A brief analysis of common defects and evaluation standards of valve appearance quality inspection

A brief analysis of common defects and evaluation standards of valve appearance quality inspection

Torque is the force that causes an object to turn. Engine torque is the torque the engine outputs from the crankshaft end. Under the condition of fixed power, it is inversely proportional to the engine speed. The faster the speed, the smaller the torque, and the larger the torque, which reflects the load capacity of the car in a certain range.
Noun explanation: torque
Torque is the force that causes an object to turn. Engine torque is the torque the engine outputs from the crankshaft end. Under the condition of fixed power, it is inversely proportional to the engine speed. The faster the speed, the smaller the torque, and the larger the torque, which reflects the load capacity of the car in a certain range.
What is the method of valve torque calculation? Valve torque is an important parameter of the valve, so many friends are very concerned about the valve torque calculation. Below, the world factory pump valve network for you to introduce the valve torque calculation in detail.
Valve torque calculation is as follows: Half the valve diameter x 3.14 square is the area of the valve plate, multiplied by the bearing pressure (that is, the pressure valve work) draw a shaft on the static pressure, multiplied by the coefficient of friction (check table of general steel friction coefficient 0.1, steel for rubber friction coefficient 0.15), the number of times the diameter of axle divided by 1000 for a quick valve torque, unit for cattle, meters, The reference safety value of electric devices and pneumatic actuators is 1.5 times of the valve torque.
When the valve is designed, the selection of the actuator is estimated, which is basically divided into three parts:
1. Friction torque of seals (sphere and valve seat)
2. Friction torque of packing on valve stem
3. Friction torque of bearing on valve stem
Therefore, the calculated pressure is usually 0.6 times the nominal pressure (about working pressure), and the friction coefficient is determined according to the material. The calculated torque is multiplied by 1.3~1.5 times to select the actuator.
Valve torque calculation should take into account the friction between the valve plate and the seat, the friction between the valve shaft and the packing, and the thrust of the valve plate under different pressure differences.
Because there are so many TYPES of disc, seat and packing, each with a different friction force, the size of the contact surface, the degree of compression, and so on. Therefore, it is generally measured by instrument rather than calculated.
The calculated value of valve torque is of great reference value, but it cannot be completely copied. Under the influence of many factors, the valve torque calculation is not more accurate than the experimental results.
Common Defects and Evaluation Standards for inspection of valve appearance quality Due to the inconsistency of product manufacturing, quality inspection and on-site acceptance standards, each standard has different judgment principles for defects, and sometimes there will be different inspection conclusions. For example, the forging valve product standard GB/T 1228-2006 allows defects within the limit size of 5% or 1.5mm, and the casting valve product standard JB/T 7927-2014 allows two examples of defects in A and B. According to the field acceptance standard SY/T 4102-2013, the outside surface of the valve shall not have cracks, trachholes, heavy skin, spots, mechanical damage, rust, missing parts and nameplates
Due to the inconsistency of product manufacturing, quality inspection and on-site acceptance standards, the determination principles of defects in each standard are different, and sometimes different inspection conclusions will appear. For example, the forging valve product standard GB/T 1228-2006 allows defects within the limit size of 5% or 1.5mm, and the casting valve product standard JB/T 7927-2014 allows two examples of defects in A and B. The valve field acceptance standard SY/T 4102-2013 stipulates that the outside surface of the valve shall not have cracks, trachholes, heavy skin, spots, mechanical damage, rust, missing parts, nameplates and paint peeling, etc. The valve quality inspection standard SH 3515-2013 stipulates that when the valve body is cast, its surface should be smooth, without cracks, shrinkage holes, tracholes, pores, burrs and other defects; when the valve body is forged, its surface should be free of cracks, interlayers, heavy leather, spots, lack of shoulder and other defects.
Oil and natural gas is flammable, explosive, and corrosive. In addition to strictly implementing the entrusted standard SH3518-2013, the valve quality inspection should also refer to the field acceptance specification of the valve and the manufacturing level of the valve. While recommending and selecting the supplier manufacturers, strengthening the factory inspection, the valve quality inspection should be based on the defect position, size and shape. And the valve working pressure, working medium, the use of the environment for comprehensive assessment, not only to ensure product quality, but also to do justice, fairness.
Appearance defect assessment
In 2014, a total of 170284 valves of various types were tested by Changqing Oilfield Technology Monitoring Center, and 5622 valves were unqualified, with an unqualified rate of 3.30%, among which 2817 valves were unqualified in appearance quality inspection, accounting for 50.11% of the total number of unqualified valves. The main trachoma, pores, cracks, mechanical damage, shrinkage, marks and body wall thickness unqualified structure and size.
1. Appearance characteristics
The main reason is that the end of the stem is not processed, the stem and the handwheel cannot be closely combined, the valve is not flexible to open and close, or the thickness of the valve wall, the diameter of the stem and the length of the structure do not meet the standard requirements. The length of the Z41H-25 DN50 gate valve is 230mm according to the standard, and the measured length is 178mm.
