Selection and technical development of valves
According to the selection standard and actual use requirements of valves, the key contents in the selection process are analyzed and introduced. Put forward the key precautions in the process of valve selection, such as pressure grade, service temperature, connection form, bonnet sealing and sealing requirements. Through the analysis of the actual accident causes of typical valves, it is required to comprehensively consider the structure, material and service conditions of valves, and highlight the importance of valve selection for safe use. This paper introduces several advanced new valve technologies at present, and compares and analyzes the advantages and disadvantages of the new valve in actual use.
In the 12th Five-Year Plan period, the petrochemical industry will have a large number of device expansion and new construction, as an important pipe fittings, the use of the number of valves has increased dramatically. But because the domestic valve manufacturers are not uniform standards, the level of material smelting, manufacturing levels vary, internal management standards are not nearly consistent, the penetration of foreign markets, the correct choice of valves to bring difficulties. At the same time, due to the deterioration of crude oil, corrosion and many other reasons caused by valve accidents also occur frequently, so the correct selection of valves is particularly important, especially the application of some new technology valves to the safe operation of the device can provide a more reliable guarantee.
The choice of valves
At present, the domestic production of valves, many enterprises, production standards to the old Ministry of Machinery Industry, Sinopec, Ministry of Water and Power standards and the current national standards, as well as foreign standards coexist in the pattern. In the selection of valves is based on the use of media, pressure, temperature set, generally first according to the pressure level, and then consider the temperature and media, while the temperature – pressure curve according to the flange of the pipeline by the flange level selection of valves. The major domestic design institutes have their own pipeline pressure level table, and the lack of authoritative unified industry standards, the choice of materials and structure of the valve in a variety of forms, bringing difficulty to the management. The choice of foreign valves in the form of basically the same, but the industry standard is more uniform, such as pipeline grade standards commonly used by the American Materials Institute (ANSI) B16.34, which on the gate valve, globe valve, check valve according to the different materials, different valve structure form prepared the selection chart, the following chart is the choice of three material types of bolted valve cover:
Gate valve, globe valve, check valve of bolted valve cover selection chart
In addition, the American Petroleum Institute (API) valve manufacturing and inspection of the more commonly used in China has relatively unified industry standards, such as API 600 is more than 1 inch (including 1 inch) flange and butt welding form, bolted and pressure-sealed valve cover steel gate valve manufacturing standards, and 1 inch below the API 602 standard manufacturing, valve inspection standards for API 598. In this standard, there are clear provisions on the ordering of valves, the selection of models, the provisions of inspection, and the signing of supplementary agreements. It should be said that foreign standards in this field are relatively unified, the rules are perfect, and the user can choose easily. Whether we are in the design department or manufacturing and sales industry are worth learning from.
A valve complete selection of requirements should include the following points:
- (1) Valve specifications (name, pressure and pressure level, diameter, number);
- (2) Valve code;
- (3) The valve application standards (flange, welding, thread, structure length, sealing surface form);
- (4) Valve structure form (stem support, bonnet structure, valve body connection);
- (5) Valve use conditions (medium, pressure, temperature);
- (6) Valve body materials (seat, spool, sealing surface, stem, packing, fasteners);
- (7) Valve cover gasket requirements;
- (8) Valve drive form and power parameters;
- (9) Other special requirements (such as structural features, application specifications, flaw detection, inspection and testing, etc.).
A few notes
Due to the wide variety of valves, the selection of valves according to the medium used in the valve, pressure, temperature as an important basis for the three parameters, the design department are more thorough consideration, the following field use according to the situation put forward some notes:
Valve connection: the main flange connection, threaded connection and welding, especially the welding valve should pay attention to the diameter of the pipeline to be welded, wall thickness, the valve body to the adaptability of the post-welding heat treatment process, especially the valve sealing material temperature resistance to be compatible with the requirements.
Valve media is an important factor in determining the choice of valve materials, in the petrochemical industry with a large number of high-sulfur crude oil refining, corrosive media valve material upgrades must be considered, especially comprehensive consideration of some carbide welded spool and seal seat against hydrogen sulfide stress corrosion, to prevent stress cracking during use.
