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

Abstract: The valves used in the thermal system of modern large-scale thermal power plants are all high temperature and high-pressure valves. How to control internal leakages of valves has become an important content of the daily technical management of thermal power plants. The reasons for internal leakages of valves for thermal systems are analyzed and relevant solutions to internal leakage of valves of thermal systems are put forward. 
 
1. Overview
In thermal power plants, valves play an important and irreplaceable role in the thermal system. According to the valve type, installed system and position, a valve mainly plays the role of cutting off, adjusting and stopping the backflow. In addition, it can also play a role in pressure stabilization, drainage, pressure reduction, backflow (such as minimum flow valves) and direction changes (such as three-way valves for water supply). In the energy-saving management of thermal power plants, internal leakage of valves is an important and long-term task. The safe, reliable and economic operation of valves is a prerequisite for the safe and economic operation of power plants. Whether internal leakage of valve management is good directly determines the operating economy of the unit and the level of saving energy and consumption reduction. In production practice, internal leakage of the three types of valves, such as drainage system valves of thermal systems, safety valves of the air exhaust and steam inlet valves of boilers' soot blowers are the most common to be seen and have the greatest impact on safety and economy. The internal leakage control of these three types of valves is studied and implemented in this article. 
 
2. The influence of internal leakage of valves

(1) The impact of internal leakage on safety

The internal leakage of the valve will make the equipment in operation unable to be completely isolated, and the safety measures cannot be separated during equipment maintenance, which not only seriously threaten the personal safety of the maintenance personnel, but also cause the necessary maintenance not to be performed, thus causing a decline in reliability. Long-term internal leakage of the valve will cause erosion and thinning of the pipeline behind the valve, and the pressure-bearing of the pipeline will decrease, which will pose a risk of a pipe burst, causing great hidden dangers to equipment and personal safety. The internal leakage of the inlet valve of the soot blower will cause chemical and mechanical losses and deterioration of materials for heating surfaces of boilers, and there is a hidden danger of causing leakage of heating surfaces.
 

(2) The impact of internal leakage on economy

Long-term internal leakage of the thermal system's valves will cause losses of steam and water, high temperature and high-pressure working media to be directly discharged to the condenser without being used, thus increasing the heat load of the condenser, a decrease in the vacuum of units and efficiency of steam turbines. When defects of the equipment are eliminated, the safety measures cannot be performed because of internal leakage of the valve, resulting in a decrease in the operating economy of the unit. At the same time, the increase in the number of valves with internal leakages will increase the cost of valve repairing, grinding and replacement.
 
(3) The impact of internal leakage on the social image
The internal leakage of the valve causes some media such as steam and water to be directly discharged to the surrounding environment. As an enterprise, especially a state-owned enterprise, while pursuing economic benefits, we should also take the responsibility for society. The internal leakage of the valve makes a part of high temperature and high-pressure steam and water directly discharge to the atmosphere, causing thermal pollution to the surrounding environment. Internal leakage of valves in desulfurization, ash removal, coal transportation and chemical systems may cause acid, lye, slurries, waste water and exhaust gas to leak, which will pollute the surrounding environment and cause adverse social impacts.
 
3. Cause analyses
According to the comprehensive statistical analysis of the causes of internal leakage of valves at the production site, it is believed that the main reasons for internal leakage of valves are as follows:
 

(1) The quality of the valve itself: there are two reasons for this. One reason is the incorrect selection of the valve's pressure and temperature grades due to the lack of knowledge and skills of the technicians; the other reason is the poor manufacturing process of the valve's manufacturer. The tightness of some valves cannot stand the test of the unit's high load pressure, resulting in no leakage at low loads and leakage at high loads.

(2) Improper operation of valves: when the valve is closed manually, it is not closed well; the sequences of the valve's opening or closing are not reasonable; the globe valve is operated at acute angles.

(3) Valve positioning: the valve's positioning is unreasonable, resulting in electric or pneumatic actuators not being closed properly. The setting of torque causes improper positioning.
(4) Poor maintenance process of valves: the repaired disassembled valve causes secondary leakage due to grinding and poor quality of reassembly; a great deviation between the calibration pressure and actual operating pressure causes internal leakage of safety valves.
(5) Impurities on the sealing surface causing improper closing: there are mainly impurities such as oxide skin generated inside the medium or iron filings left in the system after maintenance. When the valve is closed, the sealing surface will be scratched, causing the valve not to be closed well.
(6) Inadequate inspection and supervision of internal leakage of valves: there is no regular inspection, a lack of necessary supervising means and failure to deal with the leakage in time at the initial stage, thus causing the valve's damage.
 
According to the investigation and analysis of the reasons for the internal leakage of the above valves, the reasons for improper operation accounted for 25%, poor maintenance and positioning 30%, the lack of monitoring and inspection 35%, and other reasons 10%. It is initially determined that there are three main reasons for internal leakages of valves, such as improper valve operation, poor maintenance and positioning as well as bad supervising and inspection. The treatment plans are also implemented accordingly.