Principios básicos para la selección de válvulas.

En la mayoría de los casos, la selección de válvulas es una cuestión de consideración profesional para el sistema de proceso, y los principios de selección deben seguir los siguientes aspectos:

1. Cumplir con los requisitos funcionales de la operación del proceso.

Las tuberías en las instalaciones de producción industrial a menudo requieren diferentes acciones como interrupción, regulación de flujo y protección de alivio de sobrepresión de acuerdo con las necesidades de las operaciones del proceso, y se implementan a través de válvulas en las tuberías. Por lo tanto, la selección de válvulas debe cumplir primero con dichos requisitos funcionales. A continuación se muestran algunos ejemplos de ingeniería, entre otros:

1) Para los lugares donde es necesario cambiar o bloquear las tuberías, como la entrada y salida de bombas e intercambiadores de calor, aguas arriba y aguas abajo de las válvulas reguladoras y de derivación, se deben instalar válvulas de cierre;

2) Para áreas donde se requiere regulación de flujo en tuberías, las válvulas reguladoras deben instalarse verticalmente. Las válvulas reguladoras suelen estar interconectadas con medidores de nivel, medidores de flujo, termómetros, etc. para lograr control sobre la temperatura, el caudal y otros aspectos. Como tuberías de entrada y salida de hornos de calefacción, así como tuberías de fueloil y gas combustible;

3) Para situaciones previsibles en las que las tuberías puedan experimentar sobrepresión, se deben instalar válvulas de alivio de seguridad o discos de ruptura en lugares apropiados. Como las tuberías superiores de la mayoría de los recipientes a presión;

4) Las válvulas de retención deben instalarse en lugares apropiados para evitar el reflujo del medio de la tubería debido a condiciones anormales. Casi todas las salidas de bombas, puntos de mezcla de dos medios y tuberías de entrada de medios combustibles de equipos de llama abierta.

5) Para tuberías de vapor, para evitar que la generación de agua condensada afecte el funcionamiento normal de la tubería o del equipo, se deben instalar válvulas de drenaje en lugares apropiados.

2. Cumplir con los requisitos especiales de las condiciones operativas medias para válvulas.

Existen muchos subtipos de válvulas con la misma función y diferentes características. Por lo tanto, al mismo tiempo que se cumplen los requisitos funcionales, la selección de válvulas también debe considerar la influencia de otros factores, incluidas las características de flujo y el rendimiento de sellado de la válvula. A continuación se muestran algunos ejemplos de ingeniería de válvulas de cierre, entre otros:

1) Para condiciones del medio que son sensibles a la caída de presión, se deben seleccionar válvulas con menor resistencia al flujo y mejores características de flujo. Por ejemplo, hay muchas tuberías de medio de baja presión, tuberías de presión negativa, tuberías de flujo por gravedad, etc. en el dispositivo, y se deben seleccionar válvulas reductoras de baja presión en lugar de válvulas de retención con restablecimiento por resorte;

2) Para medios sensibles a la dinámica convectiva, se deben seleccionar válvulas con baja resistencia al flujo y buenas características de flujo. Por ejemplo, para condiciones de operación del medio donde puede ocurrir vaporización con fluctuaciones de presión, el flujo turbulento debe minimizarse tanto como sea posible cuando el medio fluye a través de la válvula para evitar efectos adversos en la operación causados ​​por la vaporización del medio;

3) Para condiciones de medio de alta presión, es aconsejable elegir válvulas con baja resistencia al flujo y buenas características de flujo. Porque bajo alta presión, el caudal del medio es mayor, lo que provoca una mayor reducción de la resistencia y aumenta el costo operativo del equipo de producción;

4) Para condiciones de funcionamiento medias con fluctuaciones de presión de alta frecuencia, como las válvulas de corte en las tuberías de entrada y salida de bombas y compresores alternativos, se deben seleccionar válvulas con baja resistencia al flujo y buenas características de flujo. Además de reducir la resistencia del fluido y disminuir los costos operativos del dispositivo, también pueden reducir la fuerza de excitación causada por las fluctuaciones de presión y ralentizar la vibración de la tubería;

5) Para condiciones de funcionamiento medias con requisitos especiales, el tipo de válvula debería poder adaptarse a dichos requisitos especiales. Por ejemplo, para tuberías de larga distancia con requisitos de paso de bolas, las válvulas deben poder cumplir con los requisitos de paso de bolas de la tubería;

