Control valves are a type of pipe fittings designed to control the parameters of the working environment. The concept of regulating parameters includes regulating the flow rate of the medium, maintaining the pressure of the medium within the specified limits, mixing various media in the required proportions, maintaining a given level of liquid in the vessels and some others. The control valves perform all their functions by changing the flow rate of the medium through its bore.
Depending on the specific operating conditions, various types of control of the control valves are used, most often they use external energy sources and command control from sensors that record the parameters of the medium in the pipeline . Automatic control directly from the work environment is also used. In modern industry, it is already rare, but still found, that the main way of controlling regulators in the past is manual control ( see the figure on the right ).
Depending on the parameters of the working medium ( pressure , temperature , chemical composition , etc.), different requirements are imposed on each type of regulation, which led to the appearance of many constructive types of control valves. From the point of view of automation of industrial enterprises, each of them is considered as an element of the automatic process control system , flowing with the participation of liquid and gaseous working media and regulated under the influence of received command information [1] [2] [3] .
The main types of structures
Control Valve
These devices are most widely used among various types of control valves. Most of them are very similar in design to shut-off valves , but they also have their own specific types.
In the direction of the medium flow, control valves are divided into:
- checkpoints - such valves are installed on straight sections of the pipeline, in them the direction of the flow of the working medium does not change;
- angular - change the flow direction by 90 ° ;
- three-way (mixing) - have three nozzles for connection to the pipeline (two inlet and one outlet) for mixing two media flows with different parameters into one. In plumbing, such a device is called a mixer .
The main differences of control valves are in the designs of regulatory bodies , on this basis they are divided into:
- single-seat;
- two-seaters;
- cellular;
- membrane;
- spool [1] [2] .
To control the control valves are used electric actuators , electromagnetic actuators and pneumatic actuators . So that the forces from the medium and the friction force in the guides and the seal do not lead to a decrease in the accuracy of the valve, additional devices are used - positioners [3] .
Stop valve
With the help of this device, regulation is performed according to a given characteristic, and the valve is sealed according to the tightness standards for shutoff valves, which is ensured by the special design of the plunger having a profile part for regulation, as well as a sealing surface for tight contact with the seat in the closed position; this design is a single-seat valve [1] [2] .
Mixing Valves
They are used in cases when it is necessary to mix various media in certain proportions, for example, cold and hot water , keeping a certain parameter constant (for example, temperature ) or changing it according to a given law. The difference between mixing valves and control valves is that the control action, which sets the position of the plunger in the first, determines the flow rates of two media at the same time, and not one, as in control valves [1] .
Just like control valves, mixing valves can be controlled by an electric or pneumatic actuator (see fig.).
Direct-acting pressure regulators
Direct-acting regulators serve to maintain constant pressure in the pipeline, this need may arise in real operating conditions, when pressure fluctuations of the working medium occur in it, which are unacceptable for the normal operation of the technological system or installation.
In contrast to valves of indirect action , in which for continuous regulation it is necessary to monitor the state of the parameter being monitored by special sensors and, when it deviates from the norm, give a command signal to the actuator, the direct-action regulator operates directly from the medium in the controlled section of the pipeline without the use of extraneous energy sources. In addition to such regulators, direct-acting valves include safety valves related to safety valves and non- return valves related to protective valves .
Pressure control can be carried out after the regulator (in the direction of the medium flow), in this case the regulator is called “After itself”, or before it, in this case it is called “Before itself”.
Principle of operation:
Suppose that a steady-state fluid flow through a regulator corresponds to a given nominal pressure in the pipeline, while the force from the pressure of the sensor on the sensing element is compensated by the load adjuster ( spring or load ), that is, the system is in equilibrium . When the pressure in the pipeline changes, this equilibrium is violated, and the valve shutter moves, overcoming the force from the setter, or vice versa, succumbing to it, while the degree of opening of the regulatory body, and, consequently, the flow rate of the medium, changes. As the flow rate changes, the pressure changes , and when its initial value is reached, the system again comes into equilibrium, and the shutter stops moving.
The most common direct acting regulators equipped with diaphragm actuators . The connection of the regulators to the pipeline, as a rule, is flanged , however, small diameter regulators with a threaded connection (coupling) are found [1] [2] [3] .
Level Knob
Level controllers are used in vessels used in power , refrigeration and other installations. They are controlled by a float , on command from which an additional amount of liquid is admitted (“power regulator”) or an excess amount of liquid is discharged (“overflow regulator”) - of a static and aesthetic type [1] [2] .
Other types
They can also be used as control valves, but much less frequently, other types:
- control dampers controlled by a pneumatic or electric actuator;
- regulating ball valves controlled by pneumatic, hydraulic , or electric drive;
- electric control valves [1] [3] .
See also
- Stop valves
- Safety fittings
- Protective fittings
Notes
- ↑ 1 2 3 4 5 6 7 R.F. Usvatov-Usyskin. Let's talk about fittings. - M .: Vitex, 2005.
- ↑ 1 2 3 4 5 D.F. Gurevich. Pipeline accessories. Reference manual. - L .: Engineering, 1981.
- ↑ 1 2 3 4 Pipe fittings with automatic control. Directory. Under the general editorship of S. I. Kosykh. - L .: Engineering, 1982.