The thermal loss caused by valve internal leakage in the thermal system is an important factor affecting the thermal efficiency of the boiler. Good management of valve internal leakage is an effective way to establish a long-term safety production mechanism for coal-fired power plants, improve economic operation level and establish a good social image. This article describes the effects, causes and determination methods of valve internal leakage, and proposes measures to prevent valve internal leakage.
The thermal loss caused by the internal leakage of the valve of the thermal system is an important factor leading to the increase of the boiler heat loss. Coal-fired power plants formulate measures to prevent valve leakage, master operation and maintenance skills, and treat valve leakage as a key task. It is a coal-fired power plant to establish a long-term mechanism for safe production, improve the unit's economic operation level, and establish a good society. Image effective way.
Effects of valve leakage
01 Affect safety production
Safe production is the core of the production work of coal-fired power plants. Without safe production, the continuity and high efficiency of the unit operation are no longer guaranteed. The internal leakage of the valve makes it impossible to isolate and eliminate the running equipment, threatens the safe operation of the maintenance staff, and even causes serious consequences.
02 Affect economic benefits
Internal leakage of the valve will cause the negative load rate of the machine and the operating economy to decrease. Taking a 600MW unit as an example, the internal leakage of the drain valve of the feedwater pump will seriously affect the capacity of the unit to carry the load. The internal leakage of the high-pressure heating valve on the water side causes the feed water temperature to decrease by 100 ° C, and the power consumption coal consumption will increase by 8.3g / (kW )about. In addition, the increase in the number of leaking valves will increase the cost of valve repair, grinding and replacement.
03 Impact on green management
While pursuing economic benefits, enterprises also shoulder certain social responsibilities. The internal leakage of the valves of desulfurization, ash removal, coal transportation, and chemical systems may cause leakage of acids, alkalis, slurries, waste water, and exhaust gas, and cause damage to the surrounding environment. Pollution has adverse social impacts.
Causes of valve leakage
01 Internal leakage caused by unqualified water quality
When the unit is started, especially during commissioning, due to the long-term shutdown of the system, there is a lot of rust and salt accumulation in the pipeline. This requires the fully opened system's drainage valve to be flushed during the unit startup. Impurities such as rust will be accumulated between the valve core and the valve seat. These impurities jam at the bottom of the valve core and cause the valve to close tightly, causing internal leakage.
02 Leakage due to improper operation
When the unit was started or stopped without the proper timing for opening and closing the valve, closing it too late or opening it prematurely, the high temperature and high pressure steam caused severe erosion of the valve. Shut-off valves are in a half-open and half-closed state, and valve blow-outs cause valve leakage. When the valve is closed, it is not confirmed that it is in place. For example, the drainage of the first, second, and third stages of the steam extraction pipe and the high-side water discharge packing are pressed tightly. The operator will be laborious when closing the valve. According to the number of turns of the valve stem when the valve is closed, determine whether the valve is in place. For the thermal pipelines with primary and secondary drainage valves, the prescribed sequence of opening and closing valves is not installed.
03 Influence of control part
The traditional control methods of electric valves are mechanical control methods such as valve limit switches and over-torque switches. Because these control elements are affected by ambient temperature, pressure, and humidity, the valve positioning is incorrect, spring fatigue, and uneven thermal expansion coefficients are caused. Objective factors cause internal leakage of electric valves. Pneumatic valve control components are generally equipped with cylinders with on and off functions, and the power of the cylinders is generally on-site instrument compressed air. The pressure, cleanliness of the instrument compressed air, the opening of the intake valve and the positioning of the closing feedback are all Leakage of the pneumatic door.
04 Leakage due to valve failure
During the production process of valve manufacturers, the valve materials, processing technology, and assembly technology were not tightly controlled, resulting in unsatisfactory grinding of the sealing surface and the removal of defective products such as pitting and trachoma. There is no strict control, which causes substandard products to enter the production site and cause valve leakage.
In addition, when the valve is selected incorrectly, cavitation is likely to occur, the valve suffers from severe cavitation corrosion, and the valve seat leakage can reach more than 30% of the rated flow.
judging method of valve leakage
01 Through manual observation
After the water is discharged to the valve of the pressureless funnel system, observe whether there is an uninterrupted discharge of pressured water from the water discharge funnel. If the water cannot be exhausted, it indicates that the valve is leaking. After the valve is closed, place the copper listening rod in the valve position. If there is a sound of leakage, it is a leak in the valve.
02 Judging by temperature
After the high-temperature valve is closed for 6h ~ 8h, use an infrared thermometer to measure the metal temperature between 100mm and 150mm downstream of the valve stem or valve body. If it is higher than 70 ° C, it is judged as an internal leak. After the low-temperature valve is closed, the temperature behind the valve is lower than the ambient temperature, or even frost, it is judged as internal leakage. The temperature determination method is applicable to most of the leakage valves.
