What is pipeline valve?
In the fluid piping system, valve is the control element, its main function is to isolate equipment and piping system, regulate flow, prevent backflow, regulate and discharge pressure. It can be used to control the flow of air, water, steam, corrosive media, mud, oil, liquid metal and radioactive media. Because it is very important to select the most suitable valve for the piping system, it is very important to understand the characteristics of the valve and the steps and basis for selecting the valve.
Valve material selection
The final control of the pipeline is the valve. The valve opening and closing parts control the flow pattern of the medium in the pipeline. The shape of the valve flow channel makes the valve have certain flow characteristics. This must be taken into account when selecting the most suitable valve for installation in the pipeline system.
The following are the principles to be followed when selecting valves:
Valves for closing and opening media
The flow passage is a straight through valve with small flow resistance. It is usually selected as a valve for cut-off and open medium. The downward closed valve (stop valve, plunger valve) is less selected because of its zigzag flow path and higher flow resistance than other valves. In case of high flow resistance, closed valve can be selected.
Valve for flow control
Usually choose the valve which is easy to adjust the flow as the control flow. A downward closing valve, such as a globe valve, is suitable for this purpose because its seat size is proportional to the travel of the closure. Rotary valves (plug valves, butterfly valves, ball valves) and flexure body valves (clamp valves, diaphragm valves) can also be used for throttling control, but they can only be used in a limited range of valve diameters. The gate valve uses the disc gate to make the cross cutting movement to the circular valve seat. It can control the flow well only when it is close to the closed position, so it is not usually used for flow control.
Valves for reversing and shunting
The valve can have three or more channels according to the need of reversing and shunting. Plug valve and ball valve are suitable for this purpose. Therefore, most of the valves used for reversing and diverting are one of these valves. However, in some cases, other types of valves can also be used for reversing and diverting as long as two or more valves are properly connected with each other.
Valve for medium with suspended particles
When there are suspended particles in the medium, it is most suitable to slide the valve with wiping function along the sealing surface with its closing part. If the back and forth movement of the closing member to the valve seat is vertical, particles may be clamped, so this valve is only suitable for the basic clean medium unless the sealing surface material can allow the particles to be embedded. Ball valve and plug valve can wipe the sealing surface in the process of opening and closing, so they are suitable for medium with suspended particles.
At present, no matter in petroleum, chemical industry, or other industries, the application, operation frequency and service of valves vary greatly. To control or eliminate even low-level leakage, the most important and critical equipment is valves.
Valve standard is to standardize the requirements of valve design, manufacturing, inspection, use and other links, so that both sides have a common language, can protect their own interests from the perspective of technical regulations, so that the evaluation has a unified standard. In the past, some small enterprises did not put the quality problem first. The valve body length was different, the valve material was not up to standard, and the sealing material was poor, which caused the inconvenience of installation and operation safety risk of the pipeline engineering of the user (some users were careless in purchasing, and there was no clear standard in purchasing, resulting in loopholes in purchasing and supply!).
At present, the quality of valves has the following problems:
Defects in valve casting
The valve body made of carbon steel, especially chromium molybdenum steel, is prone to sand hole, burr and fine crack during casting, and its appearance is rough. All the friends who are selling outside know that when the customer checks and accepts, the small burr will cause the customer to fail the check and acceptance, and a large number of valves will return. Many sales don’t understand. What’s the relationship between such a small burr and a small burr without affecting the use? It’s just a little ugly. But the experience of users tells them that quality is not only about valves, but also about attitude.
The tightness of valve is an important index to measure the function and quality of valve. The problem of valve internal leakage is serious, which threatens the normal operation of the whole engineering device, and it is difficult to find out whether there is internal leakage of the valve in operation. Most of the reasons for the internal leakage of the unit valve are that the sealing surface material of the valve plate and valve seat does not meet the use requirements, the processing accuracy is not enough, the wedge angle of the valve plate and valve seat of the gate valve does not match, the width of the sealing surface is not the same, the degree of coincidence is poor, the gate valve is not in place, and the valve cannot be closed. There are also packing packing assembly does not meet the requirements, resulting in leakage.
There are generally two types of valve characteristics, service characteristics and structural characteristics.
Use characteristics: it determines the main use performance and use scope of the valve, which include: Valve category (closed-circuit valve, regulating valve, safety valve, etc.); product type (gate valve, globe valve, butterfly valve, ball valve, etc.); material of main parts of the valve (valve body, bonnet, stem, disc, sealing surface); valve transmission mode, etc.
Structural characteristics: it determines some structural characteristics of the installation, maintenance and other methods of the valve, including the structural length and overall height of the valve, the connection form with the pipeline (flange connection, thread connection, clamp connection, external thread connection, welding end connection, etc.); the form of the sealing surface (insert ring, thread ring, surfacing, spray welding, valve body) ; valve rod structure (rotary rod, lifting rod), etc.
The steps and basis for selecting valves are as follows:
- 1. Define the purpose of the valve in the equipment or device, and 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 with the valve: flange, thread, welding, etc.
- 3. Determine the way to operate the valve: manual, electric, electromagnetic, pneumatic or hydraulic, electrical linkage or electro-hydraulic linkage, etc.
- 4. According to the medium, working pressure and working temperature of the pipeline, the materials of the shell and internal parts of the selected valve are determined: grey cast iron, malleable cast iron, nodular cast iron, carbon steel, alloy steel, stainless acid resistant steel, copper alloy, etc.
- 5. Select the type of valve: closed-circuit valve, regulating valve, safety valve, etc.
- 6. Determine the type of valve: gate valve, stop valve, ball valve, butterfly valve, throttle valve, safety valve, pressure reducing valve, steam trap, etc.
- 7. Determine the parameters of the valve: for automatic valves, determine the allowable flow resistance, discharge capacity, back pressure, etc., and then determine the nominal diameter of the pipe and the diameter of the valve seat hole.
- 8. Determine the geometric parameters of the selected valve: structure length, flange connection form and size, valve height direction after opening and closing, bolt hole size and quantity of connection, overall valve size, etc.
- 9. Use existing data: valve product catalog, valve product samples, etc. to select appropriate valve products.
Basis for valve selection
While knowing the steps of selecting valves, we should also know the basis of selecting valves.
- 1. The purpose, working condition and control mode of the selected valve.
- 2. Nature of working medium: working pressure, working temperature, corrosion performance, whether it contains solid particles, whether it is toxic, whether it is flammable and explosive medium, viscosity of medium, etc.
- 3. Requirements for valve fluid characteristics: flow resistance, discharge capacity, flow characteristics, sealing grade, etc.
- 4. Installation dimension and boundary dimension requirements: nominal diameter, connection method and connection dimension with pipeline, boundary dimension or weight limit, etc.
- 5. Additional requirements for reliability, service life of valve products and explosion-proof performance of electric devices.
- (note when selecting parameters: if the valve is to be used for control purposes, the following additional parameters must be determined: operation method, maximum and minimum flow requirements, pressure drop for normal flow, pressure drop when closing, maximum and minimum inlet pressure of the valve.)
