There is a problem with the instrument, and the reasons are quite complex. It is difficult to find the root cause at once. At this time, it is necessary to remain calm and analyze in stages. Firstly, analyze which unit the cause lies in, which can be roughly divided into three stages: on-site detection, intermediate transmission, and terminal display; At the same time, seasonal factors should also be considered, such as preventing high temperatures in summer and freezing in winter; When the parameters involved in the regulation are abnormal, first switch the regulator to manual mode, observe and analyze whether the system is being regulated, and then check other factors again.

Regardless of which type of instrument malfunctions, we first need to understand the production process and conditions of the installation location where the instrument is located, as well as the structural characteristics and performance of the instrument itself; Before maintenance, it is necessary to collaborate with the process personnel to analyze and determine the true cause of the instrument malfunction; At the same time, it is also necessary to understand whether the instrument comes with adjustment and interlock functions. After comprehensive consideration and careful analysis, it is necessary to maintain process stability as much as possible during the maintenance process.

1、 On site measuring instruments. Generally divided into four categories: temperature, pressure, flow rate, and liquid level

1） Common Fault Analysis of Temperature Instrument System

(1) Sudden increase in temperature: This fault is often caused by thermal resistance (thermocouple) disconnection, loose wiring terminals, (compensation) wire breakage, temperature failure, and other reasons. At this time, it is necessary to understand the location and wiring layout of the temperature, and use the resistance (millivolt) range of a multimeter to measure several sets of data at different positions to quickly identify the cause.

(2) Sudden decrease in temperature: This fault is often caused by thermocouple or thermistor short circuits, wire short circuits, and temperature failure. Starting from weak points such as wiring ports and wire bends that are prone to faults, we should investigate them one by one. The temperature on site rises while the overall control indicator remains unchanged, mostly due to the presence of liquid (water) with a lower boiling point at the measuring element.

(3) If there is a significant fluctuation or rapid oscillation in temperature, the main focus should be on checking the process operation (including the inspection and adjustment system involved in the regulation).

2） Common faults and analysis of pressure instrument system

(1) Sudden decrease or increase in pressure or no change in indicator curve: At this time, the transmitter pressure system should be checked to check whether the root valve is blocked, whether the pressure pipe is unobstructed, whether there is any abnormal medium inside the pressure pipe, whether the discharge screw is blocked, and whether the discharge valve is leaking. Freezing of winter media is also a common phenomenon. The possibility of transmitter malfunction itself is very small.

(2) Large pressure fluctuations: This situation should first be coordinated with the process personnel, usually caused by improper operation. The parameters involved in regulation should mainly be checked for the regulation system.

3） Common faults and analysis of flow meter systems

(1) The minimum flow indication value is generally caused by the following reasons: damage to the detection component (zero point too low); display problems; short circuit or open circuit in the circuit; blockage or leakage in the positive pressure chamber; low system pressure; and checking the regulator, regulating valve, and solenoid valve for the parameters involved in regulation.

(2) The maximum flow indication is mainly due to blockage or leakage of the negative pressure chamber pressure system. It is unlikely that the transmitter needs to be calibrated.

(3) Large flow fluctuations: Flow parameters are not involved in regulation, usually due to process reasons; Those involved in regulation can check the PID parameters of the regulator; Parameters with isolation tank, check whether there are bubbles in the pressure pipe, and whether the liquid level in the positive and negative pressure pipes is the same.

4） Common faults and analysis of liquid level instrument system

(1) Sudden increase in liquid level: The main inspection is to check whether the negative pressure chamber pressure system of the transmitter is blocked, leaking, collecting gas, lacking liquid, etc. The specific method of infusion is to stop the meter in the order of stopping the meter first; Close the positive and negative pressure root valves; Open the positive and negative pressure drain valves to release pressure; Open the double chamber balance container and fill the plug with liquid; Open the drain plug of the positive and negative pressure chambers; At this time, the liquid level indicator is at its maximum. Close the drain valve; Close the drain plug of the positive and negative pressure chambers; Slowly pour the same medium into the dual chamber equilibrium container, and then slightly open the drain plug for exhaust; Until it is filled, open the positive pressure chamber plug and the transmitter indicator should return to zero. Then use the transmitters in the order of the meter.