03 By changing the way the system works
Some valves have disturbed high-temperature steam before and after, such as high-by-stage large bypass, feedwater pump recirculation adjustment door, etc. Even if these valves are tight and leak-proof, their stem temperature exceeds 70 ° C. Therefore, the internal leakage determination of these valves can change the operating mode of the system. For example, the temperature of the high bypass valve is compared with the exhaust temperature of the low pressure cylinder of the turbine, and the temperature change of the high bypass valve is isolated after observing the temperature change of the high bypass valve. Close the front and rear isolation doors of the feedwater pump recirculation adjustment door and observe whether there is a change in the feedwater pump flow rate to determine whether the feedwater pump recirculation adjustment door is leaking.
04 by other means
When the drain valve or the sewage door connected to the sparse and drained mother pipe side by side, when the last valve position is close to the mother pipe, as long as any branch valve in the pipeline leaks inside, the temperature of other valves will rise to over 70 ° C. Such as boiler drainage system valves. Therefore, other methods for determining the internal leakage of these valves should be adopted. Generally, the temperature of the pipe wall in front of the valve or the temperature of the valve stem in front of the door is used to determine whether the valve is internally leaking.
Measures to prevent valve leakage
01 Raising awareness and implementing responsibilities
Establish an organization to prevent valve leakage, appoint responsible persons, fulfill both assessments, and strictly control valve inspection and acceptance. Preventing valve leaks is by no means a matter of one or two specialized workers, and should be taken as an important task for power companies to ensure that the unit's economical standards are met; professionals in the power generation department and equipment department should establish a check account for valve leak detection and management; In the maintenance of the unit, the professional should press the red dan powder to check the sealing degree of the sealing surface after the valve is overhauled, ground or repaired. At the same time, a gradual acceptance system should be implemented. If the quality of the overhaul is unqualified, it should be reworked resolutely.
02 Periodic inspection
The purpose of regular inspection of the thermal system valve is to monitor the development of internal leakage during valve operation, to provide reliable data for closing the valve during operation, and to repair the valve after shutdown. Manually isolate the door before closing the hydrophobic air operated door with severe leakage. The regular temperature measurement of the valve should be performed at least once a month. The regular temperature measurement of the valve is divided into the regular temperature measurement during operation, the temperature measurement after the startup, and the temperature measurement after the change in operating conditions, such as excessive negative separation of the unit. After the high water discharge maintenance.
After the unit is started, the electric drain valve should be manually closed immediately after the electric shutdown is completed. For internally leaking valves, multiple temperature measurement checks and tightening are performed after closing 4h ~ 6h, so that the valve may reach a tightly closed state. If the electric door closed during normal operation of the unit is opened due to changes in the operator or other abnormal reasons, it should be manually closed and checked once the system returns to normal.
03 Standard operation
When starting and stopping the unit, you should choose an appropriate timing for opening and closing the valve. If it is closed too late or opened early, high temperature and high pressure steam will cause a serious erosion of the valve;
When closing valves, shut-off valves can only be fully closed or fully open in principle, and these valves cannot be left in a half-open and half-closed state to prevent valve leakage caused by valve leakage;
For the thermal pipeline with primary and secondary drainage valves, in order to prevent the valve from leaking, the operation sequence should be: first open the door once, then open the second door, and adjust the throttling through the secondary door; when closing, first Closing the second door and closing it again, the purpose is to protect the door once;
The position of some butterfly valves should be clearly marked on the site. If these valves are closed, they should not be closed, otherwise, over-switching will cause the valve to flush;
When opening and closing different types of valves, you should select the corresponding type of wrench or operate by hand to prevent the valve from being damaged by improper force;
In order to prevent and reduce the internal leakage of electric valves, for non-automatically opened electric valves and adjustment doors, manual pressure closing is determined as the fully closed stroke, and the valve opening degree must not be reserved in principle;
For automatic opening of the electric valve and adjustment door, in the case of ensuring that any state can be automatically opened, try to close the tightly and fully in place as far as possible.
04 Rinse after shutdown
Some valves are normally-closed valves during operation, and are generally closed after opening and starting after shutdown. When the valve status is changed, the system is performed under no pressure. When conditions permit, the switch should be flushed several times under pressure.
05 Adjust valve stroke
The closing stroke of electric door and pneumatic door is best to be 0. At present, the valve is generally closed by torque. It must be carefully set to ensure that the valve can be closed tightly to prevent flushing.
06 Optimized Thermal System
In order to prevent the valve from leaking due to high pressure steam flushing, install two doors for the system that needs to be throttled. Eliminate repeated design or rarely used drainage points to reduce leakage points in system valves, such as repeated design of water supply system air discharge points, air discharge points, first and second ventilation valves of steam turbines. Add a manual door after the high school steam drain pneumatic door, and keep the manual door fully open during commissioning.
Controlling the internal leakage of the thermal system valve is the basic work of reducing the heat consumption and improving the economy of the coal-fired power plant units. The equipment management personnel do a good job in the management and maintenance of the valve internal leakage. The operating operators correctly operate and timely check the internal leakage valve to reduce or eliminate the valve. The main means of internal leakage, as long as the enterprise attaches importance to it, preventive measures are in place, and proper operation and maintenance, valve internal leakage can be greatly improved.