Other characteristics of a valve to consider are the type of connection and its size, determined by its nominal diameter.

The connection can be made by thread for valves up to 65 mm nominal diameter (DN65) or with flanges, gasket and screws for larger diameters.

The nominal diameter of a valve represents the inner diameter in mm of the pipe to which it is coupled. It corresponds, therefore, to the normalized scale for pipes.

The plug determines the flow characteristic of the valve, that is, the relationship that exists between its position (% of the degree of opening) and the % of the flow rate of the fluid. The choice of characteristic has a great influence on the stability of the process, since it represents the gain of the valve in relation to the stroke.

The characteristic curves give the flow rate that flows through the valve under a constant differential pressure. The most significant are the quick opening, the linear and the isopercentual. These characteristic curves are obtained by machining the plug so that when the stroke varies, the plug-seat passage hole configures the flow rate.

The power of a heat exchanger and its circulating flow have a logarithmic relationship. With this type of valves it will happen that for small loads, a slight opening of the valve will produce a considerable increase in power with the consequent risk of instability in the regulation loop.

The mathematical relationship of this characteristic is:

Where is the flow rate for constant pressure loss, is the valve opening, is the leakage flow rate for constant pressure loss and is the gain or proportionality constant that relates the units and margin of the valve opening to the units and margin of the flow.

The final objective is that the characteristic curves of the exchangers are compensated with the curves of the valves, achieving a constant proportionality between the degree of opening and the heat emitted by the exchanger.

As can be seen in figure b), when the valve opens at 50% of its travel, it allows 10% of the flow to pass through. In a) it is verified that 10% of the flow represents 50% of the power. Both are compensated, obtaining a linear relationship between the valve opening and the power emitted.

The mathematical relationship of this characteristic is variable depending on the manufacturer, the best known is the so-called exponential or logarithmic, also known as equal percentage.

where is the valve opening and depends on the valve adjustment range.

Taking into account the exponential function, in the theoretical characteristic, for a zero degree of opening, a zero flow rate is not obtained, but a positive value. This leakage rate with the valve closed is unacceptable, so manufacturers slightly modify the theoretical characteristic to achieve tightness in closure.

This eliminates the drawback of leakage flow, but makes that initial flow rate uncontrollable. The relationship between the maximum and minimum controllable flow rate is called the adjustment field and is a value given by the manufacturer based on the manufacturing tolerance.

The operation of a regulation valve consists of creating a specific pressure loss that varies the flow rate to the required value. The relationship between the flow and the pressure loss created,[3]​ is given by:.

[4]​ depends on the passage section of the valve and it can be verified that it has surface dimensions (), although in practice it is defined as the water flow rate in m³/h that passes through the valve with a given opening and with a differential pressure of 1 bar.

When the fluid is water, with the aforementioned units, the general formula for Kv is simplified:[5]​.

The maximum value of is obtained with the valve fully open and is then called . This value is the one given by the manufacturer for each valve and, therefore, expresses the flow rate that passes through the fully open valve when there is a differential pressure of 100 kPa between its two mouths.[6]​.

In the processes to be controlled, at 100% of the load the nominal flow must circulate and the valve, if everything is correct, must be completely open and have a differential pressure that corresponds to the installation. The flow rate in any case will be well known, but the differential pressure applied to the valve depends mainly on the actual manometric head of the pump and the head loss of the installation.

Assuming a circuit formed by a centrifugal pump, a control valve and the corresponding pipe. The pump parameters and the pressure loss absorbed by the system vary depending on the degree of valve opening. At each moment, the difference between the manometric head of the pump and the pressure loss of the pipe when the flow varies, corresponds to the pressure loss absorbed by the valve, which increases as the flow decreases.

This variation in differential pressure causes the real characteristic of the valve to deform in relation to the theoretical one, since this was generated considering a constant pressure at the valve inlet. The amplitude of the deformation depends on the relationship between and and is called authority of the valve.

The theoretical characteristic of the valve will have an authority equal to 1, that is, without variation in the differential pressure. As this ratio decreases, the authority begins to decrease, the valve curve is modified, moving further and further away from the theoretical line.[7]​ If the installation flow has been oversized, the characteristic is deformed even more.

The decrease in real authority with respect to theoretical authority,[8]​ is given by the relationship:.

In which is the overflow coefficient. When, the real authority is equal to the theoretical

.[9]​.

The valve can also be selected using abacuses provided by the manufacturers, in which a valve is chosen whose value is immediately lower than the calculated theoretical one.

It will be very common for the size of the regulation valve to be smaller than that of the pipe, although it does not have to be a norm. In any case, the size of the valve should not be less than half the size of the pipe in order to avoid the production of noise.

Opening or closing of automatic control valves is normally accomplished by electrical, hydraulic or pneumatic actuators. Typically, with a modulating valve, which can be adjusted to any position between fully open and fully closed, valve positioners are used to ensure that the valve achieves the desired degree of opening.

Air-actuated valves are typically used due to their simplicity, requiring only a compressed air supply, while electrically-actuated valves require additional wiring and switching gear, and hydraulic-actuated valves require high-pressure supply and return lines for the hydraulic fluid.

Pneumatic control signals are traditionally based on a pressure range of 3-15 psi (pressure unit) (0.2-1.0 bar), or more commonly today, an electrical signal of 4-20 mA for industry, or 0-10 V for HVAC systems. Current electrical control often includes a "Smart" communication signal superimposed on the 4-20 mA control current, so that status and verification of valve position can be signaled back to the controller. HART, Fieldbus Foundation and Profibus are the most common protocols.

