1. Why does the double seat valve easily oscillate when working at a small opening?
For a single core, the valve stability is good when the medium is flow-opening; the stability of the valve is poor when the medium is flow-closed.
The double seat valve has two spools, the lower spool is closed, and the upper spool is open, so that the flow-closed spool can easily cause the valve to vibrate when working at a small opening.
This is why the double seat valve cannot be used for small opening work.
2. Why can't the double seal valve be used as a shut-off valve?
The advantage of the two-seat valve spool is the force-balanced structure, which allows a large differential pressure, and its outstanding disadvantage is that the two sealing faces cannot be in good contact at the same time, resulting in a large leak. If it is artificially and forcibly used to cut off the occasion, it is obviously not effective, even if it has made many improvements (such as double-sealed sleeve valves), it is not advisable.
3. What is the anti-blocking performance of the straight stroke regulating valve, and the anti-blocking performance of the angular stroke valve is good?
The straight-stroke valve spool is vertically throttled, and the medium flows horizontally in and out, and the flow passage in the valve chamber is inevitably turned and inverted, making the flow path of the valve quite complicated (the shape is inverted "S" type). Thus, there are many dead zones that provide space for the precipitation of the medium, which in the long run causes blockages. The direction of the angular stroke valve throttling is the horizontal direction. The medium flows horizontally and horizontally, which makes it easy to take away the dirty medium. At the same time, the flow path is simple, and the space for the medium to precipitate is also small, so the anti-blocking performance of the angular stroke valve is good.
4. Why is the valve stem of the straight stroke control valve thinner?
It involves a simple mechanical principle: large sliding friction and low rolling friction. The valve stem of the straight stroke valve moves up and down, and the packing is slightly pressed tightly, which will tightly seal the valve stem and produce a large backlash. For this reason, the valve stem is designed to be very small, and the filler is also commonly used with a small friction coefficient of tetrafluorofiller to reduce the backlash, but the problem is that the valve stem is thin, the bend is easy, and the filler life is short. The best way to solve this problem is to use the travel valve stem, that is, the angular stroke type regulating valve. Its valve stem is 2 to 3 times thicker than the straight stroke valve stem, and the graphite packing with long life is selected. Well, the filler has a long service life, and the friction torque is small and the backlash is small.
5. Why is the cut-off pressure difference of the angular stroke type valve larger?
The cut-off pressure difference of the angular stroke type valve is large because the resultant force of the medium on the valve core or the valve plate generates a very small torque to the rotating shaft, and therefore, it can withstand a large pressure difference.
6. Why does the demineralized water medium use a rubber-lined butterfly valve and a fluorine-lined diaphragm valve with a short service life?
Demineralized media contain low concentrations of acids or bases which are highly corrosive to rubber. The corrosion of rubber is characterized by expansion, aging, and low strength. The butterfly valve and diaphragm valve with rubber lining have poor effect, and the essence is that the rubber is not resistant to corrosion. The back-lined rubber diaphragm valve is improved to a fluorine-lined diaphragm valve with good corrosion resistance, but the diaphragm of the fluorine-lined diaphragm valve can be broken by being folded up and down, causing mechanical damage and shortening the life of the valve.
The best way to do this 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 hardened as much as possible?
The shut-off valve requires that the leakage is as low as possible, the leakage of the soft-sealed valve is the lowest, and the cutting effect is of course good, but it is not wear-resistant and has poor reliability. From the double standard of small leakage, sealed and reliable, the soft seal cut is not as good as the hard seal cut. Such as full-featured ultra-light regulating valve, sealed and covered with wear-resistant alloy protection, high reliability, leakage rate of 10-7, has been able to meet the requirements of the shut-off valve.
