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日期: 2019-03-20

Faults and Analysis of Instruments

浏览次数: 10

Firstly, field measuring instruments are generally divided into four categories.

Analysis of Common Faults in One Temperature Instrument System

(1) Temperature suddenly increases: This fault is mostly caused by thermal resistance (thermocouple) circuit breaking, loose wiring terminals, (compensation) wire breaking, temperature failure and other reasons. At this time, it is necessary to understand the location and wiring layout of the temperature. The reason can be quickly found out by measuring several sets of data with Multimeter resistance (millivolt) gear at different locations.

(2) Temperature suddenly decreases: this fault is mostly caused by short circuit of thermocouple or thermal resistance, short circuit of conductor and temperature failure. We should start with the weak points which are easy to break down, such as the junction and the bend of the conductor, and investigate them one by one. The on-site temperature rises while the general control indication remains unchanged, mostly due to the low boiling point of the liquid (water) at the measuring element.

(3) Temperature fluctuation or rapid oscillation: at this time, the process operation should be mainly checked (checking and regulating system involved in regulation).

Common Faults and Analysis of Two Pressure Instrument System

(1) The pressure suddenly decreases, increases or the indicating curve remains unchanged: at this time, the pressure diversion system of the transmitter should be inspected to check whether the root valve is blocked, whether the pressure diversion pipe is unblocked, whether there is abnormal medium inside the pressure diversion pipe, blockage of the drainage wire and whether the drainage valve is leaking, etc. Medium freezing is also a common phenomenon in winter. Transmitter itself has little possibility of failure.

(2) High pressure fluctuation: This situation should be combined with technicians first, usually caused by improper operation. The parameters involved in the regulation should mainly check the regulation system.

Common Faults and Analysis of Three Flow Meter System

(1) Flow indicator value is the smallest: generally caused by the following reasons: damage to detection components (zero is too low). Short circuit or break of circuit, blockage or leakage of positive pressure chamber, low pressure of system, and check regulator, regulator and solenoid valve for parameters involved in regulation.

(2) Flow indication is the largest: the main reason is the blockage or leakage of the vacuum chamber system. Transmitters may not need to be calibrated.

(3) Flow fluctuation is large: Flow parameters do not participate in the regulation, generally for process reasons; Participating in the regulation, the PID parameters of the regulator can be checked; With the parameters of isolation tank, check whether there are bubbles in the booster tube, whether the liquid in the booster tube is as high as that in the positive and negative pressure booster tube.

Common Faults and Analysis of Four-level Instrument System

(1) Suddenly increase of liquid level: mainly check whether the negative pressure chamber pressure system of transmitter is blocked, leaked, gas gathering, lack of liquid, etc. The specific methods of irrigation are: stopping the table first according to the stopping order; closing the positive and negative pressure root valve; opening the positive and negative pressure drainage valve to relieve pressure; opening the filling plug of the double chamber balanced container; opening the drainage plug of the positive and negative pressure chamber; at this time, the liquid level indicator is the largest. Close the drain valve; Close the drain plug in the positive and negative pressure chamber; Fill the double chamber balance container slowly with the same medium, then slightly open the drain plug to block the exhaust; until filling up, open the positive pressure chamber plug at this time, and the transmitter indication should return to zero. The transmitter is then put into use in the order in which the tables are put.

(2) Suddenly smaller liquid level: mainly check whether the positive pressure chamber pressure system is blocked, leaked, gas collection, lack of liquid, balance valve is closed, etc. The specific ways to check whether the pressure diversion system is smooth are to stop the transmitter, open the sewage valve and check the sewage discharge (except for the medium that can not be leaked).

(3) The indication of the master control room is not in conformity with the liquid level on the spot: first, it is judged whether the liquid level gauge is faulty on the spot. At this time, the liquid level can be artificially increased or lowered. According to the situation of the field and the master control indication, the causes of the problem can be analyzed concretely (the closed, blocked and leakage of the root valve of the field liquid level gauge can easily lead to inaccurate on-site indication The normal level can be restored by checking zero point, range and irrigation. If it is still abnormal, the technicians can be notified to take the transmitter back for pressure adjustment.

(4) Frequent fluctuation of liquid level: Firstly, the process can be stabilized by adjusting the PID parameters after checking the feeding and discharging conditions with the technicians. Specific methods are as follows: adjust the setting value to be in accordance with the measured value, stabilize the fluctuation of the liquid level, then slowly adjust the opening of the regulating valve, so that the liquid level rises or falls slowly to meet the technological requirements, and then adjust the setting value to be in accordance with the measured value, and then adjust the setting value to be automatic after the parameters are stabilized.

In short, once some abnormal instrumentation parameters are found, it is easy to find out the problem and solve the problem by combining with technicians, starting from two aspects of process operating system and field instrumentation system, considering comprehensively and carefully, especially considering the relationship between the measured parameters and control valves, and judging the fault step by step.

