Causes and Judgment of Internal Leakages of Power Plant Valves

The thermal loss caused by the internal valve leakage of the thermal system is an important factor that affects the thermal efficiency of the boiler. The control of the internal valve leakage is an effective way for coal-fired power plants to establish a long-term mechanism for safe production, improve the level of economic operation and establish a good social image. Attention should be paid to taking measures to prevent internal leakages of valves and mastering operation and maintenance skills. Dealing with internal leakages of valves should be a key task.
 
The influences of the valve's internal leakage

1. The effect on safety in production

Safety in production is the core of the production work of coal-fired power plants. Without safety in production, the continuity and efficiency of unit operation will lose its guarantee. The internal leakage of the valve threatens the safe operation of maintenance staff, and even causes serious consequences.
 

2. The effect on economic benefits

The internal leakage of valves will cause the load rate of the machine load and operating economy to drop. Taking a 600 MW unit as an example, pour water to the entrance of the feedwater pump will cause leakage of the valve, which will seriously affect the load capacity of the unit. The internal leakage of the valve on the water side of the high-pressure heater will cause the feed water's temperature to drop by 100℃, and the coal consumption for power supply will increase by around 8.3g/(kW·h). In addition, the increase in the number of valves with internal leakage will increase costs of repair, grinding and replacement of valves.
 

3. The effect on green operation

While pursuing economic benefits, enterprises also have certain social responsibilities. Internal leakage of valves in desulfurization, ash removal, coal transportation and chemical systems may cause acid liquid, lye, slurry, waste water and exhaust gas to leak, which will cause pollution to the surrounding environment and result in a negative social impact.
 
Causes of valves' internal leakage

1. Internal leakage caused by unqualified water quality

When the unit is started, especially in the process of testing, a drain valve with a fully open system should be flushed in the starting process of the unit due to plenty of rust and salification in the pipeline that is not in operation. If the valve is not flushed well, impurities such as rust will accumulate between the valve spool and valve seat. These impurities get stuck at the bottom of the valve spool and cause the valve not to be closed tightly, thus resulting in internal leakage.
 

2. Internal leakage generated by improper operation

When the unit is started or stopped, the valve is not opened or closed at the right time. If the valve is closed too late or opened too early, the high temperature and high-pressure steam will cause serious scour of the valve. The shut-off valve is in a half-open and half-closed state, resulting in the leaking of valves. When the valve is closed, whether it is in the close position is not confirmed.
 

3. Internal leakage caused by the influence of the control part

The traditional control method of electric valves is through mechanical control methods such as valve limit switches and over-torque switches. These control elements are affected by ambient temperature, pressure and humidity, causing objective factors such as inaccurate positioning of valves, spring fatigue and uneven thermal expansion coefficient and resulting in internal leakage of the electric valve. Control elements of pneumatic valves are generally equipped with cylinders with opening and closing functions, and the power of the cylinders is generally compressed air for the instrument on site. The pressure and cleanliness of the compressed air for the instrument as well as the degree of opening and the positioning of the closing feedback of intake valves will cause internal leakage of the pneumatic valve.
 

4. Internal leakage caused by unqualified valves

The valve manufacturer does not strictly control the valve's materials, processing technology, assembly technology, etc. in the production process, resulting in unqualified grinding of the sealing surface, pitting, sand holes and other defects not being removed, and there is no strict control when professionals perform quality inspections before installation. These lead to unqualified products entering the production site and causing internal leakage of valves. In addition, when the valve type is not properly selected, it is easy to cause cavitation. The valve suffers from severe cavitation corrosion, causing leakage of the valve seat to reach more than 30% of the rated flow.
 

Judgment methods of internal leakage of valves

1. Manual observation

After draining the water to the valve of the non pressure funnel system, observe whether there is continuous discharge of pressurized water from the drain funnel. If the water cannot be drained, it indicates that the valve is leaking. After the valve is closed, place the copper listening rod at the valve position. If there is a sound, it is an internal leakage of the valve.

2. Judging by temperature

After the high temperature valve is closed for 6 to 8 hours, use an infrared thermometer to measure the metal temperature at the valve stem or parts at 100mm to 150mm downstream valve bodies. If it is greater than 70°C, it is judged as internal leakage. After the low temperature valve is closed, if the temperature behind the valve is lower than the ambient temperature, or even frost occurs, it is judged internal leakage. The temperature measurement method is applicable to most valves which suffer from internal leakage.
 

3. Changing the operating mode of the system

There are disturbing high-temperature steam before and after some valves, such as the high side first-stage large bypass, feed water pump recirculation adjustment valve, etc. Even if these valves are tight and don't leak, the valve stem's temperature exceeds 70°C. Therefore, the internal leakage determination of these valves can change the operation mode of the system. For example, compare the temperature after the high side valve being used with the exhaust temperature of the low pressure cylinder of the steam turbine or check whether the high side valve leaks by observing temperature changes behind the valve after the high side desuperheating water is isolated.
 

4. Other methods

Connect the drain valves of the main drain pipe. When the last valve position 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°C, such as valves for boiler drainage systems. Therefore, other methods should be used to determine the internal leakage of these valves. Generally, the temperature of the pipe's wall in front of the valve or the temperature of the stem is measured to determine whether there is internal leakage for valves.