Knowledge

On-site control valve instrument and measurement instrument common breakdown and analysis



First, on-site measurement instruments are generally divided into four categories.

 A temperature instrument system common fault analysis

(1) The sudden increase in temperature: This fault is caused by the thermal resistance (thermocouple) disconnection, loosening of the terminal block, (compensation) wire breakage, and temperature failure. At this time, it is necessary to understand the location and wiring layout of the temperature. Using a multimeter's resistance (millivolts) to measure several sets of data at different locations can quickly identify the cause.

(2) Temperature suddenly decreases: This fault is caused by a thermocouple or short-circuit of the thermal resistor, a short circuit in the conductor, and a temperature failure. To start with weak points that are prone to failure, such as junctions and wire bends, check each one. The temperature rises on site, and the total control instructions do not change, mostly due to the lower boiling liquid (water) at the measuring element.

(3) Large fluctuations in temperature or rapid oscillations: At this point, the process operation should be mainly checked (inspection regulation system involved in adjustment).

Two Pressure Instrumentation System Common Faults and Analysis

(1) The pressure suddenly becomes smaller, larger, or there is no change in the indicator curve: At this point, the pressure transmitter system of the transmitter should be inspected to check if the root valve is blocked, if the pressure tube is unblocked, if there is any abnormal medium inside the pressure tube, the sewer wire Plugging and drain valve leakage. Winter freezing is also a common phenomenon. The transmitter itself is unlikely to fail.

(2) Pressure fluctuations: This situation must first be combined with the craftsman, which is generally caused by improper operation. The parameters involved in the adjustment should mainly check the regulation system.

Three flow meter system common faults and analysis

(1) Flow indicator minimum: Generally caused by the following reasons: the detection element is damaged (zero point is too low; display problem; line short circuit or open circuit; positive pressure chamber is blocked or leaked; system pressure is low; parameters involved in adjustment are also checked Regulators, regulators and solenoid valves.

(2) Maximum flow indication: The main reason is that the negative pressure chamber pressure system is blocked or leaked. It may not be necessary for the transmitter to be calibrated.

(3) Large flow fluctuations: Flow parameters do not participate in the adjustment, generally for process reasons; those involved in the adjustment may check the PID parameters of the regulator; parameters of the isolation tank may check whether there are air bubbles in the pressure-increasing tube, positive and negative pressure Whether the liquid in the tube is as high.

Four Level Meter System Common Faults and Analysis

(1) The liquid level suddenly becomes larger: The main pressure transmitter in the negative pressure chamber is checked for plugging, leakage, gas gathering, and fluid shortage. The specific method of filling the liquid is: stop the watch in accordance with the stopwatch order; close the positive and negative pressure root valve; open the positive and negative pressure drain valve to relieve pressure; open the dual-chamber balance container filling liquid plug; open the positive and negative pressure chamber drain plug; At this point, the liquid level indication is maximum. Close the drain valve; close the positive and negative pressure chamber drain plug; use the same medium slowly poured into the double chamber balance container, then open the drain plug to open the exhaust; until it is full, open the positive pressure chamber plug, change The sender indicates that it should return to zero. Then use the transmitter in accordance with the order of the cast.

(2) The liquid level suddenly becomes smaller: mainly check whether the positive pressure chamber pressure system is blocked, leaked, gas collected, lack of fluid, and whether the balance valve is closed. The specific method for checking whether the lead pressure system is unblocked is to stop the transmitter, open the drain valve, and check the discharge condition (except for the medium that cannot be leaked).

(3) The main control room indication does not match the on-site liquid level: First, it is judged whether it is an on-site level gauge failure. At this time, it is possible to increase or decrease the liquid level artificially. According to the on-site and general control instructions, the specific analysis of the cause of the problem (site level gauge) Root valve closing, plugging, leaking easily cause on-site instructions are not allowed). The fluid level can be restored by checking the zero point, measuring range, and filling fluid. If it is still not normal, the technicians can be notified to monitor and reset the transmitter.

(4) Frequent fluctuations in the liquid level: First, check with the craftsman to check the feeding and discharging conditions. After determining that the process conditions are normal, it can be stabilized by adjusting the PID parameters. The specific method is: adjust the valve manual state, first adjust the set value consistent with the measured value, so that the liquid level fluctuations stabilized, and then slowly adjust the opening of the regulating valve, so that the liquid level slowly rise or fall to achieve the process requirements, and then adjust The set value is consistent with the measured value. After the parameter is stabilized, the regulating valve is automatically shot.