2. Field control instruments are mainly valves

Valve safety functions and uses can be divided into the following categories

1. Exhaust valve: Remove excess gas in pipeline, improve pipeline efficiency and reduce energy consumption.

Shunt Valve: Distribution, Separation or Mixing of Media in Pipeline.

3. Safety valve: Prevent the medium pressure in the pipeline or device from exceeding the prescribed value, so as to achieve the purpose of safety protection.

4. Check valve: Prevent medium backflow in pipeline.

5. Cut-off valve: to connect or cut off the medium flow in the pipeline.

6. Regulating valve: regulating medium pressure, flow and other parameters.

Now mainly introduce the self-supporting control valve and pneumatic control valve.

A Self-operated Pressure Regulating Valve

1. Working Principle of Self-operated Pressure Regulating Valve (Post-valve Pressure Control)

The front pressure P1 of the working medium changes to the back pressure P2 after throttling the spool and seat. P2 is put into the lower membrane chamber of the actuator through the control pipeline and acts on the top plate. The force produced is balanced with the reaction force of the spring, which determines the relative position of the valve core and seat and controls the pressure behind the valve. When the pressure P2 behind the valve increases, the force of P2 acting on the top plate also increases. At this time, the action force of the top plate is greater than the reaction force of the spring, so that the spool closes to the position of the valve seat until the action force of the top plate is balanced with the reaction force of the spring. At this time, the flow area of the spool and seat decreases and the flow resistance increases, thus reducing P2 to a set value. Similarly, when the pressure P2 behind the valve decreases, the direction of action is contrary to the above, which is the working principle of the self-operated pressure regulating valve.

2. Working Principle of Self-operated Pressure Regulating Valve (Pressure Control before Valve)

The front pressure P1 of the working medium changes to the back pressure P2 after throttling the spool and seat. At the same time, P1 acts on the top plate through the control pipeline input to the upper membrane chamber of the actuator. The force produced is balanced with the spring reaction force, which determines the relative position of the valve core and seat, and controls the pressure in front of the valve. When the pressure P1 in front of the valve increases, the force of P1 acting on the top plate also increases. At this time, the action force of the top plate is greater than the reaction force of the spring, so that the spool moves away from the seat until the action force of the top plate is balanced with the reaction force of the spring. At this time, the flow area of the spool and seat decreases, and the flow resistance decreases, thus reducing P1 to a set value. Similarly, when the pressure P1 in front of the valve decreases, the direction of action is contrary to that mentioned above, which is the working principle of the self-operated (pre-valve) pressure regulating valve.

3. Working Principle of Self-operated Flow Regulating Valve

After the controlled medium is fed into the valve, the pressure P1 before the valve is fed into the lower membrane chamber through the control pipeline, and the pressure Ps after the throttle is fed into the upper membrane chamber. The difference between P1 and Ps, i.e. Ps=P1-Ps, is called effective pressure. The thrust difference between P1 acting on the diaphragm and Ps acting on the diaphragm is balanced with the spring counterforce to determine the relative position of the valve core and seat, thereby determining the flow through the valve. When the flow through the valve increases, i.e. Ps increases, the results show that P1 and Ps act on the lower and upper diaphragm chambers respectively, which makes the spool move towards the valve seat, thus changing the flow area between the spool and the valve seat and increasing Ps. The thrust of Ps acting on the diaphragm plus the spring reaction force and the thrust of P1 acting on the diaphragm produces a balance in the new position to control the flow. On the contrary, the same is true.

Two Pneumatic Regulating Valve

Pneumatic control valve uses compressed air as power source, cylinder as actuator, and drives valve by means of electrical valve positioner, converter, solenoid valve, limit valve and other accessories, realizes switching or proportional regulation, and receives control signals from industrial automation control system to complete various process parameters such as flow rate, pressure, temperature and liquid level.

1. Classification of pneumatic control valves

Pneumatic control valve operates in two types: open and closed. Air to Open is when the pressure above the diaphragm head increases, the valve moves towards the direction of increasing opening. When the upper limit of input pressure is reached, the valve is in full open state. Conversely, when the air pressure decreases, the valve moves towards the closing direction, and when there is no air input, the valve is completely closed. So sometimes the air-opened valve is also called Fail to Close FC. The movement direction of air to Close is just opposite to that of open air. When the air pressure increases, the valve moves towards the closing direction; when the air pressure decreases or does not exist, the valve moves towards the opening direction or opens completely. It is sometimes called Fail to Open FO. The opening or closing of pneumatic control valve is usually realized by the positive and negative action of the actuator and the different assembling ways of the valve structure.

