A note for chemical workers - valve types and principles
Valves are control components in fluid delivery systems that have functions such as shut-off, regulation, diversion, backflow prevention, regulation, shunting or overflow relief.
Valves for fluid control systems range from the simplest shut-off valves to the extremely complex self-control systems available in a wide variety of sizes and sizes. Some are classified according to usage (such as chemical, petroleum, power station, etc.), some are divided by medium (such as water vapor, air valve, etc.), and some are classified according to materials (such as cast iron valves, cast steel valves, forged steel valves, etc.), According to the connection form (such as internal thread, flange valve, etc.), some according to temperature (such as cryogenic valve, high temperature valve, etc.), and some pressure points (such as vacuum valve, low pressure valve, medium pressure valve, high pressure valve , ultra high pressure valve, etc.).
At present, the most commonly used classification methods in the world and in China are divided according to principle, function and structure. General opening valve, globe valve, throttle valve, instrument valve, plunger valve, diaphragm valve, plug valve, ball valve, butterfly valve, check valve, pressure relief valve safety valve, steam trap, regulating valve, bottom valve, filter, Sewage valves, etc.
Let's take a look at the types and principles of common valves.
The gate valve, also called the gate valve, is a widely used valve. The gate valve is one of the most commonly used shut-off valves. It is mainly used to turn on or off the medium in the pipeline and is not used to regulate the flow of the medium. The range of pressures, temperatures and diameters used is very large, especially for medium and large diameter pipes. Its closing principle is that the sealing surface of the ram and the sealing surface of the valve seat are smooth, flat and uniform, which can prevent the medium from flowing through, and rely on the shape of the top mold, the spring or the ram to enhance the sealing effect. It mainly acts as a cut-off in the pipeline.
Its advantages are:
The fluid resistance is small, the opening and closing is labor-saving, and it can be used in the case of two-way flow of the medium. There is no directionality. The sealing surface is not easy to be eroded when fully opened, and the structure length is short. It is not only suitable for small valves, but also suitable for large valves.
The shut-off valve, also called the door, is the most widely used type of valve. It is popular because of the small friction between the sealing surfaces during opening and closing, which is relatively durable, the opening height is not large, and the manufacturing is easy. Convenient, not only for medium and low pressure, but also for high pressure.
Its closing principle is that, relying on the pressure of the valve bar, the sealing surface of the valve disc and the sealing surface of the valve seat are closely fitted to prevent the medium from flowing. The shut-off valve allows only one-way flow of the medium and is directional when installed. Its structural length is greater than the gate valve, and the fluid resistance is large. When it is operated for a long time, the sealing reliability is not strong.
PS: The valve values â€‹â€‹of the two types of valves, such as gate valve and globe valve, are similar, which is easy to be confused. Let's take a closer look at their differences.
[Different from the structure of the structure]:
The following picture shows a gate valve
The following picture shows a shut-off valve
[Different in principle]:
The shut-off valve is a rising valve stem type, and the hand wheel rotates and ascends together with the valve stem. The gate valve is a handwheel that rotates and the stem moves up. Flow rates vary, gate valves require full or full shutoff, while shut-off valves are not required. The shut-off valve has the specified inlet and outlet directions; the gate valve has no import and export direction requirements.
[Different from the flow]:
The shut-off valve is low in and out, and the obvious pipe is not on a horizontal line when viewed from the outside. The gate valve flow path is on the same horizontal line. The stroke of the gate valve is larger than that of the shut-off valve.
[distinguish from the sealing surface]:
The sealing surface of the shut-off valve is a small trapezoidal side of the valve core. Once the valve core is detached, it is equivalent to the valve closing (if the pressure difference is large, of course, the closing is not strict, but the anti-reverse effect is not bad), the gate valve is the sluice gate. The side is sealed, the sealing effect is not as good as the shut-off valve, and the valve core does not fall off like the shut-off valve.
[Different from the installation method]:
The flow direction of the gate valve is the same from both sides.
There are two ways to install the shut-off valve. One is that the medium can enter from below the valve core. The advantage is that the packing is not under pressure when the valve is closed, the service life of the packing can be extended, and the pipeline can be pressurized in front of the valve. In the case of the replacement of the packing; the disadvantage is that the driving torque of the valve is large, about 1 times the upper flow, the axial force of the valve stem is large, and the valve stem is easy to bend. Therefore, this method is generally only applicable to small-diameter shut-off valves (below DN50). The shut-off valves above DN200 use the medium to flow in from above (electric shut-off valves generally use medium to enter from above). The disadvantage of the way the medium enters from above is exactly the opposite of the way it enters below.
The butterfly valve is also called a butterfly valve. As its name suggests, its key components are like butterflies blowing in the wind and freely swirling. The valve flap of the butterfly valve is a disc that rotates around an axis in the valve seat. The size of the angle of rotation is the degree of opening and closing of the valve.
