Control valves. Buy a two-way and three-way control valve from LDM
Control valves are used to control the pressure of liquid and gaseous substances transmitted through pipelines. The control valve allows you to continuously or discretely regulate the flow of working fluid into the pipeline.
For systems in which it is especially important to accurately distribute the flow of the working medium, a pressure control unit is required.
This is especially true, for example, for heating networks, since the indoor climate depends on the volume of coolant entering the pipes and radiators. The throughput of the pipeline decreases or increases, respectively, as the cross-section of the hole inside the valve decreases or increases.
The problem is solved by constantly changing the capacity of the pipe through which the liquid or gas moves using a control valve.
According to their purpose, there are three main types of control valves:
- two-way through - serves only to control the flow of liquid or gas, used on straight sections of the pipeline;
- two-way corner – regulates the pressure and changes its direction, used at pipeline turning points;
- three-pass - mixes two types of working fluid into a common flow or divides one flow into two.
The simplest control valve is a straight-through valve, it consists of the following parts:
- a body in the form of a tee with a passage hole inside;
- flange or thread at the ends of the pipes;
- seal assembly that maintains the tightness of the valve;
- gate – valve regulating body;
- rod - a part used to change the position of the valve.
The flow of the working medium is regulated by changing the size of the passage opening when moving the position of the gate relative to the passage opening.
The design is partially changed and supplemented with new elements depending on the purpose of the control valve.
Note! There are shut-off and control valves that are modified so that the flow of the working medium can be completely stopped. In this case, the valve is made in such a way that in the closed position its parts are hermetically sealed.
Advantages of control valves
This type of regulator is used in domestic and industrial water and gas supply systems, heating networks and oil pipelines.
Control (shut-off and control) valves
Valves are designed to control the flow of liquid and gaseous media transported through pipelines.
Control and shut-off control valves continuously vary the flow rate of the regulated flow from a minimum when the valve is fully closed to a maximum when the valve is fully open.
Shut-off or shut-off valves do not control the regulated flow continuously, but discretely (the valve is completely open or completely closed). Both control and isolation valves have small leaks of controlled fluid when the valve is in the closed position.
It should be noted that the division of valves into control, shut-off and shut-off-control valves exists only in our country, as well as separate leakage standards for control and shut-off valves. The rest of the world simply produces control valves, the leakage of which is divided into six classes, the higher the class number, the less leakage. The last three classes refer to valves, which we call shut-off and shut-off and control valves.
The nominal bore diameter of the valve (DN) should be understood as the nominal internal diameter of the inlet and outlet pipes of the valve (in some cases, the diameter of the outlet pipe may exceed the diameter of the inlet pipe). Each value of the nominal diameter of the valve passage corresponds to the maximum possible flow rate of the regulated substance, which, in general, depends on a number of parameters (pressure drop, density, etc.). For the convenience of comparing valves and selecting the required valve size based on the results of hydraulic calculations, the concept of conditional capacity has been introduced.
The conditional capacity of the valve (Kvy) shows how much water at a temperature of 20 ° C the valve can let through when the pressure drop across it is 0.1 MPa (1 kgf/cm2) with the valve fully open.
The control valve consists of three main blocks: the body, the throttle assembly and the valve actuator. Typical pass-through design
A shut-off and control valve without an installed actuator is shown in Figure 1.
A throttle assembly is installed inside the valve body 1, consisting of a seat 2 and a plunger 3 connected to a rod 4. The seat can be made in various designs: screwed into the valve body as shown in Figure 1, pressed against the body with a special bushing, or made integral with the body .
The plunger slides along a guide made in the cover 5. A sealing gasket 6 is installed between the body 1 and the cover 5. The rod 4 is brought out through the stuffing box 7, which is a set of spring-loaded chevron rings made of fluoroplastic-4 or its modifications. An actuator is installed on cover 5, the stem of which is connected to the valve stem. The drive can be pneumatic, manual, electric or electromagnetic.
The throttle assembly is the regulating and closing element of the valve. It is in this unit that the task of changing the flow area of the valve and, as a consequence, changing its flow characteristics is implemented.
Specific bushing-seat-plunger combinations are selected based on the operating conditions of the valve: pressure drop, adjustable type
medium and its temperature, the presence of mechanical impurities, throughput, medium viscosity, etc.
In most cases, the correct direction of supply of the working fluid is important for the operation of the valve. It is marked with an arrow on the outer surface of the housings. If the medium is supplied through the left channel in the housing shown in Figure 1, then this direction of supply is called “under the shutter” (the medium approaches the plunger from below), and if the medium is supplied through the right channel, then this direction of supply is called “to the shutter” (medium presses the plunger against the seat in the closed state). The main parameters and characteristics of typical control valves produced by domestic enterprises are presented in tables 1 and 2.
