This article provides a detailed introduction to the basic knowledge of valves, giving you a comprehensive understanding of valve selection and maintenance. Valves are control components in fluid conveying systems, which have functions such as guiding, intercepting, regulating, throttling, preventing backflow, diversion, or overflow pressure relief. Valves are important control components in pipelines. It is important to choose the most suitable valve for the pipeline system, which is related to the safety, flow loss, cost, etc. of the pipeline. It is necessary to be familiar with the characteristics of the valve and the steps and basis for selecting the valve. At present, China's valve industry can produce a complete range of 12 categories, including gate valves, globe valves, check valves, ball valves, butterfly valves, throttle valves, plug valves, diaphragm valves, safety valves, regulating valves, drain valves, and special valves; The maximum working pressure is 600MPa, the maximum nominal diameter is 5350mm, the maximum working temperature is 1200 ℃, and the minimum working temperature is -196 ℃. It is suitable for media such as water, steam, oil, natural gas, highly corrosive media (such as concentrated nitric acid, medium concentration sulfuric acid, etc.), flammable media (such as benzene, ethylene, etc.), toxic media (such as hydrogen sulfide), explosive media, and radioactive media (metal sodium, primary circuit pure water, etc.). Valve pressure parts are made of cast copper, cast iron, ductile iron, high silicon cast iron, cast steel, forged steel, high and low alloy steel, stainless acid resistant steel, Hastelloy alloy, Inconel, Monel alloy, duplex stainless steel, titanium alloy, etc. And it can also support the production of various hydraulic, pneumatic, and electric valve drive devices. Faced with so many types of valves and such complex working conditions, to choose the valve product that is most suitable for installation in pipeline systems, I believe that the first step is to understand the characteristics of the valve; Secondly, one should master the steps and criteria for selecting valves; Furthermore, the principle of selecting valves should be followed.

1. The characteristics of valves include usage characteristics and structural characteristics. The usage characteristics determine the main usage performance and scope of the valve. The categories that belong to the valve usage characteristics include: closed circuit valves, regulating valves, safety valves, etc; Product types (gate valves, globe valves, butterfly valves, ball valves, etc.); The material of the main components of the valve (valve body, valve cover, valve stem, valve disc, sealing surface); Valve transmission mode, etc. Structural characteristics determine some of the structural characteristics of valve installation, maintenance, and other methods, including the length and overall height of the valve structure, and the connection form with the pipeline (flange connection, threaded connection, clamp connection, external thread connection, welded end connection, etc.); The form of sealing surface (insert ring, threaded ring, overlay welding, spray welding, valve body); The structural form of the valve stem (rotating rod, lifting rod), etc.

2. Steps for selecting valves

1. Clarify the purpose of the valve in the equipment or device, determine the working conditions of the valve: applicable medium, working pressure, working temperature, etc

2. Determine the nominal diameter and connection method of the pipeline connected to the valve: flange, thread, welding, etc

3. Determine the method of operating the valve: manual, electric, electromagnetic, pneumatic or hydraulic, electro-hydraulic linkage, etc

4. Determine the materials of the selected valve casing and internal components based on the medium transported by the pipeline, working pressure, and working temperature: cast steel, carbon steel, stainless steel, alloy steel, stainless acid resistant steel, gray cast iron, malleable cast iron, ductile iron, copper alloy, etc.

5. Choose the type of valve: closed circuit valve, regulating valve, safety valve, etc

6. Determine the type of valve: gate valve, globe valve, ball valve, butterfly valve, throttle valve, safety valve, pressure reducing valve, steam trap valve, etc

7. Determine the parameters of the valve: For automatic valves, determine the allowable flow resistance, discharge capacity, back pressure, etc. according to different needs, and then determine the nominal diameter of the pipeline and the diameter of the valve seat hole

8. Determine the geometric parameters of the selected valve: structural length, flange connection form and size, dimensions in the height direction of the valve after opening and closing, bolt hole size and quantity for connection, overall valve dimensions, etc

9. Use existing information such as valve product catalogs and valve product samples to select appropriate valve products

