7. Opening and closing times for pneumatic actuators

7. Opening and closing times for pneumatic actuators

The dynamic behavior of a pneumatic actuator is influenced by the thermodynamic processes in the diaphragm chamber volume and by the flow capacity of the accessory (positioner, solenoid valve, etc.). The air supply network pressure also plays an important roll.

Status changes in the diaphragm chamber space with the valve-setting-dependent volume usually proceed so that they approach isothermality (). The signal pressure is present in the volume . For this reason, air mass changes within the volume due to ventilation or release of air are proportional to the change in the product of (absolute pressure) and .

The actuator volume for fail-safe is dependent on the valve setting H in the following way:

(valve spring closing)

(valve spring opening)

V0 is the dead volume dependent on the actuator size Aw.

The mass flow can be determined using the DIN IEC 534 calculation equations for sizing flow capacity.

Two differential equations are obtained from these equations which can be solved for and during signal pressure change. If this is not the case (e.g. due to the pump characteristic), only incremental calculation in small time increments is possible. The first variant is implemented in the valve sizing program to estimate opening and closing times.

The true representation (of the positioner, for example) is not possible in this way, i.e. the times determined are usually smaller because in reality the positioner air capacity is reduced as the regulator deviation reduces.

Specifying the mathematical equation of the closed solution would go too far at this point.

The following times are the result:

for valve opening

for valve closing

Typical values for kv attachment devices

Accessory | Inlet air | Outlet air | Inlet air | Outlet air
Positioner 3766 | 0.081 | 0.127 | 0.7 | 0.7
Positioner 4765 | 0.081 | 0.081 | 0.7 | 0.7
Positioner 3780 / 3785 | 0.092 | 0.152 | 0.7 | 0.7
Positioner 3760 | 0.045 | 0.045 | 0.7 | 0.7
Solenoid Valve 3701 | 0.220 | 0.220 | 0.7 | 0.7
Other solenoid valves | Catalog, database | Catalog, database | 0.7 | 0.7
Fairchild 20 booster | 0.531 | 0.531 | 0.7 | 0.7
Fairchild 4500 booster | 1.968 | 1.968 | 0.7 | 0.7

Pipeline influence

The piping influence can also be determined in the valve sizing program.

( for fully-formed pipe roughness flow, (k=0.03 mm run pipes))

Dead volumes

Actuation surface | Dead volume | Actuator rated travel
80 | 130 | 15
240 | 425 | 15
350 | 690 | 15
700 | 2665 | 30
1400 | 6195 | 60
2100 | 12000 | 120
2800 | 20000 | 120