According to the basis and steps of selecting valves mentioned above, it is necessary to have a detailed understanding of the internal structure of various types of valves when selecting valves reasonably and correctly, so as to make a correct choice for the preferred valves.
Classification of valves
So far, the valve industry has been able to produce 12 categories, more than 3000 models and more than 4000 specifications of valve products, including gate valve, stop valve, throttle valve, plug valve, ball valve, electric valve, diaphragm valve, check valve, safety valve, pressure reducing valve, steam trap and emergency shut-off valve, with the maximum working pressure of 600MPa, the maximum nominal diameter of 5350mm and the maximum working temperature It is 1200 ℃, the minimum working temperature is – 196 ℃, and the applicable medium is water, steam, oil, natural gas, strongly corrosive medium (such as concentrated nitric acid, medium concentration sulfuric acid, etc.), flammable medium (such as Benben, ethylene, etc.), toxic medium (such as hydrogen sulfide), explosive medium and radioactive medium (metal sodium, primary pure water, etc.). Valve pressure parts are made of brass, cast iron, nodular cast iron, high silicon cast iron, cast steel, forged steel, high / low alloy steel, stainless steel 304, stainless and acid resistant steel, duplex stainless steel, titanium alloy, etc., and can produce various electric, pneumatic, manual and hydraulic valve driving devices. In the face of so many kinds of valves and so complex various working conditions, to select the most suitable valve products for installation of the pipeline system, first of all, we should understand the characteristics of the valve; secondly, we should master the steps and basis for selecting the valve; thirdly, we should follow the principle of selecting the valve.
Advantages and disadvantages of various valves.
1. Gate valve: gate valve refers to the valve that the closing part (gate) moves along the vertical direction of the channel axis. It is mainly used for cutting off medium on the pipeline, that is, fully open or fully closed. Generally, gate valve can not be used as regulating flow. It can be applied to low temperature pressure or high temperature and pressure, and can be made of different materials according to the valve. But the gate valve is not generally used in the pipeline for transporting mud and other media.
- ① Low fluid resistance;
- ② The torque required for opening and closing is small;
- ③ Can be used in two-way flow of media on the ring network pipeline, that is to say, the flow direction of media is not limited;
- ④ When fully open, the erosion of sealing surface by working medium is smaller than that of stop valve;
- ⑤ The body structure is simple and the manufacturing process is good;
- ⑥ The structural length is relatively short.
- ① The overall dimension and opening height are large, and the installation space is also large;
- ② In the process of opening and closing, the sealing surface is relatively rubbed, and the friction loss is large, even it is easy to cause scratch at high temperature;
- ③ Generally, gate valves have two sealing surfaces, which makes it difficult to process, grind and maintain;
- ④ The opening and closing time is long.
2. Butterfly valve: the butterfly valve is a kind of valve that uses the disc type opening and closing piece to rotate about 90 ° to open, close and adjust the fluid channel.
- ① The utility model has the advantages of simple structure, small volume, light weight and low consumption, which can not be used in large caliber valves;
- ② Quick opening and closing, small flow resistance;
- ③ It can be used for medium with suspended solid particles, and can also be used for powder and granular medium according to the strength of sealing surface. It can be applied to the two-way opening and closing and adjustment of the ventilation and dust removal pipeline. It is widely used in the gas pipeline and water channel of metallurgy, light industry, electric power, petrochemical system, etc.
- ① The flow regulation range is not large, when the opening reaches 30%, the flow will be more than 95%;
- ② Due to the limitation of butterfly valve structure and sealing material, it is not suitable for high temperature and high pressure pipeline system. Generally, the working temperature is below 300 ℃ and PN40;
- ③ The sealing performance is worse than that of ball valve and globe valve, so the sealing requirements are not very high.
3. Ball valve: it is evolved from the plug valve. Its opening and closing parts are a ball. The ball is used to rotate 90 ° around the axis of the stem to achieve the purpose of opening and closing. The ball valve is mainly used to cut off, distribute and change the direction of medium flow on the pipeline. The ball valve with V-shaped opening also has good flow regulation function.
- ① It has the lowest flow resistance (actually 0);
- ② It can be used in corrosive medium and low boiling point liquid reliably because it will not stick when working (when there is no lubricant);
- ③ In a large range of pressure and temperature, it can achieve complete sealing;
- ④ It can realize fast opening and closing, and the opening and closing time of some structures is only 0.05 ~ 0.1s, so as to ensure that it can be used in the automation system of the test bench. When the valve is opened and closed quickly, there is no impact during operation;
- ⑤ The spherical closure can be positioned automatically at the boundary position;
- ⑥ The working medium is sealed reliably on both sides;
- ⑦ When the valve is fully open and closed, the sealing surface of the ball and the valve seat is isolated from the medium, so the medium passing through the valve at high speed will not cause the erosion of the sealing surface;
- ⑧ Because of its compact structure and light weight, it can be considered as the most reasonable valve structure for low temperature medium system;
- ⑨ The valve body is symmetrical, especially the welded valve body structure, which can well bear the stress from the pipeline;
- ⑩ The closing part can withstand the high pressure difference during closing. (11) the ball valve with fully welded valve body can be directly buried in the ground, so that the internal parts of the valve are not eroded, and the maximum service life can reach 30 years. It is the most ideal valve for oil and gas pipelines.
- ① Because the most important sealing ring material of the valve seat is polytetrafluoroethylene, it is inert to almost all chemical substances, and has the comprehensive characteristics of small friction coefficient, stable performance, not easy to aging, wide range of temperature application and excellent sealing performance. But the physical characteristics of PTFE, including high expansion coefficient, sensitivity to cold flow and poor thermal conductivity, require that the design of valve seat seal must be based on these characteristics. Therefore, when the sealing material becomes hard, the reliability of the seal will be damaged. Moreover, the temperature resistance grade of polytetrafluoroethylene is low, and it can only be used under the condition of less than 180 ℃. Beyond this temperature, the sealing material will age. In consideration of long-term use, it is generally not used at 120 ℃.
- ② Its regulating performance is worse than that of stop valve, especially pneumatic valve (or electric valve).
4. Stop valve: refers to the valve where the closure (disc) moves along the center line of the valve seat. According to this movement of the disc, the change of the valve seat port is in direct proportion to the disc stroke. Because of the relatively short opening or closing stroke of the valve stem, and the very reliable cut-off function, and because the change of the valve seat port is proportional to the stroke of the valve disc, it is very suitable for the flow regulation. Therefore, this type of valve can be used for shut-off or regulation and throttling.
- ① In the process of opening and closing, the friction between the disc and the sealing surface of the valve body is smaller than that of the gate valve, so it is wear-resistant.
- ② Generally, the opening height is only 1 / 4 of the valve seat channel, so it is much smaller than the gate valve;
- ③ Generally, there is only one sealing surface on the valve body and disc, so the manufacturing process is relatively good and easy to maintain;
- ④ As the filler is generally a mixture of asbestos and graphite, the temperature resistance grade is higher. Generally, stop valves are used for steam valves.
