How do control valve manufacturers calculate and select actuators?
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This post was last edited by kylestone on 2017-7-6 at 11:51. 1. For a standard single-seat globe valve, the manufacturer selects two parameters for the actuator: the spring range is something like 0.2–1 bar, and the area of the actuator’s diaphragm is around 240 cm2. If I were to carry out my own calculations, then the maximum thrust exerted by the actuator would be F1 = 100000 Pa × 0.024 m2 = 2400 N. Then, the thrust F2 is calculated using the maximum differential pressure at full valve closure and the size of the valve core; it is necessary that F1 be greater than F2 plus the friction force caused by the packing. By the way, which one has greater friction, graphite or PTFE? And what role does the minimum value of the spring range play in the calculations? 2. As for how these two parameters, namely the spring range and the actuator diaphragm area, are selected in combination, I’ve seen that some spring ranges can reach 1.4~2.3 Bar, while some diaphragm areas can reach 700 cm2. Which one should be adjusted first when facing high thrust?
Prioritize adjusting the film, as it’s easier to adjust; springs tend to get damaged from constant adjustments, and it’s not as useful without the film
The calculation for the actuator is determined by the manufacturer; the user only needs to specify what type of actuator to use. The spring pressure range is usually 0.02 MPa to 0.1 MPa. If the thrust required is not sufficient, this value can be doubled, but it should not be too high, as the pressure resistance of the diaphragm head and diaphragm has limits.
Let me explain it to you in detail. Regarding the actuator, there are two types of actuators for diaphragm control valves: air-operated open type and air-operated closed type. Let’s start with the air-operated open type. Generally, the spring range used for such actuators is 80–240 Kpa. The value of 80 represents the closing torque applied to the valve core when the valve is closed. For example, if the diaphragm area of the actuator is 350 cm², then the output torque of the actuator will be 2800 N. In the case of an air-operated closed type actuator, springs with a pressure range of 20–100 Kpa are typically used. If the input air pressure is 0.3 Mpa, then the output torque will be 7000 N. When selecting between air-operated open type and air-operated closed type actuators, the design specifications must be followed. As for the friction force in the packing gland, the friction force of graphite is much greater than that of PTFE. Generally, PTFE is used when the operating temperature is below 250°C; flexible graphite is used for higher temperatures
Thank you for your explanation; it has helped me understand a concept that was previously unclear to me, namely how the air pressure supplied to the valve inside the cylinder, or by the actuator, changes. Do you think my current understanding is correct? The output range of the positioner should be the pressure supplied to the positioner by the filter-reductor (i.e., what you call the air source), up to 0. When the spring is in its most compressed state, the positioner outputs the entire input pressure to the cylinder. When the spring is in its minimum compressed state, the locator releases the air inside the cylinder, resulting in a pressure of 0.
You can think of it this way: if you have any questions regarding control valves in the future, feel free to ask me. I hope this can be helpful to you