Preliminary selection of valves

What is the purpose of valve preliminary selection?
The selection of valve selection is often not achieved overnight, especially for beginners. In order to calculate the discharge coefficient, estimate the noise, etc. It is necessary to select the structure of the valve body to obtain some calculation parameters. If the calculation results do not meet the requirements, the valve type should be re selected. The actuator, the form of upper valve cover and the material of valve internals can be ignored temporarily.
What is the basis of primary valve type selection?
According to the nature of the medium, process parameters, use requirements of the primary selection of valve type.
1、 Which of the following media properties should be specified
1. General fluid;
2. High viscosity, suspended solids, fibrous and other fluids;
3. Toxic or expensive fluids;
4. Corrosive fluid.
2、 Which of the following types of process parameters should be specified
1. Low pressure, small pressure difference and large flow rate;
2. Medium pressure or low pressure, large pressure difference;
3. High temperature, high pressure and high pressure difference;
4. Temperature and temperature difference;
3、 The use requirements are one or more of the following
1. The leakage is small or cut-off is required;
2. Good stability and convenient maintenance;
3. Requirements for the weight, size and volume of the control valve;
4. Action speed requirements.
What should be known about valve control medium? What else should be mastered about process parameters?
The specific name, property and main physicochemical parameters of controlled medium, density, kinematic viscosity, molecular weight, thermodynamic critical pressure, valve inlet temperature, saturated vapor pressure, specific heat capacity ratio, compression coefficient, etc. Process parameters should be generally understood: flow, pressure before valve, pressure after valve, inlet temperature, etc.
What else should be mastered about valve service conditions?
It is also necessary to master the piping form of the control valve, the pipe diameter before and after the valve, and the system resistance. The main parameters and main interference factors should be understood in the type and characteristics of the automatic control object, and the leakage and stability requirements should be understood in the regulation performance requirements.


How to choose valve positioner correctly

How do valve positioners and control valves form a loop?
The valve positioner is the main accessory of the control valve. It is used with the regulating valve to receive the output signal of the regulator, and then control the regulating valve with its output signal. After the regulating valve acts, the displacement of the valve stem is fed back to the valve positioner through the unloading device. Therefore, this kind of positioner and the regulating valve form a closed-loop loop loop.
How to classify valve positioners according to structure and working principle?
Valve positioners can be divided into pneumatic valve positioner, electro pneumatic valve positioner and intelligent valve positioner according to its structure and working principle.
What are the functions of valve positioners?
1. Improve the static characteristics of the control valve and improve the linearity of the valve position.
2. Improve the dynamic characteristics of the control valve and reduce the signal transmission lag of the control valve.
3. Change the flow characteristics of the control valve.
4. Change the response range of control valve to signal pressure to realize split range control.
4. Reverse the valve action.
Why can the valve positioner ensure the correct positioning of the control valve?
The valve positioner can increase the output power of the control valve, reduce the transmission lag of the regulating signal, speed up the moving speed of the valve stem, improve the linearity of the valve, overcome the friction of the valve stem and eliminate the influence of the unbalanced force, so as to ensure the correct positioning of the control valve.
Why should a meter be equipped with a locator under high pressure medium?
When the control valve is used in high-pressure medium, in order to prevent the fluid from leaking from the valve stem packing, the packing is often tightly capped. Therefore, a great friction force is generated in the valve stem, which is caused by the error of the stem stroke. After the positioner is used, it can overcome the effect of these friction forces and the effect of fluid imbalance force, which obviously improves the basic characteristics.
What is the function of positioner under high pressure difference?
When the pressure difference between the two ends of the control valve is greater than 1MPa, the medium will produce a large unbalanced force on the valve core, which will destroy the original working position and cause disturbance to the control system. Especially for the single seat control valve, its unbalanced force is greater than that of the double seat valve. The use of positioner can increase the output pressure, increase the output force of the actuator and overcome the unbalanced effect.


Comparison between traditional valve positioner and intelligent valve positioner

What is the difference between traditional valve positioner and intelligent valve positioner?
The valve opening signal indicated by the traditional valve positioner is actually the signal output from the indicating computer or instrument to the valve positioner, so the actual site position often deviates from the feedback position.
The intelligent valve positioner is equipped with a valve position feedback signal transmitter, which can output the actual action position signal of the valve, and can be fed back to the computer or instrument through the communication protocol. The intelligent valve positioner realizes the digital display and storage of valve position, which provides useful data for predictive maintenance and management of control valve. It also provides an alarm signal to provide fast and correct information for the operator to deal with the accident.
What is the difference of energy saving between traditional valve positioner and intelligent valve positioner?
Traditional valve positioner uses pneumatic amplifier as the controller of secondary loop. In normal operation, the pneumatic amplifier needs a certain amount of air consumption. Two or four piezoelectric valves are used in the intelligent valve positioner. When the input signal of the piezoelectric valve is 1, it is opened and when it is 0, it is closed. Therefore, compressed air is consumed only when the inlet air valve is opened, and no compressed air is consumed under other working conditions (the inlet air pressure electric valve is closed and the exhaust pressure electric valve is opened or closed). Therefore, the intelligent valve positioner has energy-saving performance.
What is the difference between traditional valve positioner and intelligent valve positioner?
In the cascade secondary ring composed of traditional valve positioner, the secondary controller is a pure proportional control law. Therefore, there is a residual difference between the valve position and the set value of the auxiliary ring, which can not guarantee the one-to-one correspondence between the input and output of the auxiliary ring. The control accuracy of traditional valve positioner is generally ± 1%.
The intelligent valve positioner adopts software control algorithm, and the proportional integral function can be used to realize the control without residual error, thus greatly improving the control accuracy. The control accuracy of intelligent valve positioner can reach ± 0.2%.
What are the advantages of intelligent valve positioner?
Through the analysis and comparison of the above theory and engineering application, the intelligent valve positioner can improve the performance of the control system and make the field adjustment more convenient.
1. The method of nonlinear characteristic compensation is extended. The nonlinear characteristics of the controlled object can be compensated by the flow characteristics of the control valve. It can be compensated by ordinary valve positioner or only valve positioner, and can be realized by intelligent digital controller in series with nonlinear link. From the point of view of reducing investment cost and maintenance, the use of intelligent valve positioner or digital controller in series to connect nonlinear compensation link has the advantages of soft connection function, which can easily change the nonlinear characteristics.
2. The type selection of control valve becomes simple. The flow characteristics of control valve can be all star flow characteristics or flow characteristics with convenient processing and high precision, so as to reduce the type of flow characteristics of control valve. If the sleeve and other valve internals are used, the valve body size can be reduced, and the type and specification of spare parts of the control valve can be greatly reduced. The sleeve valve and other control valves can also use balanced valve internals and other components for high pressure difference and noise reduction applications.
3. The valve position detection of valve positioner is non-contact, which can greatly improve the performance of the circuit.