(2) Sudden decrease in liquid level: Mainly check whether the positive pressure chamber pressure system is blocked, leaking, collecting gas, lacking liquid, and whether the balance valve is closed. The specific method to check whether the pressure system is unobstructed is to stop the transmitter, open the drain valve, and check the drain condition (except for medium that cannot leak out).

(3) The indication in the control room does not match the on-site liquid level: Firstly, it is determined whether the on-site liquid level gauge is faulty. At this time, the liquid level can be artificially increased or decreased. Based on the on-site and control indication, the specific cause of the problem can be analyzed (the root valve of the on-site liquid level gauge is closed, blocked, and leaking, which can easily cause inaccurate on-site indication). Normal liquid level can be restored by checking the zero point, range, and filling. If it is still not normal, notify the process personnel to monitor on-site and remove the transmitter for pressure calibration.

4) Frequent liquid level fluctuations: Firstly, check the feeding and discharging conditions with the process personnel to ensure that the process conditions are normal. Then, adjust the PID parameters to stabilize. The specific method is: put the control valve into manual state, first adjust the set value to be consistent with the measured value to stabilize the level fluctuation, then slowly adjust the opening of the control valve to make the level rise or fall slowly to meet the process requirements, then adjust the set value to be consistent with the measured value, and put the control valve into automatic mode after the parameters are stable.

In short, once some abnormalities are found in the instrument parameters, the first step is to work with the process personnel, starting from both the process operating system and the on-site instrument system, comprehensively considering and carefully analyzing, especially considering the correlation between the measured parameters and the control valves. By dividing the fault step by step, it is easy to identify the problem and solve it accordingly.

2、 The on-site control instruments are mainly valves

The safety functions and uses of valves can be divided into the following categories:

1: Exhaust valve: eliminates excess gas from the pipeline, improves pipeline efficiency, and reduces energy consumption.

2: Diverter valve: distributes, separates, or mixes medium in pipelines.

3: Safety valve: prevents the pressure of the medium in the pipeline or device from exceeding the specified value, thereby achieving the purpose of safety protection.

4: Check valve: prevents the backflow of medium in the pipeline.

5: Block valve: Connect or block the flow of medium in a pipeline.

6: Regulating valve: regulates the pressure, flow rate, and other parameters of the medium.

Now let's mainly introduce the self standing regulating valve and pneumatic regulating valve.

1） Self operated pressure regulating valve

1. Working principle of self operated pressure regulating valve (pressure control behind the valve)

The pre valve pressure P1 of the working medium is throttled after passing through the valve core and seat, and becomes the post valve pressure P2. P2 is input into the lower diaphragm chamber of the actuator through the control pipeline and acts on the top plate. The force generated is balanced with the reaction force of the spring, determining the relative position of the valve core and seat, and controlling the pressure behind the valve. When the pressure P2 behind the valve increases, the force exerted by P2 on the top plate also increases. At this point, the force of the top plate is greater than the reaction force of the spring, causing the valve core to close towards the valve seat position until the force of the top plate and the reaction force of the spring are balanced. At this point, the flow area between the valve core and valve seat decreases, resulting in an increase in flow resistance and a decrease in P2 to the set value. Similarly, when the pressure P2 behind the valve decreases, the direction of action is opposite to the above, which is the working principle of the self operated (behind the valve) pressure regulating valve.

2. Working principle of self operated pressure regulating valve (pre valve pressure control)

The pre valve pressure P1 of the working medium is throttled after passing through the valve core and seat, and becomes the post valve pressure P2. At the same time, P1 is input into the upper diaphragm chamber of the actuator through the control pipeline and acts on the top plate. The force generated is balanced with the reaction force of the spring, determining the relative position of the valve core and seat, and controlling the pressure in front of the valve. When the pressure P1 in front of the valve increases, the force exerted by P1 on the top plate also increases. At this point, the force of the top plate is greater than the reaction force of the spring, causing the valve core to move away from the valve seat until the force of the top plate balances with the reaction force of the spring. At this point, the flow area between the valve core and valve seat decreases, and the flow resistance decreases, thereby reducing P1 to the set value. Similarly, when the pressure P1 in front of the valve decreases, the direction of action is opposite to the above, which is the working principle of the self operated (valve front) pressure regulating valve.