- ① Because the flow direction of the medium through the valve has changed, the minimum flow resistance of the stop valve is also higher than that of most other types of valves;
- ② Due to the long stroke, the opening speed is slower than the ball valve.
5. Plug valve: it refers to the rotary valve with the closing part in the shape of a plunger. By 90 ° rotation, the channel port on the valve plug is connected or separated from the channel port on the valve body to open or close the valve. The shape of the plug can be cylindrical or conical. The principle of the ball valve is similar to that of the ball valve. The ball valve is developed on the basis of the plug valve. It is mainly used in oil field exploitation and petrochemical industry.
6. Safety valve: it refers to the overpressure protection device on the pressure vessel, equipment or pipeline. When the pressure rise in the equipment, container or pipeline exceeds the allowable value, the valve will open automatically, and then discharge in full amount to prevent the equipment, container or pipeline and pressure from rising continuously; when the pressure drops to the specified value, the valve shall close automatically and timely to protect the safe operation of the equipment, container or pipeline.
7. Steam trap: there will be some condensed water formed in the medium of conveying steam, compressed air, etc. in order to ensure the working efficiency and safe operation of the device, it is necessary to discharge these useless and harmful medium in time to ensure the consumption and use of the device. It has the following functions:
- ① It can quickly remove the generated condensate;
- ② It can prevent steam leakage;
- ③ It can remove air and other non condensable gases.
8. Pressure reducing valve: it is a valve that reduces the inlet pressure to a certain required outlet pressure through regulation, and makes the outlet pressure automatically keep stable depending on the energy of the medium itself.
9. Check valve: also known as counter flow valve, check valve, back pressure valve and check valve. These valves are automatically opened and closed by the force generated by the flow of the pipeline medium itself, belonging to an automatic valve. Check valve is used in pipeline system, its main function is to prevent medium backflow, pump and drive motor reverse rotation, as well as container medium discharge. The check valve can also be used to supply the auxiliary system whose pressure may rise above the system pressure. It can be divided into swing type (rotating according to the center of gravity) and lift type (moving along the axis).
Valve manufacturing process and processing flow
At first glance, there are not many parts of the valve, simple structure and general accuracy. In the mechanical industry, they are simple parts, but the core sealing parts of the valve are particularly demanding. Valve manufacturing process is complex and difficult. Which process characteristics should we pay attention to?
The valve materials are selected according to different applicable media. Generally, the valve materials can be divided into normal temperature, high temperature and low temperature materials, corrosion-resistant materials, and materials for low pressure, medium pressure and high pressure valves. The main materials are cast iron, cast steel, stainless steel, alloy steel, high temperature steel, low temperature steel, corrosion-resistant plastic materials, etc.
Common valve materials:
- Gray cast iron valve: gray cast iron is suitable for water, steam, air, gas, oil and other media with nominal pressure of PN ≤ 1.0MPa and temperature of – 10 ℃ ~ 200 ℃. The common grades of gray cast iron are: HT200, HT250, HT300, ht350.
- Malleable cast iron: suitable for water, steam, air and oil medium with nominal pressure of PN ≤ 2.5MPa and temperature of – 30-300 ℃. Common brands are kth300-06, kth330-08 and kth350-10.
- Ductile iron valve: suitable for water, steam, air, oil and other media with PN ≤ 1.6Mpa and temperature of – 5 ～ 80 ℃. Common brands are: QT400-15, QT450-10 and QT500-7.
- In view of the current domestic technology level, the factories are uneven, and users are often difficult to test. According to experience, it is recommended that PN ≤ 2.5MPa, and steel valve is still used as safety valve.
- Acid resistant high silicon ductile iron: suitable for corrosive medium with nominal pressure of PN ≤ 0.25MPa and temperature lower than 120 ℃.
- Carbon steel valve: suitable for water, steam, air, hydrogen, ammonia, nitrogen and petroleum products with nominal pressure of PN ≤ 32.0mpa and temperature of – 30 ～ 425 ℃. Common brands are WC1, WCB, ZG25, high quality steel 20, 25, 30 and Low Alloy Structural Steel 16Mn.
- Copper alloy valve: suitable for water, sea water, oxygen, air, oil and other media with PN ≤ 2.5MPa, as well as steam media with temperature of – 40-250 ℃. The common brands are zgnsn10zn2 (tin bronze), H62, HPb59-1 (brass), qaz19-2, qa19-4 (aluminum bronze).
- High temperature copper: suitable for steam and petroleum products with nominal pressure PN ≤ 17.0mpa and temperature ≤ 570 ℃. Common brands include ZGCr5Mo, 1cr5m0.zg20crmov, zg15gr1mo1v, 12CrMoV, wc6, wc9, etc. The specific selection must be in accordance with the valve pressure and temperature specifications.
- Low temperature steel valve: suitable for medium such as nominal pressure PN ≤ 6.4Mpa, temperature ≥ – 196 ℃ ethylene, propylene, liquid natural gas, liquid nitrogen, etc., common brands) ZG1Cr18Ni9, 0Cr18Ni9, 1Cr18Ni9Ti, zg0cr18ni9
- Stainless acid resistant steel valve: suitable for medium such as nitric acid and acetic acid with nominal pressure of PN ≤ 6.4Mpa and temperature of ≤ 200 ℃. Common brands include zg0cr18ni9ti, zg0cr18ni10 < nitric acid resistance >, zg0cr18ni12mo2ti, zg1cr18ni12mo2ti < acid resistance and urea >.
Structure of casting blank
Most of the valve blanks are thin-walled castings with complex structure, which not only require good appearance quality, but also dense internal quality and good metallographic structure, without defects such as porosity, shrinkage cavity, sand inclusion and crack. Therefore, its casting process is complex and heat treatment technology is difficult. In the mechanical industry, the casting difficulty of the blank of the pressure bearing thin shell casting of the valve is far more complex and more difficult than that of other mechanical components.
Due to the poor cutting performance of most high strength, high hardness and high corrosion-resistant materials, such as high alloy stainless steel and acid resistant steel, which have the disadvantages of high toughness, high strength, poor heat dissipation, large chip viscosity and strong processing hardening tendency, it is difficult to achieve the required dimensional accuracy and finish, which brings certain difficulties to machining tools, processes and equipment. In addition, the requirements of valve sealing surface in machining accuracy, matching angle, finish and matching sealing pair are also very high, which brings great difficulty to machining.
Process arrangement of valve parts
The number of main parts of the valve is not many, the structure is relatively simple, the processing accuracy of most dimensions is not high, and the outside is relatively rough, which gives the impression of a simple machine. In fact, the heart sealing part of the valve is extremely precise, and the “three degrees” (flatness, smoothness, hardness) of the sealing surface is very high, as well as the coincidence degree of the sealing pair composed of two sealing surfaces must reach zero to zero, so as to meet the zero leakage of the air tightness test. It is the biggest technological difficulty of valve machining to guarantee the precise zero to zero requirements of heart parts with rough benchmark.