In short, once the instrument parameters are found to be somewhat anomalous, they must first be combined with the process personnel to start with the process operating system and the field instrumentation system. Comprehensive consideration and careful analysis should be made in particular to consider the correlation between the measured parameters and the control valve. Step-by-step determination, it is easy to find the problem, to solve the problem.

Second, the scene control instrument is mainly valve
Valve safety functions and uses can be divided into the following types

1. Exhaust valve: Exclude excess gas in the pipeline, increase the efficiency of pipeline use and reduce energy consumption.

2. Diverter valve: Distribute, separate, or mix the media in the pipe.

3, safety valve: prevent the pressure of the medium in the pipeline or device exceeds the specified value, so as to achieve the purpose of safety protection.

4, check valve: prevent the media back in the pipeline.

5. Cut-off valve: switch on or cut off the circulation of the medium in the pipeline.

6, regulating valve: adjust the pressure, flow and other parameters of the medium.

Now mainly introduce self-regulating valve and pneumatic control valve.

A self-acting pressure regulator

1, the principle of self-operated pressure regulating valve (post-valve pressure control)

After the pre-valve pressure P1 of the working medium is throttled by the valve body and the valve seat, it becomes the post-valve pressure P2. P2 is input into the lower membrane chamber of the actuator through the control line and acts on the top plate. The generated force balances with the reaction force of the spring, determines the relative position of the valve core and the valve seat, and controls the pressure behind the valve. When the pressure P2 increases, the force acting on the top plate by P2 also increases. At this time, the force of the top plate is greater than the reaction force of the spring, so that the valve core is closed to the position of the valve seat until the force of the top plate is balanced with the reaction force of the spring. At this time, the flow area of the valve plug and the valve seat is reduced, and the flow resistance becomes larger, so that P2 is reduced to a set value. Similarly, when the post-valve pressure P2 decreases, the direction of action is opposite to that described above. This is the principle of the self-operating (after-valve) pressure regulating valve.

2, the principle of self-operated pressure regulating valve (pre-valve pressure control)

After the pre-valve pressure P1 of the working medium is throttled by the valve body and the valve seat, it becomes the post-valve pressure P2. At the same time, P1 is input into the upper membrane chamber of the actuator through the control pipeline and acts on the top plate. The generated force balances with the reaction force of the spring, determines the relative position of the valve core and the valve seat, and controls the pressure before the valve. When the pre-valve pressure P1 increases, the force that P1 exerts on the top plate also increases. At this time, the force of the top plate is greater than the reaction force of the spring, so that the valve core moves away from the valve seat until the force of the top plate balances with the reaction force of the spring. At this time, the flow area of the valve plug and the valve seat is reduced, and the flow resistance becomes smaller, so that P1 is reduced to a set value. Similarly, when the pressure before the valve P1 decreases, the direction of action is opposite to the above, which is the working principle of the self-operating (pre-valve) pressure regulating valve.

3, the principle of self-operated flow control valve

After the controlled medium enters the valve, the pressure P1 before the valve enters the lower chamber through the control line, and the pressure Ps after throttling the throttle enters the upper chamber. The difference between P1 and Ps is ΔPs=P1-Ps and is called the effective pressure. . The thrust generated by P1 acting on the diaphragm and the thrust generated by Ps acting on the diaphragm balance with the spring reaction force to determine the relative position of the valve plug and the valve seat, thereby determining the flow through the valve. When the flow through the valve increases, that is, △Ps increases. As a result, P1 and Ps respectively act on the lower and upper film chambers to move the spool toward the valve seat, thereby changing the flow area between the valve plug and the valve seat. As Ps increases, the thrust of the increased Ps acting on the diaphragm plus the spring reaction force and the thrust of P1 acting on the diaphragm are balanced at the new position to achieve the purpose of controlling the flow. On the contrary, the same reason.

Two pneumatic control valve

Pneumatic regulating valve uses compressed air as power source, cylinder as actuator, and uses electric valve positioner, converter, solenoid valve, limit valve and other accessories to drive the valve to realize switch or proportional adjustment, and receives industry. The control signals of the automation control system complete the adjustment of the pipeline media: flow, pressure, temperature and liquid level and other process parameters.

