Application Scope and Technical requirements of power station valves (2)

Application Scope and Technical requirements of power station valves (2)

Valve often meet the failure of the 10 practical tips, below we will say in detail.
1 Why should the cut-off valve be as hard sealed as possible?
Cut off the valve leakage requirements as low as possible, soft seal valve leakage is relatively low, cut off the effect of course, but not wear resistance, poor reliability. From the leakage and small, sealing and reliable double standard, soft seal cut off is better than hard seal cut off. Such as full-function ultra-light regulating valve, sealed and stacked with wear-resistant alloy protection, high reliability, leakage rate of 10-7, has been able to meet the requirements of the cut-off valve.
2. Why can’t the double seal valve be used as a cut-off valve?
The advantage of the two-seat valve spool is the force balance structure, allowing a large pressure difference, and its outstanding disadvantage is that the two sealing surfaces can not be good contact at the same time, resulting in large leakage. If it is artificially and forcibly used to cut off the occasion, obviously the effect is not good, even if it has made many improvements (such as double seal sleeve valve), it is not desirable.
3. Why is it easy to oscillate when the two-seat valve is small open?
For the single core, when the medium is flow open type, the valve stability is good; When the medium is flow closed, the stability of the valve is poor. The double seat valve has two spool, the lower spool is in the flow closed, the upper spool is in the flow open, so, in the small opening work, the flow closed type spool is easy to cause the vibration of the valve, this is the reason why the double seat valve can not be used for the small opening work.
4, what straight stroke regulating valve blocking performance is poor, Angle stroke valve blocking performance is good?
Straight stroke valve spool is vertical throttling, and the medium is horizontal flow into and out of the valve chamber flow channel must turn back, so that the valve flow path becomes quite complex (shape such as inverted “S” type). In this way, there are many dead zones, which provide space for the precipitation of the medium, and in the long run, cause blockage. The direction of the Angle stroke valve throttling is the horizontal direction, the medium flows in and out horizontally, and it is easy to take away the unclean medium. At the same time, the flow path is simple, and the medium precipitation space is very little, so the Angle stroke valve has good blocking performance.
5, why the straight stroke control valve stem is thinner?
Straight stroke regulating valve it involves a simple mechanical principle: large sliding friction, small rolling friction. Straight stroke valve stem up and down movement, packing slightly pressed a little, it will put the valve stem wrapped very tight, produce a large back difference. For this reason, the valve stem is designed to be very small, and the packing is commonly used with a small coefficient of friction PTFE packing, in order to reduce the backdifference, but the problem is that the valve stem is thin, easy to bend, and the packing life is short. To solve this problem, a better way is to use the travel valve stem, namely the Angle stroke type of regulating valve, its valve stem is 2 ~ 3 times thicker than the straight stroke valve stem, and the choice of long-life graphite filler, stem stiffness is good, packing life is long, the friction torque is small, small return difference.
6. Why is the cut off pressure difference of the Angle stroke valve large?
Angle stroke type valve cut off pressure difference is large, because the medium in the spool or valve plate resultant force on the rotation shaft torque is very small, therefore, it can withstand a large pressure difference.
7. Why did the sleeve valve replace the single and double seat valve but not achieve its goal?
The sleeve valve, which came out in the 1960s, was widely used at home and abroad in the 1970s. In the petrochemical plant introduced in the 1980s, the sleeve valve accounted for a larger ratio. At that time, many people believed that the sleeve valve could replace the single and double seat valve and become the second generation of products. Today, this is not the case, single seat valve, double seat valve, sleeve valve are used equally. This is because the sleeve valve only improves the throttling form, stability and maintenance better than the single seat valve, but its weight, blocking and leakage indicators are consistent with the single and double seat valve, how can it replace the single and double seat valve? So, it has to be shared.
8. Why is the service life of the desalting water medium lined with rubber butterfly valve and fluorine lined diaphragm valve short?
