Valve inspection and selection steps Valve partial failure causes and maintenance

Valve inspection and selection steps Valve partial failure causes and maintenance

Performance test: the basic performance of the valve includes strength, sealing, flow resistance, action and service life of five aspects. Valve products before the factory must be strength test and sealing performance test, for some particularly important valves, should be in batch samples for flow resistance, action and service life three aspects of performance test, for the safety valve to do open pressure, back pressure and displacement test; For the pressure reducing valve to do sensitivity test; For trap to do displacement test…
Valve inspection
The factory quality inspection of valve products should be carried out according to the relevant national standards and industry standards. If relevant certificate is required, please specify it when placing the order, and relevant fees will be charged according to the test content.
1 The factory inspection of valves usually includes the following three contents:
● Check whether the material, blank, machining and assembly of the valve meet the technical standards;
● Performance test: the basic performance of the valve includes strength, sealing, flow resistance, action and service life of five aspects. Valve products before the factory must be strength test and sealing performance test, for some particularly important valves, should be in batch samples for flow resistance, action and service life three aspects of performance test, for the safety valve to do open pressure, back pressure and displacement test; For the pressure reducing valve to do sensitivity test; For the trap to do displacement test;
● Inspection marks and identification spray paint, packaging and other aspects are in line with the relevant provisions, product certificate and product instructions and other technical documents are complete.
● Size check: control the size of the connecting end and the distance between the surfaces.
(Figure 1) Integrated ball valve side mounting type
Introduction of important inspection items in manufacturing process
● Chemical composition
Before casting, the composition of each furnace is analyzed by spectral analyzer, and the composition is qualified before casting
● Metallography, mechanical properties parameters, hardness
◆ Heat treatment (austenitic stainless steel CF8, CF8M, CF3M and other solid solution treatment; After normalizing the carbon steel), metallographic analysis is carried out and metallographic photographs are left. Unqualified not turn order
◆ When casting, each furnace has 2 standard test bars and 2 test pieces (with the same furnace products have the same furnace number to trace control), after heat treatment –
(Figure 2) two-piece ball valve
① One of the test rods was taken to do tensile test with tensile testing machine to obtain mechanical properties related parameters: tensile strength, yield strength, elongation, area reduction
② One of the specimens was tested by Brinell hardness tester to obtain the hardness HB value; If necessary, cut into impact test pieces with impact testing machine impact test, to get the impact value
(3) The remaining 1 test rod and 1 test block for reserve, together with the test has been destroyed 1 test rod and test block and the furnace material analysis test block bundled together, stored in the test rod storage rack for two years
(Figure 3) integrated ball valve top mounting type
● Electrostatic test
After valve assembly and before pressure test, in dry state, use a universal meter to test the resistance of 12 VDC, according to API608 resistance ≤10 ohms (Note: High-speed through the fluid inside the pipeline, friction is easy to produce static electricity, because for electrical insulation, soft seat, such as PTFE pipe insulation ball valve and valve body, cause local electrostatic rise or concentrated, easy to produce risk in case of spark, so must have design of conductive antistatic concentration device, API608 rules between the valve stem and valve body of conductor resistance should be less than 10 Ω)
Low torque value
After 6Kg/cm2 air pressure test of the valve, the torque value of the valve is obtained by torsion meter in clean and oil-free state
● Life test
Every new product developed, or valve body, valve cover, valve seat, valve stem and packing box structural size design changes, or valve seat, packing material changes, will use life testing machine to do life test
Valve selection steps
● The characteristics and main functions of the valve are summarized in Table 11 and Table 12
● Nominal diameter or flow – refer to the manufacturer’s catalogue to select the appropriate valve diameter
● Rated pressure — temperature — Refer to Table 3: Rated pressure of common steel valve — temperature table
● Valve terminal form — see the previous section
(FIG. 4) Three-piece ball valve
● Valve structure material — corrosion resistance, temperature. Refer to Table 4: temperature limit for valve housing material selection; Table 5: Temperature limit of special fittings of valves; Table 6: corrosion resistance table of metal materials; Table 7: List of material erosion resistance Table 8: Common soft seat materials applicable temperature
● Valve cover form — lock teeth combination type; Bolted type; Circumferential welding type; Pressure seal; Arbitrary combination
● Special structure requirements — according to the use of temperature, different places, and special requirements
■ Fire prevention and anti-static design. Special attention must be paid to the design and use of ball valves
■ Extended bonnet design. Used in refrigerating valve for conveying liquefied gas
■ Noise and cavitation limitation. Especially for the design and use of control valves
■ Reference design of expansion bag against packing leakage. ● Operation mode — see several described in section 1.1 above. Generally limited to installation environment, operation, operation conditions or times, and electric, electric drive device consideration; But because of the economy and durability of hand wheel or gear reducer, it is still used by most people.
Table 11 General valve characteristics
Causes and maintenance of partial valve failure
Packing leak
The cause of the problem
● Filler selection is not correct, and the corrosive medium, temperature, pressure does not adapt.
