【 Weak current College 】 on gate fault analysis and improvement measures

1. Overview
The gate valve to the pressure, and temperature (high low temperature), corrosion-resistant (multiple material configuration), have two-way sealed function, resistance to wear and a long life and other advantages are becoming more and more widely applied to various industrial system, implementation of the open and close the media, to the system's stability and control has played an important role.
2. classification
Domestic common standards rising stem gate gate forms category (new structure not included), its main Z40, Z41, Z44, Z42 and structure (Figure 1). User selection should be based on the principle of operating requirements, refer to select the appropriate model. And in the installation, commissioning and maintenance specifications for operation in order to avoid or eliminate accidents.
(A) Z40-wedge plate gate valve (b) parallel single Z41-gate (c) Z41-wedge type single gate (d) Z42-wedge type double gate (e) Z44-parallel double gate
3. common accident
3.1, valve stem breakage or deformation
(1) fracture
Stem breakage occurs in the upper and lower thread root, minimum cross-sectional area here, prone to the phenomenon of stress concentration and excessive. Especially when working conditions more significant deviations from the design parameters. If a power plant repeatedly occurred after DN175 electric valve opening of trapezoid threaded stem neck Office pull off by accident. The survey found, valve bonnet tightening nut loose, cover up, stem nut stuck, it is clear that this is Denso travel debugging too old, too much protection torque caused by the accident.
Another type of valve stem break accident occurs in the opening moments. Manifested in the gate has not been detached from the seat, the valve stem is up or down the thread root fracture. The reason is usually considered to be the RAM jam, this is really just a minor part of the reason or reasons. One important reason is that the valve in the closed exceptionstep-up, i.e. the valve closed, closed to the upper and lower sides of the seal between the cavity fluid pressure is far higher than the upper reaches pressure.
To produce this phenomenon for two reasons. Its a fluid is upstream in the cavity fluid heating occur, resulting in a dramatic expansion of pressure rise. The second gate close closed at the moment fluid to drain in the cavity, the cavity space is further squeeze on stem, compressibility of fluid is very limited, will make the pressure surge, this phenomenon is particularly prone to the power plant's main water valve Z962 class. At the same time, it will boost the general exception into a linear increase, far more than the stem strength design limit.
(2) deformation
Stem bent General appears in electric valve Denso debugging not at that time, such as closing torque too big and not set trip protection or disorders, its valve of destruction is very large.
3.2, leak
(1) leakage
Valve housing generally used casting processing. Although forgings can replace castings for high temperature and high pressure conditions, but the process or cost prohibitive, large-sized shells are generally used casting manufacturing. Due to the presence of porosity of castings, slag, cracks, porosity and loose a large number of defects, these defects in the operation of the valve may be exposed to a threat to the safety operation of risk. A 2007 statistics show that the valve leakage incidents, 37% for shell casting defects. The rest of the stem, the flange and flange seal failure.
The seal damage will also result in significant leakage system media. If the stem seal Department leakage by packing pressing plate of living Festival bolt clamp is not timely, caused by the event often results in serious accident. Another reason is the stem surface due process or inappropriate material chosen was medium corrosion or packing unqualified corrode stem seal surface. The valve stem and games from its surface corrosion Department brings out the filler particles, repeated appears packing space leak, this time as improper maintenance, maintenance of high pressure or high speed media in a very short time to fill out that form the leak. Similarly, to graphite stainless steel wire winding materials for seals self sealing valve and pressure to stainless steel with graphite gaskets in flange seal valve should prevent such situations from occurring. Another reason is also often cause significant leakage of media, namely packing gland and stem excessive clearance between the valve stem movement up and down, because of the gap is too large with repeated friction packing, micro-dynasty eventually led packing pressing pressure drop, causing the leak.
(2) internal leakage
Cause valve leak causes many, the new valve operation soon occur leak and off are not in place, should be sealed surface inclusions. Therefore a new installation of valve products pre-operational check carefully washed tubes can mitigate this type of accidents.
3.3, the floodgate Board failure
(1) jam
RAM jam manifested in the gate card in the body cavity of the Rails side, even with the stem detachment, largely due to the design and manufacture of bad trigger, such as rail contact width too short or too long, the rail surface roughness, etc. When RAM jam in the seat and the valve stem is forced in, on, often the damper plate t-slot fracture or deformation, avoid "exception boost" factors, temperature differential and closing force two major reasons why it is worth noting. Typical temperature condition such as a valve in the cold state close hot power, because the valve stem elongation under heat expansion so that the gate to further compact, increase the gate closing torque, leading to the gate wedge. If the thermal shutdown, when cold, because of the open side seat thermal deformation-induced crotch width larger, smaller cold rules shrinkage, thus causing the gate wedge. The above two States such as with "closingforce" too large, that is, off too tightly, as Denso driven debugging improper or inappropriate use of force as a wrench or lever, the floodgate Board to increase the chances of wedge live.