Desalting water medium contains low concentration of acid or alkali, they have a greater corrosion to rubber. The corrosion of rubber is characterized by expansion, aging and low strength. The use effect of butterfly valve and diaphragm valve lined with rubber is poor. The essence is that rubber is not corrosion resistant. After the rubber lining diaphragm valve is improved to the corrosion resistance of fluorine lined diaphragm valve, but the diaphragm of fluorine lined diaphragm valve can not stand up and down folding and broken, resulting in mechanical damage, the life of the valve is shorter. Now the better way is to use water to treat the ball valve, it can be used for 5 to 8 years.
9, why in the pneumatic valve piston actuator use will be more and more?
For pneumatic valve, the piston actuator can make full use of the air source pressure, the size of the actuator is smaller than the film, the thrust is greater, the O-ring in the piston is more reliable than the film, so it will be used more and more.
10. Why is selection more important than calculation?
Calculation and selection compared, selection is much more important, much more complex. Because the calculation is just a simple formula calculation, it does not depend on the accuracy of the formula itself, but on the accuracy of the given process parameters. The selection involves more content, a little careless, will lead to improper selection, not only cause the waste of manpower, material resources, financial resources, and the use of the effect is not ideal, bring a number of use problems, such as reliability, life, operation quality, etc.
Application scope and technical requirements of power station valves (II) The materials used for valves shall have material qualification certificates or relevant certificates: metal materials shall be marked with steel number, furnace number and batch number, and have chemical composition and mechanical properties certificates. When confined pieces of material sampling inspection results is a sample of a mechanical performance index is unqualified, should take double the amount of sample a second interview, if there is still a, this batch of parts should be heat treatment again, before the second-round exam methods such as heat treatment again the number of not more than two times (not including the number of tempering), * * * a second interview if there is still a sample, This batch of materials cannot be used.
Upper connection: Application range and technical requirements of power station valves (1)
7 Inspection and test
7.1 Material Inspection
7.1.1 Materials used for valves shall have material qualification certificates or relevant certificates: metal materials shall be marked with steel number, furnace number and batch number, and shall have chemical composition and mechanical properties certificates.
7.1.2 The materials of bearing parts shall be sampled before storage. The chemical composition shall be sampled according to the melting furnace, and the mechanical properties shall be sampled according to the batch of heat treatment. The test results shall meet the provisions of the corresponding material standards.
7.1.3 when confined pieces of material sampling inspection results is a sample of a mechanical performance index is unqualified, should take double the amount of sample a second interview, if there is still a, this batch of parts should be heat treatment again, before the second-round exam methods such as heat treatment again the number of not more than two times (not including the number of tempering), * * * a second interview if there is still a sample, This batch of materials cannot be used. When the chemical composition index of a sample is unqualified but the mechanical property index of the sample is qualified in the sampling inspection results, the disposal measures shall be decided according to the specific situation or the provisions of the material purchase contract.
7.2 Appearance quality inspection
7.2.1 The appearance quality of valve casting steel parts shall conform to JB/T 7927-1999.
7.2.2 The dimensional tolerance of the casting shall conform to the provisions of GB/T 6414-1999, but the wall thickness of the bearing part of the casting shall not have negative deviation: the casting riser shall be removed according to the prescribed gas cutting process, and the residual height after removal shall not exceed the provisions in Table 1.
Table 1 Residual height of cast steel after casting riser removal Unit mm
7.2.3 The pouring riser can be smoothed by mechanical machining. When it is at the intersection of circular arcs at the circulation position, it can be polished by grinding wheel and smoothly transition with the body surface. After eliminating the casting riser, succulent and core sand, heat treatment should be carried out according to the process. After heat treatment, sand blasting should be done to eliminate oxidized skin, sticky sand and burr.
7.2.4 Inlays (cold iron, core support, etc.) are not allowed in cast steel bearing parts.
7.2.5 The welding groove of the valve body, the welding position of the valve seat, the contact position between the valve body and the white seal ring, and the connection position with the screw thread surface of the valve body are not allowed to be defective.