● Packing installation is not correct, especially the whole packing in reserve rotation, easy to produce leakage.
● Filler beyond the use period, has been aging, loss of elasticity.
● Insufficient number of packing rings.
● Stem processing accuracy or surface finish is not enough, or ellipticity, or notch.
● Improper operation, excessive force.
Maintenance methods
● Filler material and type should be selected according to working conditions.
● Shake, the root of the coil should be placed and pressed round by round, the joint should be 30 or 45.
● Aging and damaged packing should be replaced in time.
● Filler should be installed according to the specified number of turns.
● Abide by the operation rules, except the impact type handwheel, with uniform speed and normal force operation.
● Gland bolts should be tightened evenly and symmetrically.
Leakage at gasket
why
● Gasket is not resistant to corrosion, high pressure, vacuum, high or low temperature.
● Operation is not smooth, causing valve pressure, temperature fluctuation.
● Gasket compression force is not enough.
● Improper assembly of gasket, uneven force.
● Gasket surface rough, mixed with foreign matter.
Maintenance methods
● Gasket material should be selected according to working conditions.
● Carefully adjust, smooth operation.
● Bolts should be tightened evenly and symmetrically.
● Gasket assembly should be uniform force, gasket is not allowed to lap and use double gasket.
● Pay attention to cleaning when installing gaskets, and clean the sealing surface with kerosene.
Leakage at the sealing surface
why
● The sealing surface is uneven and can not form a tight line.
● Stem and closing parts of the connection center hanging, straight or wear.
● Stem bending or assembly is not correct, so that the closing parts skewed.
● Improper selection of sealing surface material, sealing surface corrosion, erosion, wear.
● Surfacing and heat treatment not according to the rules of operation, wear, corrosion, cracks, etc.
● The sealing surface peeling off.
Maintenance methods
● Sealing surface grinding, grinding tools, abrasive agent selection is reasonable, after grinding coloring inspection, sealing surface without indentation, cracks, scratches and other defects.
● The top center of the connection between the stem and the closing part does not meet the requirements, should be trimmed, the top center should have a certain activity clearance, the axial clearance between the stem shoulder and the closing part is not less than 2 mm.
● Straighten and bend stem, adjust stem, stem nut, closing parts, seat on a common axis.
● Selection of corrosion resistance, abrasion resistance and other performance in line with the working conditions of the sealing surface.
● High temperature valve, after closing the cold shrinkage appears fine seam, after closing the interval of a certain time to close again.
● To cut off the valve valve, not allowed to use throttle valve, reducing valve, closing parts should be fully open or fully closed position, if the need to adjust the medium flow and pressure, should be set separately throttle valve and reducing valve.
● The sealing surface that cannot be adjusted should be replaced in time.
Closure piece off
why
● Bad operation, so that the closing parts stuck or more than the top dead point, joint damage fracture.
● The closing parts are not firmly connected, loose and fall off.
● The connection material is not right, can not withstand the corrosion of the medium and mechanical wear.
Maintenance methods
● To operate correctly, close the valve can not be too hard, open the valve can not exceed the top dead point, the valve fully open, handwheel should be reversed a little.
● The connection between the closing parts and the stem should be correct and firm, and there should be no return parts at the thread connection.
● The fasteners connecting the closing parts and the valve stem should withstand the corrosion of the medium, and have a certain mechanical strength and wear resistance, the closing parts fall off although rare, but it is very dangerous fault.
Stem is not flexible
why
● The valve stem and its matching parts have low machining accuracy and too small clearance.
● The axes of stem, stem nut, bracket, gland and packing are not in a straight line.
● Packing is too tight.
● Stem bent and damaged.
● Thread is not clean or rusty, poor lubrication conditions.
● Nuts loose, thread slip wire.
● The connection between valve stem and transmission device is loose or damaged.
Maintenance methods
● Improve the processing accuracy and repair quality of stem and stem nut, so that the clearance should be appropriate.
● Assembly stem and fittings, clearance is consistent, concentric, flexible rotation.
● Packing is too tight, relax the gland properly.
● Stem bending should be corrected, difficult to correct, should be replaced. Operate stem with proper closing force.
● Stem, stem nut threads should often clean and add lubricating oil, for high temperature valves, should be coated with disulfide pin or graphite powder for lubrication.
● Stem nut loose should be repaired, can not be repaired in time to replace.
● Make the nut oil smooth, good lubrication, do not often operate the valve, regular check and activity of the stem, found wear and bite phenomenon, timely repair stem nut, bracket and other accessories.
● To operate the valve correctly, closing force should be appropriate to avoid stem deformation and damage.
● After closing, when the valve is heated and extended, after the valve is closed, at a certain interval, turn the handwheel clockwise a little to prevent the stem from being killed. ,
Leakage of body and bonnet
why
● The valve body has sand hole or crack.
● Valve body tensile crack during repair welding.
Maintenance methods
● The suspected crack place will be polished, with 4% nitric acid solution etching, such as cracks can be shown.
● Excavate or replace the crack.