Steam flow is an important control parameter of the combined cycle unit, which determines the changing trend of the sliding pressure curve and affects the opening of the main steam valve. If abnormal phenomena occur, safe and reliable treatment methods must be adopted. In the process of handling, methods and methods are particularly important. The main steam flow is generally a differential pressure flowmeter . Three sets of samples are generated from one flow orifice and three differential pressure flow transmitters are connected.
1The composition and working principle of differential pressure flowmeter
　　 Steam flowmeters generally consist of flow orifices, primary valves, pressure relief lines, secondary valves, blowdown valves, balancing doors, and differential pressure transmitters . Its working principle is that the differential pressure of the flow orifice is introduced into the differential pressure transmitter through the pressure-inducing line. The pressure from the positive and negative pressure-inducing lines directly acts on the diaphragms of the transmitter's sensors on both sides of the diaphragm, and is transmitted to the measuring element through the sealing liquid in the diaphragm. The measuring element converts the measured differential pressure signal into Corresponding electrical signals are passed to the converter, which undergoes processing such as amplification to become standard electrical signals.
2 differential pressure flow meter common faults
Common faults are divided into two categories: lead pressure piping and valve faults, differential pressure transmitter faults.
2.1 Lead pressure piping and valve failure
The faults of the pilot line and the valve mainly include the plugging of the pressure-inducing pipeline, the icing of the pressure-inducing pipeline, and the leakage of the valve joint. The common plugging of pressure tube is divided into negative pressure tube plugging and positive pressure tube plugging. When the negative pressure tube is blocked and the flow rate increases and the negative pressure tube is blocked again, the flowmeter indication will increase, but the increase will decrease. When the flow rate decreases and the negative impulse tube is again clogged, the flow meter indication drops. The flow rate in the pipeline is unchanged (maintaining the original flow rate), and the flowmeter indication is unchanged. When the pressure tube is plugged. When the flow rate increases, the static pressure in the fluid pipe also increases accordingly. The increase value is set to P0. At the same time, the static pressure decreases due to the increase in flow rate, and the value is set to P1. If P0=P1, the flowmeter indication does not change; if P0>P1, the flowmeter indication increases; if P0
Leakage at valve connections can also be divided into two types of negative pressure side leakage and positive pressure side leakage. Leakage at the valve connection at the suction side increases when the flow F increases and the negative pressure pipe leaks. When the flow rate drops, the static pressure of the negative pressure pipe increases to P0, and the negative pressure pipe leaks to P1. If P0=P1, the flowmeter indication does not change; if P0>P1, the flowmeter indication increases; if P0
2.2 differential pressure transmitter failure
Differential pressure transmitter faults mainly include zero drift, diaphragm deformation or damage, and electronic component failures. Differential pressure transmitters are mainly characterized by the occurrence of measured fluctuations in the case of stable flow or the occurrence of step-by-step measurement jumps.
3 The Importance of Steam Flow Meters and Measures to Be Taken Before Common Troubleshooting
The importance of the main steam flow is mainly reflected in the sliding pressure curve, and the sliding pressure curve is related to the bypass control and the main steam valve opening. Therefore, the main steam flow is especially important to deal with the fault, in the local treatment of faults should take the necessary thermal control measures to ensure the safe and stable operation of the unit.
3.1 Importance of main steam flow
The task of the bypass control system is to ensure that the system builds pressure after the gas turbine is ignited. The steam turbine maintains the ramming pressure before it starts. The steam turbine bypasses and shuts off in a timely manner. The accident conditions are quickly opened to avoid boiler overpressure. The combined cycle waste heat boiler is designed as a 100% bypass, which can ensure that the steam generated by the waste heat boiler can be circulated through the bypass system when the steam turbine is fully cut.
The pressure control of the high-pressure bypass steam control valve is a single-loop control system. The design scheme is as follows: Before the steam turbine is started, the set value of the high-pressure bypass steam control valve follows the actual pressure and is automatically raised and lowered to maintain the rush pressure of the steam turbine. After the vehicle is completed, the set value is converted to the set value of the main steam pressure calculated by the coordinated control according to the high pressure main steam flow function, and a 0.7 MPa deviation is added to the set value to ensure that the steam turbine pressure control loop is activated. The high bypass valve ensures that it is in an automatic and fully closed state, and that all the high pressure main steam enters the turbine to do work. The high pressure main steam pressure set value sliding pressure curve is shown in the attached table 1.