2. Several common technical terms

Regulating valve is composed of actuator and body parts. Regulating valves generally adopt pneumatic film actuator, which has two kinds of action modes: positive and negative. When the signal pressure increases, the pushing-drying down mechanism is the positive one, and when the signal pressure increases, the pushing-drying up mechanism is the reaction one. Valve body parts are divided into positive and reverse assembly. When the stem moves down, the flow area between the spool and the seat decreases in the positive form, and vice versa. The function of the control valve is divided into two kinds: air-opening and air-closing. Air-opening and air-closing are composed of the positive and negative effects of the actuator and the positive and negative assemblies of the valve body parts.

Faults and Analysis of Instruments

Whether the valve is air-opened or air-closed is a comprehensive consideration in many aspects. The first consideration is process safety. After determining whether the valve is air-closed or air-opened, the role of the actuator is determined. Finally, the combination of positive and negative assembly of the valve body is determined as mentioned above.

Positive actuator means that when the gas pressure on the diaphragm increases, the actuator push rod moves towards the valve body; reaction actuator means that when the gas pressure on the diaphragm increases, the actuator push rod moves away from the valve body; and air to open and air to close valves are totally different concepts. The positive acting actuator and the positive (reverse) valve can get the air closure (air opening); on the contrary, the negative acting actuator and the reverse (front) valve can get the air closure (air opening).

The positive and negative effects of the positioner correspond to the opening and closing of the regulating valve you choose. That is to say, in order to achieve the negative feedback of the whole valve itself. The positive and negative effects of the regulator are set up for the negative feedback of the whole control loop. When the regulator is put into operation automatically, the positive and negative effects of the regulator can be embodied concretely.

The positive and negative effects of the valve positioner are determined by the air opening and closing of the control valve, and the positive and negative effects of the regulator are determined by the characteristics of each link of the control loop, so as to ensure that the control loop meets the control requirements. For example, in an automatic control system, the tuned parameters often deviate from the set values due to disturbance, that is, the tuned parameters produce deviations:

Faults and Analysis of Instruments

For the regulator, according to the unified regulation, if the measurement value increases, the output of the regulator increases, and the amplification coefficient Kc of the regulator is negative, the regulator is called a positive regulator; when the measurement value increases, the output of the regulator decreases, and the regulator is called a reaction regulator.

3. Selection of Pneumatic Regulating Valve

Before any control system is put into operation, the positive and negative effects of the regulator must be correctly selected so that the direction of the control action is correct. Otherwise, in the closed loop, the positive feedback is not negative feedback, but positive feedback. It will increase the deviation continuously and eventually lead the controlled variable to the highest or lowest limit value.

In a single loop control system, as long as the amplification factor of the regulator Kc, the amplification factor of the control valve Kv and the product of the amplification factor Ko of the controlled object are positive, the negative feedback control can be realized. The positive and negative sign of regulator, regulator valve and object amplification coefficient are as follows:

(1) The positive and negative sign of the amplification coefficient of the regulator; for the regulator, according to the unified regulation, the measurement value increases, the output increases, and the amplification coefficient Kc of the regulator is negative, which is called a positive effect. When the measured value increases, the output decreases, and the Kc is positive, it is called reaction.

(2) The magnification factor of the control valve is positive and negative; the magnification factor Kv of the control valve is defined as the positive value of the air-opening valve Kv and the negative value of the air-closing valve Kv.

(3) Positive and negative sign of the magnification coefficient of the object; the magnification coefficient Ko of the object is defined as: if the manipulation variable increases, the controlled variable increases, and Ko is positive; when the manipulation variable increases, the controlled variable decreases, and Ko is negative. It can be seen that the positive and negative signals of the regulator in the single loop control system are determined as follows: first, the positive and negative signals of the amplification coefficient Ko of the object are determined, then the positive and negative signals of the amplification coefficient Kv of the regulator are determined according to the type of the regulator selected as the air-opening or the air-closing, and finally the product of Kc, Kv and Ko should be positive, so the mode of the regulator can be determined.

In a word, the selection of gas-opening and air-closing is based on the safety of process production. When the air source is cut off, is the control valve safe in the closed position or in the open position? For example, in a heating furnace, the combustion control valve is installed in the fuel pipeline to control the fuel supply according to the temperature of the furnace or the temperature of the heated material at the outlet of the heating furnace. At this time, it is advisable to choose an air-opened valve to be safer, because once the supply of air stops, it is more appropriate for the valve to be closed than for the valve to be fully open. If the gas source is interrupted and the fuel valve is fully open, the excessive heating will be dangerous. Another example is a heat exchanger cooled by cooling water. The heat material is exchanged with cooling water in the heat exchanger and cooled. The regulating valve is installed on the cooling water pipe. The cooling water quantity is controlled by the material temperature after heat exchange. When the air source is interrupted, the regulating valve should be in the open position safer. It is better to choose the air-closed (FO) regulating valve.

