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Boiler safety valves are critical components for ensuring the safe operation of power plant systems. However, they can experience various failures due to improper maintenance, design limitations, or operational issues. Below is a detailed analysis of common problems and their solutions.
First, jamming of the moving parts in the valve is a frequent issue. This can occur due to incorrect assembly, contamination with dirt or impurities, or corrosion of components. Additionally, poor surface finish or damage to the piston chamber, such as grooves or scratches, can increase friction between moving and fixed parts. As a result, the force required to move the main safety door (f1) becomes insufficient compared to other forces (f2 + f3 + fm), leading to failure. For example, during a run-in test on furnace No. 3 in 2001, the main safety door failed to operate due to rust and debris in the piston chamber. After cleaning, repairing the chamber, and reassembling the components properly, the valve functioned normally during subsequent hydrostatic tests.
Second, excessive leakage from the piston chamber can also cause the main safety door to fail. If the piston ring or chamber is worn, the leakage increases, reducing the force (f1) acting on the piston. This imbalance (f1 < f2 + f3 + fm) prevents the valve from operating. In one case, the piston chamber clearance was found to be larger than acceptable standards, leading to significant steam leakage. The solution involved replacing damaged parts, smoothing the chamber surface, and adjusting the throttle valve to improve steam flow into the piston chamber.
Third, mismatch between the main safety valve and the impulse safety valve can lead to insufficient steam flow, preventing the main valve from opening. This often happens when the impulse valve has a smaller nominal diameter than required. For instance, after replacing a heavy hammer-type safety valve with a smaller one, the main valve failed to actuate. Adjusting the gap in the guide sleeve and increasing the flow area resolved the issue.
Fourth, delayed seating of the main safety valve after the impulse valve retracts can be caused by high residual pressure in the piston chamber. Increasing the throttle opening allows faster venting of steam, reducing pressure and enabling the main valve to seat properly.
Fifth, low back pressure on the safety valve seat can lead to excessive media loss and energy waste. This is often due to improper valve sizing or flow restrictions. Adjusting the throttle valve and optimizing the flow path can help restore proper back pressure.
Sixth, frequency jumping—where the valve repeatedly opens and closes—can be caused by high pressure fluctuations or improper valve selection. Ensuring correct valve capacity and reducing inlet resistance can mitigate this issue.
Lastly, flutter during discharge can cause mechanical fatigue and damage. This is often due to oversized discharge capacity or restricted piping. Proper valve sizing and improved piping design can prevent flutter.
In conclusion, understanding the root causes of safety valve failures and implementing timely maintenance and adjustments are essential for ensuring reliable and safe boiler operations. Even though modern safety systems include multiple layers of protection, a thorough grasp of these issues enables operators to respond effectively and maintain system integrity.