The forward and reverse effects of the controller on the valve

The forward action setting value is higher than a value, and the normal output is 0, that is, the valve is usually closed (or open), while the reverse action is opposite to the forward action. What is closed (and then opened) is the normal state of the valve. Generally 4-20mA.

For the regulator, as the input increases, the output also increases, which is a positive effect. An increase in input and a decrease in output are the reaction. For regulating valves, those whose air source enters from above the membrane head are called positive-acting regulating valves, and those whose air source enters from the bottom of the diaphragm head are called reaction-acting regulating valves. When the gas source is increased, the valve opens and the weighing gas opens the valve. When the air source is increased, the valve closes and the weighing gas closes the valve. If the regulator is positive, then the input signal increases and the output signal also increases. As for whether the valve is open or closed, it depends on the process conditions.

There are two types of regulators: forward-acting and reaction-acting regulators. The choice of the forward and reverse effects of the regulator is related to the characteristics of the controlled process and the air opening and air closing forms of the regulating valve. The controlled process can also be divided into two types: positive and negative. When the input quantity of the controlled process (material or energy passing through the regulating valve) increases (or decreases), its output (controlled parameter) also increases (or decreases). At this time, the controlled process is called a positive effect. ； On the contrary, when the input of the controlled process increases, its output decreases, and the process is called reaction. For a control system to work properly, the multiplication of the polarities of its components (expressed by their static amplification coefficients) must be positive. Since the static amplification coefficient Km of the transmitter is usually positive polarity, only the static amplification coefficient Kc of the regulator, the static amplification coefficient Kv of the regulating valve and the static amplification coefficient Ko of the process must be multiplied to a positive polarity. The polarity of each link in the control system is stipulated in this way: Positive acting regulator, that is, when the measured value of the system increases, the output of the regulator also increases, and its Kc takes a negative ； Reaction regulator, that is, when the measured value of the system increases, the output of the regulator decreases, and its Kc becomes positive. The Kv of the air-opening valve is taken as positive, and the Kv of the air-closing valve is taken as negative. The controlled process with positive action has positive Ko, and the controlled process with negative action has negative Ko. Determine the sequence of positive and negative actions of the regulator as: First, determine the air opening and air closing forms of the control valve based on principles such as process safety, and then determine its positive and negative effects based on the characteristics of the controlled process. ； Finally, the positive and negative action forms of the regulator are determined based on the above principle that the polarity multiplication of the static amplification coefficients of each link making up the system must be positive.

For terminal components, considering the safety of the equipment, there are two types: AIR TO OPEN and AIR TO CLOSE. Generally used, AIR TO OPEN is used more frequently. AIR TO OPEN 4mA close 20mA open AIR TO CLOSE 4mA open 20mA close. For the controller, there are forward and reverse actions. For example, the temperature control is different between the superheater and the pressure reducer. The superheater temperature rises. The controller output increases. The control valve opens and the water spray volume increases. The temperature drops. The pressure reducer pressure rises. The controller output decreases. The control valve closes and the pressure drops.

This depends on the process conditions, so don’t be too absolute!

A direct acting controller is a positive acting regulator, that is, when the input signal of the regulator is greater than a given value, the output of the regulator increases, and when the input signal is less than a given value, the output decreases.； Reverse acting controller refers to a reaction regulator, that is, when the input signal of the regulator is greater than a given value, the output of the regulator decreases, and when the input signal is less than a given value, the output increases. The positive and negative effects of the regulator are determined based on the characteristics of each link in the control loop, that is, the entire control loop should be a negative feedback control system.

The positive and negative effects of the controller should be considered together with whether the valve is open or closed and the characteristics of the controlled object. The general principle is to maintain negative feedback in the entire loop, that is, the transfer function is negative.

1: Determine whether to use an air-opening or an air-closing valve style based on the actual situation and from a safety first perspective 2: Determine the forward and reverse effects of the controller according to the process requirements and the selected valve style; P

If the incoming regulator is on and the process requirement is off, which components or parameters should be adjusted?

study* Yes, simple and easy to understand and remember

study* Yes, simple and easy to understand and remember