Selection of valve accessories — solenoid valve

What are the structural forms of three-way solenoid valve? For what occasion?
Three way solenoid valve can be divided into ordinary type, waterproof type, explosion-proof type and so on.
According to its action mode, it can be divided into direct acting type and pilot type. In the direct acting type, the coil pulls in the iron core to directly drive the stop valve to switch the gas path; in the pilot type, the coil pulls in the moving iron core to change the flow direction of elegant and popular air, and pushes the stop valve to switch the gas path through the compressed air.
According to its working state, it can be divided into two types: normally on type (open when power on, close when power off) and normally closed type (close when power on, open when power off).
The three-way solenoid valve is used in single acting pneumatic actuator and pneumatic diaphragm actuator to complete the automatic switching of air circuit to realize the opening and closing action of regulating valve. It is usually used in remote control, sequence control and interlocking system.
Why do the solenoid valves used in the interlock system always work in the normally energized state?
This is from the perspective of ensuring safety and reliability.
1. The interlocking system of petroleum and chemical plant is set up to ensure safety in production and prevent accidents, so the reliability of its sending message period and actuator is very high. The solenoid valve in the interlocking system doesn’t act at ordinary times, but only acts in case of accident. According to the working principle of the solenoid valve, the coil closes the moving iron core when it is powered on, and drives the valve parts to switch. When the power is off, the moving iron core and valve parts return to the original position by the action of the reset spring. If there is no power supply for a long time at ordinary times, due to rust, dirt invasion and other reasons, the moving iron core and valve parts may be stuck. In case of emergency power on, the coil will not be able to suck, and the guide finger will be out of action. If there is no power supply for a long time at ordinary times, due to the vibration of the solenoid valve, it can prevent jamming. In case of power failure due to accident, the switch can be carried out reliably by the action of return spring.
2. It is difficult to know whether the solenoid valve works normally when it is in power-off state. But usually in the power on state, once the solenoid valve itself failure, it can be checked out at any time, which is very important to ensure the reliability of the interlocking system.
3. In case of power failure, the solenoid valve can still operate reliably.
Due to the above reasons, the solenoid valve used in the interlocking system generally works under the state of constant power on, which is opposite to the situation described in the instruction manual of the solenoid valve. Therefore, the normally closed solenoid valve should be selected for the normally open situation, and the normally open solenoid valve should be selected for the normally closed situation.
What problems should be paid attention to when installing the three-way solenoid valve.
1. It should be installed vertically. The stop valve part of pilot solenoid valve should be in horizontal position.
2. Attention should be paid to the symbols of each nozzle, and do not touch it to prevent misoperation or accidents.
3. The power supply used shall meet the power requirements specified on the product name plate to prevent the coil from being burnt out.
4. The working gas source should be well purified.


Calculation of regulating valve diameter

What is the principle of caliber calculation?
In different automatic control systems, the flow, medium, pressure, temperature and other parameters vary greatly, and the flow coefficient of the control valve is tested under the pressure difference of 100KPA and the medium is normal temperature water. It is not possible to compare the actual flow with the flow coefficient of the valve (because of the different conditions of pressure difference and medium), but the value of C must be calculated. The actual parameters are brought into the corresponding C value calculation formula, and the C value is compared with the CZ value of the valve, so as to determine the valve diameter. Finally, relevant checking calculation should be carried out to further verify whether the selected valve can meet the working requirements.
What are the steps of regulating valve diameter calculation?
From the process to the final valve diameter determination, the following steps are generally required.
1、 Determine the use conditions.
1. The name, properties and main physicochemical parameters of the medium passed;
2. Understand the process parameters clearly;
3. Piping condition (pipe diameter before and after the valve, calculation of system resistance);
4. The type and characteristics of the controlled object;
5. Adjustment performance requirements, such as requirements for leakage and stability.
2、 Primary selection of valve type, and determine the flow characteristics and flow direction.
3、 Determination of calculated flow rate. According to the existing production capacity, equipment load and medium conditions, the maximum working flow Qmax and the minimum working flow qmin are determined.
4、 The decision to calculate the differential pressure. The s value is selected according to the characteristics of the system, and then the differential pressure is calculated.
5、 Kv value calculation. According to the calculated flow rate, calculated pressure difference and other relevant parameters, the kvmax of the maximum working flow which determines the regulating valve preliminarily is obtained.
6、 According to the calculated kvmax, the first gear Kv value larger than kvmax and close to kvmax is selected to obtain the caliber.
7、 Opening checking calculation.
8、 Actual adjustable ratio checking calculation. Generally, the actual adjustable ratio should be greater than 10.
9、 Pressure difference proofreading (only checking from the opening and adjustable ratio is not good, which may cause the valve to be closed and start, so this item is added).
10、 If the above checking calculation is qualified, the selected valve diameter is qualified. If it is not qualified, the caliber (and Kv value) shall be re determined or other valves shall be selected for re checking to be qualified.


Advantages and disadvantages of diaphragm valve

Diaphragm valve
The structure of diaphragm valve is very different from that of general valve. It is a new type of valve and a special type of block valve. Its opening and closing part is a diaphragm made of soft material, which separates the inner cavity of the valve body from the inner cavity of the valve cover and the driving parts. It is widely used in various fields. The commonly used diaphragm valves are rubber lined diaphragm valve, fluorine lined diaphragm valve, unlined diaphragm valve and plastic diaphragm valve. The valve core assembly is replaced by a corrosion-resistant lined valve body and a corrosion-resistant diaphragm, which uses the movement of the diaphragm to play a regulating role. The valve body material of diaphragm valve is cast iron, cast steel or cast stainless steel, and lined with various corrosion-resistant or wear-resistant materials, diaphragm material rubber and polytetrafluoroethylene. The lining diaphragm has strong corrosion resistance and is suitable for adjustment of strong corrosive media such as strong acid and alkali.
Diaphragm valve
Pneumatic diaphragm valve
Advantages of diaphragm valve
1. The structure of diaphragm valve is simple, the fluid resistance is small, and the flow capacity is larger than other types of valves of the same specification;
2. Suitable for corrosive, viscous and slurry media.
3. The valve is easy to be disassembled and maintained quickly, and the diaphragm can be replaced on site and in a short time.
4. No leakage, it can be used to adjust the medium with high viscosity and suspended particles. The diaphragm separates the medium from the upper chamber of the valve stem, so the medium without packing will not leak out.
5. The flow characteristic of the diaphragm valve is close to the quick opening characteristic, which is approximately linear before 60% stroke, and the flow rate changes little after 60%.
6. Pneumatic diaphragm valve can also be equipped with feedback signal, limiter and positioner to meet the needs of automatic control, program control or flow regulation.
7. The feedback signal of pneumatic diaphragm valve adopts contactless sensing technology. The product uses membrane type propulsion cylinder instead of piston cylinder, which eliminates the defects of piston ring easily damaged, resulting in leakage and unable to push the valve to open and close. When the air source fails, the hand wheel can be operated to open and close the valve.
Disadvantages of diaphragm valve
1. Can not be used in high pressure situation.
2. Due to the limitation of diaphragm and lining material, its pressure resistance and temperature resistance are poor. It is generally only applicable to 1.6Mpa nominal pressure and below 150 ℃.


