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Teach you valve selection, maintenance and maintenance!

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Teach you valve selection, maintenance and maintenance!

Date of release:2019-07-16 Author: Click:

Teach you valve selection, maintenance and maintenance! Chemicals must know! 10 problems often encountered in valve use?


 


1. Why is the double seat valve easy to oscillate when working with a small opening?


 


For a single core, when the medium is a flow-open type, the valve stability is good; when the medium is a flow-closed type, the valve stability is poor. The double-seat valve has two spools, the lower spool is in the flow closure, and the upper spool is in the flow opening. In this way, when working with a small opening, the flow-spool type spool will easily cause valve vibration. This is the double-seat valve. The reason why it cannot be used for small openings.


 


2. Why can't the double seal valve be used as a shut-off valve?


 


The advantage of the double-seat valve spool is the force-balanced structure, which allows a large pressure difference, and its prominent disadvantage is that the two sealing surfaces cannot be in good contact at the same time, resulting in large leakage. If it is used artificially and compulsorily for cut-off occasions, it will obviously not work well. Even if many improvements are made for it (such as double-sealed sleeve valve), it is not desirable.


 


3. What straight-line control valve has poor anti-blocking performance, and angular travel valve has good anti-blocking performance?


 


Straight stroke valve spool is vertically throttled, and the medium is flowed in and out horizontally. The flow path in the valve cavity must be turned upside down, which makes the valve's flow path quite complicated (like an inverted "S" shape). In this way, there are many dead zones, which provide space for the precipitation of the medium, which will cause clogging if it continues for a long time. The direction of the angle travel valve is the horizontal direction. The medium flows in horizontally and flows out horizontally. It is easy to take away the dirty media. At the same time, the flow path is simple and there is little space for the medium to settle. Therefore, the angle travel valve has good anti-blocking performance.


 


4. Why is the stem of the straight travel regulating valve thin?


 


It involves a simple mechanical principle: large sliding friction and low rolling friction. The stem of a straight-stroke valve moves up and down, and the packing is slightly compressed, and it will wrap the stem tightly, resulting in a large return difference. For this reason, the valve stem is designed to be very small, and the packing is usually a tetrafluoro packing with a small friction coefficient in order to reduce the backlash. However, the problem stemmed from this is that the valve stem is thin, it is easy to bend, and the packing life is short. The best way to solve this problem is to use a travel valve stem, which is an angle-stroke control valve. Its stem is 2 to 3 times thicker than a straight-stroke stem, and it uses a long-life graphite filler. Well, the filler has a long life, but its friction torque is small, and the return difference is small.


 


5. Why is the cut-off pressure difference of the angular stroke valve larger?


 


The cut-off pressure difference of the angular stroke type valve is large because the torque generated by the combined force of the medium on the valve core or the valve plate to the rotating shaft is very small, so it can withstand a large pressure difference.


 


6. Why do rubber-lined butterfly valves and fluorine-lined diaphragm valves have a short service life for desalinated water?


 


The desalinated water medium contains low concentrations of acids or alkalis, which are relatively corrosive to rubber. Corrosion of rubber manifests as swelling, aging, and low strength. The use of rubber-lined butterfly valves and diaphragm valves is poor. The essence is that rubber is not resistant to corrosion. The rear rubber-lined diaphragm valve is improved to a fluorine-lined diaphragm valve with good corrosion resistance, but the diaphragm of the fluorine-lined diaphragm valve cannot be folded up and down, causing mechanical damage and shortening the valve's life.


 


The best way now is to use a special ball valve for water treatment, which can be used for 5 to 8 years.


 


7. Why should the shut-off valve be as hard as possible?


 


The lower the leakage required by the shut-off valve, the better. The leakage of the soft seal valve is the lowest. Of course, the cut-off effect is good, but it is not wear-resistant and poor in reliability. From the double standards of small leakage and reliable sealing, the soft seal is not as good as the hard seal. Such as full-featured ultra-light regulating valve, sealed with wear-resistant alloy protection, high reliability, leakage rate of 10-7, can already meet the requirements of the shut-off valve.