Table 1.
Main parameters of shut-off and control valves
Table 2.
Conditional capacity of shut-off and control valves
ACTUATORS
Drives and actuators of shut-off and control, control and shut-off pipeline valves are designed
to convert the control signal (pneumatic, electrical or mechanical) into mechanical (linear or rotational) movement of the actuator rod and a shut-off element rigidly connected to the rod (valve, ball valve, butterfly valve, gate valve, etc.).
Actuators used to control shut-off and control valves according to the principle of operation and the type of energy used to create the necessary mechanical force on the operating valve are divided into:
Pneumatic
Electrical
Hydraulic
Combined
Pneumatic actuators
Pneumatic actuators, due to established tradition, occupy a fairly large place among drives for control valves of various types. This is primarily due to the fact that mass industrial automation until the 50s and 60s of the last century was based mainly on pneumatics. Pneumatic automated control systems today, in the era of microprocessors and the widespread use of digital electronics, look somewhat archaic, and in addition, they are quite bulky and require the organization of networks for the preparation and distribution of compressed air, which is also consumed during the operation of pneumatic systems.
At the same time, the simplicity of the design of pneumatic drives, and as a consequence of this, their fairly high reliability and maintainability, make it possible to successfully use such drives in modern automated process control systems.
Pneumatic actuators are designed to convert changes in air pressure P at the output of the regulator into movement of the regulating body - valve, damper, gate, tap, etc. The regulating body changes the flow rate of liquid, gas, steam, etc. at the control object, and thereby causing a change in the controlled process parameter.
Based on the type of drive, pneumatic actuators are divided into membrane, piston, rotary, and rotating pneumatic motors.
Diaphragm actuator (MIM)
The diagram of the membrane actuator (MIM) is shown in Figure 2. The movement of the output rod 2, connected to the regulating body, in one direction is carried out by the force that is created by the pressure P, in the other - by the force of the spring 3. The signal P enters the sealed membrane “head”, which contains a membrane made of rubberized fabric 2-4 mm thick with a rigid center. Spring 3 presses on the membrane from below. In membrane actuators (Fig. 2), the pressure of the control air acts on the membrane 4, clamped along the perimeter between the actuator covers, and creates a force that is equalized by spring 3. Thus, the stroke of the actuator rod 2 is proportional to the value of the control pressure. The stiffness and precompression of the spring determines the actuator force range and nominal stroke.
Membrane actuators are classified according to the size of the membrane “heads”. MIMS are usually supplied together
with regulatory bodies - valves. Since when pressure P is removed, the membrane always moves upward, depending on the design of the regulating body, a distinction is made between normally open NO and normally closed NC valves.
Figure 2. Diaphragm actuator mounted on a control valve:
1 - regulatory body; 2 - rod; 3 - spring; 4 - membrane; 5 - oil seal
The static characteristics of most MIMs are close to linear, however, they have a hysteresis zone of 2-15% of the largest value of P. This value depends on the friction forces in the seal 5, on the pressure drop across the control body, on the characteristics of the spring and the effective area of the membrane.
To reduce the hysteresis zone and improve the dynamic characteristics of MIMs, additional power amplifiers, called positioners, are installed on the actuator. There are positioners that operate according to a displacement compensation scheme and a force compensation scheme. In positioners of both types, the MIM is covered by negative feedback on the position of the rod, which eliminates the influence of friction forces in the stuffing box, pressure drop on the control body, etc. on the static characteristics.
At the same time, the air flow supplied to the MIM increases and the dynamic characteristics of the latter are noticeably improved.
To interface with electrical signals of control systems, electro-pneumatic positioners are used, which, in addition to improving the static characteristics of membrane actuators, ensure the conversion of the electrical signal into a pulse of control air supplied to the MIM.
The main technical characteristics of MIMs are presented in Table 3.
Table 3.
The appearance of typical MIMs installed on control valves is shown in Figure 3.
Piston pneumatic actuators
Piston pneumatic actuators (PPA) are used in cases where linear movement of the actuator rod is required
Shut-off and control valves are used to control the flow of media at industrial production facilities and household life systems. Trunk pipelines, oil and gas fields and their processing plants, steelmaking and chemical plants, wastewater treatment plants and city water supply are just a small part of the enterprises that require a huge amount of shut-off and control valves.
There are many types and modifications of shut-off and control valves. We will look at the operating principle of the most common product types such as ball valves, butterfly valves, gate valves, gate valves and diaphragm valves.
The operating principle of all the above types of shut-off valves is approximately the same. All these devices either limit the flow of the medium (air, liquids, steam, gas, solids) or completely block it. The only difference is in the design elements of the types of shut-off valves (membrane, disk, ball) with the help of which the flow is blocked.