How to select valve gasket correctly

Gasket is a mechanical seal between two objects, which is usually used to prevent pressure, corrosion, and pipeline natural thermal expansion and contraction leakage. Since the machined surface cannot be perfect, the irregularity can be filled with a gasket.
1. The correct selection of sealing gasket is the key to ensure no leakage of valve. For the same working condition, there are several kinds of pads
Notes on Selection
Correct selection of valve sealing gasket is the key to ensure no leakage of equipment. For the same working condition, there are several kinds of gaskets to choose from. According to the physical properties, pressure, temperature, equipment size, operating conditions and continuous operation period of the medium, the gasket should be selected reasonably to make full use of the characteristics of various gaskets.
When selecting gaskets, the following factors should be fully considered:
(1) It has good elasticity and resilience, and can adapt to pressure change and temperature fluctuation;
(2) It has proper softness and fits well with the contact surface;
(3) It does not pollute the process medium;
(4) It has enough toughness without damage due to pressure and tightening force;
(5) At low temperature, it does not harden and its shrinkage is small;
(6) Good processing performance, easy installation and pressing;
(7) The sealing surface is not bonded and easy to disassemble;
(8) Low price and long service life.
In the use of gasket, the pressure and temperature are mutually restricted. With the increase of temperature, after the equipment operates for a period of time, the gasket material will soften, creep and stress relaxation, and the mechanical strength will also decrease, and the sealing pressure will be reduced. vice versa. For example, it is listed in the manual that the use temperature of high-pressure asbestos rubber sheet xb450 in water and steam medium is 450 ℃ and the pressure is less than 6Mpa (when the material is used for sealing performance test, the pressure shall be kept at 440 ℃ ~ 450 ℃ and 12MPa steam for 30 minutes). But in the long-term practical use, if the temperature reaches 450 ℃, the sealing pressure is only 0.3 ~ 0.4MPa. For the gas medium with strong permeability, it is only 0.1 ~ 0.2MPa.
The above situations should be fully considered.
2. Selection of domestic gasket
(1) Industrial rubber plate
Natural rubber is suitable for water, sea water, air, inert gas, alkali, salt aqueous solution and other media, but it is not resistant to mineral oil and non-polar solvents. Its long-term service temperature is not more than 90 ℃, and its low-temperature performance is excellent. It can be used above – 60 ℃.
NBR is suitable for petroleum products, such as petroleum, lubricating oil, fuel oil, etc. the long-term service temperature is 120 ℃, if in hot oil, it can withstand 150 ℃, and the low temperature is – 10 ~ – 20 ℃.
Chloroprene rubber is suitable for seawater, weak acid, weak alkali and salt solution. It has excellent oxygen and ozone aging resistance. Its oil resistance is inferior to that of nitrile rubber and superior to other general rubber. Its long-term service temperature is lower than 90 ℃, the maximum service temperature is not more than 130 ℃, and the low temperature is – 30 ~ – 50 ℃.
There are many varieties of fluororubber, which have good acid resistance, oxidation resistance, oil resistance and solvent resistance. It can be used in almost all kinds of acid media and some oils and solvents, and its long-term use temperature is lower than 200 ℃.
As flange gasket, rubber plate is mostly used for pipeline or manhole and handhole which are often disassembled, and the pressure is not more than 1.568 MPa. Because in all kinds of gaskets, rubber gasket is the softest and has good fitting performance. It can play a sealing effect under a small preload. Because of this, when bearing internal pressure, the gasket is easy to be extruded due to its thick or low hardness.
When rubber sheet is used in organic solvents such as benzene, ketone and ether, swelling, weight gain, softening and stickiness are easy to occur, leading to sealing failure. Generally, it can not be used if the swelling degree is more than 30%.
Under the condition of low pressure (especially below 0.6MPa) and vacuum, rubber pad is more suitable. The rubber material has good compactness and low air permeability. For example, fluororubber is the most suitable sealing gasket for vacuum container, and the vacuum degree can reach 1.3 × 10-7 PA. When the rubber pad is used in the vacuum range of 10-1 ~ 10-7 PA, it needs to be baked and pumped.
(2) Asbestos rubber sheet
The price is lower than other gaskets, easy to use; the biggest problem is that although the gasket material is added with rubber and some fillers, it still can not completely fill those micro pores in collusion, and there is micro permeability. Therefore, in the highly polluting medium, even if the pressure and temperature are not high, it can not be used. When used in some high-temperature oil medium, usually in the later stage of use, due to the carbonization of rubber and filler, the strength is reduced, and the material becomes loose, which causes penetration at the interface and inside the gasket, resulting in coking and smoke. In addition, the asbestos rubber plate is easy to stick on the flange sealing surface under high temperature, which brings a lot of trouble to replace the gasket.
Under the condition of heating, the service pressure of gasket in various media depends on the strength retention rate of gasket material. There are crystal water and adsorbed water in asbestos fiber materials. At 110 ℃, 2 / 3 of the adsorbed water was precipitated, and the tensile strength of the fiber was reduced by 10%; at 368 ℃, all the adsorbed water precipitated, and the tensile strength of the fiber decreased by 20%; when the temperature exceeded 500 ℃, the crystallized water began to precipitate, and the tensile strength of the fiber was lower. The medium also has great influence on the strength of asbestos rubber sheet. For example, in aviation lubricating oil and aviation fuel, the transverse tensile strength difference of No. 400 oil resistant asbestos rubber sheet is 80%, which is due to the swelling of rubber in the plate by aviation fuel is more serious than that of aviation lubricating oil. Considering the above factors, the recommended safe use range of domestic asbestos rubber sheet xb450 is as follows: temperature 250 ℃ ~ 300 ℃, pressure 3 ~ 3.5Mpa; temperature of 400 oil resistant asbestos rubber sheet


