What is a globe valve?
Globe valve, also known as stop valve, is a forced sealing valve, so when the valve is closed, pressure must be applied to the valve disc to force the sealing surface not to leak. When the medium enters the valve from below the valve disc, the resistance to be overcome by the operating force is the friction between the valve stem and packing and the thrust generated by the pressure of the medium. The force to close the valve is greater than the force to open the valve, so the diameter of the valve stem should be larger, otherwise the failure of valve stem jacking will occur. There are three connection modes: flange connection, screw connection and welding connection. After the emergence of the self sealing valve, the medium flow direction of the stop valve changes from above the valve disc to enter the valve chamber. At this time, under the action of medium pressure, the force to close the valve is small, while the force to open the valve is large, and the diameter of the valve stem can be reduced accordingly. At the same time, under the action of medium, this form of valve is also more tight. The “three modernizations” of valves in China once stipulated that the flow direction of stop valves should be from top to bottom. When the stop valve is opened, when the opening height of the valve disc is 25% ~ 30% of the nominal diameter, the flow has reached the maximum, indicating that the valve has reached the fully open position. Therefore, the full open position of the stop valve shall be determined by the stroke of the valve disc.
The opening and closing part of stop valve (globe valve) is a plug-shaped disc. The sealing surface is a plane or sea cone, and the disc moves in a straight line along the center line of the valve seat. The movement form of valve rod (general name: concealed rod) is also of lifting rotary rod type, which can be used to control the flow of various types of fluids such as air, water, steam, various corrosive media, mud, oil products, liquid metal and radioactive media. Therefore, this type of shut-off valve is very suitable for cut-off or regulation and throttling. Because the opening or closing stroke of the valve stem of this kind of valve is relatively short, and has a very reliable cut-off function, and because the change of the valve seat opening is directly proportional to the stroke of the valve disc, it is very suitable for the regulation of flow.
Classification of stop valves
Classification by channel direction
1. Straight through stop valve
The medium inlet and outlet channels are 180 ° in the same direction, the damage degree of the flow state is small, and the pressure loss through the valve is relatively small.
2. DC stop valve
The valve stem forms a certain angle with the channel, and the sealing surface of the valve seat has a certain angle with the inlet and outlet channel. The fluid hardly changes the flow direction, which is the lowest flow resistance in the stop valve. Suitable for pipelines with coking and solid particles.
3. Angle stop valve
The medium inlet and outlet channels are not in the same direction, with an included angle of 90 °, which will produce a certain pressure drop. The biggest advantage is that they can be installed at the corner of the pipeline system, which not only saves elbows, but also facilitates operation.
Classification by screw position
1. Upper threaded stem stop valve
The valve rod thread is outside the shell and is not in direct contact with the working medium. The valve rod thread is not corroded by the medium, which is convenient for lubrication and labor-saving operation.
2. Lower threaded stem stop valve
The valve stem thread is inside the valve body and in direct contact with the working medium. The valve stem thread is easy to be corroded by the medium and cannot be lubricated. It is mostly used in occasions with small nominal diameter and low medium working temperature.
Classification by use
1. Fluorine lined stop valve
Fluorine lined valve, also known as fluorine plastic lined corrosion-resistant valve, is to place polytetrafluoroethylene resin (or profile processed) on the inner wall of steel or iron valve pressure parts by molding (or inlay) (the same method is applicable to the lining of various pressure vessels and pipeline accessories) or the outer surface of valve internals, Using its unique performance in anti-corrosive medium, it is made into all kinds of valves and pressure vessels.
Fluorine lined stop valve fluorine lined stop valve is applicable to aqua regia, sulfuric acid, hydrochloric acid, hydrofluoric acid and various organic acids, strong acids and strong oxidants of various concentrations at – 50 ~ 150 ℃, as well as strong alkali organic solvents of various concentrations and pipelines of other corrosive gases and liquid media.