Test and inspection of valves
The valve is an important opening and closing and regulating element of the pressure pipeline, and the operating conditions of the pressure pipeline are very different, such as high temperature, high pressure, low temperature, deep cold, flammable and explosive, highly toxic and strong corrosion. However, the test and inspection conditions of valve manufacturing can not meet the same requirements of the working conditions. The international and domestic valve test standards stipulate that the gas or water is used as the medium for the test under the conditions close to the normal temperature. There is a fundamental hidden danger, that is, the valve products passing the normal factory test may not meet the use requirements due to material selection, casting quality and seal damage under harsh actual working conditions, and there will be major quality accidents. It’s no wonder that some old valve experts who have worked for a lifetime are more restrained and worried as they get older.
Valve manufacturing process
Step 1: valve body manufacturing.
Valve body (casting, sealing surface overlaying).
Purchase of castings (as per the standard) ➱ incoming inspection (as per the standard) ➱ weld overlay groove ➱ ultrasonic flaw detection (as per the drawing) ➱ weld overlay and post weld heat treatment ➱ finishing ➱ grinding sealing surface ➱ hardness inspection and dye detection of sealing surface.
Step 2: valve internals manufacturing process.
A. Internal parts such as valve disc and valve seat to be hardfacing.
Purchase of raw materials (according to the standard) ➱ incoming inspection (according to the standard) ➱ production of blank (round steel or forging, according to the drawing process requirements) ➱ rough machining of ultrasonic flaw detection surface (when required by the drawing) ➱ rough machining of weld overlay groove ➱ surfacing and post weld heat treatment ➱ finishing of all parts ➱ grinding of sealing surface ➱ hardness inspection and dye detection of sealing surface.
Purchase of raw materials (according to the standard) ➱ incoming inspection (according to the standard) ➱ production of blank (round steel or forging, according to the drawing process requirements) ➱ rough machining of weld overlay groove ➱ surfacing and post weld heat treatment ➱ finishing of all parts ➱ grinding of outer circle ➱ surface treatment of valve rod (nitriding, quenching, chemical coating) ➱ final treatment (polishing, grinding, etc.) ➱ grinding of sealing surface ➱ hardness inspection and dye detection of sealing surface.
C. Internal parts without hardfacing.
Purchase of raw materials (according to the standard) ➱ incoming inspection (according to the standard) ➱ production of rough (round steel or forging, according to the drawing process requirements) ➱ rough machining of ultrasonic flaw detection surface (when required by the drawing) ➱ finishing of all parts.
Step 3: Fastener manufacturing.
Fastener manufacturing standard dl439-1991.
Purchase of raw materials (according to the standard) ➱ incoming inspection (according to the standard) ➱ making rough (round steel or forging, according to the drawing process requirements) and sampling for necessary inspection ➱ rough processing ➱ finishing ➱ spectral inspection.
Step 4: General Assembly.
Pick up parts ➱ clean, clean ➱ rough assembly (according to the drawing) ➱ hydrostatic test (according to the drawing and process) ➱ after qualified, disassemble and clean ➱ final assembly ➱ and electrical assembly or actuator commissioning (for electric valves) ➱ paint packaging ➱ shipping.
Step 5: production and inspection process
- 1. All kinds of raw materials purchased by the company.
- 2. Use spectrum analyzer to test raw materials, and print out the test report of raw materials for backup.
- 3. Cut raw materials with cutting machine.
- 4. The inspector shall inspect the cutting diameter and length of raw materials.
- 5. The forging workshop processes the raw materials by forging and forming.
- 6. The inspector inspects the size of the blank.
- 7. The worker is cutting the edge of the blank.
- 8. Sanding workers sandblast the rough surface.
- 9. The inspector shall conduct surface treatment inspection after sand blasting.
- 10. Workers machine the blanks.
- 11. Valve body sealing thread processing – the employees conduct the post processing inspection on the products after processing by the self inspection and inspection personnel.
- 12. Valve body connection thread processing.
- 13. Middle hole processing.
- 14. The inspector shall conduct the general inspection.
- 15. Qualified semi-finished products are sent to semi-finished products warehouse.
- 16. Semi finished products are electroplated.
- 17. Semi finished product plating surface treatment inspection.
- 18. Inspection of all kinds of accessories (ball, stem, seal valve seat).
- 19. Product assembly is carried out in the General Assembly Workshop – the inspectors of the assembly line inspect the products.
- 20. After assembly, the product goes through pressure test and drying to the next process.
- 21. The general assembly workshop shall carry out product packaging – the packaging line inspector shall inspect the sealing, appearance and torsion of the product. Unqualified products must not be packed.
- 22. Qualified products are packed in bags and sent to the finished product warehouse.
- 23. All inspection records will be classified and stored in the computer for inquiry at any time.
- 24. Qualified products are sent to domestic and foreign countries through containers.
Selection and installation of chemical pipeline valves
In chemical production, it is very common to install valves in pipelines. There are many types of valves. In what occasions, which type of valves are selected, some of which are not fully understood, and even where the selected valves should be installed, the performance of the valves is different, and the installation location is certainly different. These will give common users headache. The following is an introduction based on these situations The installation position and selection of valves in the engineering pipeline shall be made into specific items:
Selection principle of valves used in water supply pipeline
1. When the pipe diameter is not more than 50 mm, stop valve should be used. When the pipe diameter is more than 50 mm, gate valve and butterfly valve should be used.
2. The gate valve should be used for the parts with small flow resistance (such as water pump suction pipe).
3. The stop valve should be used on the pipe section which is often opened and closed.
4. The multi-function valve should be used on the outlet pipe of water pump with larger diameter.
5. Gate valve and butterfly valve shall be used on the pipe section with two-way flow of water, and stop valve shall not be used.
6. When the flow and water pressure need to be adjusted, the regulating valve and stop valve should be used.
7. Butterfly valve and ball valve should be used for the parts with small installation space.
Setting position of valves on water supply pipeline
- 1. Household pipes, water meters and branch risers (the bottom of the riser, the upper and lower ends of the vertical circular pipe network riser), and the water supply pipes of residential quarters are from the lead-in pipe section of the municipal water supply pipes.
- 2. The nodes of outdoor circular pipe network in residential quarter shall be set according to the separation requirements. When the annular pipe section is too long, the sectional valve should be set.
- 3. The starting point of the water distribution pipe from the indoor water supply pipe to the household, public toilet, etc. shall be set when there are 3 or more water distribution points on the water distribution branch pipe.
- 4. The outlet pipe of the water pump, the suction pump of the self filling water pump, and the lower part of the water supply pipe network should be provided with a drain valve.
- 5. The branch pipe of the ring pipe network, the connecting pipe through the branch pipe network, the starting point of the branch pipe or the connecting pipe from the main water supply pipe of the residential quarter, and the water supply pipe of the equipment (such as heater, cooling tower, etc.).
- 6. The water inlet and outlet pipes, discharge pipes and some accessories of the water tank, such as the front of the automatic exhaust valve, pressure relief valve, water hammer eliminator, pressure gauge, water hydrant, etc., the front and back of the pressure relief valve and backflow preventer, etc.
Check valve for water supply pipeline
On the inlet pipe; on the inlet pipe of the closed water heater or water equipment; on the outlet pipe of the water pump; on the outlet pipe section of the water tank, water tower and highland pool that the inlet and outlet pipes share one pipe.
