1, Introduction
Safety valve is a very important protective valves, widely used in a variety of pressure vessels and piping systems, when the pressure in the system pressure exceeds the set value, it can automatically open to the media excess emissions into the atmosphere and going to ensure the pressure vessel and the safe operation of pipeline systems, to prevent accidents from occurring, and when the pressure back down to work within the system pressure, or slightly lower than the pressure of work and can shut down automatically. The reliability of safety valves to work directly related to the equipment and the safety of persons must be given due importance.
2, safety valve Common Cause analysis and solution
2.1, valve leakage
In the device under normal operating pressure, valve flap and valve seat sealing surface took place more than permitted levels of leakage, valve leakage will not only cause loss of media. In addition, the media will continue to leak to the destruction of hard sealing material, but the common safety valve sealing surface is metal on metal materials, while striving to do clean formation, but to bring pressure in the medium under other circumstances to definitely do not drain very difficult. Therefore, the working fluid is a steam safety valve, the required pressure value, if the naked eye can not see in the export side, it can not find leak, they maintain that sealing performance is qualified. Usually caused by valve leakage due mainly to the following three cases:
One scenario is that dirt impurities fell sealing surface, the sealing surface pad to live, resulting in a gap between the spool and the valve seat to valve leakage. Way to eliminate this fault is to remove all the dirt fall to the sealing surface and impurities, the general size of the repair in the boiler ready to shutdown, the first security door to do to run ram tests, if found to have carried out the dissolution of leakage after shutdown maintenance, if it is point furnace after running ram the security gate leakage tests found that the estimated result of this is the case, you can run for 20 minutes before the cooling ram to run the rudder once, on the sealing surface to erosion.
Another situation is that the sealing surface damage. Sealing surface damage caused by the following main reasons: First, bad sealing surface material. For example, in 3 ~ 9, the main security door furnace due to years of maintenance, the main security gate valve core and valve seat sealing surface generally has been developed very low, so that the hardness of the sealing surface also greatly reduced, resulting in sealing performance degradation, eliminating the The best way is to the phenomenon of the original sealing surface turning down, and then re-surfacing by drawing processing, to improve the surface hardness of the sealing surface. Note that in the process is bound to ensure the process quality, such as the sealing surface cracks, defects such as trachoma must go on to the turning re-processing. The new processing spool valve seat must comply with drawing requirements. Currently in use YST103 common steel electrode surfacing processing spool sealing surface effect will be better. Second, repair of poor quality, spool valve seat grinding not meet quality standards, to eliminate this failure is based on degree of injury after the use of grinding or turning the sealing surface grinding method of repair.
Another reason causing valve leakage is due to improper assembly or the part size inappropriate. Spool valve seat in the assembly process is not completely right or in conjunction with a translucent surface phenomenon, or a spool valve seat sealing surface too broad and is not conducive to sealing. Elimination method is to check with the space around the spool size and uniformity to ensure the top hole and the sealing surface spool Far from degrees, check the gap does not allow lifting valve core ministries; according to drawings called for a suitable reducing the width of the sealing surface to achieve effective sealing .
2.2, valve leakage with face
Refers to the upper and lower surface of the valve between the combination of the leakage phenomenon, the main reason for this leakage has the following aspects: First, with surface force bolt tight enough or tight side, resulting in poor combination of surface sealing. Elimination method is to adjust bolt tight edge, according to a certain time in the tight bolts right angle to a tight manner, preferably around the edges tight while measuring gap, the bolt tight to bear the fixed date, and make joint surface around the gap unanimously. Second, the combination of surface toothed valve gasket does not meet the standards. For example, the toothed gasket slight radial grooves, parallel degree of difference, too sharp or too sloping tooth defects can cause seal failure. So that combination of surface valve leakage. In the overhaul of spare parts at the time of good quality, use of standard tooth profile gasket on the occurrence of this phenomenon can be avoided. Third, body surface flatness with poor or live in a hard pad of impurities caused by seal failure. As the body of the combination of poor surface flatness caused by the leakage of body surface with the elimination method is to re-grind the valve with the disintegration of surface until it meets the quality standards. As the impurities cushion seal failure caused by living in the valve assembly combination of carefully cleaning the surface to avoid falling into impurity.
2.3, impulse action emperor safety valve safety valve does not move
This phenomenon is often referred to as the main security door and refuse to move.
