Nuclear valves, special conditions because of its use, complexity, poor, coupled with towers, so the requirement is higher.
1, nuclear power valves working conditions:
Nuclear valve complex in addition to its environmental conditions outside, the transport of radioactive medium and temperature, pressure, level of harshness is also very special.
Nuclear main valve transmission medium: saturated steam, condensate, heavy radioactive water vapor, radiation corrosive, radioactive medium, sulfuric acid and alkali, carbon dioxide, sodium, helium, oil, vacuum and other fluids.
Back on the road in the working conditions of large diameter valve is the most complex nuclear power plant at this stage than the power plant steam parameters on the steam parameters (pressure 22.5MPa, temperature 565 ℃) to be low, but nuclear power plant operating conditions are too complex more. In liquid metal fast neutron reactor coolant device, the highest steam parameters (temperature steam turbine before 600 ℃, pressure of 14.0MPa).
2. Fault type nuclear power valve
In the nuclear power plant running on the system of valves, the most common type of fault has the following four:
① stem leakage
② seat leakage
③ optional electric actuator torque is too large and off the sealing surface damage caused by high
④ external leakage
3. Technical requirements for nuclear power valve
Nuclear power valve operation under actual working conditions, nuclear power, its technical characteristics and requirements of the valve than the thermal valve higher. In addition to the technical requirements of nuclear power valve the technical requirements of conventional valves, but also consider the media focused on the pollution of impurities, ambient temperature, operating temperature, humidity, radiation, DC power supply and voltage fluctuation, the seismic stability and vibration conditions technical requirements, security levels and so on.
① design of nuclear power valve
a) Strength Design
Nuclear power valve design, strength calculation is essential. In addition to the conventional strength calculation, finite element analysis and seismic calculation analysis, a nuclear safety valve, also called for: a film stress of the limit calculation, a thin film stress + bending stress limit calculation, and the loop start - circulatory arrest increases the time limits of secondary stress amplitude calculation, in addition to the loop start - plus a lockout condition other than changes in secondary stress range limit calculation, fatigue analysis.
b) structural design
Most of the nuclear power system to carry the radioactive medium, does not allow any leakage, so the structural design of the valve packing, bellows, the valve seat seal structure design is more important (body shape design, provides the ASME standard.)
Abroad, filling the general structure of a multiple seal, Ω ring seal between the fill and the folder structure and the disc spring of the stuffing box seal structure. Generally use the combination of bellows bellows seal structure. Important high-pressure valves, valve seat with forged structure. In addition, the valve and pipeline connection with butt or socket welded structure.
② nuclear material valve
Nuclear power valve material must have good resistance to corrosion, radiation, shock and resistance to intergranular corrosion. Under normal circumstances:
a) pressure parts must be ASME BPVC-Ⅱ-D-1 Table 2A and Table 2B in the material requirements
b) stem and the bearing bolts often used precipitation hardening steel
c) multi-purpose graphite fiber packing, pure asbestos or graphite
③ Nuclear power valve actuator
Nuclear power valve actuator performance and quality are very important and critical, must have a safe operation reliability and should be able to withstand temperature, pressure, humidity, radiation, earthquake damage, chemical pollution and the changes for maximum power, water loss in the event of failure and must be the case, can still work within the prescribed period (generally the standard is 14 days).
In addition to the electric drive components require O-ring seal to isolate the external environment, the drive designer should also consider the status of the nuclear electrical fast operation with high-pressure valve problems (common standards, CL1500 14"Following the American standard valve, and its fast operation time is 10 seconds).
④ valve testing and inspection of nuclear power
a) need for conventional nuclear power valve pressure test - shell test, strength test valve, the seal test, the valve seat seal test, packing test;
b) with the implementation of institutions, such as electric valve, pneumatic valve for seismic test;
c) forms for all operating life test static pressure valve;
d) the importance of a loop through the valve must also be cold, hot and LOCA accidents (ie, LOCA) test.
(U.S. ASME, Japan, JEM and other standards, testing and examination of the above described in detail, and provides assessment criteria.)