Note: it is not necessary to install check valve in the pipe section with pipeline backflow preventer.
The position where the water supply pipeline is provided with exhaust device.
- 1. For intermittent water supply network, automatic exhaust valve shall be set at the end and high point of the network.
- 2. For the pipe section with obvious fluctuation and accumulation of air in the water supply network, automatic exhaust valve or manual valve has been set at the peak point of the section for exhaust.
- 3. When the air pressure water supply device adopts the automatic air supply type air pressure water tank, the high point of its water distribution pipe network shall be equipped with an automatic exhaust valve.
Construction scheme of valve and pipeline installation
Direction and position
Many valves have directionality, such as stop valve, throttle valve, pressure reducing valve and check valve. If they are installed reversely, the service effect and life (such as throttle valve) will be affected, or they will not work at all (such as pressure reducing valve), or even cause danger (such as check valve). For general valves, there is a direction mark on the valve body; if not, it shall be correctly identified according to the working principle of the valve. The valve cavity of the stop valve is not symmetrical. The fluid should be allowed to pass through the valve port from bottom to top, so that the fluid resistance is small (determined by the shape), the opening is labor-saving (due to the upward pressure of the medium), and the medium is not pressurized after the closing, which is convenient for maintenance. This is the reason why the stop valve cannot be reversed. Other valves have their own characteristics.
Also called gate plate valve, it is a valve that uses the gate to control the opening and closing, and adjusts the pipeline flow and the opening and closing pipeline by changing the cross section. Gate valves are mostly used for full open or full close operation of fluid medium.
Gate valve installation generally has no direction requirements, but can not be inverted.
Is the use of disc to control the opening and closing of the valve. By changing the clearance between the valve disc and the valve seat, that is, changing the size of the channel section to adjust the medium flow or cut off the medium channel. Pay attention to the flow direction of the fluid when installing the stop valve.
The principle to be followed when installing the stop valve is that the fluid in the pipeline passes through the valve hole from bottom to top, which is commonly known as “low in and high out”. It is not allowed to install the valve reversely.
Check valve, also known as check valve and check valve, is a kind of valve that automatically opens and closes under the action of the pressure difference between the front and the back of the valve. Its function is to make the medium flow in one direction only and prevent the medium from flowing backward.
According to the different structure of check valve, there are lift type, swing type and butterfly clamp type. Lift check valve and horizontal and vertical points.
When installing the check valve, pay attention to the flow direction of the medium, and do not install it reversely.
Pressure reducing valve
- 1. The vertically installed pressure reducing valve group is generally set at a suitable height from the ground along the wall; the horizontally installed pressure reducing valve group is generally installed on the * operation platform.
- 2. The profile steel shall be respectively loaded into the wall outside the two control valves (commonly used for stop valves) to form a bracket, and the bypass pipe shall also be stuck on the bracket for leveling and alignment.
- 3. The pressure reducing valve shall be installed vertically on the horizontal pipeline without tilting. The arrow on the valve body shall point to the flow direction of the medium and shall not be installed reversely.
- 4. Stop valves and high and low pressure gauges shall be installed on both sides to observe the pressure changes before and after the valve. The pipe diameter behind the pressure reducing valve shall be 2 × 3-3 × larger than the inlet diameter in front of the valve, and the bypass pipe shall be installed for maintenance.
- 5. The equalizing pipe of membrane type pressure reducing valve shall be connected to the low-pressure pipe. For low-pressure pipeline, safety valve shall be set to ensure the safe operation of the system.
- 6. When it is used for steam decompression, a drain pipe shall be set. For the pipeline system with high degree of purification, a filter shall be set in front of the pressure reducing valve.
- 7. After the installation of the pressure reducing valve group, the pressure reducing valve and safety valve shall be tested, flushed and adjusted according to the design requirements, and the adjusted marks shall be made.
- 8. When flushing the pressure reducing valve, close the inlet valve of the pressure reducer and open the flushing valve for flushing.
- 1. Block valve (stop valve) shall be set before and after, and filter shall be set between drain valve and front block valve to prevent dirt in condensate from blocking drain valve.
- 2. Check pipe shall be set between the drain valve and the back block valve to check whether the drain valve works normally. If the check pipe is opened for a large amount of steam, it means that the drain valve is broken and needs to be repaired.
- 3. The bypass pipe is set to discharge a large amount of condensate during startup and reduce the drainage load of the drain valve.
- 4. When the drain valve is used to drain the condensed water of the thermal equipment, it shall be installed at the lower part of the thermal equipment to make the condensate pipe return vertically and connect it to the steam trap to prevent water storage of the thermal equipment.
- 5. The installation position shall be as close as possible to the drainage point. If the distance is too far, air or steam will accumulate in the slender pipe in front of the drain valve.
- 6. When the horizontal pipeline of main steam pipe is too long, the drainage problem shall be considered.
- 1. Before installation, the product must be carefully inspected to check whether there is a certificate of conformity and product manual, so as to make clear the constant pressure at the time of delivery.
- 2. The safety valve shall be arranged near the platform as far as possible for inspection and maintenance.
- 3. The safety valve shall be installed vertically, the medium shall flow out from the bottom to the top, and the verticality of the valve rod shall be checked.
- 4. In general, block valve cannot be set at the front and back of safety valve to ensure safety and reliability.
- 5. Relief of safety valve: when the medium is liquid, it is generally discharged into the pipeline or closed system; when the medium is gas, it is generally discharged to the outdoor atmosphere.
- 6. The oil and gas medium can be discharged into the atmosphere generally. The vent pipe outlet of the safety valve shall be 3M higher than the surrounding Zui high structure, but the following situations shall be discharged into the closed system to ensure safety.
- 7. The diameter of the population pipeline, Zui small, shall be equal to the inlet diameter of the valve; the diameter of the discharge pipe shall not be less than the outlet diameter of the valve, and the discharge pipe shall be led to the outdoor, and installed with elbows, so that the pipe outlet faces the safety zone.
- 8. During the installation of safety valve, when the connection between safety valve and equipment and pipeline is open hole welding, its open hole diameter shall be the same as the nominal diameter of safety valve.
The installation position of the valve must be convenient for operation: even if the installation is temporarily difficult, the long-term work of the operator should also be considered. It is better to make the hand wheel of the valve flush with the chest (generally 1.2m away from the operating floor), so that the opening and closing of the valve is more economical. The hand wheel of landing valve shall face up, and do not tilt to avoid awkward operation. The valve of the wall leaning machine and the equipment shall also leave room for the operator to stand. It is not safe to operate in the sky, especially in acid and alkali, toxic media, etc. Do not install the gate upside down (i.e. the hand wheel is downward), otherwise, the medium will be left in the valve cover space for a long time, which is easy to corrode the valve stem, and is forbidden by some process requirements. At the same time, it is inconvenient to change the packing. Open stem gate valve shall not be installed underground, otherwise exposed stem will be corroded due to humidity. When installing the lift check valve, make sure that the valve disc is vertical, so that the lift is flexible. The swing check valve shall be installed with its pin shaft horizontal so that it can swing flexibly. The pressure relief valve shall be installed vertically on the horizontal pipeline and shall not tilt in all directions
The installation and construction must be careful to avoid hitting the valve made of brittle materials.
