Control and regulation of the main technological parameters: flow rate, level, pressure and temperature

The set of single operations forms specific technological processes. In the general case, the technological process is carried out by means of technological operations that are carried out in parallel, sequentially or in combination, when the beginning of the next operation is shifted relative to the beginning of the previous one.

Process management is an organizational and technical problem and today it is solved by creating automatic or automated process management systems.

The purpose of technological process control can be: stabilization of some physical quantity, its change according to a given program or in more complex cases optimization of some summarizing criterion, highest productivity of the process, lowest cost of the product, etc.

Typical process parameters subject to control and regulation include flow rate, level, pressure, temperature and a number of quality parameters.

Closed systems use the current information about the output values, determine the deviation ε (T) controlled value Y (t) from its determined value Yo) and take actions to reduce or completely eliminate ε(T).

The simplest example of a closed system, called a deviation control system, is the system for stabilizing the water level in the tank, shown in Figure 1. The system consists of a two-stage measuring transducer (sensor), a device 1 control (regulator) and an actuator mechanism 3, which controls the position of the regulating body (valve) 5.

Rice. 1. Functional diagram of the automatic control system: 1 — regulator, 2 — level measuring transducer, 3 — drive mechanism, 5 — regulating body.

Flow control

Flow control systems are characterized by low inertia and frequent parameter pulsations.

Typically, flow control restricts the flow of a substance using a valve or gate, changing the pressure in the pipeline by changing the speed of the pump drive or the degree of bypass (diverting part of the flow through additional channels).

The principles of application of flow regulators for liquid and gaseous media are shown in figure 2, a, for bulk materials — in figure 2, b.

Rice. 2. Flow control schemes: a — liquid and gaseous media, b — bulk materials, c — media ratios.

In the practice of automation of technological processes, there are cases when it is necessary to stabilize the flow ratio of two or more media.

In the scheme shown in Figure 2, c, the flow to G1 is the master, and the flow G2 = γG — slave, where γ — the flow rate ratio, which is set in the process of static regulation of the regulator.

When the master flow G1 changes, the FF controller proportionally changes the slave flow G2.

The choice of the control law depends on the required quality of parameter stabilization.

Level control

Level control systems have the same characteristics as flow control systems. In the general case, the behavior of the level is described by the differential equation

D (dl / dt) = Gin — Gout +Garr,

where S is the area of ​​the horizontal part of the tank, L is the level, Gin, Gout is the flow rate of the medium at the inlet and outlet, Garr — the amount of medium increasing or decreasing the capacity (can be equal to 0) per unit time T.

The constancy of the level indicates the equality of the quantities of supplied and consumed liquid. This condition can be ensured by influencing the supply (Fig. 3, a) or the flow rate (Fig. 3, b) of the liquid. In the version of the regulator shown in Figure 3, c, the results of measurements of liquid supply and flow rate are used to stabilize the parameter.

The liquid level pulse is corrective, excluding the accumulation of errors due to inevitable errors that occur when the supply and flow rate change. The choice of the regulation law also depends on the required quality of parameter stabilization. In this case, it is possible to use not only proportional but also positional controllers.

Rice. 3. Schemes of level control systems: a — with an effect on the power supply, b and c — with an effect on the flow rate of the medium.

Pressure regulation

Constancy of pressure, like constancy of level, indicates the material balance of the object. In the general case, the change in pressure is described by the equation:

V (dp / dt) = Gin — Gout +Garr,

where VE is the volume of the apparatus, p is the pressure.

Pressure control methods are similar to level control methods.

Temperature control

Temperature is an indicator of the thermodynamic state of the system. The dynamic characteristics of the temperature control system depend on the physico-chemical parameters of the process and the design of the apparatus. The peculiarity of such a system is the significant inertia of the object and often of the measuring transducer.

The principles of implementation of thermoregulators are similar to the principles of implementation of level regulators (Fig. 2), taking into account the control of energy consumption in the facility. The choice of the regulatory law depends on the momentum of the object: the greater it is, the more complex the regulatory law. The time constant of the measuring transducer can be reduced by increasing the speed of movement of the coolant, reducing the thickness of the walls of the protective cover (sleeve), etc.

Regulation of product composition and quality parameters

When adjusting the composition or quality of a given product, a situation is possible when a parameter (for example, grain moisture) is measured discretely. In this situation, the loss of information and the reduction of the accuracy of the dynamic adjustment process are inevitable.

The recommended scheme of a regulator that stabilizes some intermediate parameter Y (t), the value of which depends on the main controlled parameter — the product quality indicator Y (ti) is shown in Figure 4.

Rice. 4. Scheme of the product quality control system: 1 — object, 2 — quality analyzer, 3 — extrapolation filter, 4 — computing device, 5 — regulator.

Computing device 4, using a mathematical model of the relationship between the parameters Y (t) and Y (ti), continuously evaluates the quality rating. The extrapolation filter 3 gives an estimated product quality parameter Y (ti) between two measurements.