Analysis of technical problems faced by valve installation and packing replacement in power station

Analysis of technical problems faced by valve installation and packing replacement in power station

The position of valve installation must be convenient for operation; Even if the installation is temporarily difficult, it is necessary to consider the long-term work of the operator. It is better to take the valve handwheel and chest (generally 1.2 meters away from the operation floor), so that it is easier to open and close the valve. Ground valve handwheel should be up, do not tilt, so as to avoid awkward operation. The valve of the wall machine depends on the equipment, but also to leave room for the operator to stand. To avoid the operation of the sky, especially acid and alkali, toxic media, otherwise very unsafe. Gate valve should not be reversed (i.e. hand wheel down), otherwise the medium will be retained in the valve cover space for a long time…
Valve installation
Once the valve is correctly selected, it must be properly installed, maintained and operated to maximize its efficiency. The quality of valve installation directly affects the use, so careful attention must be paid.
(1) Direction and position
Many valves have directional, such as globe valve, throttle valve, pressure reducing valve, check valve, etc., if installed in reverse, it will affect the use effect and life (such as throttle valve), or does not work at all (such as pressure reducing valve), or even cause danger (such as check valve). General valves, direction signs on the valve body; In case there is no, it should be identified correctly according to the working principle of the valve. The valve chamber of the globe valve is asymmetric, so that the fluid should be passed through the valve port from bottom to top, so that the fluid resistance is small (determined by the shape), open labor saving (due to the medium pressure up), after closing the medium does not press the packing, easy maintenance. This is why the globe valve can not be installed. Other valves have their own characteristics.
The position of valve installation must be convenient for operation; Even if the installation is temporarily difficult, it is necessary to consider the long-term work of the operator. It is better to take the valve handwheel and chest (generally 1.2 meters away from the operation floor), so that it is easier to open and close the valve.
Ground valve handwheel should be up, do not tilt, so as to avoid awkward operation. The valve of the wall machine depends on the equipment, but also to leave room for the operator to stand. To avoid the operation of the sky, especially acid and alkali, toxic media, otherwise very unsafe. Gate valve do not reverse (that is, hand wheel down), otherwise it will make the medium retained in the valve cover space for a long time, easy to corrosion of the stem, and for some process requirements taboo. It is extremely inconvenient to change the packing at the same time.
Open stem gate valves, do not install underground, otherwise damp will corrode exposed stem.
Lift check valve, installation to ensure that the disc vertical, in order to lift flexible.
Swing check valves should be installed with horizontal pin shaft for flexible swing.
The pressure relief valve should be installed in an upright position on a horizontal pipeline, and should not be tilted in any direction.
(2) Construction operations
Installation and construction must be careful, do not hit the brittle material made of the valve. Before installation, the valve should be inspected to check the specifications and identify whether there is any damage, especially for the stem. Also turn a few times to see if it is skewed, because in the process of transportation, ** easy to hit the valve stem. Also *** valve debris. When the valve is hoisted, the rope should not be tied to the handwheel or stem to avoid damage to these parts, but should be tied to the flange.
For the pipeline connected to the valve, be sure to clean. Compressed air can be used to blow away iron oxide, sand, welding slag and other debris. These sundries, not only easy to scratch the sealing surface of the valve, including large particles of sundries (such as welding slag), but also plugging the small valve, so that it fails.
Install screw valve, should be the sealing packing (thread and lead oil or ptfe raw material belt), package in the pipe thread, do not get to the valve, so as not to the valve memory product, affecting the flow of media.
When installing flanged valves, tighten bolts symmetrically and evenly. Valve flanges and pipe flanges must be parallel, and the clearance is reasonable to avoid excessive pressure or even cracking of the valve. For brittle materials and low strength of the valve, especially attention. The valve to be welded with the pipe should be spot-welded first, then fully open the closing parts, and then welded to death.
(3) Protection facilities
Some valves also require external protection, which is insulation and cooling. Heat tracing steam piping is sometimes added to the insulation layer. What kind of valve should be insulated or cold, according to the production requirements. In principle, where the valve medium to reduce the temperature too much, will affect the production efficiency or frozen valve, you need to keep heat, or even heat; Where the valve exposed, adverse to production or cause frost and other adverse phenomena, you need to protect cold. Insulation materials are asbestos, slag wool, glass wool, perlite, diatomite, vermiculite and so on; Keep cold material has cork, perlite, foam, plastic to wait. Water and steam valves that are not used for a long time must be released.
(4) By-passes and instruments
Some valves have bypasses and gauges in addition to the necessary protection. A bypass is installed to facilitate the maintenance of trap. Other valves are also installed by bypass. Bypass installation depends on valve condition, importance and production requirements.
(5) Replacement of filler
Stock valves, some packing is not good, and some do not match with the use of media, which needs to replace the packing. Valve manufacturers can not consider the use of thousands of units of different media, the stuffing box is always filled with ordinary packing, but when using, must let the filler and the medium to adapt.