1. Classification of pneumatic control valve

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

2, common several professional terms

The regulating valve has two parts: the actuator and the body part. Pneumatic diaphragm actuators are generally used for regulating valves. Their working modes are positive and negative. When the signal pressure increases, the actuator that pushes down is a positive acting actuator. When the signal pressure increases, the actuator that pushes up moves is the reaction actuator. Valve body parts are divided into two types, positive and negative. When the valve stem is moved down, the flow area of the valve plug and the valve seat is reduced to be installed, and vice versa. The function of the regulating valve is divided into two kinds: air opening and air closing. The air opening and closing are formed by the positive and negative action of the actuator and the positive and negative components of the valve body.

The air-opening or air-closing of a regulating valve is a comprehensive consideration in many aspects. The first consideration is the safety of the process. After determining whether the air or air is released, the function of the actuator is determined. Finally, the positive and negative installation of the valve body is determined. The combination method is as described above.

Positive acting actuator means that when the gas pressure on the diaphragm increases, the actuator rod moves toward the valve body; the counteracting actuator means that when the gas pressure on the diaphragm increases, the actuator rod moves away from the valve body; (Air to open) Air-to-close valves are completely different concepts. Positive-acting actuators and positive-fit (reverse-loaded) valves are gas-closed (air-opened); conversely, reactive-actuated actuators and counter-mounted (positive-mounted) valves are gas-closed (air-opened).

The positive and negative effects of the positioner correspond to the air opening and closing of the control valve you have chosen. That is to say in order to achieve the negative feedback of the entire valve itself. The positive and negative action of the regulator is used to set the negative feedback of the entire control loop. When the regulator is automatically operated, the positive and negative actions of the regulator can be embodied.

The positive and negative actions of the valve positioner are determined according to the air-opening of the control valve. The positive and negative actions of the regulator are determined according to the characteristics of each link of the control circuit, and it is necessary to ensure that the control circuit meets the control requirements. For example, to implement negative feedback control, in the automatic control system, the adjusted parameter often deviates from the set value due to the influence of interference, ie, the adjusted parameter generates a deviation.

For the regulator, in accordance with the unified provisions, if the measured value increases, the regulator output increases, the regulator amplification coefficient Kc is negative, the regulator is called a positive acting regulator; measured value increases, the regulator output decreases, Kc is a positive regulator that is called a reaction regulator.

3, the choice of pneumatic valve

Before any control system is put into operation, the positive and negative actions of the regulator must be properly selected so that the direction of control action is correct. Otherwise, in the closed loop it is not negative feedback but positive feedback. It will continue to increase the deviation, and ultimately must The controlled variable will be directed to the highest or lowest limit.

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

(1) The sign of the controller amplification factor; for the regulator, according to the unified regulations, the measured value increases, the output increases, and the controller amplification factor Kc is negative, which is called positive action. The measured value increases, the output decreases, and Kc is positive, which is called reaction.

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

(3) The sign of the zoom factor of the object; the zoom factor Ko of the object is defined as: If the manipulated variable increases, the controlled variable also increases, Ko is positive; the manipulated variable increases, the controlled variable decreases, and Ko is negative. It can be seen that the determination method of the positive and negative action of the regulator of the single-loop control system is as follows: first determine the sign of the object amplification factor Ko, and then determine the positive and negative values of the control valve amplification factor Kv according to the selection of the control valve as air-open or air-off. No. The final product of Kc, Kv, and Ko should be positive, and the mode of operation of the regulator can be determined.

In short, the choice of air-opening is based on the perspective of safety in process production. When the air supply is shut off, is the regulator valve in a closed position safe or open position safe? For example, in a furnace combustion control, a regulating valve is installed on a fuel gas line, and the supply of fuel is controlled in accordance with the temperature of the furnace or the temperature of the heated material at the outlet of the heating furnace. At this time, it is better to use the air opening valve to be safer, because once the air source is stopped, the valve is closed more properly than the valve is fully open. If the air supply is interrupted and the fuel valve is fully opened, it is dangerous to overheat. Another example is a heat exchange device that is 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 cooling water quantity is controlled by the material temperature after heat exchange. When the air supply is interrupted, the regulating valve should be in the open position to be safer, and the air-closed (FO) control valve should be selected.

4, pneumatic valve maintenance

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 valve.

A. Check points during maintenance

Check the inner wall of the valve: In the occasion of high pressure difference and corrosive medium, the diaphragm of the inner wall of the valve and the diaphragm valve is often subjected to the impact and corrosion of the medium, and the pressure resistance and corrosion resistance must be checked;

b. Check valve seat: Because the medium penetrates during work, the inner surface of the thread used to fix the valve seat is susceptible to corrosion and causes the valve seat to loosen;

c. Checking the valve plug: The valve plug is one of the movable parts of the regulating valve. It is seriously eroded by the medium. During the inspection, it is necessary to carefully check whether the various parts of the valve plug are corroded and worn, especially in the case of high pressure difference. The wear of the core due to cavitation is even more serious due to cavitation. Damaged spools should be replaced; check the seal packings.