2. Inspection method
The valve structure can be inspected by visual inspection. The wall thickness of the valve body is generally measured by ultrasonic thickness meter, and the length of the structure is generally measured by vernier calipers, tape measures, depth rulers and other tools and instruments. The measured part should be polished smooth when the wall thickness is measured, so as not to affect the accuracy of the test. Body small wall thickness generally appears on both sides of the flow passage or the bottom of the body.
3. Defect assessment
Valves WITH nonconforming VALVE STRUCTURE, body wall thickness, length OF structure, AND STEM diameter are directly deemed NONconforming.
Trachoma and stoma
Shrinkage and porosity
1. Appearance characteristics
Shrinkage and porosity are generally located in the solidified part of the casting valve (hot joint) or the structural mutation part. Shrinkage and loose inner surface without oxidation color, irregular shape, rough pore wall accompanied by many impurities and small pores.
2. Inspection method
Shrinkage and loose appearance are not easy to find, and leakage generally occurs in the process of pressure test. During the test, attention should be paid to the shrinkage parts of the pouring mouth, riser and valve body of the valve. After the test, the above parts should be touched by hand to prevent defects from being missed due to paint cover-up.
3. Defect assessment
Shrinkage is easy to cause discontinuity of valve structure, shrinkage or loose should be judged as unqualified diameter.
The crack
1. Appearance characteristics
The crack generally appears in the hot joint part of the two walls of the forging valve body and the structural mutation part, such as the flange root and the convex surface of the outer wall of the valve body. The depth of the crack is shallow, generally based on hair lines. The shape of the hot crack is tortuous and irregular, the gap is wide, the cross section is seriously oxidized, and the crack is not metallic luster, and the crack occurs and develops along the grain boundary. The cold crack is usually straight, the metal surface of the crack is not oxidized, and the crack often extends through the grain to the whole section.
2. Inspection method
In addition to visual inspection, magnetic powder or osmotic inspection can also be used for cracks on the valve surface.
3. Defect assessment
The existence of cracks reduces the bearing cross-sectional area of the valve, and the crack ends form sharp notches, and the stress is highly concentrated, which is easy to expand and lead to failure. Usually obviously visible cracks are not allowed, regardless of their location and size are judged as unqualified. After the crack is found, it can be polished with grinding wheel. If it is confirmed that the crack has been completely eliminated, the valve surface is not damaged, and the thickness is thinner and not obvious, it can be judged as qualified, otherwise it will be treated as a return.
Mechanical damage
1. Appearance characteristics
Mechanical damage is the valve in the process of transportation, handling, lifting, stacking and so on knock damage, or cutting, cutting and other processing damage, such as convex or plane sealing flange sealing surface scratch, indentation, casting riser gas cutting surface and forging edge cutting defects formed by not processing. These defects reach a certain depth, will also affect the quality and life of the valve.
2. Inspection method
Mechanical DAMAGE TO THE VALVE SURFACE CAN BE DETECTED BY VISUAL INSPECTION, AND THE DEPTH OF THE DEFECT CAN BE MEASURED WITH a WELD INSPECTION RULER OR DEPTH RULER.
3. Defect assessment
Radial scratches, mechanical damage and defects on the sealing surface of convex or plane sealed flanges, as well as scratches and bumps on the two sides of the ring connected flange sealing surface groove, will affect the sealing property of valve flanges and are generally not allowed to exist. Flange is not sealed, body and cover surface scratches and mechanical damage as long as the depth is within the allowance range, do not affect the overall quality of the valve, can be accepted as qualified products. However, sharp scratches must be polished smooth to prevent stress concentration.
Valve body identification and others
The main body wall thickness, the length of the structure is unqualified or the nominal pressure of the body on the die casting, the trademark exists the phenomenon of alteration, the inspection process should prevent the plate or low pressure valve instead of high pressure valve. For example, the nominal pressure “25″ cast on the valve body of Z41H-25 DN50 valve has been altered, and the thickness of the valve body has been measured to be 7.8mm, which is not in line with the stipulation of 8.8mm for the valve used in the petrochemical industry. It belongs to the 1.6mpa valve instead of the 2.5mpa valve after polishing the mark.
conclusion
The pressure test can only be carried out after the appearance quality of the valve passes the inspection. If the appearance quality is not qualified, the valve will leak during the test at the least, and the cracking accident will occur at the most. If the defect is not determined, it will cause unnecessary waste and even quality disputes. Therefore, different valve function and reliability requirements are not the same, acceptable defects are not the same, the determination of valve surface defects should be based on the use of the valve, the type of defects, location, size and other comprehensive analysis, in order to scientific, fair, fair quality inspection, to meet the needs of oil and gas field engineering construction.