3. Working principle of self operated flow regulating valve

After the controlled medium is input into the valve, the pressure P1 in front of the valve is input into the lower membrane chamber through the control pipeline. The pressure Ps after throttling by the throttle valve is input into the upper membrane chamber. The difference between P1 and Ps, i.e. △ Ps=P1 Ps, is called the effective pressure. The balance between the thrust generated by P1 acting on the diaphragm and the thrust difference generated by Ps acting on the diaphragm and the spring reaction force determines the relative position of the valve core and valve seat, thereby determining the flow rate through the valve. When the flow rate through the valve increases, that is, Δ Ps increases, P1 and Ps act on the lower and upper diaphragm chambers respectively, causing the valve core to move towards the valve seat, thereby changing the flow area between the valve core and the valve seat, increasing Ps. The thrust of the increased Ps acting on the diaphragm, combined with the spring reaction force and the thrust of P1 acting on the diaphragm, is balanced at a new position to achieve the purpose of controlling the flow rate. On the contrary, the same applies.

2） Pneumatic control valve

Pneumatic control valve is a type of valve that uses compressed air as its power source, a cylinder as its actuator, and accessories such as electrical valve positioners, converters, solenoid valves, limit valves, etc. to drive the valve and achieve on/off or proportional regulation. It receives control signals from industrial automation control systems to adjust various process parameters such as flow rate, pressure, temperature, and liquid level of pipeline medium.

1: Classification of pneumatic control valves.

The action of the pneumatic control valve is divided into two types: air opening and air closing. Air to Open is when the air pressure on the membrane head increases, the valve moves in the direction of increasing opening. When the input air pressure reaches the upper limit, the valve is in a fully open state. Conversely, when the air pressure decreases, the valve moves in the closing direction, and when there is no input air, the valve is fully closed. Therefore, sometimes pneumatic valves are also known as fail to close (FC) valves. The direction of the Air to Close action is exactly opposite to that of the Air to Open action. When the air pressure increases, the valve moves in the closing direction; When the air pressure decreases or does not exist, the valve opens in the direction of opening or fully opens. Therefore, it is sometimes referred to as Fail to Open FO. The pneumatic opening or closing of a pneumatic control valve is usually achieved through the positive and negative action of the actuator and different assembly methods of the valve state structure.

2: Common professional terms

The regulating valve consists of two parts: the actuator and the valve body components. Regulating valves generally use pneumatic diaphragm actuators, which have two modes of action: positive and negative. When the signal pressure increases, the actuator that pushes the stem downwards is the positive action actuator, and when the signal pressure increases, the actuator that pushes the stem upwards is the negative action actuator. The valve body components are divided into two types: forward and reverse installation. When the valve stem moves downwards, the reduced flow area between the valve core and the valve seat is considered as a positive installation, and vice versa. The operation of regulating valves can be divided into two types: air opening and air closing. Air opening and air closing are composed of the positive and negative actions of the actuator and the forward and reverse assembly of the valve body components.

Whether to open or close the regulating valve is a comprehensive consideration from multiple aspects. Firstly, process safety is the main consideration. After determining whether to close or open the valve, the function of the actuator is determined, and finally the combination of the valve body's forward and reverse installation is determined as described above.

Positive action actuator refers to the push rod of the actuator moving towards the valve body when the gas pressure on the diaphragm increases; Reaction actuator refers to the movement of the actuator push rod away from the valve body when the gas pressure on the diaphragm increases; Air to open and air to close valves are completely different concepts. The positive actuator and the forward (reverse) valve are air closed (air open); On the contrary, the reaction actuator and the reverse (forward) valve can achieve gas closure (gas opening).