Table 1 High Pressure Main Steam Pressure Setpoint Sliding Pressure Curve
Cold
High pressure main steam flow (kg/s)
0
47.25
75
100
High pressure main steam pressure (Mpa)
5.0
5.0
9.6
9.6
Warm state
High pressure main steam flow (kg/s)
0
47.25
75
100
High pressure main steam pressure (Mpa)
7.0
7.0
96
9.6
Two-by-one mode
Cold
High pressure main steam flow (kg/s)
0
47.25
75
100
High pressure main steam pressure (Mpa)
5.0
5.0
12.5
12.5
Warm state
High pressure main steam flow (kg/s)
15
45
75
150
High pressure main steam pressure (Mpa)
7.5
8.0
11
12.5
If the flow rate measurement is larger than the actual situation, the impact on the control of the high pressure bypass is mainly that the set value of the high pressure main steam pressure is too high, which affects the normal operation of the protection of the high pressure bypass. The effect on the main steam regulating valve is that the set value of the high pressure main steam pressure is too high, the main steam regulating door will be turned off, affecting the normal power generation of the steam turbine, and may even cause non-stop of host protection actions.
If the flow rate measurement is smaller than the actual situation, the impact on the control of the high pressure bypass is mainly that the set value of the high pressure main steam pressure is too small, and the high pressure bypass will open the valve and release the pressure. If the deviation is large, the turbine operation will be safe. The influence of the main steam regulating valve is that the high pressure main steam pressure setting value is too high, the main steam regulating door will open large, affecting the boiler water level control of the waste heat boiler, and may even cause the host protection action to stop.
3.2 Measures to be taken before common faults are dealt with If a steam flow meter fails, the situation can be divided into three types: one fault, two faults, and three faults. The thermal control measures taken according to the fault conditions are also different. The following will take the pressure piping insulation measures in place, resulting in its icing as an example, respectively, for the above three cases are discussed.
3.2.1 Treatment measures for a fault When a differential pressure type flow transmitter freezes, the specific conditions may be divided into two types. One is the positive pressure side first freezing, the negative pressure side after the ice, a For the negative pressure side of the first ice, positive pressure side after the ice. When the first condition occurs, the flow rate display will rise rapidly. When the negative pressure side re-ices, the flow rate display will decrease. When the positive pressure side and the negative pressure side all freeze, the flow will show zero. When the second condition occurs, the flow rate display will drop rapidly. When the ice is re-frozen on the positive pressure side, the flow rate display will decrease. When the negative pressure side and the positive pressure side are all frozen, the flow rate will show zero.
When this happens, thermal control should take logical enforcement measures in a timely manner. The specific measures for forcing are bad quality judgments for forced flow display and differential pressure measurement. For the specific flow-rate setting value, consult the operating personnel and determine the value of the forced setting according to the local processing progress. If the method of adopting the forced large value should be based on this, a certain offset should be added. If the method of adopting the strong set value, the method should be adopted. Based on this, a certain amount of bias is reduced to ensure that the differential pressure type flow transmitter without freezing is selected in the logic three-fetching, so that the value of normal measurement can be adjusted and protected. See Figure 1 for details.
3.2.2 Handling of Two Failures When a second differential pressure type flow transmitter becomes iced, corresponding measures should be taken in accordance with the situation when the first ice is forcibly set. If the first station adopts the method of forcing a large value, the second station shall adopt the method of forcing a small value; if the first station adopts the method of forcing a small value, the second station shall adopt the method of forcing a large value. In this way, it is ensured that the two differential pressure type flow transmitters that are frozen will not be selected in the logic three-fetching, so that the normal measured values ​​can participate in adjustment and protection. The specific operation is shown in Figure 1.
3.2.3 Handling of three failures When the third differential pressure type flow transmitter is frozen, the current value must be immediately forcibly set to ensure normal operation of the unit. Then according to the actual situation, take follow-up measures.