B. Daily maintenance of pneumatic control valve

When the regulating valve adopts graphite-asbestos as the filling material, a lubricant should be added on the packing for about three months to ensure that the regulating valve is flexible and easy to use. If the filler pressure cap is found to be very low, fillers should be added. If it is found that the polytetrafluoroethylene dry filler is hardened, it should be replaced in time. Attention should be paid to the operation of the regulating valve in the inspection, check the valve position indicator and regulator. The output is consistent; for the regulator valve with a positioner, the gas source should be checked frequently, and the problem should be solved in a timely manner; the hygienic condition of the regulating valve and the integrity of the components should be maintained.

III Common Faults and Causes
(I) Failure of the regulating valve and its causes

1. No signal, no air source.

  the reason:

1 The air source is not open;

2 Dirty air source, resulting in blockage of the gas source pipe or blockage of the filter and the pressure relief valve (special attention is paid to the fact that the winter air source is icing with water);

3 compressor failure so that the pressure of the air source is low;

4 Air source leakage.

2. There is a source of gas, no signal.

  the reason:

1 regulator failure, 2 gas source leakage; 3 valve positioner leakage; 4 control valve diaphragm damage.

3. Locator without air source.

  the reason:

1 Filter clogged; 2 Pressure reducing valve malfunctioning; 3 Pipe leaking or blocked.

4. The positioner has a gas source without output.

  the reason:

1 Positioner orifice block; 2 amplifier failure; 3 nozzle blocking.

5. There is a signal, no action.

  the reason:

1 spool off, 2 spool stuck; 3 stem bent; 4 spring breaking actuator.

(B) The malfunction of the regulating valve and its causes

1. The source pressure is not stable.

  the reason:

1 gas source leakage; 2 pressure relief valve failure.

2. The signal pressure is not stable.

  the reason:

1 The time constant of the control system (T=RC) is inappropriate; 2 the regulator output is not stable.

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

  the reason:

1 The ball valve of the amplifier in the positioner is loosely shut off due to the wear of the contaminants. If the air consumption increases, the output oscillation will occur.

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

3 output pipe, line leak; 4 actuator rigidity is too small.

(c) Failure and Causes of Regulating Valve Vibration

1. The regulator valve vibrates at any opening.

  the reason:

1 The support is unstable; 2 There is a vibration source nearby; 3 The valve plug and the bush wear badly.

2. The regulator valve vibrates near the fully closed position.

  the reason:

1 Larger control valve, often used in a small opening; 2 single seat valve medium flow and closed in the opposite direction.

(4) malfunctions and causes of slow motion of the regulating valve

1. The stem is only dull when operated in one direction.

  the reason:

1 Diaphragm leakage in pneumatic diaphragm actuators; leakage of "O" seals in 2 actuators.

2. The valve stem is dull when it is reciprocated.

  the reason:

1 There is sticky matter in the valve body; 2 There is a problem with the packing, the pressure is too tight or needs to be replaced.

(5) Faults and Causes of Large Leakage of Valves that Have Closed in Place

1. The amount of leakage is large when the valve is fully closed.

  the reason:

1 The valve core is worn, the internal leakage is serious, and the 2 valve is not adjusted well.

2. The valve does not reach the fully closed position.

  the reason:

1 medium pressure difference is too large, the actuator stiffness is small, the valve is not strict; 2 foreign objects in the valve; 3 bushing sintering.

(6) The flow adjustable range becomes smaller

The main reason is that the spool is corroded and the adjustable minimum flow rate is increased.

Shanghai KS Flow Control Equipments Co.,Ltd is a valve manufacture with more than 30 years of valve manufacturing experience, which set up in 1984,specializing in Ball Valve,Gate Valve, Globe Valve, Check Valve and Butterfly Valve.

The company has advanced processing and manufacturing equipment, more than 70 CNC machine tools and machining centers, three spray lines. The company has established a complete physical and chemical laboratories, with direct reading spectrometer, spectrum alloy analyzer, helium mass spectrometer leak detector, electronic universal testing machine, coordinate measuring instrument, nondestructive testing, torque life test, high and low temperature comprehensive performance test device Etc.