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How to deal with the internal leakage of valves in power plant?

The heat loss caused by the valve leakage of thermal system is an important factor affecting the thermal efficiency of boiler. The treatment of valve leakage is an effective way for coal-fired power plants to establish a long-term mechanism of safe production, improve the level of economic operation and establish a good social image. This paper describes the influence, cause and judgment method of valve internal leakage, and puts forward the measures to prevent valve internal leakage.

The heat loss caused by valve internal leakage of thermal system is an important factor leading to the increase of boiler heat loss. It is an effective way for coal-fired power plants to establish a long-term mechanism of safety production, improve the economic operation level of units and establish a good social image to formulate measures to prevent and control valve leakage, master operation and maintenance skills, and take the treatment of valve leakage as a key work.

Influence of valve internal leakage

Affect safety production

Safety production is the core of coal-fired power plant production. Without safety production, the continuity and efficiency of unit operation will be lost. The internal leakage of the valve makes the equipment in operation unable to be isolated and eliminated, threatens the safe operation of the maintenance staff, and even causes serious consequences.

Impact on economic benefits

The internal leakage of valve will lead to the decrease of load rate and operation economy. Taking a 600 MW unit as an example, the internal leakage of the discharge valve of the filter screen of the feed water pump will seriously affect the capacity of the unit with load. The internal leakage of the valve at the water side of the high-pressure heating system will lead to an increase of about 8.3 g / (kW · h) of coal consumption for power supply when the feed water temperature decreases by 100 ℃. In addition, the increase in the number of internal leakage valves will increase the cost of valve repair, grinding and replacement.

Impact on green operation

Enterprises in the pursuit of economic interests at the same time also shoulder a certain social responsibility, desulfurization, ash removal, coal handling, chemical system valve leakage may cause acid, alkali, slurry, waste water, waste gas leakage, cause pollution to the surrounding environment, have adverse social impact.

Causes of valve internal leakage

Internal leakage caused by unqualified water quality

During the start-up of the unit, especially during the commissioning period, due to the long-term shutdown of the system, there is a lot of rust and salt accumulation in the pipeline, which requires the flushing of the drain valve of the fully open system during the start-up of the unit. If the flushing is not complete, there will be rust and other impurities between the valve core and the valve seat. These impurities jam at the bottom of the valve core, resulting in the valve not closed tightly, causing internal leakage.

Internal leakage caused by improper operation

When the unit is started or stopped, there is no proper time to open or close the valve. If the valve is closed too late or opened too early, high temperature and high pressure steam will cause serious erosion to the valve. The shut-off valve is in the state of half open and half closed, and the blow loss of the valve leads to the internal leakage of the valve. When the valve was closed, it was not confirmed that it was closed in place. For example, the drainage of the first, second and third sections of the steam extraction pipeline and the drain packing of the high-pressure water side were pressed tightly, so the operators would take more effort to close the valve. The operators did not strictly follow the operation requirements to judge whether the valve was closed in place according to the number of turns of the stem thread when the valve was closed. For the thermal pipeline with primary and secondary drain valves, there is no specified operation sequence to open and close the valve.

Influence of control part

The traditional control mode of electric valve is through the mechanical control mode such as valve limit switch and over torque switch. Because these control elements are affected by the environment temperature, pressure and humidity, the valve positioning error, spring fatigue, uneven thermal expansion coefficient and other objective factors cause the internal leakage of electric valve. Pneumatic valve control components are generally equipped with open and close function of the cylinder, and the power of the cylinder is generally the instrument compressed air on site. The pressure and cleanliness of the instrument compressed air, the opening of the intake valve and the positioning of the close feedback will lead to the leakage of the pneumatic valve.

Internal leakage caused by unqualified valve

In the production process, the valve manufacturer did not strictly control the valve material, processing technology and assembly technology, resulting in unqualified grinding of the sealing surface and failure to eliminate the products with defects such as pockmarks and trachomas. However, the quality inspection of professional personnel before on-site installation was not strictly controlled, resulting in unqualified products entering the production site and causing internal leakage of the valve.
In addition, when the valve is not properly selected, it is easy to produce cavitation phenomenon, and the valve suffers from serious cavitation corrosion. The leakage of the valve seat can reach more than 30% of the rated flow.

Judgment method of valve internal leakage

Through artificial observation

After the valve of the non pressure funnel system is closed, observe whether the funnel at the water outlet continuously discharges the water with pressure. If the water can not be discharged completely, it indicates that the valve has internal leakage. After the valve is closed, put the copper listening stick in the valve position. If there is a sound of leakage, it is the internal leakage of the valve.

It is determined by temperature

After the high-temperature valve is closed for 6h-8h, measure the metal temperature at the downstream of valve stem or valve body by infrared thermometer. If it is higher than 70 ℃, it is judged as internal leakage. When the low-temperature valve is closed, the temperature behind the valve is lower than the ambient temperature, or even frosting, it is judged as internal leakage. The temperature measurement method is suitable for most internal leakage valves.

By changing the operation mode of the system

There are disturbed high-temperature steam before and after some valves, such as high-pressure bypass, feed water pump recirculation regulating valve, etc. even if these valves are tight without leakage, the stem temperature of these valves also exceeds 70 ℃. Therefore, the internal leakage judgment of these valves can change the operation mode of the system, such as comparing the temperature behind the high bypass valve with the exhaust temperature of the low pressure cylinder of the steam turbine, observing the temperature change behind the valve after isolating the desuperheating water of the high bypass valve to judge whether the high bypass valve is internal leakage, and observing whether the flow of the feed pump changes by closing the isolation valves before and after the feed pump recirculation regulating valve to judge whether the feed pump recirculation regulating valve is internal leakage It’s missing the net.