Newly built gas turbine load - boiler steam flow curve. According to the recent historical trend, the turbine load-boiler steam flow curve is summarized and the POLY function is used to achieve the corresponding replacement of the gas turbine load and the main steam flow. This curve can meet the load adjustment of the network adjustment. The reason for selecting the recent historical trend is that the other factors affecting the main steam flow (for example, ambient temperature, humidity, and turbine exhaust temperature, etc.) are less transformed, and the main influencing factor of the change in the main steam flow is the gas turbine load. Gas turbine load - reliability of the boiler steam flow curve.
However, due to the recent trend may not be able to collect relevant data of part of the load section, the necessary remedial measures should be taken in the part beyond the existing data load, and the main steam flow of another heat recovery boiler operating at the same load can be referenced to compensate for this. Insufficient data available. In order to ensure the reliability of data switching, the switching conditions should be rigorous. The load of the existing function is increased by 25 MW on the lower line and the load on the other is greater than 100 MW. The advantage of such a setting is that when another running unit suddenly fails or trips, the data can be switched to the function output in time to ensure the safe operation of the unit. Specific operation shown in Figure 2. 
In the figure, the analog switching block RAXFR is set to a reasonable switching rate to avoid the interference caused by the large signal fluctuations in the switching process on the normal operation of the unit. Added three to switch the output of the neutralization function. The original three-input exit adds a corresponding analog switching block RAXFR to realize the switching of the original three-neutralization-neutralized gas turbine load-boiler steam flow curve, and sets a reasonable switching rate to prevent the disturbance caused by the switching rate.
4 Common Troubleshooting Methods for Steam Flow Meters When the treatment measures are completed, the local equipment failure should be further analyzed and dealt with for different types of failures so that relevant failures can be handled properly.
4.1 Pressure pipe plugging Under normal circumstances, the plugging of the pressure pipe is mainly caused by the occasional discharge of particulates from the pressure pipe of the differential pressure transmitter. When the pressure tube is blocked, close the valve once and use a wire or wire to block it. If it is impossible to open, open the valve and flush it with steam pressure. After the flushing with steam is still not resolved, the relevant cutting and welding tools should be used, and the plugged part of the impulse pipe should be replaced to restore normal operation.
4.2 Pressure-inducing piping Ice formation and pressure-inducing piping ice formation occurs only during extremely cold weather or when the insulation measures are not in place. The treatment measures are to put the relevant insulation power supply in time so that the heating cable is heated. Check whether the corresponding insulation layer thickness meets the relevant design requirements, or if the local insulation layer is damaged. If the temperature caused by the occurrence of the insulation is low, a correspondingly thick insulation middle layer should be installed in time.
The precautionary measures for icing in pressure piping are to check whether the insulation measures of the relevant equipment are in place during the winter and whether there is any damage during long-term operation and repair in time if any.
4.3 Leakage at the valve connection Leakage at the valve connection is mostly a leak at the joint between the valve and the pipe. When the valve joint leaks, the seepage or steam leakage phenomenon at the weld can be seen on the site. If it is a leakage before the valve, professional plugging company needs to be handled. If it is a valve leakage, you can close the valve and re-weld. Plugging. If there is a special case of leakage of the pressure tube after a valve, replace the leak.
4.4 Differential Pressure Transmitter Faults In the normal operation of differential pressure transmitters, the most common faults are zero drift, diaphragm deformation or damage, and electronic component failure. Zero point drift and electronic component failures that can be directly judged in the field can be determined only after the diaphragm has been calibrated. The zero drift is determined by opening the balance valve and closing the secondary and negative pressure side valves. At this time, the differential pressure transmitter output current is 4m A, and the screen shows 0. If it is not 4m A, it means there is a drift in the zero point and the zero point of the differential pressure transmitter needs to be adjusted. Electronic component failures are primarily judged by display and measurement.
5 Conclusion When the implementation of a corresponding measure of failure occurs, the disturbance caused by the control of the unit due to the steam flowmeter is reduced; when the implementation of the three simultaneous failures avoids the unplanned shutdown, sufficient processing for the local equipment is won. time.
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