The flow characteristics of the control valve mainly include four flow characteristics: straight line, equal percentage, quick opening and parabola.
When installed in the actual control process, the differential pressure of the valve will change with the change of the flow rate. That is, when the flow rate is small, the pressure loss of the piping part is small, and the differential pressure of the valve will increase, and the differential pressure of the valve will decrease when the flow rate is large. This valve characteristic, which is different from the inherent characteristic, is called the effective flow characteristic.
The internal valve of the quick start feature is disc shaped and is used primarily for the opening/closing action.
The flow control characteristics of the control valve spool surface shape valve are determined by the flow characteristics of the valve and the combination of process piping, pumps, etc., and are selected in the table below according to the proportion of valve pressure loss in each control object and system .
Control object Proportion of valve pressure loss in the system Flow characteristics of the valve
Flow control or liquid level control Below 40% Equal percentage
flow control or liquid level control Above 40% Linear
pressure control or temperature control Below 50% Equal percentage
pressure control or temperature control Above 50% Linear
Since the pressure loss of the piping increases in proportion to the square of the flow rate, if the characteristics of the valve body show a simple linear change, the differential pressure of the valve increases when the flow rate is small, and the flow rate becomes large when the valve is slightly opened. When the flow rate is large, the differential pressure of the valve decreases. The flow rate cannot be directly proportional to the opening of the valve. For this reason, the purpose of designing the equal percentage characteristic is to add the characteristics of the piping and the pump to realize the flow control that is independent of the flow rate and only changes in proportion to the valve opening.
The operation of
the piping system and the pressure loss control valve
can be selected according to the combination of the drive unit and the valve body.
Combination of drive unit and valve body and valve action (example of single-seat valve)
Valve action includes three types: direct action, reverse action, and hold-type action. The direct action mode of pneumatic drive such as diaphragm type and cylinder type is a method of closing the valve by increasing the air pressure signal, also known as “AIR TO CLOSE”. The reverse action method is to open the valve by increasing the air pressure signal, also known as “AIR TO OPEN” or “AIRLESS TO CLOSE”. Electrically operated signals can be converted to pneumatic signals by the positioner. When the operation signal is interrupted or the air source is interrupted or the power is cut off, please consider the safety and rationality of the procedure and choose to close or open the valve.
For example, when controlling the amount of acid through a valve in the process of mixing water and acid, it is safe and reasonable to close the acid control valve when the electrical signal line is disconnected or the air signal piping leaks, the air source is interrupted, or the power is cut off. Reverse action valve.
Post time: Aug-31-2023