Before installation, check the valve, check the specification and model, and identify whether there is any damage, especially for the valve stem. Also rotate a few times to see if it is skewed, because in the process of transportation, it is most likely to hit the skewed stem. Also remove the debris from the valve.
When lifting the valve, the rope shall not be tied on the hand wheel or stem, so as to avoid damage to these parts, and it shall be tied on the flange.
The pipeline connected to the valve must be cleaned. Compressed air can be used to blow away iron oxide scraps, mud sand, welding slag and other sundries. These sundries are not only easy to scratch the sealing surface of the valve, but also can block the small valve and make it invalid. When installing the screw valve, the sealing packing (thread hemp and aluminum oil or polytetrafluoroethylene raw material belt) shall be wrapped on the pipe thread, and shall not be put into the valve, so as to avoid the accumulation in the valve and the influence of medium circulation.
When installing the flange valve, pay attention to tighten the bolts symmetrically and evenly. The valve flange and the pipe flange must be parallel with reasonable clearance, so as to avoid excessive pressure or even cracking of the valve. For brittle materials and low strength valves, special attention should be paid. For the valve to be welded with the pipe, spot welding shall be carried out first, then the closed part shall be fully opened, and then the valve shall be welded.
Some valves must also have external protection, which is heat preservation and cold insulation. Sometimes hot steam mixing pipeline is added in the insulation layer. What kind of valve should be insulated or cold, according to production requirements.
In principle, if the medium in the valve reduces the temperature too much, which will affect the production efficiency or freeze the valve, it needs to keep warm, or even mix with heat; if the valve is exposed, which is adverse to production or causes frost and other adverse phenomena, it needs to keep cold. Thermal insulation materials include asbestos, slag cotton, glass wool, perlite, diatomite, vermiculite, etc. cold insulation materials include cork, perlite, foam, plastic and so on.
Bypass and instrumentation
Some valves, in addition to the necessary protection facilities, also have bypass and instruments. Bypass is installed. Convenient for maintenance of drain valve. Other valves also have bypass valves. Whether to install bypass depends on the valve condition, importance and production requirements.
Inventory of valves, some packing has been difficult to use, some with the use of media is not consistent, which requires the replacement of packing.
The valve manufacturer can’t consider using different media of thousands of doors and thousands of types. The stuffing box is always filled with ordinary packing, but when using, the packing must be suitable for the media.
When replacing the packing, press it in circle by circle. Each circle of joint should be 45 degrees, and the circle and the circle should be 180 degrees apart. The packing height should consider the space for the gland to continue to press. At present, it is necessary to make the lower part of the gland press the packing chamber to a proper depth, which can generally be 10-20% of the total depth of the packing chamber. For valves with high requirements, the joint angle is 30 degrees. The joints between the rings shall be staggered by 120 degrees. In addition to the above fillers, according to the specific conditions, three pieces of laminated polytetrafluoroethylene ring (resistant to strong corrosive media below 200 ℃) nylon bowl ring (resistant to ammonia and alkali below 120 ℃) and other shaped fillers can also be used (resistant to weak alkali below 60 ℃, oil crystal below 80 ℃, and various corrosive media below 150 ℃). A layer of polytetrafluoroethylene raw material tape is wrapped outside the ordinary asbestos packing, which can improve the sealing effect and reduce the electrochemical corrosion of the valve rod.
When pressing the seasoning, rotate the valve rod at the same time to keep it uniform around and prevent it from being too dead. Tighten the gland evenly without tilting.
Large diameter and large valves
Engineering pipe diameter comparison table (commonly used):
- 1 inches =25.4 mm =8 English score
- 1/2 is four points (4 points) DN15
- 3/4 is six points (6 points) DN20
- 2 in charge of DN8
- 4 in charge of DN15
- 6 in charge of DN20
- 1 ‘DN25 1.2’ DN32 1.5 ‘DN40 2’ DN50 2.5 ‘DN65
- 3 ‘DN80 4’ DN100 5 ‘DN125 6’ DN150 8 ‘DN200
- 10 ‘DN250 12’ DN300
The expression of 2.4.2 diameter should meet the following requirements:
- pipes for water and gas transportation (galvanized or non Galvanized), cast iron pipes and so on shall be expressed in nominal diameter DN.
- Seamless steel pipe, welded steel pipe (straight or spiral seam), copper tube, stainless steel pipe and other pipes, diameter should be expressed by outer diameter * wall thickness.
- The diameter of reinforced concrete (or concrete) pipes, clay pipes, acid resistant ceramic pipes and cylinder pipes should be expressed in D of inner diameter.
- Plastic pipes, pipe diameters should be expressed in accordance with product standards.
- When the design is used to represent the diameter of the nominal diameter DN, the nominal diameter DN and the corresponding product specifications shall be given.
- Specification for rigid polyvinyl chloride pipe for building drainage (de 5836.1-92 (nominal diameter) * e (nominal wall thickness) (GB 5836.1-92)
- The specification of polypropylene (PP) pipes for water supply is expressed by de * e (nominal outer diameter * wall thickness).
The difference between DN and De:
DN refers to the nominal diameter of the pipeline.
Note: This is neither outer diameter nor inner diameter; it should be related to the early development of the pipeline engineering and the British units; it is usually used to describe the galvanized steel pipe, and the corresponding relationship with the British unit is as follows:
- 4 in charge: 4/8 inch: DN15;
- 6 in charge: 6/8 inch: DN20;
- 1 inch tube: 1 inches: DN25;
- Inch two tube: 1 and 1/4 inches: DN32;
- Inch half tube: 1 and 1/2 inch: DN40;
- Two inch tube: 2 inches: DN50;
- Three inch tube: 3 inches: DN80 (many places are also labeled DN75);
- Four inch tube: 4 inches: DN100;
De mainly refers to the outer diameter of pipes, which are generally marked by De, and need to be marked as X wall thickness.
It is mainly used for Description: seamless steel pipe, PVC and other plastic pipes, and other pipes that require clear wall thickness.
Take galvanized welded steel pipe as an example, using DN and De two annotation methods are as follows:
- DN20 De25X2.5mm
- DN25 De32X3mm
- DN32 De40X4mm
- DN40 De50X4mm
We are used to use DN to label welded pipes, and seldom use De to mark pipes without involving wall thickness.
But marking plastic pipes is another matter. It is still related to industry habits. In actual construction, the 20, 25, 32 pipes we refer to simply refer to De, rather than DN, where there is a difference between specifications. It is very easy to make losses in the course of purchase and construction without clear.
The connection between two kinds of piping materials is more than thread connection and flange connection. Other connections are rarely used.
Galvanized steel pipe and PPR pipe can adopt the above two kinds of connections. Only less than 50 of the pipe is more convenient with thread, and flange is more reliable than 50.