Tripping the main security door in the boiler is running against is a very large, the equipment is a major hidden danger seriously affect the safe operation of equipment, once in the operation of pressure vessels and piping in the medium pressure in excess of rating, the main security door no action, super-pressure operation of the equipment can easily result in equipment damage and major incidents.
In the analysis of the main security door refused to move because, first an analysis of the action principle of the main security door. Shown in Figure 1, when the pressure within the pressure vessel rose to the entire impulse pressure relief valve, safety valve impulse action, medium from the container through the main valve piston pipeline rushed indoors, in the Pistons will have a tiny room expansion Buck, if this time the piston chamber pressure of P1, the piston throttling area of Shs, this time acting on the piston on the f1 as follows:
f1 = P1 × Shs ... ... ... ... ... ... ... ... (1)
If the media inside the pressure vessels at this time the pressure for P2, spool area of Sfx, then the media at this time an upward force on the spool f2 as follows:
f2 = P2 × Shx ?..............( 2)
Usually a safety valve piston diameter than the large diameter spool, so equation (1) and type (2) Shs> Sfx? P1 ≈ P2
If the spring through the stem upward pull on the spool set f3 and to moving parts and the friction between the fixed parts (mainly between the piston and piston chamber friction) set fm, the action of the main security door pre-conditions: only acting on the piston slightly greater than the force acting on spool on the f1 to f2 and the spring force upward through the stem upward pull on the spool f3 and the moving parts and the friction between the fixed parts (mainly the piston and the the friction between the piston chamber) fm sum, that is: f1> f2 + f3 + fm to start when the main security door.
Through practice, the main security door refused to move primarily to the following three factors:
First, there is jamming the valve phenomena of moving parts.
This may be due to improper assembly, dirt and impurities mixed or parts corrosion; piston chamber surface finish is poor, surface damage, there are grooves hard points, such as deficiencies. This allows moving parts and fixed parts fm between the friction increases, other conditions remain unchanged under the f1 <f2 + f3 + fm Therefore, the main security doors refused to move.
For example, in 2001, before the No. 3 furnace overhaul of the main security door overheating ram test run, there have been the main security doors refused to move. Maintenance inspection found that when the disintegration of the piston there is plenty of rust and dirt and impurities, the piston in the piston can not exercise indoors, resulting in the main security doors refused to move.
Maintenance when piston ring and piston expansion chamber had rust treatment room on the piston grooves and other defects of the grinding, assembly before the piston interior wall evenly coated with lead powder, and strictly in sequence of the valve assembly. In the boiler water pressure tests, flushing of the pulse tube, and then the main security door and the impulse valve connected to the last point furnace overhaul safety valve when running ram test again as usual.
Second, is the main security door leaks a large quantity of the piston chamber. When the valve piston chamber large amount of gas leakage, the equation (1) the role of the f1 a smaller force on the piston, other conditions remain unchanged under the f1 <f2 + f3 + fm Therefore, the main security doors refused to move. Large amount of gas leakage caused by the piston chamber and the valve itself is the main reason for the air tightness and piston rings do not meet size requirements or piston ring wear and tear is too large not meet sealing requirements are.
For example, 3 ~ 9, keeper of the piston valve is a piston ring quality requirements of the edges and corners should be smooth, free state open space of not more than 14, assembled open space △ = 1 ~ 1.25, piston and piston chamber space B = 0.12 ~ 0.18, piston rings and piston chamber gap of S = 0.08 ~ 0.12, piston rings good contact with the piston chamber, the transmittance should be no larger than the circumference of a / 6. Interior of the piston requirement is that the interior of the groove depth of the piston shall not exceed 0.08 ~ 0.1mm, the ellipticity does not exceed 0.1mm, taper degree of not more than 0.1mm, should be smooth without scratches, but the disintegration of each furnace maintenance check found that the main Security door piston rings, piston and piston chamber maintenance procedures do not meet the requirements, the current general piston and piston chamber gap are S ≥ 0.20, and the defects in the surface of the piston chamber is more serious, seriously affected the piston chamber of the steam density nature, resulting in leakage of steam too large piston chamber.
Way to eliminate this defect are: piston interior surface treatment, to replace qualified piston and piston rings, where the impulse throttle safety device system, off a small throttle opening, increasing the access to the main security door into the piston chamber steam, in the conditions allow, under the impulse can also be a safety valve by increasing the stroke to increase access to the main security door into the piston chamber means to promote the main steam safety valve action.
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