The basic classification knowledge of valves

Valve is used to control the medium in the pipeline, with a movable mechanism of mechanical products. Its basic function is to connect or cut off the flow of pipeline medium, change the flow direction of medium, adjust the pressure and flow of medium, and protect the normal operation of pipeline and equipment.
Valve type
With the development of modern science and technology, valves are widely used in T industry, construction, agriculture, national defense, scientific research and people’s life. Valves have become indispensable general mechanical products in various fields of human activities.
Industrial valve was born after the invention of steam engine. In recent ten years, due to the needs of petroleum, chemical industry, power station, metallurgy, shipbuilding, nuclear energy, aerospace and other aspects, higher requirements have been put forward for the valve, which urges people to study and produce high parameter valve FJ. The working temperature ranges from – 269 ℃ to 3430 ℃, and the working pressure from 1.33 × l0 ^ – 8 to 1460mpa. The nominal size of the valve ranges from micrometer gauge valve to industrial pipeline valve with nominal size of more than ten meters and weight of dozens of tons. The driving mode has developed from manual operation to electric, pneumatic and hydraulic operators to program control, numerical control and remote control.
It can be seen from the above description that the valve is widely used. However, if the valve is not properly selected, used and maintained, resulting in valve leakage, fire, explosion, poisoning, scald accidents, energy waste, equipment corrosion, material consumption increase, environmental pollution, and even shutdown, the consequences will be unimaginable. How to correctly understand, select, install, use and maintain valves is an urgent problem for users and engineers.
Classification of valves
With the continuous improvement of process flow and performance of various complete sets of equipment, the types of valves are also constantly changing and increasing. There are many kinds of valve classification methods, the classification methods are different, the results are not the same, several commonly used classification methods are as follows.
1.2.1 classification by use and function
The valves can be classified into block valve, check valve, diverter valve, regulating valve, safety valve, other special purpose valve and multi-purpose valve.
Block valve – mainly used to cut off or connect the medium in the pipeline. Such as gate valve, globe valve, ball valve, plug valve, butterfly valve, etc. Check valve type – used to prevent backflow of medium. Such as check valve, etc. Shunt broad class — used to change the flow direction of medium in the pipeline, which plays the role of distributing, dividing or mixing medium. Such as two-way or four-way plug valve T-way or four-way ball valve, distribution valve, etc.
Regulating valve – mainly used to regulate the flow and pressure of medium. Such as regulating valve, pressure reducing valve, flow valve, balance valve, etc. Safety valve – used for overpressure safety protection, discharge excess medium to prevent the pressure from exceeding the rated value of safety valve. When the pressure returns to normal, the valve will be closed to prevent the medium from flowing out. Such as a variety of safety valves, torrent valves, etc.
Other special valves such as steam trap, vent valve, slag discharge valve, blowdown valve, pigging valve, etc.
Multi purpose valves, such as stop check valve, check ball valve, stop check safety valve, etc.
1.2.2 classification by power source
According to the power source, the valve can be divided into automatic valve and drive valve. Automatic valve – a valve that operates by itself depending on the capacity of the medium (liquid, air, steam, etc.). Such as safety valve, check valve, pressure reducing valve, steam trap, emergency shut-off valve, etc.
Actuated valve – a valve operated by manual, electrical, pneumatic, or hydraulic power. Such as gate valve, globe valve, ball valve, butterfly valve, diaphragm valve, etc.
1.2.3 classification by main technical parameters
(1) Classification by nominal size
The valve can be divided into small diameter valve and medium diameter valve according to the nominal size
Door, large diameter valve and extra large diameter valve.
Small diameter valve – valve with nominal size ≤ Dn40.
Medium diameter valve is a valve with nominal size DN, DN50 ≤ DN ≤ DN300.
Large diameter valve – valve with nominal size DN, DN350 ≤ DN ≤ DN1200.
Extra large diameter valve – the valve with nominal size greater than or equal to dn 1400.
(2) Classification by nominal pressure
According to the nominal pressure, the valve can be divided into vacuum valve, low pressure valve, medium pressure valve, high pressure valve and ultra-high pressure valve.
Low vacuum valve – 10 ^ 5 ~ 10 ^ 2PA.
Medium vacuum valve – 10 ^ 2 ~ 10 ^ – 1pA.
High vacuum valve – 10 ^ – 1 ~ 10 ^ – 5pa.
Ultra high vacuum valve – 10 ^ – 5 PA.
Low pressure valve – valve with nominal pressure ≤ PN16.
Medium pressure valves – PN16 & lt; valves with nominal pressure ≤ PN100.
High pressure valves – PN100 & lt; valves with nominal pressure ≤ pn1000.
Ultra high pressure valve – nominal pressure & gt; pn1000 valve


The advantages and disadvantages of all valves are here

1. Valve selection and setting location:
(1) Valves used on water supply pipelines are generally selected according to the following principles:
1. When the pipe diameter is not more than 50mm, a stop valve should be used; when the pipe diameter is more than 50mm, a gate valve or butterfly valve should be used
2, when the flow and water pressure need to be adjusted, a regulating valve and a stop valve should be used
3. For parts that require low flow resistance (such as on the suction pipe of a pump), gate valves should be used
4. Gate valves and butterfly valves should be used on pipe sections where water flow needs to flow in both directions, and stop valves should not be used
5. Butterfly valves and ball valves should be used in places where the installation space is small
6. ​​It is advisable to use shut-off valves on pipe sections that are frequently opened and closed
7, a multi-function valve should be used on the outlet pipe of the pump with a larger diameter
(2) Valves should be installed at the following locations on the water supply pipeline:
1. The water supply pipeline of the residential quarter is from the introduction pipe section of the municipal water supply pipeline
2. The nodes of the outdoor loop pipe network of the residential quarter should be set up according to the separation requirements. When the annular pipe section is too long, a segmented valve should be installed
3. The beginning of the branch pipe connected to the main water supply pipe of the residential area or the beginning of the household pipe
4. Household pipes, water meters and branch risers (the bottom of the riser, the upper and lower ends of the riser of the vertical ring pipe network)
5. The branch pipes of the ring-shaped pipe network and the connecting pipes through the branched pipe network
6. ​​The end of the water distribution pipe connecting the indoor water supply pipe to the households, public toilets, etc., and the water distribution point on the water distribution branch pipe is set when there are 3 or more
7. Outlet pipe of water pump, suction pump of self-filling water pump
8. The inlet and outlet pipes and drain pipes of the water tank
9. The water inlet and make-up pipes of equipment (such as heaters, cooling towers, etc.)
10. Water distribution pipes for sanitary appliances (such as urinals, washbasins, showers, etc.)
11. Some accessories, such as automatic exhaust valve, pressure relief valve, water hammer eliminator, pressure gauge, sprinkler, etc., before and after pressure reducing valve and backflow preventer, etc.
12. A drain valve should be installed at the lowest point of the water supply network