By other means

When the last valve is close to the main pipe, as long as any branch valve in the pipeline leaks, the temperature of other valves will rise to more than 70 ℃, such as the valve of boiler drainage system. Therefore, other methods should be used to determine the internal leakage of these valves. Generally, the temperature of the pipe wall in front of the valve or the temperature of the valve stem in front of the primary valve should be measured to determine whether the valve is internal leakage.

Measures to prevent valve leakage

Raise awareness and implement responsibility

Establish an organization to prevent and control the internal leakage of the valve, appoint a responsible person, fulfill the two complete assessment, and strictly control the valve maintenance and acceptance. The prevention and control of valve leakage is by no means a matter of one or two professional workers, which should be taken as an important work for power enterprises to ensure the unit economy up to the standard; the professional staff of power generation department and equipment department should establish the account of valve leakage inspection and treatment; in the unit maintenance, after the valve maintenance, grinding or repair, the professional staff should press red lead powder to check the combination degree of the sealing surface, and implement step by step inspection According to the system of level acceptance, the unqualified maintenance quality should be reworked.

Regular inspection

The purpose of regular inspection of valves in thermal system is to monitor the development of internal leakage during valve operation, and to provide reliable data for valve closing during operation and valve maintenance after shutdown. Manual valve isolation shall be performed before the pneumatic drain valve with serious leakage is closed. Regular temperature measurement of valve should be conducted at least once a month. Regular temperature measurement of valve includes regular temperature measurement during operation, temperature measurement after startup, temperature measurement after recovery of working condition change, such as excessive negative separation of unit. After the high-pressure heater is discharged for maintenance.
After the unit is started, the electric drain valve should be manually closed once after the electric closing. For the valve with internal leakage, after closing for 4H ~ 6h, it is possible to make the valve close tightly. If the electric valve closed during the normal operation of the unit is opened due to the change of the operating party or other abnormal reasons, it shall be manually pressed and closed once and checked after the system returns to normal.

Standard operation

  • When the unit starts and stops, the appropriate time to open and close the valve should be selected. If the valve is closed too late or opened too early, the high temperature and high pressure steam will cause serious erosion to the valve;
  • When opening and closing the valve, the shut-off valve can only be fully closed or fully opened in principle, and these valves cannot be in the state of half open and half closed, so as to prevent the valve internal leakage caused by blowing loss;
  • For the thermal pipeline with primary and secondary drain valves, in order to prevent the internal leakage of the valve, the operation sequence should be as follows: open the primary valve first, then open the secondary valve, and adjust the throttle through the secondary valve; close the secondary valve first, then close the primary valve when closing, so as to protect the primary valve;
  • Some butterfly valves should clearly mark the switch position on site. If these valves are closed in place, they should not be closed. Otherwise, excessive switch will cause valve scouring;
  • When opening and closing different types of valves, the corresponding type of wrench or manual operation should be selected to prevent the valve from being damaged by improper force;
  • In order to prevent and reduce the internal leakage of the electric valve, for the electric valve and regulating valve that are not opened automatically, the manual pressing and closing in place is set as the full closing stroke. In principle, the valve opening shall not be reserved;
  • For the automatic opening of electric valves and adjustment doors, under the condition of ensuring that they can be opened automatically in any state, the full closing stroke should be set as close and in place as possible.

Flushing after shutdown

Some valves are normally closed during operation. They are usually closed when they are opened and started after shutdown. When the valve state changes, they are carried out under the system pressureless state. If impurities accumulate in the valve core, the valve core cannot be closed tightly. Therefore, when conditions permit, these valves should be opened and closed several times under pressure for flushing.

Adjust valve stroke

The best closing stroke of electric valve and pneumatic valve is 0. At present, the closing position of general valve is determined by torque. It must be carefully adjusted to ensure that the valve can be closed tightly to prevent scouring.

Optimization of thermal system

In order to prevent valve leakage caused by high-pressure steam scouring, two valves are added to the system requiring throttling adjustment. Cancel the repeated design or rarely used water drainage points to reduce the internal leakage points of system valves, such as the repeated design water and air drainage points of water supply system, the first and second ventilation valves of steam turbine, etc. The manual valve is added after the pneumatic valve of high and medium pressure steam drainage, and the manual valve is kept fully open during commissioning.

Concluding remarks

The treatment of valve leakage in thermal system is the basic work of reducing heat consumption and improving economy of coal-fired power plant units. Equipment management personnel should do a good job in the treatment and maintenance of valve leakage. Correct operation and timely inspection of valve leakage by operating personnel are the main means to reduce or eliminate valve leakage. As long as the enterprise attaches importance to it, preventive measures are in place, and proper operation and maintenance is carried out, valve leakage can be eliminated It has been greatly improved.

Source: Network Arrangement – China Valve Supplier:

(Yaang Pipe Industry is a leading manufacturer and supplier of nickel alloy and stainless steel products, including Super Duplex Stainless Steel Flanges, Stainless Steel Flanges, Stainless Steel Pipe Fittings, Stainless Steel Pipe. Yaang products are widely used in Shipbuilding, Nuclear power, Marine engineering, Petroleum, Chemical, Mining, Sewage treatment, Natural gas and Pressure vessels and other industries.)

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how to deal with the internal leakage of valves in power plant - How to deal with the internal leakage of valves in power plant?
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How to deal with the internal leakage of valves in power plant?
This paper describes the influence, cause and judgment method of valve internal leakage, and puts forward the measures to prevent valve internal leakage.
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