The sealing form of valve cover generally has gasket, pressure tight seal, threaded seal and welded seal, which must be reasonably selected according to the use of the situation, and the pressure tight seal is more reliable under high pressure. The following table is the selection of API valve gasket:
|Valve class||Large cover form||Gasket type|
|CL150||Bolted joint||Asbestos (cast iron valve), asbestos winding|
|CL300||Bolted joint||Asbestos winding flexible graphite winding|
|CL 600、CL 800||Bolted joint||Soft iron, 5cr- 0 5Mo、Aust. s. s. octagonal metal pad|
|C L≥900||Pressure||Soft iron, 5cr- 0 5Mo、Aust. s. s|
In the localization of imported valves, special attention should be paid to the pounds of imported valves can not be directly and simply converted to pressure level. For example: 1500 pounds of imported valves, simple conversion 1500/14.34/9.8 = 10.67MPa, while the actual medium requirements are not high, the temperature ≤ 200 ℃, in the pressure of 20.0Mpa pipeline still use 1500 pounds of valves. Therefore, in lieu of a comprehensive consideration of temperature, pressure and the use of media, according to the characteristics of the domestic valve selection.
With the strengthening of social awareness of environmental protection, the growth of the operating cycle of the refinery, the valve sealing is more important. Valve sealing includes internal and external sealing, internal sealing refers to the seal of the spool; external sealing refers to the large cover, stem seal. These seals must be achieved from the processing accuracy and structural form of attention, while in the sealing materials to actively use new technologies. Such as: winding pad, enhanced graphite pad, bellows and the application of graphite packing with metal wire. In addition is to strengthen corporate standards, requiring corporate standards higher than industry standards, and strive for “zero” leakage. The following table is a corporate standard:
|Valve size||ANSI B16. 104 specified internal leakage standard||Internal leakage standard specified in API598||Internal leakage standard specified by a foreign brand|
With the reduction of enterprise labor costs and industrial automation requirements are increasingly high, the valve automatic switch must also be considered. Such as: the use of electric, pneumatic service clothing, not only to achieve the automatic switching of the valve, and can achieve remote control, for the device provides a broader prospect of comprehensive computer control.
Typical valve accident analysis
In a hydrocracking unit of a company, the acid water extraction line of the high pressure separator was perforated due to corrosion of the line and the valve had internal leakage. Only 10 days after the valve was put into use, it was found that the weld seam of the replacement valve was cracked and the acidic water with high concentration of hydrogen sulfide was sprayed out as far as 8 meters at high pressure, but it was found in time and handled properly without causing a major accident.
Afterwards, the structure of the valve and the valve leakage causes were analyzed: the leakage mainly occurred at the weld of the valve seat and valve cover connection, as follows:
Figure.1 Leaking valve weld cracking profile
Figure.2 The valve seat and valve cover weld at the cross-sectional diagram
It is clear from the figure that the crack is from the interface between the valve seat and the valve cover, where just like a gap, the stress is obviously concentrated, cracking in the direction of 45 ° through the entire interface, indicating that the tensile stress plays a major role.
After the comparison of the leaking valve with the original design of the valve structure, as well as the anatomy of the leaking valve and the analysis of the cracked fracture, the following conclusions were formed:
Leaking valve body and cover between the gasket. Sealing is achieved through the interface between the valve seat and the cover of the outer surface of the weld, the deepest weld is 3.5 mm, apparently the weld is only sealing welding, but the lack of gasket, sealing welding for strength welding is the direct cause of valve cracking.
Valve seat and valve cover interface connection of the seal weld there is a gap, the stress is obviously concentrated, the weld is carbon steel, and microscopic bubbles, the strength is very low. In operation, under the combined action of high pressure and corrosive acidic water, the final shear stress corrosion cracking.
According to the conclusion of the above analysis, such valves are inevitable accidents, the selection of valves, the change must be careful, especially in high-pressure, toxic media pipeline should be used pressure sealing structure of high-pressure valves, the structure of the valve, material, use conditions should be considered to ensure the safe use.
A company in the new device and valve maintenance process due to the positioning error of the electric actuator, resulting in valve stem bending. The main reason is that the electric actuator and the valve are from different suppliers, so the selection must be set according to the relevant switching stroke and torque parameters of the valve manufacturer, and the technical data must be retained after the assembly and commissioning are qualified by the valve manufacturer to provide accurate technical parameters for inspection and maintenance.