Advantages and disadvantages of valves

(3) Check valves should generally be selected according to factors such as their installation location, water pressure before the valve, sealing performance requirements after closing, and the size of the water hammer caused by closing
1. If the water pressure in front of the valve is small, it is advisable to use swing, ball and shuttle check valves
2. When the airtight performance after closing is tight, a check valve with closing spring should be selected
3. When it is required to weaken the shut-off water hammer, a quick-closing muffler check valve or a slow-closing check valve with a damping device should be selected
4. The valve break or spool of the check valve should be able to close automatically under the action of gravity or spring force
(4) Check valves shall be provided on the following pipe sections of the water supply pipeline:
on the introduction pipe; on the inlet pipe of a 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 where the inlet and outlet pipes are combined.
Note: The pipe section equipped with the pipe backflow preventer does not need to install a check valve.
(5) Exhaust devices should be installed in the following parts of the water supply pipeline:
1. For the water supply pipe network used intermittently, the end and highest point of the pipe network should be equipped with automatic exhaust valves
2. There are obviously undulating pipe sections in the water supply network that accumulate air, and automatic exhaust valves or manual valves have been set up at the peak of this section.
3. Air pressure water supply device. When automatic air-supply air pressure water tank is used, the highest point of the water distribution pipe network should be equipped with an automatic exhaust valve
2. Advantages and disadvantages of various valves:
1. Shanghai Chuanhu Gate Valve: Gate valve refers to a valve whose closing member (gate) moves along the vertical direction of the channel axis. It is mainly used as a cut-off medium in the pipeline, that is, fully open or fully closed. Generally, the gate valve cannot be used as an adjustment flow. It can be applied to low temperature and pressure as well as high temperature and high pressure, and can be based on different materials of the valve. But gate valves are generally not used in pipelines that transport mud and other media
①The fluid resistance is small;
②The torque required for opening and closing is small;
③It can be used on the ring network pipeline where the medium flows in both directions, that is to say, the flow direction of the medium is not restricted;
④When fully open, the erosion of the sealing surface by the working medium is smaller than that of the stop valve;
⑤The body structure is relatively simple, and the manufacturing process is better;
⑥The structure length is relatively short.
①The overall dimensions and opening height are large, and the installation space required is also large;
②In the process of opening and closing, the sealing surface is relatively rubbed by people, and the abrasion is large, even at high temperature, it is easy to cause abrasion;
③Generally, gate valves have two sealing surfaces, which adds some difficulties to processing, grinding and maintenance;
④Long opening and closing time.
2. Butterfly valve: A butterfly valve is a valve that uses a disc-type opening and closing member to reciprocate about 90° to open, close and adjust the fluid channel.
①Simple structure, small size, light weight, saving consumables, do not use in large-diameter valves;
②Rapid opening and closing, low flow resistance;
③It can be used for media with suspended solid particles, and it can also be used for powder and granular media depending on the strength of the sealing surface. It can be applied to the two-way opening and closing and adjustment of ventilation and dust removal pipelines, and is widely used in gas pipelines and waterways in metallurgy, light industry, electric power, and petrochemical systems.
①The flow adjustment range is not large, when the opening reaches 30%, the flow will enter more than 95%;
②Due to the limitation of the structure of the butterfly valve and the sealing material, it is not suitable for use in high temperature and high pressure piping systems. The general working temperature is below 300℃ and below PN40;
③The sealing performance is worse than that of ball valves and globe valves, so it is used in places where the sealing requirements are not very high.
3. Ball valve: evolved from a plug valve, its opening and closing part is a sphere, which uses the sphere to rotate 90° around the axis of the valve stem to achieve the purpose of opening and closing. The ball valve is mainly used for cutting off, distributing and changing the flow direction of the medium in the pipeline. The ball valve designed as a V-shaped opening also has a good flow adjustment function.
①has the lowest flow resistance (actually 0);
②Because it will not get stuck when working (when there is no lubricant), it can be reliably used in corrosive media and low-boiling liquids;
③In a larger pressure and temperature range, it can achieve complete sealing;
④It can realize fast opening and closing, and the opening and closing time of some structures is only 0.05~0.1s to ensure that it can be used in the automation system of the test bench. When opening and closing the valve quickly, the operation has no impact;
⑤The spherical closing piece can be automatically positioned on the boundary position;
⑥The working medium is reliably sealed on both sides;
⑦When fully open and fully closed, the sealing surface of the ball and 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 compact structure and light weight, it can be considered as the most reasonable valve structure for cryogenic medium system;
⑨The valve body is symmetrical, especially the welded valve body structure, which can withstand the stress from the pipeline well;
⑩The closing piece can withstand the high pressure difference when closing. ⑾The ball valve with fully welded body can be directly buried in the ground, so that the internal parts of the valve are not corroded, and the maximum service life can reach 30 years. It is the most ideal valve for oil and natural gas pipelines. It is a professional manufacturer of Shanghai Chuanhu Valve Ball Valve.
①Because the main seat sealing ring material of the ball valve is polytetrafluoroethylene, it is inert to almost all chemical substances, and has a small friction coefficient, stable performance, not easy to age, wide temperature application range and excellent sealing performance The comprehensive characteristics. However, the physical properties of PTFE, including high expansion coefficient, sensitivity to cold flow and poor thermal conductivity, require the design of valve seat seals to focus on these characteristics. Therefore, when the sealing material becomes hard, the reliability of the seal is impaired. Moreover, PTFE has a low temperature resistance grade and can only be used at less than 180°C. Above this temperature, the sealing material will deteriorate. In the case of long-term use, it will generally only be used at 120°C.
②Its regulation performance is worse than that of globe valves, especially pneumatic valves (or electric valves).

Advantages and disadvantages of valves
4. Cut-off valve: refers to a valve whose closing part (disc) moves along the center line of the valve seat. According to this movement of the valve disc, the change of the valve seat port is proportional to the valve disc stroke. Since the opening or closing stroke of the valve stem of this type of valve is relatively short, and it has a very reliable cut-off function, and because the change of the valve seat port is in direct proportion to the stroke of the valve disc, it is very suitable for flow adjustment. Therefore, this type of valve is very suitable for cutting off or regulating and throttling.
①During the opening and closing process, 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.
②The opening height is generally only 1/4 of the valve seat passage, so it is much smaller than the gate valve;
③Usually there is only one sealing surface on the valve body and the disc, so the manufacturing process is relatively good and easy to maintain;
④Because the filler is generally a mixture of asbestos and graphite, the temperature resistance level is higher. Generally steam valves use stop valves.
①As 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;
②Due to the longer stroke, the opening speed is slower than that of the ball valve.
5. Plug valve: refers to a rotary valve with a plunger-shaped closing part. The passage port on the valve plug is communicated with or separated from the passage port on the valve body through a 90° rotation to realize opening or closing. The shape of the valve plug can be cylindrical or conical. The principle is basically similar to that of the ball valve. The ball valve is developed on the basis of the plug valve. It is mainly used for oilfield exploitation, but also for petrochemical industry.
6. Safety valve: refers to the pressure vessel, equipment or pipeline, as an overpressure protection device. When the pressure in the equipment, container or pipeline rises above the allowable value, the valve automatically opens, and then the full amount is discharged to prevent the equipment, container or pipeline and the pressure from continuing to rise; when the pressure drops to the specified value, the valve should automatically Close in time to protect the safe operation of equipment, containers or pipelines.
7. Steam trap: Some condensed water will be formed in the medium of conveying steam, compressed air, etc. In order to ensure the working efficiency and safe operation of the device, these useless and harmful media should be discharged in time to ensure the consumption and consumption of the device. use. It has the following functions: ①It can quickly remove the condensed water produced; ②Prevent steam leakage; ③Exclude 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 adjustment, and relies on the energy of the medium itself to automatically maintain a stable outlet pressure.
9, check valve: also known as reverse valve, check valve, back pressure valve and one-way valve. These valves are automatically opened and closed by the force generated by the flow of the medium in the pipeline, and belong to an automatic valve. The check valve is used in the pipeline system, and its main function is to prevent the medium from flowing back, preventing the pump and the drive motor from reversing, and releasing the container medium. Check valves can also be used to supply pipelines for auxiliary systems whose pressure may rise above the system pressure. They can be divided into swing type (rotating by the center of gravity) and lifting type (moving along the axis).