2. National standard stop valve
The national standard stop valve is the most commonly used product in the stop valve series. The API American Standard stop valve implements ANSI and API standards. The sealing surfaces of the valve disc and valve seat are made of Stellite cobalt based cemented carbide with different hardness. It has reliable sealing, high hardness, wear resistance, high temperature resistance, corrosion resistance, good scratch resistance and long service life.
3. Needle valve
Needle valve is a small diameter instrument valve, which plays the role of opening and closing and accurate flow control in the instrument measurement pipeline system. It mainly includes power station instrument series, oilfield special series, American standard series, etc.
4. Low temperature stop valve
Low temperature stop valve usually refers to the valve whose working temperature is lower than – 110 ℃, which is widely used in liquefied natural gas, liquefied petroleum gas and other low-temperature industries.
At present, globe valves with applicable temperature up to – 196 ℃ can be manufactured. All parts are pretreated with liquid nitrogen at low temperature to completely avoid seal deformation and leakage during use.
5. Plunger stop valve
Plunger valve plunger valve is also known as plunger stop valve. Its sealing surface adopts radial sealing structure. Two elastic sealing rings sleeved on the polished plunger compress the elastic sealing ring around the plunger through the load applied on the valve cover through the connecting bolts between the valve body and the valve cover to realize the sealed stop valve.
6. Insulation jacket stop valve
The insulation jacket stop valve adopts the insulation jacket design, and the external heat source is used to heat the outside of the valve, which can effectively prevent the temperature loss of the medium when passing through the valve and ensure the temperature of the medium.
7. Special stop valve for oxygen
The special stop valve for oxygen is made of silicon brass or stainless steel with excellent material. It has the advantages of high mechanical strength, wear resistance and good safety.
It has the best explosion-proof and flame-retardant performance and eliminates the unsafe factors on the oxygen pipeline. It is widely used in the pipe network of oxygen consuming projects such as iron and steel, metallurgy, petrochemical and chemical industry. In addition to the functions of ordinary valves, it also has its own characteristics.
Strict oil prohibition measures shall be adopted during manufacturing, and all parts shall be strictly degreased before installation.
Working principle of stop valve
In short, the principle of the stop valve is to use the torsion of the valve stem to give a downward pressure to the sealing surface, and rely on the pressure of the valve rod to make the sealing surface of the valve disc closely fit with the sealing surface of the valve seat, so as to prevent the inflow of media or adjust the flow rate of media. The opening and closing part of the stop valve is a cylindrical disc, the sealing surface is flat or conical, and the disc moves in a straight line along the center line of the fluid. The movement mode of the valve rod includes lifting rod type (the valve rod is lifted and the handwheel is not lifted) and lifting rotating rod type (the handwheel rotates and lifts with the valve rod, and the nut is set on the valve body). The stop valve is a forced sealing valve, so when the valve is closed, pressure must be applied to the valve disc to force the sealing surface not to leak.
We assume that there is a pipe with a cover. The water flows in from the bottom of the pipe and flows out to the nozzle. The cover of the outlet nozzle is equivalent to the closing part of the stop valve. If the pipe cover is lifted up by hand, the water flows out. If the pipe cover is covered by hand, the water stops flowing, which is equivalent to the working principle of the stop valve.
By rotating the hand wheel, the stop valve makes the valve stem rise and fall, driving the valve disc to make a linear displacement parallel to the medium flow direction, so as to achieve the purpose of opening and closing. The hand wheel rotates clockwise, and the working medium enters from the channel below the valve disc. At this time, the applied sealing force must be equal to or greater than the sum of the specific pressure generated on the sealing surface and the upward force of the medium.
When the stop valve is opened, when the opening height of the valve disc is 25% ~ 30% of the nominal diameter, the flow has reached the maximum, indicating that the valve has reached the fully open position. Therefore, the full open position of the stop valve shall be determined by the stroke of the valve disc.
1. The stop valve only allows one-way flow of medium and has directionality during installation.
2. The function of the stop valve is either 0 or 1, either open or closed. In other words, the stop valve is only applicable to full opening and full closing, and regulation and throttling are not recommended.
