Instrument failure and fault cause analysis.

Instrument failure and cause of failure.

Chemical instrumentation is very important for chemical production, and the instruction information it provides has a key guiding role. Based on the practical experience, this paper analyzes the common four chemical instrumentation faults and discusses the causes of the faults.

The cause of abnormal indication of chemical instrument.

Due to chemical production operation pipelining, streamline, continuity and totally enclosed and other characteristics, technology personnel needs through the observation condition of instrument display of various process parameters such as reaction temperature, material flow, pressure and liquid level of container, the composition of raw materials, etc.) to determine process production is normal, the quality of the product is unqualified, according to the instrument instructions to add quantity or production, or even stop. The indication of the instrument failure is abnormal, and most of it comes from the field instrument, so it is very important to maintain and repair the field instrument operation. Field meter measurement parameters are generally divided into four categories: temperature, pressure, flow rate and liquid level. According to the different measurement parameters, different field instrument failures should be analyzed.

The specific ideas are:

On the analysis of the system fault before check the field instruments, to site operation workers understand the parameters of the production load and raw material, the changes of view fault instrument recording curve, to carry on the comprehensive analysis, to determine the cause of the problem. If the meter records curve is a dead line, or the record curve is originally fluctuating, now suddenly becomes a straight line, the fault is likely in the instrumentation system. Because most of the current meter is DCS system, the parameter changes can be very sensitive to the computer. At this time, the process parameters can be artificially changed to see the curve changes. If there is no change, it can be concluded that there is a problem in the instrument system. If the curve can track the change of process parameters, the fault of instrument system can be eliminated. When changing the process parameters, it is found that the record curve mutates or jumps to the maximum or minimum, and the fault is often in the instrument system. Failure occurs before the instrument recording curve has been normal, fluctuate after record curve is no rule or control system to realize automatic control, manual operation can't control, even the fault may be caused by the improper operation process. When DCS is found to be abnormal, it can go to the field to check the indicator value of the field instrument. If there is a large difference, it is likely that the instrumentation system fails.

In conclusion, it is necessary to pay special attention to the characteristics of the controlled object and the control valve when analyzing the cause of the fault in the field, which may be the cause of the fault of the field instrument system.

Main failure analysis of chemical instrument.

In chemical production, the parameters of instrument display mainly include flow, temperature, liquid level and pressure.

1. Temperature measuring instrument failure.

The temperature measuring instrument failure is mainly indicated by the temperature is zero, high or low. Common temperature indicating instrument mainly include thermal resistance and thermocouple, thermocouple measuring instrument, for example, in the process of fault judgment can be disconnected from the thermocouple, adopt and thermocouple thermoelectric properties similar to sub compensating lead wires, when the meter can display circuit is normal under the condition of room temperature. In addition, it can also be used to measure the electric potential of the thermocouple at either end of the thermocouple and check the thermocouple resistance to determine whether there is a short circuit or short circuit phenomenon. Fault is found in the thermocouple is usually short circuit or open circuit, such as wiring by corrosion or virtual, often the uniformity of process medium, protect casing surface structure, liquid in the face of thermocouple submerged exceptions can cause thermocouple's own instructions.

2. Flow measurement instrument failure.

The flow meter can be divided into velocity method, volumetric method and quality method according to its own measuring principle. Due to the variety of flow meters and different working principles, the performance of the faults is very diverse. The fault of flow meter is mainly manifested as fluctuating, large or small. And causes of failure is involved in many aspects, such as the medium of the Reynolds number, viscosity, density and so on can affect the parameters of the instrument accurately display changes, when no sufficient compensation in the process of measuring gas temperature, pressure, design temperature stress and temperature stress is easy to appear not consistent, which can make the flow indicator error; The transmission link of the measurement fails; The pressure pipe is leaking, the crystallization is blocked, the media cannot fill the pipe and the pipeline is vibrating, etc. The circuit of the transmitting signal is loose, corroded or disturbed. The circuit board of the instrument itself is damaged or the film box is deformed.

3. Failure of pressure measuring instrument.

The working principle of the pressure measuring instrument is that when the pressure changes, the stress deformation of the measuring element is different, and the stress deformation is converted into an electrical signal for transmission. The deformation of the diaphragm or the blockage of the guide tube is a frequent failure of the pressure gauge. Due to the high frequency of pressure switch in the process of pressure measurement, whether there is loose or corrosion in the wiring place, or whether the contact is good will cause the instrument failure.

4. Malfunction of liquid level measuring instrument.

Liquid level measuring instrument according to the working principle of the different can be divided into float or rely on buoyancy principle for measuring buoy, rely on the principle of liquid column differential pressure or electrical measurement of radio frequency admittance, rely on wave transmission of ultrasonic, radar, etc. The failure of liquid level measurement instrument includes unstable fluctuation of liquid level, indicating high level of liquid level or low level. In the fault handling of liquid level measuring instrument need to take into account the process change the density of the medium itself, because of false boiling liquid level, at the same time to consider to guide the phenomenon such as crystallization pressure tube, jams and vaporization.

Prevent chemical instrumentation failure.

Due to the chemical instrumentation multi-purpose in a dangerous situation (inflammable, explosive, high temperature and high pressure) or conditions (highly toxic, strong corrosion and harmful), so in order to strengthen the fault handling of chemical instrumentation, actively prevent the occurrence of major accidents. The most effective way to prevent chemical failures is to strengthen the sense of responsibility of the staff, and implement the hierarchical management of instrument equipment and preventive maintenance. This method is mainly based on the specific production status of the enterprise, and the meter is divided into different grades according to the importance and danger degree of the equipment. For important equipment or high risk equipment maintenance once a week or only on cycle shorter, once found down trend or equipment, to maintain the equipment in a timely manner, in the case of guarantee the normal work of the above equipment, staff on other devices not periodic surveillance method of maintenance and regular maintenance, prevent chemical instrument failure.