How to choose valve materials under high temperature conditions?

The high temperature working conditions mainly include sub-high temperature, high temperature class I, high temperature class II, high temperature class III, high temperature class IV, and high temperature class V, which will be introduced separately below.

1. Sub-high temperature

Sub-high temperature means that the operating temperature of the valve is in the region of 325 to 425°C. If the medium is water and steam, WCB, WCC, A105, WC6 and WC9 are mainly used. If the medium is sulfur-containing oil, mainly use C5, CF8, CF3, CF8M and CF3M with anti-sulfide corrosion. They are mostly used in the atmospheric and vacuum units and delayed coking units of oil refineries. At this time, valves made of CF8, CF8M, CF3 and CF3M are not used for corrosion resistance of acid solutions, but used for sulfur-containing oil and oil and gas pipelines. In this working condition, the upper limit of the maximum operating temperature of CF8, CF8M, CF3 and CF3M is 450°C.

2. High temperature class I

The valve’s working temperature is 425~550℃, it is high temperature class I (referred to as PI class). The main material of the PI-level valve is “high-temperature grade I medium carbon chromium nickel rare earth titanium high-quality heat-resistant steel” based on CF8 in the ASTM A351 standard. Because PI grade is a specific name, the concept of high temperature stainless steel (P) is included here. Therefore, if the working medium is water or steam, although high-temperature steel WC6 (t≤540℃) or WC9 (t≤570℃) can also be used, although high-temperature steel C5 (ZG1Cr5Mo) can also be used for sulfur-containing oil, They cannot be called PI grades here.

Electric high temperature control valve

Electric high temperature control valve (material WC6, with high temperature heat sink)

3. High temperature class Ⅱ

The working temperature of the valve is 550~650℃, which is set as high temperature level II (referred to as PII level). The PⅡ high temperature valve is mainly used in the heavy oil catalytic cracking unit of the refinery. It contains high temperature lined wear-resistant gate valves used in three-rotation nozzles and other parts. The main material of the PⅡ valve is the “high temperature grade Ⅱ medium carbon chromium nickel rare earth titanium tantalum reinforced heat-resistant steel” based on CF8 in the ASTM A351 standard.

4. High temperature Ⅲ grade

The working temperature of the valve is 650~730℃, which is set as high temperature grade III (referred to as PⅢ grade for short). PⅢ high temperature valves are mainly used in large-scale heavy oil catalytic cracking units in refineries. The main material of PⅢ grade high temperature valve is “high temperature grade Ⅲ medium carbon chromium nickel molybdenum rare earth titanium tantalum reinforced heat-resistant steel” based on CF8M in ASTMA351 standard.

5. High temperature grade IV

The working temperature of the valve is 730~816℃, which is set as high temperature grade IV (referred to as PIV grade). The upper limit of the working temperature of the PIV valve is 816℃ because the maximum temperature provided in the standard ASMEB16134 pressure-temperature grade selected for the valve design is 816℃ (1500υ). In addition, after the working temperature exceeds 816°C, the steel is close to entering the forging temperature zone. At this time, the metal is in the plastic deformation zone, and the metal has good plasticity, and it is difficult to withstand high working pressure and impact without being deformed. The main material of the PⅣ valve is CF8M in the ASTM A351 standard, which is the basic “high temperature grade IV medium carbon chromium nickel molybdenum rare earth titanium tantalum reinforced heat-resistant steel”. Heat-resistant stainless steels such as F310 (with C content ≥01050%) and F310H in the CK-20 and ASTMA182 standards.

6. High temperature grade V

The working temperature of the valve is above 816℃, referred to as PⅤ, PⅤ high temperature valve (used as a shut-off valve, not a regulating butterfly valve) must adopt special design methods, such as lining heat insulation lining or passing water or air The normal operation of the valve can only be ensured by cooling. Therefore, the upper limit of the working temperature of the PⅤ high temperature valve is not stipulated. This is because the working temperature of the control valve is not solved by materials alone, but by special design methods, and the basic principles of the design methods are the same. According to the working medium, working pressure and special design method adopted, the PⅤ high temperature valve can select reasonable materials that can satisfy the valve. In the PⅤ high temperature valve, usually the flue flapper valve or the flapper or butterfly plate of the butterfly valve often use the high temperature alloy HK-30 and HK-40 in the ASTM A297 standard. They can resist oxidation and reducing gases below 1150℃. Corrosion resistant, but cannot withstand impact and high pressure loads.