6) Para medios con alta viscosidad o canales de transporte de polvo, la válvula seleccionada no debe tener esquinas muertas que permitan que el medio permanezca, como la ranura del asiento de una válvula de compuerta, para evitar que el medio bloquee las partes de cierre de la válvula. y hacer que la válvula pierda su función adecuada;

7) Para medios altamente peligrosos, como los medios extremadamente peligrosos definidos en GB/T5044, se deben seleccionar válvulas con buenas propiedades de sellado, como válvulas de fuelle y válvulas de diafragma;

8) Debido a la influencia de la temperatura ambiental, es previsible que se produzca vaporización en el medio. Por lo tanto, no es aconsejable utilizar válvulas que formen cámaras de válvulas cerradas ni instalar sistemas automáticos de alivio de presión para las cámaras de válvulas.

3. Deben tenerse en cuenta consideraciones económicas
Economy is an unavoidable consideration for the construction of industrial production facilities, and valve selection is no exception. The economy of valve selection is reflected in two aspects, one is the price of the valve itself, and the other is the impact of the valve on the operating cost of the device. For example, using butterfly valves instead of gate valves can effectively reduce the application cost of large diameter low and medium pressure valves. For example, ball valves are relatively heavy, so their prices are often higher than gate valves and globe valves, especially large diameter valves. The price of ball valves is much higher than other types of shut-off valves. Therefore, if there are no other special requirements, other types of shut-off valves should be used as much as possible. For example, for ball valves and butterfly valves, the price of their metal sealing types is much higher than that of non-metallic sealing types. Therefore, if there are no other special restrictions, non-metallic sealing types should be used as much as possible. Sometimes, the cost of a valve is at odds with its performance, and it is not an either or choice. In this case, the balance between the usability, reliability, and price of the valve should be comprehensively considered to ultimately determine the most cost-effective valve type.

Characteristics and applications of commonly used shut-off valves

1、The characteristics and selection of gate valves:

Compared with globe valves, gate valves have lower flow resistance, lower opening and closing force, smaller structural dimensions, and reliable sealing. However, they are prone to forming a closed valve chamber when the valve is closed, so they are not suitable for media that are affected by environmental temperature and undergo vaporization and pressure increase. When the gate valve is partially opened, the medium will generate eddy currents on the back of the gate plate, which can easily cause erosion and vibration of the gate plate. The sealing surface of the valve seat is also prone to damage, so it is generally not used for throttling. Compared with ball valves, gate valves have higher flow resistance, cannot be lined, are not suitable for ball passing, have slower opening speed, but are lightweight, have lower opening and closing force, reliable sealing, easy operation and maintenance, lower processing requirements, and are inexpensive. Compared with butterfly valves, gate valves have lower flow resistance, lower opening and closing force, reliable sealing, lower processing requirements, but cannot be lined, slow opening speed, heavy weight, and are prone to liquid and gas accumulation in the valve chamber. They are cheaper but more expensive than soft sealed butterfly valves. Based on the characteristics summarized above, the following principles should be when selecting gate valves:

1) They should be the preferred valve for pipeline cutting;

2) Almost not limited by temperature and pressure, but for large diameters (such as DN1000 and above), medium and low pressure applications (CL300 and below), it is more economical to choose butterfly valves, and butterfly valves are lightweight and take up less space;

3) The opening height of the valve has a non-linear proportional relationship with the flow area, and is generally only suitable for fully open or fully closed situations, and should not be used for regulation;

4) There is a closed valve chamber, which is not suitable for use in viscous medium conditions and medium conditions containing solid particles. In low-temperature medium environments or medium conditions where vaporization may occur, automatic pressure relief devices should be installed in the valve chamber, or globe valves, butterfly valves, etc. should be used instead;

5) When there is a requirement for ball passing in the pipeline, a full bore plug valve or ball valve should be used;

6) The valve body should not be lined;

7) Can be used in conjunction with bellows, but the stem stroke is longer than that of globe valves, so it is better to use globe valves in combination with bellows;

8) Compared with ball valves, the flow state is poor and it is not suitable for pressure pulsation medium conditions and medium conditions that may cause cavitation;

9) Compared with ball valves, the flow state is poor and it is not suitable for high-pressure oxygen conditions;

10) Compared with ball valves, the flow state is poor and it is not suitable for medium conditions that may cause erosion or erosion corrosion;

11) During shutdown, there may be residual media in the valve chamber, so it is not suitable for use in highly toxic and corrosive liquid medium conditions or for taking measures to vent and drain;

12) The valve seat groove forms a dead angle for medium retention, so it is not suitable for powder conveying or medium working conditions where crystallization and solidification may occur.

