously overloaded or the load is short-circuited; the rated current of the thermal relay does not match the actual load current; the motor is operated too frequently; the action mechanism of the thermal relay is not working properly, which makes the thermal element overloaded for a long time and cannot protect the thermal relay; the connecting wire of the thermal relay is in poor contact Cause heat to burn out.

(2) Troubleshooting methods for common faults of thermal relays:

①The specifications of the thermal relay should be selected appropriately. If it is too small, the thermal relay will malfunction, and the main current will easily burn out the main circuit of the thermal relay; if it is too large, the thermal relay will not operate. Therefore, when selecting a thermal relay, it is best to make the intermediate value within the rated adjustment range of the thermal relay coincide with the rated current value of the controlled motor load, so that the thermal relay has a certain current adjustment range.

② Check the circuit fault, and replace the appropriate thermal relay after eliminating the short circuit fault.

③Change the way of operating the motor to reduce the number of times to start the motor.

④Replace with a sensitive and qualified thermal relay.

⑤Try to remove the oxide layer of the wiring head and the thermal relay terminal, and re-press the main wiring of the thermal relay.

3. Excessive head resistance

An overload of the air compressor can also cause the air switch to trip. The overload of the air compressor is usually due to the excessive resistance of the head of the air compressor, which causes the starting current of the air compressor to be too large, causing the air switch to trip.

The role of lubricating oil in the screw air compressor is very important. Its role is mainly manifested in four aspects: cooling, lubrication, sealing, and noise reduction. The quality of the oil and the amount of oil directly affect the air compressor. The normal operation of the air compressor and the service life of the air compressor, especially the impact on the head part of the air compressor is the most obvious.

There are three main situations that lead to increased nose drag:

(1) Lubricating oil has high viscosity.

(2) The coking of the machine head; the coking of the machine head causes the friction resistance of the rotor of the air compressor to increase when it is running, and the current of the air compressor increases accordingly;

(3) Too much lubricating oil in the machine head will also lead to increased resistance. After the air compressor was shut down, the oil cut-off valve was not closed in time, and a large amount of lubricating oil entered the machine head, which caused the starting resistance to be too large when the air compressor was restarted.

The processing methods are:

(1) Manually turn the air compressor to check whether the head and the motor can rotate freely, so as to judge whether the resistance increases. When the ambient temperature decreases, the viscosity of the oil increases, and the resistance also increases, the starting current of the motor becomes larger, and the starting time of the air compressor is prolonged. The ambient temperature at that time can be determined according to the exhaust gas temperature.

Prolong the start-up time of the motor on the air switch under the condition of ensuring safety, so as to meet the extension of the start-up time of the air compressor when the oil viscosity increases, so as to ensure the normal start-up of the air compressor. Do not extend too much, otherwise it will affect other electrical equipment.

(2) Oil coking in the machine head is a phenomenon that often occurs in screw air compressors when the oil quality is poor or the temperature is high. Due to the coking of the machine head, the frictional resistance increases, the load of the unit increases, and the current increases. In severe cases, the screw and the rotor are bonded together by oil coke, and the unit cannot rotate freely.

(3) The function of the oil cut-off valve is to cut off the oil injection pipe in time when the air compressor is shut down, so as to prevent the residual pressure in the oil and gas barrel from pressing a large amount of lubricating oil into the machine head, resulting in too much lubricating oil in the machine head. The most direct way to solve the problem of excessive lubricating oil in the machine head is to open the machine head, release part of the lubricating oil in the machine head, and repair or replace the oil cut-off valve at the same time.

4. Failure of unloading valve

Air compressor unloading valve function: When the air consumption changes, the intake valve adjusts the opening degree of the valve body through the control system to match the air consumption. ).

The most intuitive way to judge whether the unloading valve is closed or not is to remove the air filter, and you can clearly see whether the intake baffle of the unloading valve is closed or open. If the intake baffle is stuck due to mechanical impurities, etc., the unit can be opened smoothly after cleaning; if the intake baffle is stuck on the guide rod, grind the guide rod with sandpaper and apply a small amount of grease on the guide rod; If the unloading valve is seriously damaged, the unloading valve needs to be replaced.

5. Power failure

(1) First of all, check whether the power connection of the air compressor is correct, whether the power switch is closed, in case someone closes it by mistake; whether the power fuse is blown, and whether the power supply voltage reaches 90% or more of the rated voltage.

(2) Check whether all components of the pressure relay of the air compressor power supply are damaged, and whether the given value of the pressure relay is appropriate.

 

Sollant Group-Professional Compressed Gas Solutions Provider