III. Basis for selecting valves

1. The purpose, operating conditions, and control method of the selected valve

2. Properties of working medium: working pressure, working temperature, corrosion performance, whether it contains solid particles, whether the medium is toxic, whether it is flammable and explosive, viscosity of the medium, etc

3. Requirements for valve fluid characteristics: flow resistance, discharge capacity, flow characteristics, sealing level, etc

4. Requirements for installation dimensions and external dimensions: nominal diameter, connection method and size with pipeline, external dimensions or weight limitations, etc

5. Additional requirements for the reliability, service life, and explosion-proof performance of valve products and electric devices (when selecting parameters, it should be noted that if the valve is to be used for control purposes, the following must be determined)

Additional parameters: operating method, maximum and minimum flow requirements, normal flow pressure drop, pressure drop when closing, maximum and minimum inlet pressure of the valve Based on the above criteria and steps for selecting valves, it is necessary to have a detailed understanding of the internal structure of various types of valves when selecting valves reasonably and correctly, in order to make the right decision on the preferred valve.

The ultimate control of pipelines is valves. The valve opening and closing components control the flow direction of the medium in the pipeline, and the shape of the valve flow channel gives the valve certain flow characteristics. When selecting the valve that is most suitable for installation in the pipeline system, this must be considered.

The significance of correctly selecting valves

The correct selection of valves is crucial for ensuring the safe production of the device, improving the service life of valves, and meeting the long-term operation of the device. The main cause of many valve accidents is improper valve selection, such as using cast iron valves in cold areas. Due to the accumulation of water in the oil in the valve body, the valve is prone to freezing and cracking during winter. In some pump outlet valves, due to certain reasons, small pipe gate valves are often used to regulate flow in situations where the required process is low and the pump power is high. Due to the local opening of the gate valve plate, vibration is generated, which accelerates the wear of the sealing surface between the gate plate and the valve seat, making it easy to cause valve leakage. In addition, the quality of valves also has a significant impact on production and use.

If a major explosion accident occurs in a fertilizer plant, the cause of the accident is determined to be a responsibility accident caused by serious quality issues with the valve. The cause of the explosion was that the second stage outlet valve could not withstand normal working pressure and suddenly burst, causing the check valve to malfunction. The 0.75MPa semi water gas in the system quickly overflowed and mixed with the surrounding air to form an explosive gas space, resulting in a gas chemical explosion. According to testing, multiple indicators of the valve do not meet the relevant national standards. In addition, both of these are valves that use water, steam, and oil as media, and are non combustible gas specific valves. The design department's selection was also incorrect. Therefore, the correct selection of valves is crucial for device production, safety, long-term operation, and improving efficiency.

Principles for selecting valves

Valves used in petrochemical production facilities should meet the following basic principles

reliability

Petrochemical production requires continuous, stable, and long-term operation. Therefore, it is required that the valves used should have high reliability, a large safety factor, and should not cause major production safety and injury accidents due to valve failures; Meeting the requirements of long-term operation of the device, long-term continuous production is the benefit; In addition, reduce or avoid "running, emitting, dripping, and leaking" caused by valves, create clean and civilized factories, and implement HSE management.

Meet process production requirements

Valves should meet the requirements of operating medium, pressure, temperature, and usage, which is also the most basic requirement for valve selection. For example, if a valve is required to provide overpressure protection and discharge excess media, safety valves and relief valves should be selected; If it is necessary to prevent medium backflow during operation, a check valve should be used; To automatically discharge condensate, air, and other non condensable gases continuously generated in steam pipelines and equipment, while also preventing steam from escaping, a drain valve should be selected. In addition, for corrosive media, corrosion-resistant materials such as valve bodies should be selected.

Convenient operation, installation, maintenance and repair

After the valve is safe, it should be able to enable operators to correctly identify the valve direction, opening signs, and indication signals, facilitating timely and decisive handling of various emergency faults. Meanwhile, the selected valve type should have a simple structure and be easy to install, maintain, and repair.

