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Prevention Of Various Valve Leakage Problems
- Jun 01, 2018 -

The valve is usually composed of valve body, valve cover, packing, gasket, sealing surface, valve stem, transmission and other common parts. The following describes the causes of common failures of common parts and the methods for preventing and eliminating failures.

    First, the valve body and valve cover leakage

    Causes of leaks in the valve body and valve cover

    The manufacturing quality is not high, and there are defects such as trachoma, loose tissue, and clips on the body of the valve body and the valve cover. Iron castings are common;

Cold cracking in the sky, poor welding, defects such as clips, incomplete penetration, stress cracks, etc., cast iron valves are damaged by impact by heavy objects;

    Prevention and elimination methods:

    1. Improve the quality of castings and strictly test the strength before installation;

    2. For cast iron valves with temperatures below zero and below zero, valves that should be insulated or mixed and used to stop the use of hot water should exclude water. The welds of valve bodies and valve covers made of welded joints should be carried out in accordance with the relevant welding procedures. After welding, inspection inspections and strength tests shall be carried out;

    3. It is forbidden to stack heavy objects on the valve. It is not allowed to hit the cast iron and non-metal valves with the hand hammer. There should be brackets for the installation of large-diameter valves.

    Second, the leakage at the filling

    Outer leakage of the valve, the proportion of filling at the largest, resulting from:

    1, the wrong choice of filler, not resistant to media corrosion. High pressure or vacuum, high temperature or low temperature of the valve cannot be used;

    2. The installation of packing does not have defects such as small generation, spiral coiling, poor joints, and loosening and loosening. The packing material has exceeded the service life, and it has been aged and lost its elasticity. The accuracy of the valve bore is not high, and there is bending, corrosion, wear, etc. defect;

    3, the number of packing ring is not enough, the gland is not pressed, the bolts and other parts are damaged so that the gland cannot be pressed;

    4, improper operation, excessive force and so on;

    5. When the glands cover different shafts, the gap between the gland and the valve stem is too small or too large, causing the valve stem to wear and the packing to be damaged;

    Prevention and elimination methods:

    1. The material and type of packing should be selected according to working conditions;

    2, according to the relevant provisions of the correct installation. Shaking, packing should be placed on a circle by pressing, the joint should be 300 or 450;

    3, the use of long, aging, damaged filler should be replaced;

    4. The stem should be straightened and repaired after it is bent and worn. Serious damage should be replaced;

    5, packing should be installed in accordance with the provisions of the number of turns, the gland should be tightened symmetrically and evenly, the pressure sleeve should be pre-tightened gap more than 5 mm;

    6. Damage to glands, bolts, and live parts should be promptly repaired or replaced;

    7, should abide by the operating procedures, in addition to the impact of the hand wheel in the normal speed of operation with uniform speed;

    8, should be evenly and symmetrically tighten the gland bolt, the gap between the gland and the stem is too small, should increase its clearance, the gap between the gland and the stem is too large, should be replaced cover;

    Third, leakage at the gasket

    The reasons are:

    1. The gasket is not selected correctly, it is not resistant to the corrosion of the medium, it is not resistant to the use of high pressure or vacuum, high temperature or low humidity;

    2, the operation is not stable, causing valve pressure, temperature fluctuations, especially temperature fluctuations;

    3, the gasket pressing force is not enough or no pre-tightening gap at the connection;

    4, improper gasket assembly, uneven force;

    5, static sealing surface processing quality is not high, rough surface, uneven, horizontal scratches, seals are not parallel to each other and other defects;

    6, static sealing surface and gasket is not clean, mixed with foreign matter;

    Prevention and elimination methods:

    1. The material and plastic type of the gasket should be properly selected according to the working conditions;

    2, careful adjustment, smooth operation.

    3. Tighten the bolts evenly and symmetrically. If necessary, use a torque wrench. The pre-tightening force should meet the requirements and must not be too small or too large. Flange and threaded connections should have a certain pre-tightening clearance;

    4. The gasket assembly should be in right alignment and the force is uniform. The gaskets must not be overlapped and double gaskets must be used.

    5, static sealing surface corrosion, damage, processing quality is not high should be repaired, polished, coloring checks to make static;

    6. The cover meets the relevant requirements;

    7, when installing the gasket should pay attention to the use of clean kerosene seal surface cleaning, gasket should not fall;

    Fourth, the sealing surface leakage

    The valve sealing surface is the core of the valve. There are four types of leakage in the valve, namely the leakage of the sealing surface, the leakage of the sealing ring, the leakage of the closing element and the leakage of the embedded foreign material between the sealing surfaces.

    Leakage of the sealing surface for the following reasons:

    1, the sealing surface is uneven, can not form a seal line;

    2. The connection between the valve stem and the closing part is suspended, irregular or worn;

    3, the valve stem is bent or improperly assembled so that the closing member is skewed or missing;

    4. The sealing surface material is not properly selected or the valve is not selected according to working conditions. The sealing surface is prone to corrosion, erosion, and wear;

    5, surfacing and heat treatment is not according to the rules of operation, because the hardness is too low to produce wear, corrosion due to burning of alloying elements, cracks due to excessive internal stress generated defects;

    6, the surface treatment of the sealing surface peeling or excessive grinding, losing the original performance;

    7. The sealing surface is not closed tightly or the slits appear due to cold shrinkage after closing, resulting in middle erosion;

    8. The shut-off valve is used as a throttle valve and a pressure reducing valve. The sealing surface is damaged by erosion;

    9. The valve has reached the fully closed position and the excessive closing force is applied continuously. Including the incorrect use of long lever operation, the sealing surface is crushed and deformed.