2、The characteristics and selection of globe valves:

Compared with gate valves, globe valves have a certain regulating effect and are often used to regulate the bypass of valve groups; The flow resistance of the shut-off valve is relatively high, and it needs to overcome the resistance of the medium when closing. Therefore, a large torque is required to close the valve. Generally, the maximum diameter it is suitable for is DN250 (NPS10); The globe valve does not have a sealed valve chamber, so it is more suitable for medium conditions where vaporization may occur due to the influence of ambient temperature. Compared with ball valves, it has higher flow resistance, cannot pass the ball, slower opening speed, but is lightweight, reliable in sealing, easy to operate and maintain, has lower processing requirements, and is cheaper in price. Compared with butterfly valves, it has reliable sealing, low processing requirements, but slow opening speed, heavy weight, and easy accumulation of liquid and gas in the valve chamber. It is cheaper but more expensive than soft sealed butterfly valves.

Based on the characteristics summarized above, the selection of globe valves should follow the following principles:

1) high flow resistance, strong closing force, higher price than gate valves but lower than metal sealed ball valves, and less use for cutting off main pipelines;

2) Although the Y-shaped globe valve reduces the degree of flow obstruction of the medium and lowers the flow resistance of the medium, it can be used as a main shut-off valve in pipelines. However, other drawbacks are still evident and it is not the preferred main shut-off valve;

3) Almost unrestricted by temperature and pressure. However, due to its high closing force, it is not suitable for pipelines of DN250 (NPS10) and above;

4) The opening height of the valve is linearly proportional to the flow rate of the medium, so it is commonly used to cut off the bypass of the regulating valve, or in situations where more accurate flow control is required, such as hose station terminals, domestic water terminals, venting and drainage terminals, etc;

5) Complex flow channels and severe erosion: not suitable for medium conditions containing solid particles;

6) Curved flow channels are not suitable for pressure pulsation conditions;

7) Curved flow channels are not suitable for high-pressure oxygen medium conditions;

8) Curved flow channels cannot be used for working conditions with ball passing requirements; 9) Without a sealed valve chamber, suitable for working conditions of low-temperature media or media that may undergo vaporization;
10) Cannot or should not be lined;
11) Due to the short stroke of the valve stem and the type of lifting valve stem, it is a better valve type for use with bellows.

3、Characteristics and selection of ball valves

The biggest feature of selecting ball valves are that among many types of shut-off valves, they have the lowest fluid resistance and the best flow characteristics. At the same time, they also have the characteristic of rapid opening and closing with 90 ° rotation. But ball valves are heavy, expensive, and have relatively large structural dimensions, so they are not suitable for pipelines with too large diameters. Compared with butterfly valves, ball valves do not have the problem of insufficient closure due to reverse torque caused by medium pressure, so their sealing performance is more reliable than butterfly valves. Based on these characteristics, the selection of ball valves should follow the following principles:

1) The valve resistance drop should be minimized, and it can even form an almost complete channel with the connecting pipe, making it suitable for medium conditions that are sensitive to resistance drop;

2) Valves are the heaviest and most expensive, and the larger the valve size, the more obvious this characteristic becomes. Therefore, when choosing a ball valve for larger sizes, economic considerations should be taken into account;

3) For small-sized valves, although soft sealed ball valves are much cheaper, they are limited by temperature and pressure.

4) Fixed ball valves have a sealed valve chamber. When used in low-temperature medium conditions or medium conditions where vaporization may occur, a relief device should be installed, or globe valves, butterfly valves, etc. should be used instead;

5) Suitable for cutting off pipelines with ball passing requirements;

6) Can be lined;

7) Suitable for pressure pulsation medium conditions and medium conditions that may generate cavitation;

8) Suitable for high-pressure oxygen medium conditions;

9) Suitable for medium conditions that may cause erosion or erosion corrosion;

10) Suitable for multi-channel structures, where one valve can obtain 2-4 different flow channels, thus reducing the number of valves used.