 


8. Why does the sleeve valve replace the single or double seat valve but not as expected?


 


Sleeve valves, which came out in the 1960s, were used extensively at home and abroad in the 1970s. Sleeve valves accounted for a large proportion of petrochemical equipment introduced in the 1980s. At that time, many people believed that sleeve valves could replace single and double seat valves. And become the second generation product. Until now, this is not the case, and single-seat, double-seat, and sleeve valves have been used equally. This is because the sleeve valve only improves the throttle form, stability and maintenance better than the single seat valve, but its weight, anti-blocking and leakage indicators are consistent with the single and double seat valve, how can it replace the single and double seat valve How about it? Therefore, they can only be used together.


 


9. Why is selection more important than calculation?


 


Compared with calculation and selection, selection is much more important and more complicated. Because the calculation is just a simple formula, it is not in the accuracy of the formula itself, but in the accuracy of the given process parameters. The selection involves a lot of content. A little carelessness will lead to improper selection, which will not only cause waste of manpower, material resources and financial resources, but also result in unsatisfactory use, which will lead to some use problems, such as reliability, life, and operation. Quality, etc.



10. Why is the use of piston actuators more and more in pneumatic valves?



For pneumatic valves, the piston actuator can make full use of the pressure of the air source, making the size of the actuator smaller than the diaphragm type, the thrust is greater, and the O-ring in the piston is more reliable than the diaphragm, so its use will be more and more many.


Steps and basis of valve selection before purchase:


In fluid pping systems, valves are control elements whose main role is to isolate equipment and piping systems, regulate flow, prevent backflow, regulate and drain pressure. Since it is very important to select the most suitable valve for the piping system, it is also important to understand the characteristics of the valve and the steps and basis for selecting the valve.


The valve industry so far has been able to produce a full range of gate valves, globe valves, throttle valves, plug valves, ball valves, electric valves, diaphragm valves, check valves, safety valves, pressure reducing valves, steam traps and emergency shut-off valves. And other 12 major categories, more than 3,000 models and more than 4,000 specifications of valve products; the maximum working pressure is 600MPa, the maximum nominal diameter is 5350mm, the maximum working temperature is 1200 ° C, the minimum working temperature is -196 ° C, and the applicable medium is water. , Steam, oil, natural gas, highly corrosive media (such as concentrated nitric acid, medium concentration sulfuric acid, etc.), flammable media (such as stupid, ethylene, etc.), toxic media (such as hydrogen sulfide), explosive media, and radioactive media ( Sodium metal,-circuit pure water, etc.).

Material of valve pressure-bearing parts: cast copper, cast iron, nodular cast iron, high silicon cast iron, cast steel, forged steel, high and low alloy steel, stainless acid-resistant steel, Hastelloy, Inconel, Monel alloy, duplex stainless steel , Titanium alloy, etc. And can produce a variety of electric, pneumatic, hydraulic valve drive devices. Faced with so many types of valves and so complicated various working conditions, to choose the valve product that is most suitable for installation in the piping system, I think that we should first understand the characteristics of the valve; secondly, we must master the steps and basis for selecting the valve; Follow the principle of selecting a valve.

1. There are generally two types of valve characteristics, use characteristics and structural characteristics.


Use characteristics: It determines the main use performance and use range of the valve. The valve use characteristics are: the type of valve (closed valve, regulating valve, safety valve, etc.); product type (gate valve, globe valve, butterfly valve, ball valve, etc.) Materials for the 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 valve installation, repair, maintenance and other methods. The structural characteristics are: the structural length and overall height of the valve, and the connection form with the pipeline (flange connection, screw connection, clamp connection) , External thread connection, welding end connection, etc.); the form of the sealing surface (inlay ring, thread ring, overlay welding, spray welding, valve body); valve stem structure (rotary rod, lifting rod) and so on.


2. The steps and rationale for selecting a valve are as follows:


⑴Selection steps


1. Define the purpose of the valve in the equipment or device, and determine the working conditions of the valve: applicable media, working pressure, working temperature, etc.


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


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


4. Determine the material of the casing and internal parts of the selected valve according to the medium, working pressure, and working temperature of the pipeline: gray cast iron, malleable cast iron, ductile cast iron, carbon steel, alloy steel, stainless acid-resistant steel, copper alloy, etc.