    10, the sealing surface wear is too large and the phenomenon of dropped, that is, the seal can not be closely sealed;

    Prevention and elimination methods:

    1, when the sealing surface grinding, the grinding tools, abrasives, abrasive cloth sandpaper and other objects should be used to make a reasonable grinding method is correct, should be colored after grinding inspection seal surface should be no indentation, cracks, scratches and other defects;

    2. The connection between the valve stem and the closing part shall meet the design requirements. If the top core does not meet the requirements, it shall be trimmed. The top core shall have a certain clearance, especially the axial clearance between the stem shoulder and the closing part shall not be less than 2 Mm;

    3. Straightening of the valve stem should be straightened. After the stem, stem nut, closing member and valve seat are adjusted, they should be on a common axis;

    4. When selecting the valve or replacing the sealing surface, the working conditions should be met. After the sealing surface is processed, the corrosion resistance, resistance, and scratch resistance are better.

    5. The surfacing and heat treatment process shall meet the technical requirements of the regulations and specifications. The sealing surface shall undergo acceptance after processing and shall not have any defects that may affect its use;

    6. The surface of the sealing surface such as flaming, nitriding, penetrating shed, plating and other processes must be strictly in accordance with the technical requirements of its procedures and specifications. The penetration layer of the grinding sealing surface should not exceed one third of the layer to damage the plating and the permeable layer. Seriously, the coating and the permeation layer should be removed and resurfaced. The sealing surface of the high-frequency surface fire can be repeated fire repair;

    7. The valve should be marked with closed or open, and should be promptly repaired if it is not closed tightly. For high-temperature valves, some cracks that occur after closing and shrinking should be closed more than once after closing.

    8. Valves used as shut-off valves are not allowed to be used as throttle valves and pressure reducing valves. When closed parts are to be in fully open or fully closed positions, the flow and pressure of medium must be adjusted. Throttle valves and subtraction should be set separately. Pressure valve

    9. The front opening and closing of the valve shall be in accordance with the section “Operation of the valve”. The closing force of the valve is appropriate. The diameter of the hand wheel is less than 320 mm. Only one person is allowed to operate. A hand wheel equal to or greater than 320 mm in diameter allows two people to operate or one person. Leverage within 500 mm of lever operation;

    10. After the water line is dropped, it should be adjusted, and the sealing surface of the sealing surface that cannot be adjusted should be replaced;

    V. Leakage at the sealing ring connection

    The reasons are:

    1, the sealing ring is not strict;

    2, the seal ring and body welding, welding combination bad;

    3, sealing ring connection thread, screws, pressure ring loose;

    4, the sealing ring connection surface is eroded;

    Prevention and elimination methods:

    1. Leakage of the sealing ring should be filled with adhesive or pressure and then fixed;

    2. If the sealing ring is surfacing or the welding connection is poor, it shall be re-welded according to the welding specification. When the surfacing cannot be repaired, the original surfacing layer shall be removed and the surfacing and processing shall be re-established according to the specifications.

    3. The sealing ring connected with threads, screws and pressure rings shall be loose, shall be removed for cleaning, replace the damaged screws, press rings, and grind the sealing ring and the connection seat and reassemble. Repairing and damaging large rivers by welding, bonding, etc.;

    4, the sealing ring connection surface is corroded, available grinding, bonding, welding repair, can not be replaced when the seal ring;

    Six, closed parts off leak

    The reasons are:

    1, poor operation, so that the closure was stuck or exceeded the top dead point, the connection is broken;

    2, the closure is not firmly connected, loose and fall off;

    3, the choice of connection material is not right, can not withstand the corrosion of the media and mechanical wear;

    Prevention and elimination methods:

    1. To operate properly, close the valve can not be too much force, open the valve can not exceed the top dead center, after the valve is fully open, the handwheel should be reversed a little;

    2. The connection between the closing part and the valve stem should be correct, firm, and the connection part of the thread connection should have the retraction part;

    3. The fasteners used to connect the closing part to the valve stem shall withstand the corrosion of the medium, and have certain mechanical strength and wear resistance. Although the closure is detached, it is rare, but it is a very dangerous failure;

    7. Leakage of foreign objects embedded between sealing surfaces

    The reasons are:

    1. Insufficiently open or closed sealing surfaces can easily contaminate some dirt;

    2, the media is not clean, containing abrasive particles, rust, welding burrs, and even equipment and devices on the bolts and other parts, stuck in the sealing surface;

    3, the media itself has hard particles;

    Prevention and elimination methods:

    1. Infrequently opened and closed valves, when conditions allow, should close or open the valve frequently, leave a slit when closing, repeat several times, so that the sediment on the sealing surface is washed away by high-speed fluid, then, According to the original open and closed state restore;

    2. Before the valve should be installed sewage, filtration and other devices, or regularly open the valve plug. Do not forcibly close the mixing surface between the sealing surfaces, such as shovels, bolts, etc. Use the method of opening thin seams to wash away these foreign materials, and remove the bonnet from the dog's foreign bodies that are difficult to wash away with the medium;

    3. For mediums with hard pellets, it is generally not appropriate to use a valve, and valves that are made of soft materials such as plug valves, ball valves, and sealing surfaces should be used as much as possible.