4. Maintenance of pneumatic control valve

Pneumatic control valve is very important to ensure the normal operation and safe production of process equipment. Therefore, it is necessary to strengthen the maintenance of pneumatic control valves.

A. Key Inspection Sites in Maintenance

A. Inspection of the inner wall of the valve body: In the case of high pressure difference and corrosive medium, the inner wall of the valve body and the diaphragm of the diaphragm valve are often impacted and corroded by the medium. It is necessary to focus on checking the pressure and corrosion resistance.

B. Inspection seat: The internal surface of threads used to fix the seat is susceptible to corrosion due to the infiltration of working medium, which makes the seat relaxed.

C. Inspection of valve core: The valve core is one of the movable parts of the control valve, which is seriously eroded by the medium. During maintenance, it is necessary to carefully check whether the parts of the valve core are corroded and worn. Especially in the case of high pressure difference, the wear of the valve core is more serious because of cavitation. The valve core with serious damage should be replaced; the sealing filler should be checked.

B. Daily maintenance of pneumatic control valve

When graphite-asbestos is used as filler in the control valve, lubricating oil should be added to the filler once in about three months to ensure the flexible and easy use of the control valve.

If the packing pressure cap is found to be very low, the filling should be supplemented. If the PTFE dry filling is found to be hardened, the filling should be replaced in time; the operation of the control valve should be paid attention to during the inspection tour, and the output of the valve position indicator and the regulator should be checked; the air source of the control valve with the locator should be checked regularly, and the problems should be found and dealt with in time; the sanitation and all parts of the control valve should be maintained regularly. It's complete and easy to use.

Three common failures and their causes

(1) Failure and Reasons of Regulating Valve Not Acting

1. No signal, no gas source.

Reason:

(1) The gas source is not open;

(2) The dirty air source causes the blockage of the air source pipe or the blockage of filters and pressure relief valves (special attention should be paid to the freezing of the air source with water in winter);

(3) Compressor failure makes the pressure of gas source low;

(4) Gas source main pipe leakage.

2. Gas source, no signal.

Reason:

Regulator failure, gas source pipe leakage, valve positioner leakage, and valve diaphragm damage.

3. The locator has no air source.

Reason:

(1) filter blockage; (2) decompression valve failure; (3) pipeline leakage or blockage.

4. The locator has air source but no output.

Reason:

(1) blockage of orifice of locator; (2) failure of amplifier; (3) blockage of nozzle.

5. Signal and no action.

Reason:

The valve core falls off, the valve core is stuck, the valve stem is bent, and the spring of the actuator is broken.

(2) Faults and causes of unstable operation of control valves

1. Gas pressure is unstable.

Reason:

1. Leakage of main pipe of gas source; 2. Fault of pressure relief valve.

2. Signal pressure is unstable.

Reason:

The time constant of the control system (T=RC) is not appropriate; the output of the regulator is unstable.

3. The pressure of the gas source is stable and the signal pressure is stable, but the action of the control valve is still unstable.

Reason:

The ball valve of the amplifier in the positioner is not tightly closed by the abrasion of dirt, and the output oscillation will occur when the gas consumption is especially increased.

The nozzle baffle of the amplifier in the positioner is not parallel, and the baffle can not cover the nozzle.

(3) Leakage of the output pipe and line; and (4) too small rigidity of the actuator.

(3) Faults and Causes of Vibration of Regulating Valves

1. The control valve vibrates at any opening.

Reason:

The support is unstable, the vibration source is nearby, and the wear of valve core and bushing is serious.

2. The control valve vibrates when it is close to the full closed position.

Reason:

The control valve is selected to be large and often used at a small opening; and the medium flow direction of the single seat valve is opposite to the closing direction.

(4) Faults and Causes of Slow Action of Regulating Valves

1. Stem is dull only in one direction.

Reason:

The leakage of diaphragm in pneumatic film actuator and O-type seal in actuator are discussed.

2. The valve stem is blunt in reciprocating motion.

Reason:

There are sticky clogging in the valve body; and there are problems with packing, which is too tight or needs to be replaced.

(5) Faults and Causes of Large Leakage of Regulating Valve Closed in Position

1. When the valve is fully closed, the leakage is large.

Reason:

The valve core is worn out and the internal leakage is serious. The valve is not properly adjusted and tightly closed.

2. The valve can not reach the full closed position.

Reason:

The pressure difference of the medium is too large, the rigidity of the actuator is small, and the valve is not tightly closed; there are foreign bodies in the valve; and the bushing is sintered.

(6) Reducing the adjustable range of flow

The main reason is that the spool is corroded smaller, so that the adjustable minimum flow rate becomes larger.

 


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