Selection and performance of commonly used materials for valves

Common materials for valves
(1) Cast iron
1. Gray cast iron, such as HT200, HT lung 0, etc., is suitable for the nominal pressure not greater than PN16, and the working temperature is -10~100℃. For oils and general liquid media (water, steam, petroleum products, etc.) between C, the nominal pressure is not greater than PN10. The working temperature is -10~200℃. Mediums such as steam, general gas, coal gas, ammonia gas between C (ammonia, alcohol, vinegar, aldehydes, rewards, friezes and other low corrosive media). It is not suitable for media such as hydrochloric acid and nitric acid. But it can be used in concentrated sulfuric acid because concentrated sulfuric acid can produce a passivation film on the metal surface to prevent the corrosion of cast iron by concentrated sulfuric acid.
2. Malleable cast iron, such as KTH350-10, KTH450-06, etc., is suitable for steam, general gas, liquid, oil and other media with nominal pressure not greater than PN25 and working temperature between -10 and 300℃. Its corrosion resistance is similar to gray cast iron.
3. Nodular cast iron, such as QT400-I5`QT450-I0, etc., is suitable for the nominal pressure not greater than FN25, and the working temperature is 10-300. The medium between C and steam, general gas and oil. It has strong corrosion resistance and can work in a certain concentration of sulfuric acid, nitric acid, and acid salt. But it is not resistant to corrosion by chloric acid, strong alkali, hydrochloric acid and ferric chloride hot solution. Avoid sudden heat and cold during use, otherwise it will break.
4. Nickel cast iron has stronger alkali resistance than gray iron and ductile iron valves. It is used in dilute sulfuric acid, dilute hydrochloric acid and caustic alkali. Fetter iron is an ideal valve material.
(2) Carbon steel
Carbon steel includes WCA, WCB and WCC, etc., which are suitable for media such as steam, non-corrosive gas, petroleum and related products whose working temperature is between -29 and 425℃.
(3) Stainless steel
1.304 series stainless steel Yi Yin is suitable for steam, non-corrosive gas, petroleum and related products and other media with a working temperature of 196~650℃, and corrosive media with a working temperature of -30~200℃. It has excellent atmospheric resistance and can withstand nitric acid and other oxidizing agents. It can resist the corrosion of alkali, water, salt, organic acid and other organic compounds. It is not resistant to the corrosion of non-oxidizing acids such as sulfuric acid and hydrochloric acid, but it is not resistant. Non-drying hydrogen chloride, oxidizing chloride and organic acids such as oxalic acid and lactic acid. A series of stainless steels containing 2% to 3% aluminum on the basis of one, its corrosion resistance is superior to a series of stainless steels, and its candle resistance in non-oxidizing acids and hot organic acids and chlorides It is better than Luofeng stainless steel and has better pitting corrosion resistance. @Containing iron or cone 32I, 347 series stainless steel has stronger resistance to the corrosion of the crystal mesh. @The 904L series stainless steel with high chromium and high Yang has higher corrosion resistance than ordinary stainless steel. It can be used to treat sulfuric acid, phosphoric acid, sulphuric acid, sulfurous acid, organic acid, alkali, salt solution, hydrogen sulfide, etc., and can even be used for High temperature occasions under certain concentrations. But it is not resistant to the corrosion of concentrated or hot hydrochloric acid and wet rats, chlorine, ulcer, iodine, aqua regia, etc.
(4) Copper alloy
Copper alloy is mainly suitable for valves with nominal pressure not greater than PN25 and operating temperature between -40 and 180℃ for oxygen and seawater pipelines. It has good corrosion resistance to water, seawater, various salt solutions, and organic matter. It has good sweet corrosion resistance to sulfuric acid, phosphoric acid, acetic acid, dilute hydrochloric acid, etc. that do not contain oxygen or oxidants, and has good resistance to alkalis. But it is not resistant to the corrosion of oxidizing acids such as nitric acid and concentrated sulfuric acid. Corrosion of molten metal, sulfur and sulfide. Avoid contact with ammonia, which can cause stress corrosion cracking of copper and copper alloys. It should be noted when selecting, the corrosion resistance of copper alloys varies to a certain degree.
(S) Aluminum alloy
Aluminum alloy has good corrosion resistance to strong oxidizing concentrated nitric acid, and can withstand organic acids and solvents. But it is not resistant to corrosion in reducing media, strong acids and strong alkalis. The higher the purity of aluminum, the better the corrosion resistance, but the lower the strength, it can only be used as a valve or valve lining with very low pressure.
(6) Qin alloy
Qin alloy is mainly suitable for the nominal pressure not greater than PN25 and the working temperature is between 30 and 316. C between seawater, chloride, oxidizing acid, organic acid, alkali and other media. Qin is an active metal, which can form an oxide film with good corrosion resistance at room temperature. It is resistant to corrosion by sea water, various chlorides and hypochlorites, wet chlorine, oxidizing acids, organic acids, alkalis, etc. But it is not resistant to the corrosion of relatively pure reducing acids such as sulfuric acid and hydrochloric acid, but it is resistant to corrosion by nitric acid containing oxidants. Qin alloy valves have good resistance to pitting corrosion, but stress corrosion will occur in red fuming nitric acid, chloride, methanol and other media.
(7) Cha alloy
Arrow is also an active metal. It can form a compact oxide film. It has good corrosion resistance to nitric acid, complex acid, lye, molten alkali, salt solution, urea, sea water, etc., but it is not resistant to hydrogen acid and concentrated sulfuric acid. The corrosion of aqua regia is not resistant to the rot of wet chlorine and oxidizing metal chlorides.
(8) Ceramics
Ceramics are mainly made of silicon dioxide by melting and sintering, such as zirconia. Alumina, silicon oxide, etc., in addition to high wear resistance, temperature resistance, and heat insulation properties, it also has high corrosion resistance. In addition to being intolerant to oxyfluoric acid, fluorosilicic acid and strong alkalis, it can resist heat and concentration. Nitric acid, hydrochloric acid. Aqua regia, salt solution and organic solvent, ceramic valves are generally used in pipelines with a nominal pressure not greater than PN16. Such valves, such as using other materials,
When selecting, consider other materials-energy.
(9) FRP
The performance of FRP varies with its adhesive. The epoxy resin glass fiber reinforced plastic can be used in hydrochloric acid, phosphoric acid, dilute sulfuric acid and some organic acids. The β-broken glass pot has better corrosion resistance.
Its alkali resistance, acid resistance and comprehensive corrosion resistance are generally suitable for pipelines with a nominal pressure not greater than PN16.
(10) Plastic
The most important feature of plastic valves is strong corrosion resistance, and even has advantages that metal material valves cannot be equipped.-Not suitable for pipelines with a nominal pressure not greater than PN6. With different types of plastics, their corrosion resistance
big different.
1. Nylon, also known as polyamide, is a thermoplastic and has good seed resistance. It can withstand the corrosion of dilute acid, salt, and alkali, and has good corrosion resistance to economics, rewards, thieves, vinegar, and soy. . But not resistant to strong acid and oxidation
Corrosion by acid, cool and formic acid. Polyvinyl chloride is a thermoplastic, with excellent corrosion resistance. It can withstand acids, alkalis, salts and organics. It is not resistant to the corrosion of concentrated nitric acid, fuming sulfuric acid, stuffed liver, rewards, halogenated, aromatic halo, etc.
2. Polyethylene has excellent corrosion resistance. It has good corrosion resistance to non-oxidizing acids such as hydrochloric acid, dilute sulfuric acid, hydrogen double acid, dilute nitric acid, alkali, salt solution and organic solvents at room temperature. But it is not resistant to the corrosion of concentrated phosphoric acid, concentrated sulfuric acid and other strong oxidants.
3. Polypropylene is a thermoplastic, its corrosion resistance is similar to that of polyethylene, slightly better than polyethylene wax. It can withstand most organic acids, inorganic acids, alkalis, salts, nitric acid, fuming sulfuric acid, chlorine reef Strong oxidizing acids such as acids have poor corrosion resistance.
4. Phenolic plastics are resistant to the corrosion of non-oxidizing acids such as hydrochloric acid, dilute sulfuric acid, phosphoric acid, and salts. But they are not resistant to strong oxidizing acids such as nitric acid and bribing acid. Alkali and some organic solvents.
5.Polytetrafluoroethylene has very excellent corrosion resistance. It can withstand almost all chemical media except for molten metal, potassium, sodium, triple complex chlorine, oxygen trifluoride at high temperature, and high flow rate liquid recombination. Rot