When replacing the filler, press in round and round. Each ring joint is appropriate to 45 degrees, the ring and ring joint stagger 180 degrees. The height of packing should consider the room for further compression of the gland. At present, the lower part of the gland should be pressed to an appropriate depth of the packing chamber, which can generally be 10-20% of the total depth of the packing chamber.
For demanding valves, seam Angle is 30 degrees. The joints between the rings are staggered by 120 degrees. In addition to the above packing, but also according to the specific situation, the rubber O ring (natural rubber resistance to 60 degrees Celsius weak alkali, butanol rubber resistance to 80 degrees Celsius oil products, Fluorine rubber resistant to a variety of corrosive media below 150 degrees Celsius) three stacked polytetrafluoroethylene ring (resistant to strong corrosive media below 200 degrees Celsius) nylon bowl ring (resistant to ammonia, alkali below 120 degrees Celsius) and other forming filler. A polytetrafluoroethylene (PTFE) raw tape is wrapped outside the ordinary asbestos coil, which can improve the sealing effect and reduce the electrochemical corrosion of the stem.
When pressing the packing, turn the stem at the same time to keep it even around and prevent too much death. Tighten the gland evenly and not tilt.
There are several indexes to measure valve quality: sealing reliability, action response ability, strength, stiffness and life, etc. The valve is considered as the basic unit in the whole thermal equipment system, and there are fluid-structure coupling vibration and vibration control requirements. To ensure these indicators, the following major problems need to be solved first.
1 Control (determine the reliability of valve action)
The failure of the control system of the main steam valve and reheat steam valve is one of the five major steam turbine accidents, which is mainly manifested in the valve opening does not conform to the design, including the failure of transmission mechanism, advance of stroke and lag, which affect the strength and vibration of the valve. Valve opening control directly affects the working condition of steam engine, so it is highly valued and has become one of the most important problems in the research.
In recent years, in the study of valve reliability, intelligent valve is the main direction of research, intelligent valve has the function of self-judging working conditions, and real-time self-regulation. The key component of intelligent valve is digital positioner. Digital positioner uses microprocessor to position the valve actuator accurately, monitor and record the relevant data of the valve.
2 Strength (should meet the life and rigidity requirements)
The frequent start of the unit on the strength of the valve and the service life of the valve is particularly prominent, especially with the control valve of the steam turbine, the focus of the previous research on the valve control problem, now it seems that the strength of the problem can not be ignored. Carolann Giovando, deputy editor of Power Engineering magazine, writes that researchers should not focus exclusively on control problems, but on strength, life and sealing, which are essential for valve operation.
(1) Due to the frequent start-up of the unit, the original main steam valve may not meet the new operation requirements. Because the general main steam valve is designed according to the basic load, the design process only according to static pressure, temperature, creep assessment of its strength, there is no low cycle fatigue life problem. Now the working conditions change, the original design may not meet the requirements. Therefore, it is necessary to consider the low cycle fatigue life design in the design process, so that the design condition is consistent with the operation condition, in order to achieve the purpose of extending the life.
(2) Due to the inaccuracy of the actuator stroke control, the spool has impact load on the seat. There have been power plants seat fragmentation, fragmentation block was rushed into the turbine, resulting in a sharp decline in turbine output, rotor serious damage of the fault.
In addition, for high pressure valves, as well as cavitation phenomenon, original casting defects of valve body, valve body after crack life analysis and prediction are worth further study.
3 vibration
Valve opening changes, poor dynamic performance of the actuator and valve leakage are the cause of vibration, vibration damage to the valve itself is very small, but the impact on the whole unit is great, in the low-frequency oscillation.
The low-frequency oscillation of the unit is divided into two kinds: one is the oil film oscillation, which is produced by the oil film supporting the bearing in the unit’s speed up or no-load operation; The other is steam oscillation, which is more complex than oil film oscillation. It vibrates under the action of steam excitation force and often occurs after the unit is loaded. Valve opening change and leakage are important causes of steam oscillation. Data show that the United States and Germany have steam oscillation crash accidents, China has also happened 50 MW and 200 MW turbine crash accidents, because of the lack of real-time data records, so the cause of failure cannot be determined, but suspected to be related to two low-frequency oscillations. Thus, the elimination and reduction of steam oscillations is very important, which depends on a systematic study of valve opening changes and the excitation forces generated by leakage. The probability of steam oscillation can be reduced by properly designing the valve opening and closing stroke.
4 Leakage (internal leakage and external leakage)
(1) Leakage is not only the cause of vibration, but also causes pollution and energy loss. To solve the leakage problem, to a certain extent, the system can avoid vibration, but also prolong the life of equipment, improve efficiency.
(2) The life of the high-pressure valve of the supercritical unit is sometimes very short, and the packing must be replaced after several times of starting. It is necessary to study new sealing packing or design new effective sealing form to prolong the life and improve the operation reliability of this kind of high pressure valve. -
At present, the level of the complete set of valves continues to improve, only to solve the above problems well, in order to ensure the comprehensive performance of the valve and better overall quality.