Advantages and disadvantages of stop valve
Advantages of stop valve
a. Compared with the gate valve, the structure of the stop valve is relatively simple. Usually, there is only one sealing surface on the valve body and disc, with small sealing area, saving valuable materials and low cost. Therefore, the manufacturing process is relatively good and easy to maintain.
b. The wear and scratch of the sealing surface are light, and the sealing performance is good. During opening and closing, there is no relative sliding between the valve disc and the sealing surface of the valve body (except the conical sealing surface), so the wear and scratch are not serious, the sealing performance is good and the service life is long.
c. The opening height is small, which is generally only 1/4 of the diameter of the valve seat channel. The structure is compact and saves installation space.
Disadvantages of stop valve
a. The length of the structure is large.
b. The opening and closing torque is large and the opening and closing is laborious. When closing, because the movement direction of the valve disc is opposite to the action direction of the medium pressure, the force of the medium must be overcome, so the opening and closing torque is large.
c. High flow resistance. The medium channel in the valve body is tortuous, with large flow resistance and power consumption, especially in hydraulic devices. Among all kinds of block valves, the flow resistance of stop valve is the largest.
d. The medium flow direction is limited. When the medium flows through the stop valve, it can only flow from bottom to top or from top to bottom at the valve seat channel, so the medium can only flow in one direction and cannot change the flow direction.
e. When the valve is closed, solid particles in the fluid medium may be clamped between the sealing surfaces.
f. Inconvenient installation.
Operating characteristics of stop valve
The sealing form of the stop valve is forced sealing. When the valve is closed, sufficient force must be applied to the valve disc to achieve the sealing above the specific pressure. Since the sealing force and medium pressure are on the same axis, when the medium enters from the lower part of the valve disc (low in and high out), because the medium pressure is opposite to the sealing force, the valve closing torque is much greater than the opening torque, and the maximum stress borne by the valve stem is compressive stress, it must have sufficient strength and rigidity, otherwise the valve stem will produce flexible deformation. For this reason, when the diameter specification of the stop valve is greater than DN150 (nps16), in order to reduce the valve operating torque and reduce the valve stem diameter, the medium flow direction is usually changed from the upper part of the valve disc (high in and low out). At this time, the medium pressure is the same as the sealing force direction. Under the medium pressure, the valve is easier to seal, the closing torque becomes smaller and the opening torque increases, The maximum stress borne by the valve stem is tensile stress, and the diameter of the valve stem can be reduced accordingly.
When the stop valve is opened, when the opening height of the valve disc reaches 25% ~ 30% of the nominal size of the valve, the flow has reached the maximum, which indicates that the valve has reached the fully open position. Therefore, the fully open position of the stop valve should be determined according to the stroke of the valve disc.
When the stop valve is closed and reopened, it is similar to the forced sealing gate valve, that is, after the valve is closed, sufficient operating force shall be applied on the sealing surface to achieve sealing. Therefore, the closing torque of the valve shall be increased to the specified value on the basis of the operating torque. When the valve is opened again, due to overcoming the influence of static friction, thermal expansion and other factors, the opening torque of the valve is usually greater than the closing torque in order to reliably open the valve. Therefore, it should be considered in the design.
In order to reduce the high impact pressure formed in the opening and closing valve, the stop valve should be operated slowly. In a sense, it should produce a change rate consistent with the flow rate.
The operating torque of the stop valve presents the following characteristics.
① When “low in and high out”, the valve starts to close from the fully open position. The resistance to be overcome by the operating force is the friction resistance of the valve stem and packing and the thrust caused by the medium pressure on the section (axial) of the valve disc. With the decline of the valve disc, the fluid forms a pressure difference before and after the valve disc to prevent the decline of the valve disc, and this resistance will increase rapidly with the decline of the valve disc, As the valve is completely closed, the valve rod applies a forced sealing force, and the pressure difference before and after the valve disc reaches the maximum working pressure of the medium. At this time, the valve rod bears the maximum closing torque. During the opening of the valve, the thrust formed by the medium pressure or the pressure difference before and after the valve disc is the same as the opening direction of the valve, but it should be pointed out that at the moment of opening, the opening operation torque may exceed the closing torque because it is necessary to overcome the large static friction torque between the sealing surfaces.