The ball valve is one of the most reliable elements of shut-off valves. Valves of this type provide a very good possibility of completely shutting off the flow if the shut-off element is turned a quarter turn (90°). The advantages of the ball valve also include low closing time and low probability of leakage in case of seal wear
Ball valves can be divided into partial bore and full bore. A partial bore valve in the open state has a passage diameter smaller than the diameter of the pipeline, a full bore valve has a passage diameter equal to the diameter of the pipeline. A full bore ball valve is more efficient because... allows the pressure drop across the valve to be minimized.
Ball valves are only recommended for use in the fully open or fully closed position. They are not designed for precise flow control, or for operating in a partially open position, as this would create excessive pressure on part of the housing, which could lead to its deformation. Deformation of the housing leads to leaks and breakdowns.
In the "open" position |  |
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In the "closed" position |  |
A butterfly valve regulates the flow using a special element - a disk mounted on a shaft and rotating around its axis. Just like a ball valve, a butterfly valve is capable of closing in a fairly short time, since the disk makes the same 90 ° rotation, which is why this valve is also called a quarter-turn.
Depending on the position of the disk and shaft relative to the body, butterfly valves can be three-eccentric or two-eccentric. A valve with offset eccentricity means that the axis of the disk is shifted relative to the geometric axis of the body, which ensures a tighter fit of the disk to the valve seal, and therefore eliminates leaks.
Butterfly valves are characterized by simplicity of design, light weight, and compact dimensions. But the materials used in the manufacture of valves may limit their use in very high temperatures or extremely aggressive environments. This mainly concerns valve seals made of polymer materials.
In the "open" position |  |
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In the "Closed" position |  |
The shut-off and control valve is suitable for use in various process facilities, with the exception of large diameter pipelines, to control and regulate the flow of the medium.
The principle of operation of valves is not very different from the principle of operation of other shut-off and control valves. The advantages of these valves are that the valve stroke is short for full opening; accordingly, such a valve usually has small dimensions and an acceptable weight. The valve also has high tightness and no friction between the valve seal and the seat, which significantly reduces their wear.
The disadvantages of this type of valves are strong hydraulic resistance and, accordingly, large energy losses, limitation of the maximum diameter of pipelines on which they can be installed, as well as the existence of stagnant zones (due to the S-shaped internal cross-section) where impurities can accumulate and garbage.
In the "open" position |  |
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In the "closed" position |  |
The design of a gate valve resembles a sluice - the flow is regulated by dividing it using a metal plate - a gate. A gate valve is one of the simplest devices for regulating flow.
Gate valves, depending on the design of the locking element, can be wafer-type, double-sided or knife-type.
The advantages of a gate valve include the fact that this type of valve, when open, does not contain any elements that impede the flow.
In the "open" position |  |
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Step 1 |  |
Step 2 |  |
In the "closed" position |  |
Diaphragm valves use a flexible membrane (diaphragm) as the shut-off element, a "pinch" method to stop the flow of the valve using the flexible membrane.
One of the advantages of a diaphragm valve is that the components of the valve itself are separated from the medium flow, which in the case of aggressive media increases the service life of the valve, subject to regular maintenance and timely replacement of the membrane.
These types of valves are generally not suitable for aggressive environments and environments with high temperatures; they are mainly used for plumbing systems.
Below is a video that clearly shows the principle of operation of a three-eccentric butterfly valve
Seat control valve (linear)— made on the basis of a seat valve. Regulation is carried out by changing the flow area between the valve and the seat. This type of control valve is called linear because it is controlled by electric actuators with a progressive movement of the stem. The universal design of the control valve allows you to create almost any flow characteristic due to modifications of the valve and seat, and the excellent control characteristics and simple design of the control valve with seat valve have contributed to its widespread use in building engineering systems. The only drawback of linear valves is the complex shape of the flow part, which is unsuitable for use with viscous media.
Ball control valve (rotary)— made on the basis of a ball valve. Regulation is carried out by changing the flow area by rotating the ball around an axis perpendicular to the direction of water flow. The flow section of the ball can be round or another shape. Rotary control valves of this type are called because they are controlled by actuators with radial rotation of the stem. Ball control valves are used in conjunction with high closing force rotary actuators and are controlled by radial movement of the stem. The disadvantages of ball control valves are the need to use expensive electric drives with high closing force and the difficulty of creating a linear or equal percentage flow characteristic - as a result, low control accuracy. The advantages include the simple shape of the flow part, suitable for use with viscous working media.
According to the presence of a protective function, control valves are divided into:
- Normally open - when the power is turned off, the flow area is opened.
- Normally closed - when the power is turned off, they block the flow.
- Without a protective function - when the power is turned off, the electric drive stops.