The characteristics and selection of butterfly valves: Butterfly valves have the characteristics of rapid opening and closing with a 90 ° rotation, as well as the advantages of light weight and small structural dimensions (especially for clamp type butterfly valves). But its sealing reliability is relatively poor, so it is commonly used in industrial production equipment for medium and low pressure and medium low temperature medium working conditions. With the increasing scale of petrochemical production facilities, replacing gate valves with butterfly valves is an inevitable trend. Due to the emergence of dual eccentric high-performance butterfly valves and triple eccentric metal hard sealed butterfly valves, they have effectively solved the problems of thermal expansion compensation and wear compensation. Therefore, butterfly valves are gradually being used in more demanding medium operating conditions. Based on these characteristics, the selection of butterfly valves should follow the following principles:

1) Compared with gate valves, globe valves, ball valves, plug valves, etc., butterfly valves have the smallest structural dimensions, lightest weight, and cheaper price. Therefore, for large-sized pipelines, using butterfly valves instead of other shut-off valves is very advantageous in reducing the construction cost of the equipment;

2) Compared with gate valves, globe valves, ball valves, plug valves, etc., its sealing reliability is the worst, so it is not suitable for medium working conditions with high temperature (such as 200 ℃ and above) and high pressure (such as CL600 and above), except for special butterfly valves produced by special factories;

3) For soft sealed butterfly valves, although the price may be cheaper, they are limited by temperature and pressure.

4) No sealed valve chamber, suitable for low-temperature medium conditions or medium conditions where vaporization may occur;

5) Cannot be used for cutting off pipelines with ball passing requirements;

6) Can be lined;

7) Not suitable for pressure pulsation medium conditions and medium conditions that may cause cavitation;

8) Not suitable for high-pressure oxygen medium conditions;

9) Not suitable for medium conditions that may cause severe erosion or erosion corrosion;

10) Compared to gate valves and ball valves, butterfly valves are not suitable for pipeline systems with strict pressure loss requirements due to their greater pressure loss.

Selection of attribute parameters for commonly used valves

There are many parameters for describing the properties of valves, but they can generally be divided into four categories: basic parameters, structural parameters, main component materials, and special requirements. Only by selecting and determining these parameters can the properties of the applied valve be accurately expressed. The attribute parameters are generally the information content used in the long description of the valve bill of materials, and also the information content expressed in the valve data table. Among them, the basic parameters are the fundamental parameters that describe the valve and are the core parameters that determine the usability and reliability of the applied valve. They cannot be selected by default. Structural parameters are also important parameters for describing valves, but some of them can be selected by default. Most valve manufacturers have sufficient experience to provide reasonable structural configurations based on the basic parameters and medium conditions proposed by the purchaser. The main component materials are also important parameters for describing valves, which can affect the performance and safety of the valve. Therefore, the author does not recommend default selection. Special requirements refer to those requirements for valves that have significant individual characteristics and can be simply described. If these requirements are universal or require lengthy descriptions, they are often given in the supporting project procurement technical regulations, or simply given in the valve material table and valve data table, while being detailed in the project procurement technical regulations.

1、Principles for selecting basic parameters
The basic attribute parameters of valves include valve type, nominal diameter, valve body material, valve product standards, and nominal pressure rating, totaling five items. Among them, the selection of valve types has been introduced earlier, so only the selection of the other four parameters will be introduced here.
1) Nominal diameter: flow rate, flow velocity, pressure drop, economy, standardized series.
2) Main materials: castings, forgings, others.
3) Valve product standards: The selection of valve product standards is the basis for determining the nominal pressure rating of valves. Common standards include American Mechanical Engineers' Standards (ASME), American Petroleum Institute Standards (AP), American Society of Manufacturers' Standards (MSS), British Standards (BS), German Standards (DI), European Union Standards (EN), and International Organization for Standardization (ISO) standards, among others. Domestic standards include Chinese National Standards (GB), Machinery Industry Standards (JB), and others. Not all of the above standards have such consistency. Valves are required to have equivalent pressure bearing capacity and adaptability to the medium with other pipeline components, and to have equivalent connection dimensions with other connected pipeline components.
4) Nominal pressure rating: The pressure and temperature ratings given by product standards, the influence of operating media, and the influence of additional forces on piping systems.