8. Why does the sleeve valve replace the single and double seat valve but does not have the wish?
The sleeve valve that was introduced in the 1960s was widely used at home and abroad in the 1970s. The proportion of the sleeve valve in the petrochemical plant introduced in the 1980s was large. At that time, many people believed that the sleeve valve could replace the single and double seat valves. Become a second generation product. This is not the case today, single-seat valves, double-seat valves, and sleeve valves are equally used. This is because the sleeve valve only improves the throttling form, stability and maintenance better than the single seat valve, but its weight, anti-blocking and leakage indicators are consistent with single and double seat valves. How can it replace single and double seat valves? What? Therefore, they can only be used together.
9. Why is selection more important than calculation?
Compared with the selection, the selection is much more important and more complicated. Because the calculation is just a simple formula calculation, it is not in itself the accuracy of the formula, but whether the given process parameters are accurate. There are many contents involved in the selection. If you are not careful, it will lead to improper selection, which not only causes waste of manpower, material resources and financial resources, but also has unsatisfactory effect on use, which brings about some use problems such as reliability, longevity and operation, quality and so on.
10. Why is there more and more piston actuators used in pneumatic valves?
For pneumatic valves, the piston actuator can make full use of the air supply pressure, making the actuator smaller than the diaphragm type, the thrust is larger, and the O-ring in the piston is more reliable than the film, so its use will become more and more many.
Steps and basis for valve selection before purchase:
In fluid piping systems, valves are control elements whose primary function is to isolate equipment and piping, regulate flow, prevent backflow, regulate and drain pressure. Since it is important to select the most suitable valve for the piping system, it is important to understand the characteristics of the valve and the steps and basis for selecting the valve.
The valve industry has been able to produce a wide 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. 12 categories, more than 3,000 models, more than 4,000 valves; the highest working pressure is 600MPa, the maximum nominal diameter is 5350mm, the maximum working temperature is 1200 °C, the minimum working temperature is -196 °C, the 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 ( Metal sodium, - circuit pure water, etc.).
Valve bearing material cast copper, cast iron, ductile iron, high silicon cast iron, cast steel, forged steel, high and low alloy steel, stainless acid resistant steel, Hastelloy, Inconel, Monel, duplex stainless steel , titanium alloy, etc. And can produce a variety of electric, pneumatic, hydraulic valve drive devices. Faced with such a large number of valve types and such complicated working conditions, it is necessary to select the most suitable valve product for the piping system. I believe that the characteristics of the valve should be understood first; secondly, the steps and basis for selecting the valve should be mastered; Follow the principle of selecting a valve.
1. There are two general characteristics of the valve, the use characteristics and structural characteristics.
Use characteristics: It determines the main performance and scope of use of the valve. It belongs to the valve characteristics: valve category (closed valve, regulating valve, safety valve, etc.); product type (gate valve, globe valve, butterfly valve, ball valve, etc.) The material of the main parts of the valve (valve body, valve cover, valve stem, valve disc, sealing surface); valve transmission mode. 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 pipe (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 stem structure (rotating rod, lifting rod).
2. The steps and basis for selecting a valve are as follows:
(1) Selection steps
1. Defining the purpose of the valve in the equipment or device, determine the working conditions of the valve: applicable medium, working pressure, working temperature, etc.
2. Determine the nominal diameter and connection method of the pipe connected to 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.
4. Determine the material of the casing and internal parts of the selected valve according to the medium, working pressure and working temperature delivered by the pipeline: gray cast iron, malleable cast iron, ductile iron, carbon steel, alloy steel, stainless acid-resistant steel, copper alloy, etc.
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 the parameters of the valve: For automatic valves, determine the allowable flow resistance, discharge capacity, back pressure, etc. according to different needs, and then determine the nominal diameter of the pipe and the diameter of the seat hole.
7. Determine the geometric parameters of the selected valve: structural length, flange connection form and size, dimensions of the valve height direction after opening and closing, bolt hole size and number of connections, and overall valve dimensions.
8. Use the existing information: valve catalogue, valve product samples, etc. to select the appropriate valve products.
(2) The basis for selecting the valve While understanding the steps of selecting the valve, you should further understand the basis for selecting the 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 flammable, explosive medium, viscosity of the medium, etc.