② When “high in low out”, the valve starts to close from the fully open position. The resistance to be overcome by the operating force is still the above two forces. With the decline of the valve disc, the pressure difference between the fluid before and after the valve disc is conducive to the valve closing until the valve is closed. When the valve is opened, the situation is just the opposite. Because the thrust caused by the medium pressure and the pressure difference before and after the valve disc is opposite to the valve opening direction, the operating torque required for the valve opening process is much larger than the closing torque.
Maintenance of stop valve
For the stop valve in operation, various valve parts shall be complete and intact. Bolts on flanges and supports are indispensable, and threads shall be intact without looseness. If the fastening nut on the hand wheel is found to be loose, it shall be tightened in time to avoid wearing the connection or losing the hand wheel and nameplate. If the hand wheel of the stop valve is lost, it is not allowed to replace it with a monkey wrench, and it shall be equipped in time. The packing gland shall not be skewed or have no pre tightening clearance. For the stop valve in the environment easily contaminated by rain, snow, dust, wind and sand, the valve stem shall be equipped with a protective cover. The scale on the stop valve shall be complete, accurate and clear. The lead seal, cap and pneumatic accessories of the stop valve shall be complete and intact. It is not allowed to knock, stand or support heavy objects on the stop valve in operation; In particular, the maintenance of non-metallic valves and cast iron valves during valve grease injection shall be prohibited. The professional maintenance of valves before welding, before production and after production plays a vital role in serving the valve in production and operation. Correct, orderly and effective maintenance will protect the valve, make the valve function normally and prolong the service life of the valve. Valve maintenance seems simple, but it’s not. There are often neglected aspects of work.
First, the problem of grease injection quantity is often ignored when the stop valve is greased. After greasing and refueling, the operator selects the valve and greasing connection mode, and then carries out greasing operation. There are two situations: on the one hand, the amount of grease injection is small, the grease injection is insufficient, and the wear of the sealing surface is accelerated due to the lack of lubricant. On the other hand, excessive grease injection causes waste. The reason is that the sealing capacity of different stop valves is not accurately calculated according to the valve type and category. The sealing capacity can be calculated according to the size and type of stop valve, and then an appropriate amount of grease can be injected reasonably.
Second, when the stop valve is greased, the pressure problem is often ignored. During the greasing operation, the greasing pressure changes regularly in peak and valley. The pressure is too low, the seal leaks or fails, the pressure is too high, the grease injection port is blocked, the grease in the seal is hardened, or the sealing ring is locked with the valve ball and valve plate. Usually, when the grease injection pressure is too low, the injected grease mostly flows into the bottom of the valve cavity, which usually occurs in small gate valves. If the grease injection pressure is too high, on the one hand, check the grease injection nozzle and replace it if the grease hole is blocked; On the other hand, for lipid hardening, use cleaning fluid to repeatedly soften the failed sealing grease and inject new grease for replacement. In addition, the sealing model and sealing material also affect the grease injection pressure. Different sealing forms have different grease injection pressure. Generally, the grease injection pressure of hard seal is higher than that of soft seal. The ball valve is generally in the open position during maintenance, and it can be closed for maintenance under special circumstances. Other valves cannot be treated as open. The gate valve must be closed during maintenance to ensure that the lubricating grease fills the sealing groove along the sealing ring. If it is opened, the sealing grease will directly fall into the flow channel or valve cavity, resulting in waste.
After installation, the stop valve shall be inspected regularly. The main inspection items are as follows:
(1) Wear of sealing surface of stop valve.
(2) Wear of trapezoidal threads of valve stem and valve stem nut.
(3) Whether the packing is outdated and invalid. If it is damaged, it shall be replaced in time.
(4) After the maintenance and assembly of the stop valve, the sealing performance test shall be carried out.
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