The positive and negative effects of the locator correspond to the air opening and closing of the regulating valve you have selected. That is to say, it is set up to achieve negative feedback of the entire valve itself. The positive and negative effects of the regulator are set to provide negative feedback to the entire control loop. Only when the regulator is put into automatic mode can the positive and negative effects of the regulator be specifically reflected.

The positive and negative effects of the valve positioner are determined based on the opening and closing of the regulating valve, while the positive and negative effects of the regulator are determined based on the characteristics of each link in the control circuit to ensure that the control circuit meets the control requirements. For example, in the implementation of negative feedback control, in automatic control systems, the adjusted parameters often deviate from the set values due to interference, that is, the adjusted parameters produce deviations:

For regulators, according to unified regulations, if the measured value increases, the regulator output increases, and the regulator amplification factor Kc is negative, then the regulator is called a positive acting regulator; As the measured value increases, the output of the regulator decreases, and Kc is a regular regulator. This regulator is called a reactive regulator.

3: Selection of pneumatic control valve.

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

In a single loop control system, negative feedback control can be achieved as long as the product of the amplification factor Kc of the regulator, the amplification factor Kv of the regulating valve, and the amplification factor Ko of the controlled object is positive. The positive and negative signs of regulators, regulating valves, and object amplification factors are specified as follows:

(1) The sign of the amplification factor of the regulator; For regulators, according to unified regulations, when the measured value increases and the output increases, the amplification factor Kc of the regulator becomes negative, which is called positive effect. When the measured value increases and the output decreases, Kc becomes positive, which is called a reaction.

(2) The positive and negative signs of the amplification factor of the regulating valve; The amplification factor Kv of the regulating valve is defined as positive for the gas opening valve Kv and negative for the gas closing valve Kv.

(3) The sign of the magnification factor of the object; The amplification factor Ko of an object is defined as: if the manipulated variable increases, the controlled variable also increases, and Ko is positive; The manipulated variable increases, the controlled variable decreases, and Ko is negative. From this, it can be seen that the method for determining the positive and negative effects of the regulator in a single loop control system is as follows: first, determine the sign of the amplification factor Ko of the object, and then determine the sign of the amplification factor Kv of the regulating valve based on whether the regulating valve is selected as air open or air closed. Finally, the product of Kc, Kv, and Ko should be positive to determine the mode of action of the regulator.

In short, the selection of gas opening and closing is based on the safety perspective of process production. When the gas source is cut off, is it safe for the regulating valve to be in the closed position or the open position? For example, in the combustion control of a heating furnace, the regulating valve is installed on the fuel gas pipeline to control the supply of fuel based on the temperature of the furnace or the temperature of the heated material at the outlet of the heating furnace. At this point, it is safer to choose a gas valve because once the gas supply stops, it is more appropriate to keep the valve closed than fully open. If the gas source is interrupted and the fuel valve is fully opened, it can cause excessive heating and pose a danger. For example, in a heat exchange device cooled by cooling water, the hot material is cooled by heat exchange with the cooling water in the heat exchanger. The regulating valve is installed on the cooling water pipe, and the temperature of the material after heat exchange is used to control the amount of cooling water. When the gas source is interrupted, the regulating valve should be in the open position for safety, and it is recommended to use an air tight (FO) regulating valve.

4: Maintenance of pneumatic control valves: Pneumatic control valves are of great significance in ensuring the normal operation and safe production of process equipment. Therefore, it is necessary to strengthen the maintenance of pneumatic control valves.

A、 Key inspection areas during maintenance

a. Check the inner wall of the valve body: In situations with 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. Check the valve seat: Due to the infiltration of the medium during operation, the threaded inner surface used to fix the valve seat is prone to corrosion, causing the valve seat to loosen;

c. Check the valve core: The valve core is one of the movable components of the regulating valve, which is severely eroded by the medium. During maintenance, it is necessary to carefully check whether all parts of the valve core are corroded or worn, especially under high pressure differentials. The wear of the valve core is more severe due to cavitation caused by cavitation. Severely damaged valve cores should be replaced; Check the sealing packing.

