Nuclear power valves, due to their special, complex and harsh service conditions, coupled with large amount and wide range, have higher requirements.
1. Working Conditions of Nuclear Power Valves.
Transport medium of nuclear power valves mainly contains: saturated steam, condensate, radioactive heavy water steam, irradiation corrosive, radioactive medium, sulfuric acid and alkali, carbon dioxide, sodium, helium, oil, vacuum and other fluids.
2. Common Fault of Nuclear Power Valves
There are four types of fault that are the most common of valves in operation of
nuclear power system.
① stem leakage
② seat leakage
③ sealing surface damage caused by too large optional actuator and too high closing moment.
④ external leakage
3. Technical Requirements of Nuclear Power Valves
According to the actual operation conditions of nuclear power valves, their technical characteristics and requirements are higher than those of thermal power valves. In addition to technical requirements of conventional valves, nuclear power valves should be focused on impurity pollution of medium, ambient temperature, operating temperature, humidity, radiation, DC power supply and voltage fluctuations, stability under seismic and vibration, security level and so on.
① Design of Nuclear Power Valves
a) Strength Design
Strength calculation is essential for the design of nuclear power valves. In addition to conventional strength calculation, finite element analysis and seismic calculation analysis, a top-class nuclear safety valve must be operated a film stress limit calculation, a film stress + bending stress limit calculation, range ability limit calculation and fatigue performance analysis.
b) Structural Design
Because most of transmission medium in nuclear power system is radioactive, any leak is not allowed. Thus, the valve packing bellows and seal valve seat is particularly important for structural design.
For valve packing, foreign countries generally use multiple sealing structure, Ω ring seal structure and stuffing box seal structure which has disc springs between packing layers. A combination of bellows seal structure is commonly used for valve bellows. For important high-pressure valves, their seats should be made with forging structure. In addition, butt or socket welded structures should be adopted for the connections between valve bodies and pipes.
② Material of Nuclear Power Valves
Materials of nuclear power valves must have good corrosion resistance, radiation resistance and intergranular corrosion resistance. Under normal circumstances:
a) Pressure parts must be adopted materials meeting ASMEBPVC-
Ⅱ-D-1 requirements.
b) Stems and bearing bolts are often made of precipitation harden steel
c) Packings are mostly used graphite fiber, pure asbestos or expanded graphite.
③ Actuator of Nuclear Power Valves
Performance and quality of nuclear power valve actuator is very important and critical, which must have reliability for safe operation. It should be able to withstand temperature, pressure, humidity, radiation, earthquake damage, chemical pollution and maximum change of power, and can still work within the prescribed time limit (general 14 days) in the event of faulty due to water loss.
④ Testing and Inspection of Nuclear Power Valves
a) Conventional hydraulic test – case test, valve strength test, seal test, seat seal test and packing test;
b) Seismic tests for implementing agencies, such as
electric,
pneumatic actuators.
c) Static pressure life tests for valves in all forms of operations.
d) Cold, hot and LOCA accident tests for major loop valves.