Several new technologies valve applications
Orbital ball valve (lifting lever ball valve, see Figure 3)
Orbital ball valve is a single-end sealed ball valve, due to the use of a patented spiral slide stem, so that the sealed ball in the switch first tilted away from the seat, so as to achieve the ball and seat without friction between the work, with the advantages of flexible switching, long service life, good sealing performance, the ball sealing surface has a self-cleaning effect. This kind of valve because of the real zero leakage, switching torque is very small is widely used in gas media, the valve can reach ANSI 150-4500 CLASS, but in the selection of the following points should be noted:
The use of media impurities should not be too dirty, to prevent damage to the sealing surface of the valve ball;
Installation location should not have throttling and often violent vibration, because the valve ball in the fully open position, to avoid the ball due to external vibration on the valve stem has abrasive marks;
The selection should pay attention to whether the full diameter, if necessary, should be specifically stated.
Double shut-off plug valve (see Figure 4)
Double shut-off plug valve is a double-ended sealed valve, using a sliding dovetail coupling, so that the sealing slider in the rotation of the plug with the valve body, so as to achieve the sealing slider and the seat without friction between the work, with the advantages of flexible switching, double-ended sealing performance, is to avoid the use of complex double valve structure of the safety isolation means. The following points should be noted when selecting:
- The medium used should be relatively clean to prevent jamming of the sealing slider;
- Due to the use of flexible soft seal, the use temperature should not be too high. 5.
Three eccentric metal butterfly valve (see Figure 5)
Three eccentric metal butterfly valve is now mainly used in large-diameter occasions to replace the gate valve. The valve adopts three eccentricities of the centerline of the stem and sealing surface, the centerline of the stem and valve body, and the centerline of the sealing surface of the valve plate and the valve body, eliminating the friction between the valve plate seal and the valve seat, making the valve to ensure the seal at the same time, extending the service life. The valve is widely used due to its small size and small switching torque. However, because the valve plate in the center of the pipeline, the flow performance will be affected to a certain extent. 6.
One-way stop valve (see Figure VI)
One-way stop valve is a combination of piston lift check valve and stop valve, in the fully closed state of its role with the stop valve exactly the same, fully open and become a piston lift check valve, the biggest advantage is that a valve can be used twice, replacing the complex combination of check valve and stop valve or gate valve. Currently is widely used in the compressor and high-pressure pump outlet. But the biggest shortcoming is only fully open and fully closed two states.
Fast blind valve (see Figure 7)
The quick blind valve is a device that uses a cam mechanism to achieve rapid blind sealing of the pipeline. The valve uses a patented cam mechanism to amplify the torque to achieve the lifting of the blind. It has the advantages of easy operation, online gasket replacement and complete isolation, etc. It is mainly used in the isolation of high-risk media.
Vane check valve (see Figure 8)
Vane check valve is also known as gas check valve. The use of spring-controlled closure, before the backflow can be quickly closed, with rapid closure, reliable, small impact force, small installation position, suitable for stable operation under changing conditions and other characteristics. But the cleanliness of the medium requires high.
Figure.3 Orbital ball valve
Figure.4 Double shut-off plug valve
Figure.5 Three eccentric metal butterfly valve
Figure.6 One-way stop valve
Figure.7 Fast blind valve
Figure.8 The valve piece check valve
As a large number of important fittings and valves used in the field of petrochemicals, we must strengthen the management, to understand some of the relevant knowledge, for the stability of the device, long, full, excellent operation has a very realistic significance. The development of the national economy, the petrochemical industry must come to the forefront of international competition, we must be in the design and selection, valve orders, acceptance and installation, use of maintenance and other aspects of good quality, the choice of high-quality, reliable valves to reduce maintenance, to ensure the reliability of the valve seal, to ensure the long-cycle operation of the device, in order to improve the overall efficiency of enterprises and good social benefits.
Source: China Valves Manufacturer – Yaang Pipe Industry Co., Limited (www.sfutube.com)
(Yaang Pipe Industry is a leading manufacturer and supplier of nickel alloy and stainless steel products, including Stainless Steel Valves, Stainless Steel Flanges, Stainless Steel Pipe Fittings, Stainless Steel Pipe. Yaang products are widely used in Shipbuilding, Nuclear power, Marine engineering, Petroleum, Chemical, Mining, Sewage treatment, Natural gas and Pressure vessels and other industries.)
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