Advantages and disadvantages of different valves Large diameter valves why difficult to switch? How to solve it?

Advantages and disadvantages of different valves Large diameter valves why difficult to switch? How to solve it?
Classification by use and function
The truncation valve is mainly used to truncate or connect the medium flow. Including gate valve, globe valve, diaphragm valve, ball valve, plug valve, disc valve, plunger valve, ball plug valve, needle type instrument valve, etc.
Regulating valve is mainly used to adjust the flow of the medium, pressure, etc. Including regulating valve, throttle valve, reducing valve and so on.
Check valves are used to prevent media backflow. Check valves of various construction are included.
Diverter valves are used to separate, distribute or mix media. Including a variety of structure of the distribution valve and trap, etc.
Safety valve for medium overpressure when the safety protection. All types of safety valves are included.
Classified by main parameters
(a) according to the pressure classification
Vacuum valve operating pressure below standard atmospheric pressure valve.
Low pressure valve Valve with nominal pressure PN less than 1.6MPa.
Medium pressure valve nominal pressure PN2.5~6.4MPa valve.
High pressure valve nominal pressure PN10.0~80.0MPa valve.
Super high pressure valve nominal pressure PN greater than 100MPa valve.
(2) according to the medium temperature classification
High temperature valve t valves greater than 450C.
Medium temperature valve 120C less than T 450C valve.
Normal temperature valve -40C < T < 120C valve.
Low temperature valve -100C less than T less than -40C valve.
Temperature valve T less than -100C valve.
(3) according to the valve body material classification
Non-metallic material valve: such as ceramic valve, FRP valve, plastic valve.
Metal material valve: such as copper alloy valve, aluminum alloy valve, lead alloy valve, titanium alloy valve, Monel alloy valve
Cast iron valve, carbon steel valve, cast steel valve, low alloy steel valve, high alloy steel valve.
Metal body lined valve: such as lined lead valve, lined plastic valve, lined enamel valve.
General classification system
This classification method is not only according to the principle, function and structure, is the most commonly used classification method at home and abroad. Generally, there are gate valves, globe valves, throttle valves, instrument valves, plunger valves, diaphragm valves, plug valves, ball valves, butterfly valves, check valves, pressure reducing valves, safety valves, traps, regulating valves, bottom valves, filters, blowdown valves, etc.
Why is it difficult to switch large caliber valves? How to solve it?
Analysis of the reasons for the difficulty in opening and closing large diameter valves
The average adult’s horizontal limit output force is 60-90kg, depending on the different physique.
The general flow direction of the globe valve is designed to be low and high. When the valve is closed, the human body pushes the handwheel horizontally to rotate, so that the valve disc moves downward and closes. At this time, the combination of three aspects of force needs to be overcome, namely:
1) axial jacking force Fa;
2) Friction force between packing and valve stem Fb;
3) The stem contacts the disc core with friction force Fc
The total torque is M=(Fa+Fb+Fc)R
It can be seen that the larger the caliber is, the larger the axial jacking force is. When it is close to the closed state, the axial jacking force is almost close to the actual pressure of the pipe network (because P1-P2P1, P2=0 when it is closed).
For example, a DN200 caliber globe valve is used on a 10bar steam pipe, and its * term closes the axial thrust Fa= 10R2 =3140kg. The horizontal circumferential force required for closing is close to the limit of the horizontal circumferential force that normal human body can output. Therefore, it is very difficult for a person to completely close the valve under this working condition.
Of course, some factories suggest to install this kind of valve in reverse, which solves the problem of difficult to close, but there is the problem of difficult to open after closing.
Cause analysis of large diameter globe valve prone to internal leakage
Large caliber stop valve is generally used in the boiler outlet, the main cylinder, steam director and other positions, these positions have the following problems:
1) The pressure difference at the outlet of the boiler is generally large, so the steam flow rate is also larger, and the erosion and destruction effect on the sealing surface is also large. In addition, the boiler combustion efficiency can not be 100%, which will cause the boiler outlet steam water content is larger, easy to produce cavitation and cavitation damage to the valve sealing surface.
2) On the outlet of the boiler and the cut-off valve near the cylinder, because the steam just out of the boiler, there is intermittent overheating phenomenon, in the process of saturation, if the boiler water softening treatment is not too good, often precipitate part of the acid and alkali substances, on the sealing surface will cause corrosion and erosion; Some crystallizable substances may also be attached to the valve sealing surface crystallization, resulting in the valve can not be tightly sealed.
3) The inlet and outlet valve of the sub-cylinder, because the steam consumption after the valve is caused by production requirements and other reasons, the steam consumption is large and small, in the case of large change in the flow rate, it is easy to produce flash, cavitation and other phenomena, thus causing erosion, cavitation and other damage to the sealing surface of the valve.
4) Generally, when large pipe diameter pipelines are opened, it is necessary to preheat the pipeline, and the preheating process generally requires a small flow of steam to pass through, so that the pipeline is slowly and evenly heated to a certain degree before the cut-off valve can be fully opened, so as to avoid excessive expansion caused by rapid heating of the pipeline and damage to part of the connection. But in this process, the valve opening is often very small, resulting in erosion rate is far greater than the normal use effect, seriously reduce the service life of the valve sealing surface.
Large diameter stop valve switch difficult solution
1) First of all, it is recommended to choose bellows sealing globe valve to avoid the friction resistance influence of plunger valve and packing valve and switch more easily.
2) the spool seat must be made of materials with good erosion resistance and wear performance, such as Siteli carbide;
3) It is recommended to use double disc structure, not due to small opening caused by excessive erosion, affect the service life and sealing effect.

Post time: Aug-05-2022

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