The butterfly valve has the characteristics of light weight, saves materials compared with other valves, has a simple structure, is quick to open and close, can be used for cutting and throttling, has small fluid resistance, and is labor-saving. The butterfly valve can be made into a large diameter. Where a butterfly valve can be used, it is best not to make the gate valve, because the butterfly valve is more economical than the gate valve and has good regulation. At present, butterfly valves are widely used in hot water pipes.
The ball valve works by rotating the valve to make the valve unblocked or blocked. The ball valve switch is light in weight, small in size, can be made into a large diameter, reliable in sealing, simple in structure, convenient in maintenance, and the sealing surface and the spherical surface are often closed, and is not easily eroded by the medium, and is widely used in various industries.
The plug valve relies on the plug body to rotate around the center line of the valve body to achieve the purpose of opening and closing. Its role is to cut, divide and change the flow direction of the medium. The structure is simple, the size is small, only 90 degrees of rotation is required during operation, and the fluid resistance is not large. The disadvantage is that the switch is laborious, the sealing surface is easy to wear, and it is easy to get stuck at high temperature, which is not suitable for adjusting the flow rate.
Plug valves, also known as cocks, cocks, and turns. It has many types, including straight-through, three-way and four-form. Plug valves are best suited for use as a shut-off and closing medium as well as for shunting, but can sometimes be used for throttling depending on the nature of the application and the erosion resistance of the sealing surface.
The check valve is a valve that automatically opens and closes by the force of the fluid itself, and its function is to prevent the medium from flowing backward. Its name is many, such as check valve, check valve, single flow door and so on. According to the structure, it can be divided into two categories.
Lift type: The flap moves along the vertical centerline of the valve body. There are two types of check valves: one is horizontal, mounted on a horizontal pipe, the valve body is similar in shape to a shut-off valve, and the other is vertical and mounted in a vertical pipe.
Swing type: The valve flap rotates around the pin shaft. These valves have single, double and multiple lobes, but the principle is the same.
Pressure reducing valve
The pressure reducing valve is a valve that adjusts the inlet pressure to a certain required outlet pressure and relies on the energy of the medium itself to automatically maintain the outlet pressure. From the point of view of fluid mechanics, the pressure reducing valve is a throttling element whose local resistance can be changed, that is, by changing the throttling area, the flow velocity and the kinetic energy of the fluid are changed, resulting in different pressure loss, thereby achieving the purpose of decompression. Then, relying on the adjustment of the control and regulation system, the fluctuation of the post-valve pressure is balanced with the spring force, so that the post-valve pressure remains constant within a certain error range.
The pressure reducing valve mainly controls the fixed outlet pressure of the main valve, and the main valve outlet pressure is not changed by the inlet pressure change, and the outlet pressure is not changed due to the change of the main valve outlet flow rate. Applicable to industrial water supply, fire water supply and domestic water pipe network system.
A flow valve is a valve that controls the flow of a fluid by changing the throttle section or the length of the throttle. The throttle valve and the check valve are connected in parallel to form a one-way throttle valve. The throttle valve and the one-way throttle valve are simple flow control valves. In the hydraulic system of the fixed pump, the throttle valve and the relief valve cooperate to form three throttling speed control systems, that is, the throttle throttle speed regulation System, return oil throttle control system and bypass throttle speed control system. The throttle valve has no negative flow feedback function and cannot compensate for the instability caused by the load change. It is generally only used in applications where the load does not change much or the speed stability is not high.
Since the flow rate of the throttle valve depends not only on the area of â€‹â€‹the orifice, but also on the pressure difference before and after the orifice, the rigidity of the valve is small, so it is only suitable for the load change of the actuator is small and the speed stability is not high. occasion.
For the throttle speed control system with large load variation of the actuator and high speed stability requirements, the throttle valve must be pressure compensated to keep the differential pressure before and after the throttle valve constant, so as to achieve flow stability.
The safety valve is a special valve in which the opening and closing member is in a normally closed state under the action of an external force. When the pressure of the medium in the equipment or the pipeline rises above a specified value, the medium is prevented from exceeding the specified value by discharging the medium outside the system. . Safety valves are automatic valves, mainly used in boilers, pressure vessels and pipelines. The control pressure does not exceed the specified value, which plays an important role in protecting personal safety and equipment operation. Note The safety valve must be pressure tested before it can be used.
The safety valve is a special branch of the valve family. Its speciality is because it is different from other valves only as a switch, and more importantly, it is used to protect the safety of the equipment. Safety valves are generally divided into spring-type safety valves and lever-type safety valves and pulse-type safety valves according to the structure. Among them, spring-type safety valves are most commonly used; they are divided into threaded safety valves and flange safety valves according to the connection method. Safety valve diameter is generally not large, commonly used between DN15mm-DN80mm, more than 150mm is generally called large diameter safety valve.
