Internal Leakages of Valves in Thermal Systems of Large-scale Thermal Power Plants (Part Two)

4. Comprehensive treatment plans
According to the above analysis of the reasons for internal leakage of valves in the thermal power plant's thermal system, combined with the differences in the purpose, structure, and operating conditions of each valve, a targeted comprehensive treatment plan is proposed for implementation.
 
4.1 Optimization and improvement of the valve's operation
(1) The closing stroke of electric valves and pneumatic valves is preferably 0. Nowadays, the closing position of the general valve is determined by torque. It is recommended that closing of the stroke should be used, and provide corresponding support and guidance according to the equipment manufacturer's technology. The stroke must be set to ensure that the valve can be closed well to prevent the sealing surface from being flushing.
(2) For shut-off valves, in principle, they can only be in the fully open or fully closed state. It is forbidden to make shut-off valves at a half-open or half-close state when they are in operation, especially the use of the steam trap for small flow sewage discharge, and the globe valve working as a regulator. Prevent the valve core from internal leakage of the valve due to chemical and mechanical loss.
(3) When manually operating different types of valves, select the corresponding valve wrench to avoid damage to the small valve by operating it with a large valve's wrench and a large valve not being closed well with a small valve's wrench.
(4) The isolation valve should not be operated for a long time at acute angles as far as possible. When the valve is closed manually, it must be closed well to prevent the valve from being blushing.
 
4.2 Optimization and improvement of maintenance and positioning processes
(1) When the valve is disassembled and overhauled, the quality of the sealing surface should be checked and accepted. When the valve core and valve seat are checked, both the surface finish and the valve's sealing line should be checked.
(2) Make the positioning accurate; the maintenance and thermal control personnel should communicate well, and the appropriate torque of valve actuators should be adjusted to ensure that the electric or pneumatic actuator can be closed well.
(3) Perform the pressure test of the valve of the drainage system in combination with the water pressure test of the boiler, and check the quality of maintenance.
(4) Take timely measures during the initial period of the internal leakage of valves to avoid it being more serious, for example, isolate and repair valves for fuel atomized steam systems, valves for heater systems, and intake valves for soot blowers in time; as for globe valves with impurities inside, open them to flush impurities and then close them.

(5) Conduct a comprehensive inspection of valves of the thermal system, forming an inspection report for internal leakage of valves, and develop a targeted plan, such as grinding medium and low pressure valves with slight internal leakage and inlet valves of soot blowers, changing medium and low pressure valves with serious internal leakage, disassembling and grinding safety valves and steam traps in the process of maintenance.

 
4.3 Optimization and improvement of supervision and inspection methods
(1) Install online temperature monitoring devices for all the valves of the boiler's drainage system and introduce the DCS system, so that the status of the internal leakage of the drain valve can be absolutely clear, which not only provides a basis for maintenance, but also makes the leakage status of the drain valve public, convenient for all technicians to supervise and manage the leakage of the drain valve.
(2) Make the inspection of internal leakage of valves regular work; conduct monthly inspection on safety valves and steam valves, and include internal leakage in the maintenance plan when internal leakage is found.
(3) Improve the inspection method of internal leakage of steam valves of soot blowers, putting stem after disassembling the plug instead of temperature measurement.
(4) Use temperature measuring tools, infrared imagers, listening needles, and other methods to check internal leakage of valves.
 
5. Implementation of the plan
In 2019, the company dealt with 20 valves that have internal leakage. Up to now, the thermal system valves of the two units have reached zero leakage, and the effect is remarkable.
 
5.1 Direct results
Impacts of steam and water losses: According to the Energy-saving Management Measures,   the coal consumption of the steam turbine is calculated as 0.02g/kWh. A total of 20 valves that leak internally are treated, and the coal consumption of the power supply is reduced by 0.4g/kWh. Changes in water replenishment rates in units: After the special treatment for internal leakage of valves, the plant's water replenishment rate continues to improve. From June to October, the water replenishment rate decreased by 0.16% year-on-year; the coal consumption for power supply is reduced by 0.02g/kWh. The coal consumption is reduced by 0.42 g/kWh. The annual power generation is 5.4 billion kWh; 2,268 tons of standard coal are saved annually, and the annual economic benefit is 1.7 million RMB.
 
5.2 Indirect results
(1) Through the implementation of the subject, the employees have been trained in valves' selection, operation and maintenance processes, and they have a more comprehensive and in-depth understanding of the thermal system.
(2) Carried out the valve technology competition to achieve the purpose of training.
(3) Taking the opportunity to inspect and discover the on-site installation violations, the operating platform of the economizer and drain valve of the water wall was modified.
(4) Formed a management system; perfected the regular inspecting work of internal leakage of valves, and created a file of systems.
(5) Continued to summarize the experience of dealing with internal leakages of valves, which can be used to improve the life of the valve's equipment and reduce the failure rate.
 
6. Conclusion
The number of internal leakages of the thermal system's valve was effectively reduced by implementing comprehensive treatment of internal leakage of valves; the safety and economy of the equipment were improved, and the expected goals were achieved.