7.2.6 Steel castings shall have no defects such as pores, shrinkage holes, shrinkage porosity, sand and cracks.
7.2.7 The outer surface of the forgings is not allowed to have cracks, folds, forging wounds, marks, slag inclusion and other defects. For the surface to be processed, such as the above defects but not completely removed after processing, only after the approval of the technical department, it is allowed to use.
7.3 Ray Detection
7.3.1 Detection Parts
7.3.1.1 Ray inspection shall be carried out on the body groove of steel castings welded to pipelines meeting any of the following conditions. The range of penetration is 1.5T ~50mm from the end face of the groove, and the two values are small, as shown in FIG. 1
A) Pipes with an outer diameter greater than 426mm(water pipe greater than 273mm) and a wall thickness greater than 20mm;
B) Pipes with wall thickness greater than 40mm(water pipe greater than 30mm) and outside diameter greater than 159mm.
1 – the body; 2 – pipe.
DW – outside diameter of pipe; T — wall thickness of pipe connected to valve.
FIG. 1 Range of penetration
7.3.1.2 Butt weld of the valve.
7.3.1.3 Repair parts to be inspected by ray after welding.
7.3.2 Detection timing, method and acceptance standard
7.3.2.1 The ray detection of the groove is generally carried out before the groove processing.
7.3.2.2 The X-ray inspection method of valve groove and repair welding part of cast steel shall comply with the provisions of Grade A in GB/T 5677-1985. Valve butt welds shall be radiographed in accordance with GB/T 3323-1987 Class AB.
7.3.2.3 Valve groove and repair welding parts of cast steel parts shall be evaluated according to GB/T 5677-1985 and qualified in the third level. Valve butt welds shall be evaluated according to GB/T 3323-1987, Grade 2 qualified.
7.4 Magnetic particle or permeation detection
7.4.1 Detection parts
7.4.1.1 Parting surface, casting riser, stress concentration, intersection of different surfaces and parts with doubt on quality of alloy steel valve body.
7.4.1.2 Groove surface of alloy steel cast steel valve body.
7.4.1.3 Fillet weld on the bearing part of the valve.
7.4.1.4 Parts of shell and other parts that need magnetic powder or penetration inspection after welding.
7.4.1.5 Surfacing sealing face of steam valve with nominal pressure PN≥MPa or working temperature T ≥450℃. The number of samples tested in each batch of valves is:
A) For DN≥50mm, it shall be 100% of the total number of valves in this batch
B) DN < 50mm, not less than 5% of the batch of valves, but not less than one. In the *** sampling test, if there is one unqualified, double should be taken for the second sampling test; If there are still unqualified products in the second sampling test, each product shall be tested one by one.
7.4.2 Test timing, method and acceptance standard
7.4.2.1 For parts to be machised, magnetic particle or penetration inspection shall be carried out after the final machining.
7.4.2.2 Magnetic particle detection method shall comply with the relevant provisions of GB/T 9444-1988. The penetration test method shall comply with the relevant provisions of GB/T 9443-1988.
7.4.2.3 Parts requiring magnetic powder or penetration testing and sealing surface of the valve shall be evaluated and accepted according to the corresponding standards stipulated in 7.4.2.2 of this standard, and the third level shall be qualified.
7.5 Assembly and performance inspection
7.5.1 All parts of the valve shall be inspected by the quality inspection department before assembly, and unqualified parts shall not be assembled. Alloy steel parts shall be 100% spectrally checked and marked to ensure that they are not confused with parts of other materials.
7.5.2 The sealing surface shall be guaranteed to have enough hardness according to the design drawing or see Appendix D. The sealing surface after grinding is not allowed to have cracks, depressions, pores, spots, scratches, nicks and other defects. The sealing surface should ensure that the radial anastomosis is not less than 80%