    Eight, valve stem operation is not flexible

    Reasons for inflexible valve stem operation:

    1. The precision of the valve stem and its mating parts is low, the clearance is too small, and the smoothness is poor;

    2. The stems, stem nuts, brackets, glands, packings, etc. are not properly assembled and their axes are not on a straight line;

    3, packing pressure too tight, hold the valve stem;

    4, valve stem bending;

    5, trapezoidal thread is not clean, full of dirt and abrasive particles, poor lubrication conditions;

    6, nut loose trapezoidal thread slide wire;

    7. The sliding stem nut and the sliding part of the bracket have poor lubrication conditions. In the middle, the abrasive grains are mixed into the abrasive grains to cause them to wear or die, or they rust and die due to long-term operation.

    8, poor operation, the stem and related parts deformation, wear and damage;

    9, the valve stem and the transmission connection loose or damaged;

    10. The stem is dead or the closure is stuck;

    Prevention and elimination methods:

    1. Improve the machining accuracy and repair quality of the valve stem, stem nut and its fittings, and the clearance for mutual cooperation should be appropriate, and the smoothness should meet the requirements;

    2. When assembling the valve stem and its connecting parts, the assembly should be correct, the gap should be the same, the concentricity should be maintained, and the rotation should be flexible. Brackets, glands, etc. are not allowed to be skewed;

    3, after the packing pressure is too tight, should properly relax the cover, you can eliminate the packing hold the valve stem phenomenon;

    4, the valve stem should be corrected for bending, difficult to correct, should be replaced. When operating the valve stem, the closing force is appropriate and cannot be too large;

    5. The threads of the stem and stem nut should be cleaned and lubricated frequently. For high temperature valves, the vulcanized pin or graphite powder should be coated for lubrication.

    6, the valve stem nut should be repaired loose, can not be more than the stem nut and slide wire trapezoidal thread, should be replaced;

    7, all the year round so that the valve nut at the oil flow, sliding surface clean, good lubrication, infrequently operated valve, should check and activity on the valve stem, found that there is wear and die, should repair the stem nut, Brackets and other accessories;

    8, to properly operate the valve, the closing force should be appropriate, so as not to cause the valve stem and related components to produce deformation and damage, affect the normal operation of the valve stem;

    9. The stem and handwheel, handle and other transmissions are connected correctly and firmly, and they are found to be loose or worn and repaired in time;

    10. The correct operation of the valve is an effective measure to prevent the valve stem from dying and closing the device. If the valve part is easily heated and bulged after it is closed, the valve shall be unloaded once after the valve is closed for a certain period of time, that is, the counterclockwise direction of the hand wheel shall be reversed a little to prevent the valve stem from dying;

    Nine, the common failure of the drive device

    Damage to handwheels, handles and wrenches, causes:

    1. Damage to the handwheel, handle or wrench caused by the use of long bars, pipe wrenches or the use of impact tools;

    2. The fastener of hand wheel, handle or wrench is loose;

    3, hand wheel, handle and wrench and valve stem connector plus square hole, keyway or thread wear, can not transmit torque;

    Prevention and elimination methods:

    1. The correct use of handwheels, handles and wrenches, prohibit the use of long levers, pipe wrenches and impact tools;

    2. Fasteners that connect handwheels, handles, and spanners should be lost and damaged. For valves that are more vibrating, and where looseness is easily loosened, they should be replaced with flexible washers, etc.;

    3, hand wheel, handle and wrench joint wear should be repaired, repair more difficult, should be fixed or replaced by adhesive;

    10. Gears, worm gears and worm gears are not flexible

    The reasons are:

    1. Incorrect assembly of gears, worm gears and worm gears;

    2. The parts made of the transmission mechanism have low machining accuracy and poor smoothness;

    3, shaft and bushing (bearing parts> clearance is too small, poor lubrication, was worn or killed;

    4. The second gear is not clean, and the poorly toothed part is jammed by the foreign matter and the tooth part is worn out or the tooth shaft is bent;

    5, gear, worm gear and worm positioning screws, tight circle loose, key pin damage is not good;

    Prevention and elimination methods

    1. The assembly of gears, worm gears and worm gears is reasonable and the clearance is appropriate. Refer to the disassembly and assembly chapter of the valve;

    2. Improve the machining accuracy of the transmission mechanism components. Check the quality of the components before assembly.

    3, the vehicle by the appropriate clearance and bushing (bearing) clearance; Lubricating Road communication, regular fuel, the wear parts should be repaired; serious wear and tear should be replaced.