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


6. Determine valve parameters: For automatic valves, first determine the allowable flow resistance, discharge capacity, back pressure, etc., and then determine the nominal diameter of the pipeline and the diameter of the valve seat hole.


7. Determine the geometric parameters of the selected valve: structural length, flange connection form and size, height dimension of the valve after opening and closing, size and number of bolt hole connections, overall valve dimensions, etc.


8. Use existing information: valve product catalogs, valve product samples, etc. to select appropriate valve products.


The basis for selecting a valve. While understanding the steps for selecting a valve, you should further understand the basis for selecting a valve.


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


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



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


4. Installation and external dimensions requirements: nominal diameter, connection method and connection size with pipes, external dimensions or weight restrictions. ⑤ Additional requirements for the reliability, service life and explosion-proof performance of electric devices. (It should be noted when selecting the parameters: if the valve is to be used for control purposes, the following additional parameters must be determined: operating method, maximum and minimum flow requirements, pressure drop during normal flow, pressure drop when closed, maximum and minimum inlet of the valve Pressure.) According to the above-mentioned basis and steps for selecting a valve, it is necessary to understand the internal structure of various types of valves in detail in order to properly and properly select a valve in order to make the correct choice for the preferred valve. The ultimate control of the pipeline is the valve. The valve opening and closing element controls the flow mode of the medium in the pipeline. The shape of the valve flow channel makes the valve have certain flow characteristics. This must be considered when selecting the valve that is most suitable for installation in the pipeline system.


3. The following are the principles to be followed when selecting a valve:


⑴ Valves for cut-off and open media The flow passage is a straight-through valve, which has a small flow resistance, and is usually selected as the valve for cut-off and open media. Down-closed valves (stop valves, plunger valves) are less commonly used because of their tortuous flow paths and higher flow resistance than other valves. Where higher flow resistance is allowed, closed valves can be used.


 Valve for controlling flow rate Generally, a valve which is easy to adjust the flow rate is selected for controlling flow rate. Down-closed valves, such as globe valves, are suitable for this purpose because their seat size is proportional to the travel of the closure. Rotary valves (plug valves, butterfly valves, ball valves) and flexure valve (clamp valves, diaphragm valves) can also be used for throttling control, but usually only within a limited range of valve diameters. The gate valve uses a disc-shaped gate plate to make a cross-cutting movement to the circular valve seat. It can control the flow better only when it is close to the closed position, so it is usually not used for flow control.


(3) Valve for reversing and diverting According to the need of reversing and diverting, this valve can have three or more channels. Plug valves and ball valves are more suitable for this purpose, so most of the valves used for reversing and diverting use 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 to each other.


Valve with suspended particles When the medium has suspended particles, it is most suitable to use a valve with a wiper function whose sliding member slides along the sealing surface. If the back-and-forth movement of the closing member to the valve seat is vertical, particles may be trapped. Therefore, this valve is only suitable for basically clean media unless the sealing surface material allows the particles to be embedded. The ball valve and the plug valve have a wiping effect on the sealing surface during the opening and closing process, so it is suitable to be used in the medium with suspended particles. At present, no matter in the pipeline system of petroleum, chemical industry or other industries, the valve application, operating frequency and service are ever-changing. To control or prevent even the smallest leakage, the most important and critical equipment is the valve. The ultimate control of the pipeline is the valve. The service and reliable performance of the valve in various fields are unique.


Valve inspection and maintenance after purchase


Insufficient repair and maintenance of the valve caused the valve to leak or fail to be repaired; the valve was not regularly overhauled and tested, and even used for many years without cleaning, pressure testing and technical appraisal. Oil, oil channeling; the valve has not been closed after maintenance, or the valve has not been blocked after the valve has been removed; the valve dust blue gasket uses non-oil and pressure-resistant materials. Therefore, we must strengthen the inspection of the valve and strive to prevent problems before they occur.


Main contents of valve inspection:


1. Whether the valve stem dynamic seal and flange gasket static seal are leaking


2. Whether the opening and closing status is normal


3. Are there any abnormalities such as damage and leakage in the valve body? 4. Turn the normally-open or normally-closed valve 1 or 2 times or do a lift test


5. Lubricate the normally open or normally closed valve stem


6.Check and debug the power head and electrical system of the pneumatic valve


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