Regulating valve, also known as control valve, in the field of industrial automation process control, by accepting the control signal output from the control unit, by means of power operation to change the final control element of the process parameters such as medium flow, pressure, temperature, liquid level. It usually consists of an actuator and a valve. According to the characteristics of the stroke, the regulating valve can be divided into straight stroke and angular stroke; according to the power used by the actuator, it can be divided into three types: pneumatic regulating valve, electric regulating valve and hydraulic regulating valve; according to its function and characteristics There are three kinds of linear characteristics, equal percentage characteristics and parabolic characteristics. The regulating valve is suitable for air, water, steam, various corrosive media, mud, oil and other media. English name: control valve, the number is usually the beginning of FV. Regulating valves are commonly classified: pneumatic regulating valves, electric regulating valves, hydraulic regulating valves, and self-operated regulating valves.
Below we will analyze in detail the accident case of a major leak caused by valve breakage.
On X-X Day, a certain oil company's oil tank valve lost more than 1,560 tons of gasoline. Because it is located in the Gobi Desert, the leaked oil all penetrated into the ground.
(1) Basic situation
The oil depot is 2. 9km away from the petrochemical plant. It is connected by two D/V200 underground pipelines and crosses two roads. A flat-plate valve well is provided at each end of the road. On December 9, 1981, the oil depot signed an oil transfer procedure with the refinery, which was based on the refinery tank measurement. The warehousing leader responsible for the oil pumping took two green workers' inspection lines from the pumping station and arranged two telephone operators to look at the telephone; the metering squad leader arranged the metering work. Two shifts from 9:00 to 19:00. The first shift is from 9:00 to 15:00. The oil is transferred at 12:00 (oil temperature is -4.7 Â°C), the oil before the oil is 277 mm high, the liquid level is 607 mm at 13:45, and the height is 330 mm; at 14:40, the liquid level is 1204 mm higher. 597mm. At 15:00, the second shift takes over. At 15:50, the liquid level is 1626mm higher, which is 422mm higher. At 16:45, the liquid level is 1964mm higher and 338mm higher. The oil inlet speed starts to decrease from 15:00 to 00. After the oil depot leader heard the abnormality of the oil delivery, he instructed the warehousing chief to inspect it. The chief said that he â€œknowsâ€ and did not actually check it. After leaving the post for private affairs, I will not return until 23:00. On the 10th, the 15:00 chief and a metering staff went to the refinery for measurement and found a problem. At this time, two young workers were patrolled. When inspecting the second road at 500m from the oil depot, it was found that the valve in the valve well broke 3cm, and the oil ran out of the ground and penetrated into the ground.
(II) Analysis of reasons and lessons (mainly from the aspect of valve quality and selection)
This is a mechanical accident involving a valve break (a technical accident), which is essentially a serious liability accident. Its main lesson:
1. Poor valve quality
In the valve body of the valve, there are three cracks in the four reinforcing ribs, all perpendicular to the direction of the pulling force. Cast iron valves, according to national technical standards, should be able to withstand 337t damage pull, valve bolts (M2012) should be able to withstand 229t damage pull, pipeline (Î¦219) should be able to withstand 201t damage pull. That is, under the action of breaking the tensile force, the first step should be the steel pipe, followed by the 12 bolts, and the valve again. The tensile strength test (a set of three test bars) of the batch of valves was 8.0, 8.5, 8.5 kg/cm2, respectively, which was less than 8.3 times of the 20kg/cm2 specified by the state. After sampling and analysis, the sulfur and carbon content exceeds the regulations, and the silicon and manganese contents are less than the specified, resulting in a decrease in tensile strength.
After the accident, the oil depot was inspected by 70 DN200 valves. Among them, 12 of the valve bodies had cracks; 8 of the 38 valves in the oil pump room had cracks; 5 of the 8 valves outside the oil pipeline were 5 Only the valve body ribs have cracks. The poor quality of the valve was the main cause of the accident.
2. Design problem
In general, there are not many problems in design. But from a management perspective, design should also draw some useful lessons from it. If the oil tank is buried in the underground pipeline, the designed soil depth is 8. 5m, and the low-lying unevenness factor is not considered. 5ç±³ã€‚ After the completion of the project, the depth of the cover is less than 8. 5m. The valves in the oil inlet and outlet valves and the valve wells shall be high-pressure valves in cast steel, and the actual design is cast iron low-pressure valves. The valve well is a flat cover type. There is no inspection room and step. It is difficult to check the valve condition during actual operation. If some compensation measures can be taken in the valve well, the valve quality is poor and it is not necessarily broken. It is broken and does not break 3cm. Wide crack.
Finally, I will attach a variety of CAD valve icons to everyone:
1) Quick-latch adapter is designed to offer easier, faster and safer makeup for casing;
2) Be able to couple with two plugs simultaneously, simplifying cementing operation and preventing cement pollution;
3) Positive lever indicates plug releasing;
4) Balance pipe is developed.
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