Note: if there are two metal pipes connected with different materials, we should consider whether there will be galvanic reaction. Otherwise, it will accelerate the corrosion rate of the active metal pipes. It is better to use flanged connections and separate the two kinds of metal with insulating materials such as rubber gaskets.
Wall thickness of galvanized steel pipe:
- D 15 – 2. 7 5 mm, DN20-2.75, DN25-3.25, DN32-3.25
- DN40-3.5, DN50-3.5, DN70-3.75, DN80-4, D 100 100 mm.
How big is the size of the water pipe?
See table below: 1. Pipe size inch mm comparison table.
|Pipe size inch mm comparison table|
|Diameter inch||1/4 “||3/8 “||1/2 “||3/4 “||1 “||1.2 “||1.5 “||2 “||2.5 “|
|Outer diameter mm||Thirteen point seven||Seventeen point one four||Twenty-one point three||Twenty-six point seven||Thirty-three point four||Forty-two point two||Forty-eight point three||Sixty point three||Seventy-three|
|Diameter inch||3 “||4 “||5 “||6 “||8 “||10 “||12 “||14 “||16 “|
|Mm in diameter||DN80||DN100||DN125||DN150||DN200||DN250||DN300||DN350||DN400|
|Outer diameter mm||Eighty-eight point nine||One hundred and fourteen point three||One hundred and forty-one point three||One hundred and sixty-eight point three||Two hundred and nineteen point one||Two hundred and seventy-three||Three hundred and twenty-three point eight||Three hundred and fifty-five point six||Four hundred and six point four|
|Note: 1 inches =25.4 mm =8 1 points =1/8 “2 points =1/4” 3 points =3/8 “4 points =1/2” 6 points =3/4 “8 =1”|
2. In detail, how much valve diameter (Interface) should be used for large water pipes?
|The pipe diameter DN (nominal diameter) corresponds to the outer diameter of tube (mm):|
|Small diameter series||18||25||32||38||45||57||73||89||108||133|
|Large diameter series||22||27||34||42||48||60||76||89||114||140|
|Small diameter series||159||219||273||324||360||406||457||508||610|
|Large diameter series||168||219||273||325||377||426||480||530||630|
3. For large water pipes (large diameter), the valve diameter (Interface) should be controlled in detail.
|Pipe diameter DN (nominal diameter) corresponds to outer diameter of pipe (mm).|
|pressure||Matching pipe outer diameter||Nominal pressure||Matching pipe outer diameter|
|Large diameter series outside diameter||Small outer diameter series outside diameter||Large diameter series outside diameter||Small outer diameter series outside diameter|
|DN15||Phi 22||Phi 18||DN125||Phi 140||Phi 133|
|DN20||Phi 27||Phi 25||DN150||Phi 168||Phi 159|
|DN25||Phi 34||Phi 32||DN200||Phi 219||Phi 219|
|DN32||Phi 42||Phi 38||DN250||Phi 273||Phi 273|
|DN40||Phi 48||Phi 45||DN300||Phi 324||Phi 325|
|DN50||Phi 60||Phi 57||DN350||Phi 356||Phi 377|
|DN65||Phi 76||Phi 73||DN400||Phi 406||Phi 426|
|DN80||Phi 89||Phi 89||DN450||Phi 457||Phi 480|
|DN100||Phi 114||Phi 108||DN500||Phi 508||Phi 530|
The reference table is only a complete reference to the pipe diameter.
Relationship between valve diameter and media flow rate
- The flow and flow rate are mainly determined by the valve diameter, and also related to the resistance of the valve’s structural type to the medium, and at the same time, it has certain internal relations with the pressure, temperature and the concentration of the medium.
- The flow area of valve is directly related to flow velocity and flow rate, and flow velocity and flow volume are two interdependent quantities. When the flow rate is constant, the flow rate is large, and the area of the runner can be smaller. Conversely, the flow path area is large, its flow velocity is small, the flow area is small, and its flow velocity is large.
- The flow velocity of the medium is large, and the valve diameter can be smaller, but the resistance loss is large, and the valve is easy to damage. The flow rate is large, which causes electrostatic effect and danger to inflammable and explosive medium. The flow velocity is too small, the efficiency is low, and the economy is not economical. For large viscosity and explosive medium, a smaller flow rate should be taken. Oil and a large viscosity liquid flow with viscosity, generally 0.1 ~ 2m/s.
- Generally speaking, the flow rate is known and the velocity can be determined by experience. The nominal diameter of the valve can be calculated by flow rate and flow rate.
- The valve diameter is the same, its structural type is different, the fluid resistance is also different. Under the same conditions, the greater the drag coefficient of valve, the more the flow and flow rate of fluid flow through the valve. The smaller the resistance coefficient, the less the flow and flow rate of fluid passing through the valve.
- The flow rate in various media is shown below.
Conditions of use
DN > 200
DN > 200
Low pressure steam
P < 1 (absolute pressure)
Medium pressure steam
=1.0 ~ 4 (absolute pressure)
High pressure steam
=4.0 ~ 12 (absolute pressure)
=0 ~ 0.05 (gauge pressure)
2.5 to 15
Semi water gas
=0.1 ~ 0.15 (gauge pressure)
=5 ~ 10 (absolute pressure)
Rho < 0.01 (gauge pressure)
0.05 to 0.3
Concentration 0 to 30%
Concentration 88% to 93%
One point two
One point five
Water and viscosity
=0.1 ~ 0.3 (gauge pressure)
0.5 to 2
Supervisor =0.3 (gauge pressure)
1.5 to 3.5
Boiler feed water
0.5 to 1.5
0.2 to 0.5
Seawater and slightly alkaline water
< 0.6 (gauge pressure)
1.5 to 2.5
- The unit of DN value is mm.
- The unit of value is MPa.
- The resistance coefficient is small, only in the range of 0.1 to 1.5. The large diameter gate valve has a resistance coefficient of 0.2 to 0.5, and the resistance coefficient of the gate valve is larger.
- The drag coefficient is much larger than the gate valve, usually between 4~7. The Y type cut-off valve (DC) has the lowest drag coefficient between 1.5 and 2, and the resistance coefficient of forged steel cut-off valve is the largest, even up to 8.
- The resistance coefficient depends on the structure: the swing check valve is usually about 0.8 to 2, of which the resistance coefficient of the multi valve swing check valve is large, and the lift check valve has the largest resistance coefficient, up to 12.
- The resistance coefficient is small, usually about 0.4 to 1.2.
- The drag coefficient is generally around 2.3.
- The resistance coefficient is small, generally less than 0.5.
- The resistance coefficient is the smallest, generally around 0.1.
- The resistance coefficient of the valve is the value of the valve in full open state.
Valve diameter selection should take into account the valve machining accuracy and size deviation, and other factors. Valve diameter should have a certain amount of affluence, usually 15%. In actual work, the valve diameter depends on the diameter of the process pipeline.
11 taboos for valve and pipeline installation
The installation and use of valves and pipes must be in accordance with the operation specifications, otherwise accidents or even accidents may occur. Below for you to bring the valve, pipeline installation and use of 11 taboos, I hope you like.