Other characteristics of a valve to consider are the type of connection and its size, determined by its nominal diameter.

The connection can be made by thread for valves up to 65 mm nominal diameter (DN65) or with flanges, gasket and screws for larger diameters.

The nominal diameter of a valve represents the inner diameter in mm of the pipe to which it is coupled. It corresponds, therefore, to the normalized scale for pipes.

The plug determines the flow characteristic of the valve, that is, the relationship that exists between its position (% of the degree of opening) and the % of the flow rate of the fluid. The choice of characteristic has a great influence on the stability of the process, since it represents the gain of the valve in relation to the stroke.

The characteristic curves give the flow rate that flows through the valve under a constant differential pressure. The most significant are the quick opening, the linear and the isopercentual. These characteristic curves are obtained by machining the plug so that when the stroke varies, the plug-seat passage hole configures the flow rate.

The power of a heat exchanger and its circulating flow have a logarithmic relationship. With this type of valves it will happen that for small loads, a slight opening of the valve will produce a considerable increase in power with the consequent risk of instability in the regulation loop.

The mathematical relationship of this characteristic is:

Where is the flow rate for constant pressure loss, is the valve opening, is the leakage flow rate for constant pressure loss and is the gain or proportionality constant that relates the units and margin of the valve opening to the units and margin of the flow.

The final objective is that the characteristic curves of the exchangers are compensated with the curves of the valves, achieving a constant proportionality between the degree of opening and the heat emitted by the exchanger.

As can be seen in figure b), when the valve opens at 50% of its travel, it allows 10% of the flow to pass through. In a) it is verified that 10% of the flow represents 50% of the power. Both are compensated, obtaining a linear relationship between the valve opening and the power emitted.

The mathematical relationship of this characteristic is variable depending on the manufacturer, the best known is the so-called exponential or logarithmic, also known as equal percentage.

where is the valve opening and depends on the valve adjustment range.

Taking into account the exponential function, in the theoretical characteristic, for a zero degree of opening, a zero flow rate is not obtained, but a positive value. This leakage rate with the valve closed is unacceptable, so manufacturers slightly modify the theoretical characteristic to achieve tightness in closure.

This eliminates the drawback of leakage flow, but makes that initial flow rate uncontrollable. The relationship between the maximum and minimum controllable flow rate is called the adjustment field and is a value given by the manufacturer based on the manufacturing tolerance.

The operation of a regulation valve consists of creating a specific pressure loss that varies the flow rate to the required value. The relationship between the flow and the pressure loss created,[3]​ is given by:.

[4]​ depends on the passage section of the valve and it can be verified that it has surface dimensions (), although in practice it is defined as the water flow rate in m³/h that passes through the valve with a given opening and with a differential pressure of 1 bar.

When the fluid is water, with the aforementioned units, the general formula for Kv is simplified:[5]​.

The maximum value of is obtained with the valve fully open and is then called . This value is the one given by the manufacturer for each valve and, therefore, expresses the flow rate that passes through the fully open valve when there is a differential pressure of 100 kPa between its two mouths.[6]​.

In the processes to be controlled, at 100% of the load the nominal flow must circulate and the valve, if everything is correct, must be completely open and have a differential pressure that corresponds to the installation. The flow rate in any case will be well known, but the differential pressure applied to the valve depends mainly on the actual manometric head of the pump and the head loss of the installation.

Assuming a circuit formed by a centrifugal pump, a control valve and the corresponding pipe. The pump parameters and the pressure loss absorbed by the system vary depending on the degree of valve opening. At each moment, the difference between the manometric head of the pump and the pressure loss of the pipe when the flow varies, corresponds to the pressure loss absorbed by the valve, which increases as the flow decreases.

This variation in differential pressure causes the real characteristic of the valve to deform in relation to the theoretical one, since this was generated considering a constant pressure at the valve inlet. The amplitude of the deformation depends on the relationship between and and is called authority of the valve.

The theoretical characteristic of the valve will have an authority equal to 1, that is, without variation in the differential pressure. As this ratio decreases, the authority begins to decrease, the valve curve is modified, moving further and further away from the theoretical line.[7]​ If the installation flow has been oversized, the characteristic is deformed even more.

The decrease in real authority with respect to theoretical authority,[8]​ is given by the relationship:.

In which is the overflow coefficient. When, the real authority is equal to the theoretical

.[9]​.

The valve can also be selected using abacuses provided by the manufacturers, in which a valve is chosen whose value is immediately lower than the calculated theoretical one.

It will be very common for the size of the regulation valve to be smaller than that of the pipe, although it does not have to be a norm. In any case, the size of the valve should not be less than half the size of the pipe in order to avoid the production of noise.

Opening or closing of automatic control valves is normally accomplished by electrical, hydraulic or pneumatic actuators. Typically, with a modulating valve, which can be adjusted to any position between fully open and fully closed, valve positioners are used to ensure that the valve achieves the desired degree of opening.

Air-actuated valves are typically used due to their simplicity, requiring only a compressed air supply, while electrically-actuated valves require additional wiring and switching gear, and hydraulic-actuated valves require high-pressure supply and return lines for the hydraulic fluid.

Pneumatic control signals are traditionally based on a pressure range of 3-15 psi (pressure unit) (0.2-1.0 bar), or more commonly today, an electrical signal of 4-20 mA for industry, or 0-10 V for HVAC systems. Current electrical control often includes a "Smart" communication signal superimposed on the 4-20 mA control current, so that status and verification of valve position can be signaled back to the controller. HART, Fieldbus Foundation and Profibus are the most common protocols.