Precautions for the selection of conventional valves

(1) Requirements for the use of valves

①Ordinary gate valves, ball valves, and globe valves are strictly prohibited for adjustment according to their structural characteristics. But in process design, it is generally used for adjustment. Due to adjustment and use, the valve seal is in a throttling state for a long time, impurities in the oil scouring the seal, damaging the sealing surface, resulting in lax closing or because the operator has damaged the sealing surface to achieve the seal, causing the valve to pass or open .
②The valve installation position is unreasonable. When the medium used contains impurities, a filter or filter screen is not installed at the front end of the valve, causing impurities to enter the valve, causing damage to the sealing surface, or deposits of impurities on the bottom of the valve, causing the valve to be closed loosely. And produce leakage.

(2) Consider from the perspective of process requirements

① For corrosive media, if the temperature and pressure are not high, non-metallic valves should be used as much as possible; if the temperature and pressure are high, lining valves should be used to save precious metals. When choosing non-metal valves, economic rationality should still be considered;
For a medium with a higher viscosity, if a smaller flow resistance is required, a direct-flow stop valve, gate valve, valve, plug valve and other valves with a small flow resistance should be used. Valves with small flow resistance and low energy consumption; when the medium is a special medium such as oxygen or ammonia, the corresponding special valve for oxygen or ammonia should be selected.
② Directional valves should not be used for dual-flow pipelines, but non-directional valves should be used. For example, after the heavy oil pipeline of an oil refinery stops running, the pipeline must be purged with steam in the reverse direction to prevent the heavy oil from solidifying and blocking the pipeline. It is not appropriate to use a stop valve here, because the sealing surface of the stop valve is easily eroded when the medium flows in the reverse direction. It also affects the efficiency of the valve, and it is better to use a gate valve.
③For some media with crystallization or deposits, stop valves and gate valves should not be used, because their sealing surfaces are easily worn by crystals or deposits. Therefore, it is more appropriate to use a ball valve or a plug valve; a flat gate valve is optional, but a jacket valve is best.
④In the selection of gate valve, the single gate with exposed rod is more suitable for corrosive media than the double gate with dark rod; the single gate is suitable for medium with high viscosity; the adaptability of double wedge gate to high temperature and deformation of sealing surface It is better than a wedge-type single gate, and will not cause jamming due to temperature changes, especially compared to
Rigid single gate is more superior.
⑤The valves on general water and steam pipes can be cast iron valves. However, if the steam is stopped in the outdoor steam pipes, the condensate will freeze and the valves will be damaged by freezing. Therefore, in cold areas, the valve should be made of cast steel, low-temperature steel, or effective heat preservation measures.
⑥For highly dangerous and highly toxic media or other harmful media, valves with bellows structure should be used to prevent the media from leaking from the packing.
⑦Gate valves, globe valves and ball valves are the most used valves among the valves, and comprehensive consideration should be given to them when selecting them. The gate valve has strong flow capacity and low energy consumption for the conveying medium, but has a large installation space. The shut-off valve has a simple structure and is easy to maintain, but has a large flow resistance. The ball valve has the characteristics of low flow resistance and fast opening and closing, but the use temperature is limited. For petroleum products and other media with relatively high viscosity, considering the strong flow capacity of gate valves, gate valves are mostly used. In water and steam pipelines, stop valves are used, and the pressure drop is not large. Therefore, stop valves are used in water, steam and other medium pipelines. There are many applications, and ball valves can be used under the conditions permitting.

(3) From the perspective of easy operation

①For large-diameter valves and long-distance, high-altitude, high-temperature, and high-pressure occasions, electric and pneumatic valves should be used. For flammable and explosive occasions, Luo
Explosion-proof devices are used, and hydraulic and pneumatic devices are used for safety and reliability.
② For valves that require quick opening and quick closing, butterfly valves, ball valves, plug valves or quick opening gate valves should be selected according to the needs, and general gate valves and stop valves should not be used. In situations where the operating space is restricted, open-stem gate valves should not be used. Dark-stem gate valves should be used, but butterfly valves are best. Use rising stem gate valve,

(4) Considering the accuracy of regulating flow

When it is necessary to adjust the flow rate accurately, a regulating valve should be used. When the accuracy of the flow rate is reduced, a needle valve or a throttle valve should be used. When it is necessary to reduce the pressure after the valve, a pressure reducing valve should be used, and when the stability of the pressure after the valve is to be maintained, a pressure regulator valve should be used.

(5) Considering temperature and pressure resistance

High-temperature and high-pressure media often use castings of chromium-molybdenum steel and chromium-molybdenum-vanadium steel. For ultra-high-temperature and high-pressure media, the corresponding forgings should be considered. The comprehensive performance of forgings is better than castings, and the temperature and pressure resistance capabilities are also superior to castings.

(6) Considering cleanability

In the production and transportation of food and biological engineering, the requirements for valves on the process pipeline need to consider the cleanliness of the medium, and general gate valves and globe valves cannot be guaranteed. In terms of cleanability, no valve can be compared with a diaphragm valve.
①Diaphragm valve has a simple structure, with various valve bodies and diaphragm materials, which can be widely used in the fields of food and biological engineering, and also suitable for some difficult and dangerous media.
a. Only the valve body and diaphragm are in contact with the flow, and all other parts are isolated. The valve can be thoroughly sterilized with steam.
b. Possess its own detergency capability.
c. It can be repaired online. Therefore, the diaphragm valve has become the most widely used valve in the food and biological C. engineering fields.
②Bottom valve.
In the case of strict sterilization requirements, the discharge valve at the bottom of the storage tank has almost no choice. The bottom valve is directly welded to the bottom head of the storage tank during equipment manufacturing. It is very different from the usual method of making a nozzle at the bottom of the tank and then connecting the valve to the nozzle. When the valve is closed, the valve core It is level with the inner bottom of the storage tank, so it effectively eliminates the dead corners in the tank, so that all the liquid in the tank can be fully mixed during the fermentation process, and the unique steam sealing system greatly reduces product contamination Possibility.