Economy

Pay attention to saving investment and reducing device costs. Today's investment is tomorrow's cost. For general occasions, domestically produced valves should be selected to meet usage requirements; If several different types of valves can meet the usage requirements, low-cost and structurally simple valves should be selected; High grade materials such as Cr Mo steel, stainless steel, and babbitt alloy should not be used if the general materials can meet the usage requirements. Valves, like other mechanical products, also require maintenance and upkeep. If this work is done well, it can extend the service life of the valve. The following will introduce the maintenance and upkeep of the valve

Storage and maintenance

*The purpose of storage and maintenance is to prevent valve damage or reduce quality during storage. In fact, improper storage is one of the important reasons for valve damage

*Valve storage should be well-organized, with small valves placed on shelves and large valves neatly arranged on the warehouse floor. They should not be piled up or stacked randomly, and the flange connection surface should not come into contact with the ground. This is not only for the sake of aesthetics, but also to protect the valve from damage

*Due to improper storage and handling, such as broken handwheels, misaligned valve stems, loose or missing fixing nuts between handwheels and valve stems, unnecessary losses should be avoided

*For valves that will not be used temporarily in the short term, asbestos packing should be removed to avoid electrochemical corrosion and damage to the valve stem

*Valves that have just entered the warehouse should be inspected. If rainwater or dirt enters during transportation, they should be wiped clean before being stored

*The inlet and outlet of the valve should be sealed with wax paper or plastic sheets to prevent dirt from entering

*The processing surface of valves that can rust in the atmosphere should be coated with anti rust oil for protection

*Valves placed outdoors must be covered with rainproof and dustproof materials such as felt or tarpaulin. The warehouse storing valves should be kept clean and dry

Usage and maintenance

*The purpose of use and maintenance is to extend the service life of the valve and ensure reliable opening and closing

*The valve stem thread often rubs against the valve stem nut and needs to be coated with a little yellow dry oil, molybdenum disulfide or graphite powder for lubrication

*Valves that are not frequently opened or closed should also be regularly rotated with the handwheel, and lubricant should be added to the valve stem threads to prevent jamming

*For outdoor valves, protective sleeves should be added to the valve stem to prevent rain, snow, dust, and rust

*If the valve system is mechanically idle, lubricating oil should be added to the gearbox on time

*Always keep the valve clean

*Regularly inspect and maintain the integrity of other components of the valve. If the fixing nut of the handwheel falls off, it should be matched completely and cannot be used properly. Otherwise, it will grind the four directions on the upper part of the valve stem, gradually losing the reliability of the fit, and even unable to start

*Do not rely on valves to support other heavy objects, and do not stand on valves

*The valve stem, especially the threaded part, should be wiped regularly. Lubricants that have been contaminated with dust should be replaced with new ones, as the dust contains hard debris that can easily wear the threads and valve stem surface, affecting its service life

Maintenance and upkeep of fillers

*The packing directly affects the key sealing components of whether leakage occurs during valve opening and closing. If the packing fails and causes leakage, the valve is also considered to have failed. Especially for valves in urea pipelines, the packing is prone to aging due to its high temperature and corrosion. Strengthening maintenance can extend the lifespan of fillers

*When the valve leaves the factory, in order to ensure the elasticity of the packing, it is generally based on static pressure testing without leakage. After the valve is installed in the pipeline, due to temperature and other factors, external leakage may occur. At this time, it is necessary to tighten the nuts on both sides of the packing gland in a timely manner, as long as there is no external leakage. If external leakage occurs again in the future, tighten it again and do not tighten it all at once to prevent the packing from losing elasticity and sealing performance

*Some valve packing contains molybdenum disulfide lubricating paste. After using it for several months, the corresponding lubricating grease should be added in a timely manner. When it is found that the packing needs to be supplemented, the corresponding packing should be added in a timely manner to ensure its sealing performance

Maintenance of transmission parts

During the opening and closing process of the valve, the lubricating grease originally added will continue to drain, and factors such as temperature and corrosion will also cause the lubricating oil to continuously dry up. Therefore, the transmission part of the valve should be regularly inspected, and if oil shortage is found, it should be replenished in a timely manner to prevent increased wear due to a lack of lubricant, resulting in inflexibility of the transmission or failure of the jamming case