3. Requirements for valve fluid characteristics: flow resistance, discharge capacity, flow characteristics, seal rating, etc.
4. Installation size and dimensions: nominal diameter, connection to the pipe and connection size, dimensions or weight restrictions.
5. Additional requirements for the reliability, service life and explosion-proof performance of the valve. (When selecting parameters, note that if the valve is to be used for control purposes, the following additional parameters must be determined: method of operation, maximum and minimum flow requirements, pressure drop for normal flow, pressure drop at shutdown, maximum and minimum inlet for valve Pressure.) According to the above-mentioned basis and steps of selecting the valve, the internal structure of each type of valve must be carefully understood when selecting the valve reasonably and correctly, so that the correct choice can be made for the preferred valve. The final control of the pipe is the valve. The valve opening and closing member controls the flow pattern of the medium in the pipe. The shape of the valve flow path allows the valve to have a certain flow characteristic, which must be taken into account when selecting the most suitable valve for the pipe system.
3. The following are the principles to follow when selecting a valve:
(1) The valve flow path for the cut-off and open medium is a straight-through valve with a small flow resistance, and is usually selected as a valve for the cut-off and open medium. Down-closed valves (shut-off valves, plunger valves) are less preferred because of their tortuous path and higher flow resistance than other valves. Where a higher flow resistance is allowed, a closed valve can be used.
(2) The valve for controlling the flow rate usually selects a valve that is easy to adjust the flow rate as the control flow rate. A down-closed valve (such as a shut-off valve) is suitable for this purpose because its seat size is proportional to the stroke of the closure. Rotary valves (plug valves, butterfly valves, ball valves) and flexing valve body valves (clamping valves, diaphragm valves) can also be used for throttling control, but are usually only available in a limited range of valve sizes. The gate valve is a cross-cut movement of the circular valve seat with a disc-shaped shutter. It can control the flow better only when it is close to the closed position, so it is usually not used for flow control.
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(3) Valves for commutating splits these valves may have three or more passages depending on the need for commutating splits. Plug valves and ball valves are more suitable for this purpose, so most of the valves used for commutating splits select one of these types of valves. However, in some cases, other types of valves may be used for commutation splitting by simply interconnecting two or more valves.
(4) Valves for media with suspended particles when the medium has suspended particles; it is most suitable to use a sliding valve with a sliding member along the sealing surface. If the closing movement of the closure to the valve seat is vertical, it is possible to hold the particles, so that the valve is only suitable for substantially clean media unless the sealing surface material allows for the incorporation of particles. The ball valve and the plug valve have a wiping action on the sealing surface during the opening and closing process, so it is suitable for use in a medium with suspended particles. At present, in the petroleum, chemical and other industries, the valve application, operating frequency and service are ever-changing. To control or eliminate even the slightest leakage, the most important and critical equipment is the valve. The ultimate control of the pipe is the valve, and the service and reliable performance of the valve in all areas is unique.
Valve inspection and maintenance after purchase
Valve maintenance and repair is not timely, resulting in valve failure to repair leakage or switch failure; valve is not regularly repaired pressure test, even for many years without cleaning, pressure test and technical identification, resulting in debris deposited in the valve, closed tight, severe seepage Oil, eucalyptus oil; the valve is not closed after inspection, or the valve is not blocked after the valve is removed; the valve dust gasket uses oil-resistant and non-pressure-resistant materials. Therefore, it is necessary to strengthen the inspection of the valve and strive to prevent it from happening.
The main contents of the valve inspection:
1. Whether the valve stem dynamic seal and the flange gasket static seal are leaking
2. Is the opening and closing state normal?
3. The valve body has abnormal phenomena such as damage and leakage.
4. Rotate the normally open or normally closed valve for 1 to 2 turns or 1 lift test.
5. Lubricate the valve stem of normally open or normally closed valve
6. Check and debug the power head and electrical system of the pneumatic valve