Taboo 1: the necessary quality inspection is not carried out before the valve installation.
Consequence: during the operation of the system, the valve switch is not flexible, the closing is not strict and the phenomenon of water (steam) leakage occurs, causing rework and repair, even affecting the normal water supply (steam).
Measures: pressure strength and tightness test shall be conducted before valve installation. 10% of each batch (of the same brand, specification and model) shall be selected for the test, and no less than one. For the closed-circuit valves installed on the main pipe to cut off, strength and tightness tests shall be conducted one by one. The valve strength and tightness test pressure shall comply with the provisions of code for acceptance of construction quality of building water supply and drainage and heating engineering (GB 50242-2002).
Taboo 2: the specification and model of the installed valve do not meet the design requirements. For example, the nominal pressure of the valve is less than the system test pressure; when the pipe diameter of the water supply branch pipe is less than or equal to 50 mm, the gate valve is used; the stop valve is used for the dry and vertical pipes of hot water heating; the butterfly valve is used for the water suction pipe of the fire pump.
Consequence: affect the normal opening and closing of the valve and adjust the resistance, pressure and other functions. Even cause system operation, valve damage forced to repair.
Measures: be familiar with the application scope of various valves, and select the specifications and models of valves according to the design requirements. The nominal pressure of the valve shall meet the requirements of system test pressure. According to the requirements of construction specification: stop valve shall be used when the diameter of water supply branch pipe is less than or equal to 50mm; gate valve shall be used when the diameter is greater than 50mm. Gate valve shall be used for hot water heating dry and vertical control valve, and butterfly valve shall not be used for fire pump suction pipe.
Taboo 3: wrong valve installation method. For example, the water (steam) flow direction of stop valve or check valve is opposite to the sign, the valve stem is installed downward, the check valve installed horizontally is installed vertically, the handle of rising stem gate valve or butterfly valve has no space for opening and closing, and the valve stem of concealed valve does not face the inspection valve.
Consequence: valve failure, switch maintenance difficulties, stem down often cause water leakage.
Measures: install in strict accordance with the valve installation instructions. The rising stem gate valve shall have enough stem extension opening height. The handle rotation space shall be fully considered for the butterfly valve. The stem of various valves shall not be lower than the horizontal position or downward. The concealed valve shall not only be provided with an inspection valve meeting the opening and closing requirements of the valve, but also the valve stem shall face the inspection valve.
Taboo 4: ordinary valve flange is used for butterfly valve flange.
Consequence: the size of butterfly valve flange is different from that of common valve flange. Some of the inner diameter of the flange is small, while the valve disc of butterfly valve is large, which results in failure to open or hard opening and damages the valve.
Measures: the flange plate shall be processed according to the actual size of butterfly valve flange.
Taboo 5: during pipeline welding, the staggered joint of pipes after butt joint is not on a central line, no gap is left for butt joint, no groove is cut for thick wall pipes, and the width and height of weld do not meet the requirements of construction specifications.
Consequence: if the pipe is not in the same center line, it will directly affect the welding quality and appearance quality. No gap shall be left for butt joint, no groove shall be cut for thick wall pipe, and when the width and height of weld do not meet the requirements, the welding cannot meet the strength requirements.
Measures: after the butt joint of the welded pipe, the pipe shall not be staggered. It shall be on a central line. The butt joint shall have a gap. The thick wall pipe shall be beveled. In addition, the width and height of the weld shall be welded according to the specification requirements.
Taboo 6: the pipeline is directly buried in frozen soil and untreated loose soil, and the spacing and position of pipeline buttresses are improper, even in the form of dry bricks.
Consequence: the pipeline is damaged in the process of backfill compaction due to unstable support, resulting in rework and repair.
Measures: the pipeline shall not be buried on frozen soil and untreated loose soil. The distance between buttresses shall meet the requirements of construction specifications, and the supporting pad shall be firm, especially at the pipeline interface, which shall not bear shear force. Brick buttresses shall be built with cement mortar to ensure integrity and firmness.
Taboo 7: the material of the expansion bolt for fixing the pipe support is poor, the hole diameter for installing the expansion bolt is too large or the expansion bolt is installed on the brick wall or even the light wall.
Consequence: the pipe support is loose, the pipe is deformed or even falls off.
Measures: qualified products must be selected for expansion bolts. If necessary, samples shall be taken for test and inspection. The hole diameter for installing expansion bolts shall not be greater than 2mm of the outer diameter of expansion bolts. Expansion bolts shall be applied to concrete structures.
Taboo 8: the strength of flange plate and gasket for pipeline connection is not enough, and the connecting bolt is short or the diameter is thin. Rubber pad is used for heat pipe, asbestos pad is used for cold water pipe, double-layer pad or bevel pad is used, and flange pad protrudes into the pipe.
Consequence: the flange connection is not tight, even damaged, and leakage occurs. When the flange gasket protrudes into the pipe, it will increase the flow resistance.
Measures: the flange plate and gasket used for the pipeline must meet the requirements of the design working pressure of the pipeline. Rubber asbestos pad shall be used for flange gasket of heating and hot water supply pipes; rubber pad shall be used for flange gasket of water supply and drainage pipes. The gasket of the flange shall not protrude into the pipe, and the outer circle shall be suitable to the bolt hole of the flange. No inclined pad or several pads shall be placed in the middle of the flange. The diameter of the bolt connecting the flange shall be less than 2mm than the flange hole diameter, and the length of the protruding nut of the bolt rod shall be 1 / 2 of the nut thickness.
Taboo 9: during the hydrostatic strength test and tightness test of the pipeline system, only the pressure value and water level change are observed, and the leakage inspection is not enough.
Consequence: leakage occurs after the operation of the pipeline system, affecting normal use.
Measures: when the pipeline system is tested according to the design requirements and construction specifications, in addition to recording the pressure value or water level change within the specified time, it is necessary to carefully check whether there is leakage.
Taboo 10: the pipeline system is not washed carefully before completion, and the flow and speed cannot meet the requirements of pipeline flushing. It even uses the hydraulic strength test to drain water instead of flushing.
Consequence: if the water quality fails to meet the operation requirements of the pipeline system, the pipeline section will be reduced or blocked.
Measures: flush with the maximum designed juice flow or water flow rate not less than 3m / s in the system. The water color and transparency of the outlet shall be consistent with that of the inlet through visual inspection.
Taboo 11: carry out water pressure test under negative temperature during winter construction.
Consequence: due to the water pressure test, the pipe quickly freezes and damages the pipe.
Measures: try to carry out water pressure test before construction in winter, and blow out the water after pressure test, especially the water in the valve must be cleaned, otherwise the valve will freeze and crack. During the water pressure test in winter, the project shall be carried out under the positive indoor temperature, and the water shall be blown clean after the pressure test. When the hydrostatic test cannot be carried out, compressed air can be used for the test.
Source: Network Arrangement – China Valve Manufacturer – Yaang Pipe Industry Co., Limited (www.metallicsteel.com)
(Yaang Pipe Industry is a leading manufacturer and supplier of nickel alloy and stainless steel products, including Super Duplex Stainless Steel Flanges, 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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