B、 Daily maintenance of pneumatic control valve

When the regulating valve uses graphite asbestos as the packing, lubricating oil should be added to the packing once every three months to ensure the flexibility and usability of the regulating valve. If the packing cap is found to be pressed very low, the packing should be replenished. If the PTFE dry packing is found to have hardened, it should be replaced in a timely manner; Attention should be paid to the operation of the regulating valve during the patrol inspection, and the valve position indicator and regulator output should be checked for consistency; Regularly check the air source for regulating valves with locators and promptly address any issues found; Regularly maintain the hygiene of the regulating valve and ensure that all components are intact and functional.

3） Common faults and their causes

（1） Malfunctions and causes of the regulating valve not functioning;

1. No signal, no gas source.

Reason: ① The gas source is not turned on;

② The air source is dirty, causing blockage of the air source pipe or filter, pressure reducing valve (especially pay attention to the freezing of water in the air source in winter);

③ Compressor malfunction causes low air source pressure;

④ The gas source main pipe is leaking.

2. There is a gas source but no signal.

Reason: ① Regulator malfunction, ② Air source pipe leakage; ③ Valve positioner leaks air; ④ The regulating valve diaphragm is damaged.

3. The locator has no air source.

Reason: ① Filter blockage; ② Pressure reducing valve malfunction; ③ Pipeline leakage or blockage.

4. The locator has a gas source but no output.

Reason: ① The throttle hole of the locator is blocked; ② Amplifier malfunction; ③ The nozzle is blocked.

5. There is a signal but no action.

Reason: ① Valve core detachment, ② Valve core jamming; ③ Valve stem bending; ④ The actuator spring is broken.

（2） Malfunctions and causes of unstable operation of regulating valves;

1. Unstable gas source pressure.

Reason: ① Leakage of gas source main pipe; ② Pressure reducing valve malfunction.

2. The signal pressure is unstable.

Reason: ① Inappropriate time constant (T=RC) of the control system; ② The regulator output is unstable.

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

Reason: ① The ball valve of the amplifier in the locator is not tightly closed due to wear and tear from dirt, and when the air consumption increases significantly, it will produce output oscillation;

② The nozzle baffle of the amplifier in the locator is not parallel, and the baffle cannot cover the nozzle;

③ Leakage of output pipes and wires; ④ The rigidity of the executing mechanism is too low.

（3） Malfunctions and causes of regulating valve vibration;

1. The regulating valve vibrates at any opening.

Reason: ① Unstable support; ② There is a vibration source nearby; ③ The valve core and liner are severely worn.

2. The regulating valve vibrates when approaching the fully closed position.

Reason: ① The regulating valve is selected too large and is often used with a small opening; ② The flow direction of the medium in a single seat valve is opposite to the closing direction.

（4） Malfunction and Causes of Slow Action of Regulating Valve

1. The valve stem is only sluggish when moving in one direction.

Reason: ① Leakage of the diaphragm in the pneumatic diaphragm actuator; ② The "O" seal in the actuator leaks.

2. The valve stem exhibits sluggishness during reciprocating motion.

Reason: ① There is adhesive blockage inside the valve body; ② There is a problem with the packing, it is pressed too tightly or needs to be replaced.

（5） Fault and cause of large leakage despite the regulating valve being fully closed;

1. When the valve is fully closed, there is a large amount of leakage.

Reason: ① The valve core is worn and has severe internal leakage, ② The valve is not properly adjusted and cannot be tightly closed.

2. The valve cannot reach the fully closed position.

Reason: ① The pressure difference of the medium is too large, the rigidity of the actuator is small, and the valve is not tightly closed; ② There are foreign objects inside the valve; ③ Lining sintering.

（6） The adjustable range of traffic becomes smaller.

The main reason is that the valve core is corroded and becomes smaller, thereby increasing the adjustable minimum flow rate.

Reprinted from official account: coal chemical engineer

Mega-tek Instrument Classroom