(2) bump crack
Gate upas (or bottom deformation) mostly in Z944/Z964 parallel-plate products, the floodgate Board bottom directly impact occurs, many at the bottom of the valve body due to improper electric valve Denso, close travel debugging does not limit or the limit is extremely prone to malfunction.
(3) of the sealing surface crack
Seal the surface cracks were observed in the alloy material valve, often because of unreasonable result. Reasonable selection and reasonable control of the welding parameters can be avoided.
4. analysis and strategy
4.1, design and manufacturing
① in national standardOn the gate without opening marking requirements, common valve can only be judged by hand, the open degree in many occasions is reasonable. Recommendations stem tip design limit device, to prevent excessive valve closed.
② stem strength more reduced, lower load capacity. If the threaded neck roots, etc., these parts in the cavity anomalies often among the first fracture pressure. Recommended DN350 above electric valve stem diameter should be appropriate to increase, while processing Assembly should raise bracket and axiality and reasonable choice of electric equipment of torque values.
③ existing gate national standards in the cavity anomalies boost phenomenon is not involved, the design is not made within the bypass or external bypass requirements, leading to boost cannot drain, the use of accidents often occur. Recommended temperature difference is large, with flat plate valve suitable.
④ use aluminum bronze manufacturing stem nut, when Denso speed faster, or more frequently when you open and close quickly polished surface, cause the thread to the valve. When the appropriate proposed design has increased the number of teeth, efficient lubrication, and regular maintenance replacement. For important valve should adopt reasonable packing seal combination, important tie surface gaps (e.g. self sealing valve of body and valve are covered with gap, valve stem and the packing pressing and covering with gap, packing gland and valve are covered with gap, etc.) and location of the tolerance (e.g. valve cover and stand up and concentricity, flange screw position, etc.) should be strictly controlled.
⑤ to present technological level, to recommend the critical high-temperature and high-pressure valve shell adopts all forged or forging welding structure manufacturing. Valve parts and materials should be strictly in accordance with the relevant criteria for selection, testing and treatment.
⑥ guides rapidly processing should be removed, both retaining full joint surface to ensure serious offset gate will not be separated from the rail.
⑦ suitable seal the surface material and the hardness of the match, machining guarantee roughness, flatness and anastomosis degree consistent with the requirements. The stem surface treatment to meet the design requirements that prevent its surface rust, corrosion, or coating peeling early.
4.2, installation and debugging
① recommended in important high pressure downstream of large diameter valves increased bypass device to reduce on-off pressure differential, ease off instant gate on the sealing surface abrasion, at the same time to decrease the Denso torque, lower manufacturing costs.
② in the installation safety valve is a valid control exception boost, you can protect the valve open and close parts, whilst maintaining the safe operation of the system. In addition in the upstream side and in the cavity between the installation outside the bypass or gate River a lateral pressure relief holes are effective and feasible means.
③ pipeline flush thoroughly, so that the tube impurities (such as iron, welding slag, welding head, nut, etc) rolling wound sealing surfaces.
④ recommended gate valve stem vertical tilt up installation, the installation will increase the chance of gate stuck. Horizontal installation should be mounting bracket support Denso, and correction of the stem at the time of installation.
⑤ Electric valve installation debugging should be professionals seriously as required. Opening of the tour is too big or too high to protect torque is pull off the stem or stem nut jammed and so on. Right hand rotation for the open position, with torque protection, general protection torque control in 20% to 30%. Close the trip is too large, the torque of consequences is the gate in the seat, and wedge live excessive torque may lead to pull off the valve, the brake plate t-slot fracture, stem nut jammed and so on.
The correct approach is to use the hand wheel set off on the right after the Gate 1 ~ 3mm (mainly on high pressure valve), this avoids the exception in the opening moments of the step-up result is too large. Flat gate valve closes the trip too large will often (or deformation) gate. Debugging add decided its close location to travel, and do not need too much off torque.
4.3, operation and maintenance
① recommended torque is greater than m of 400N · valve should use force amplifier design, such as gears, striking hand wheel and belt bearing stem nut, etc. DENSO selection should be set aside 30% to 50% torque margin, does not recommend the use of pneumatic-wedge gate valves.
② the valve seat sealing design run for a period of time should be preloaded bolts fastening, prevent the pressure wave caused when the cover down, washed out media leaking seals. In response to the packing pressing bolts and tighten the bolt in the flange pressure for normal maintenance, timely fastening. On the bearing body, various types of thread Department regularly lubricating, timely removal of dust and dirt. Periodic action test (important valve should always test the action performance and close, preventing rust jam, discover hidden),periodic maintenance (important valve should periodically check for and repair, maintenance of its sealed surface, driving mechanism, seals and other important key parts), the establishment of equipment maintenance records.