2. Principles for selecting structural parameters
There are many structural parameters of valves, some of which are conventionally matched, while others can be chosen by engineering designers. Sometimes, it is not appropriate for valve manufacturers to choose all structural parameters entirely. 1) Valve body structure: globe valve, check valve, ball valve, butterfly valve, structural dimensions.
2) Valve flow aperture: Commonly used valve flow apertures are classified into standard type, full bore type, necking type, and other types.
The so-called standard aperture refers to the valve aperture defined by ASME B1616.34 and GB/T12224. The so-called full bore refers to the valve aperture that is basically the same as the inner diameter of the connecting pipe.
The so-called necking refers to the valve aperture where the flow diameter of the valve core is one or more levels lower than the standard aperture.
Examples of full bore applications, but not limited to: high-pressure (referring to pressure levels of CL600 and above) pipelines at the outlet of compressors or pumps, reciprocating compressor or pump outlet pipelines, pipelines that may experience strong cyclic acid corrosion, pipelines that may experience strong carbon dioxide corrosion, pipelines that may experience strong urea corrosion, high-temperature and high-pressure oxygen pipelines, media environments containing solid particles, high-pressure steam pipelines, and pipelines with ball passing requirements.
3) Connection types: flange connection, welding connection, threaded connection, short pipe connection, other connection types.
4) Internal component types: gate valve, globe valve, check valve, ball valve, butterfly valve, plug valve, diaphragm valve.
5) Valve cover types: bolted valve cover, pressure sealed valve cover, extended valve cover/stem, other types. Not all valves have valve covers.
6) Bracket types: open pole bracket (OS&Y), concealed pole bracket (IS&Y). Valve brackets are not pressure bearing components. Not all valves require brackets, such as butterfly valves.
7) Operation type: manual (handle, handwheel, gear, sprocket, etc.), electric, pneumatic, etc.
8) Bypass structure: The bypass has two main functions: firstly, for pipelines with high temperatures, when the valve is opened, the high-temperature medium will cause thermal shock to downstream pipelines or equipment and may cause damage; Secondly, for high-pressure or large-diameter valves, in order to balance the pressure on both sides of the valve disc and reduce the opening torque, a bypass is also required.
9) Emptying and Drainage: For valves with upper or lower valve chambers, if the valve chamber structure size is large and even if pipeline blowing measures are taken, the residual medium in the valve chamber cannot be completely blown away, and the residual medium is flammable, explosive, toxic, and corrosive, which affects the maintenance and repair of the pipeline (including the valve itself), it is necessary to consider setting up valve upper chamber venting (for gas media) or lower chamber drainage (for liquid media). On site emission issues.
10) Lifting ears and feet: In engineering practice, it is common to encounter situations where the valve stem or handwheel is used as the lifting force point during valve lifting, resulting in damage to the valve stem or handwheel. Valves with a nominal diameter greater than or equal to DN200 should consider installing dedicated lifting lugs to prevent the occurrence of the above situation. Valves that meet the above conditions should consider setting up dedicated valve feet to prevent damage from collisions with hard concrete floors or platforms.
11) Other structures: live load structure, low dissipation structure, corrugated pipe sealing, fire safety and anti-static structure, vacuum structure.

3、Principles for selecting materials for the three main components：

1) Material interno: el estándar API600 define los componentes internos de las válvulas de compuerta, incluido el vástago de la válvula, la superficie de sellado del asiento de la válvula, la superficie de sellado de la placa de la válvula, la superficie de sellado superior y otros medios de contacto. También se proporcionan el número de pieza interna de API y los principios de selección.

2) Material no metálico del disco/asiento de la válvula: El material debe tener estabilidad química en las condiciones del medio de aplicación. En las condiciones del medio de aplicación, el material debe tener las propiedades mecánicas básicas requeridas teniendo en cuenta el principio de economía.

3) Pernos y juntas de tapa de válvulas: principios para seleccionar pernos y juntas de tapa de válvulas.

4) Empaquetadura del vástago de la válvula: El vástago de la válvula es un componente móvil y la empaquetadura no solo debe tener un buen rendimiento de sellado, sino también un pequeño coeficiente de fricción. La selección del empaque está relacionada con las propiedades del fluido del proceso, la temperatura y la presión.