Actuator for valves
In general, there are three main types of actuators for valves:
- Electrically operated
- Pneumatically operated
- Hydraulically operated
The electric actuator is an electric motor which can open, close or control a valve by turning the motor. This is in most cases a quarter-turn actuator which operates a quarter-turn valve (such as a ball valve, butterfly valve or plug valve). But it can also be a linear electric actuator which, for example, pulls up the slide of a gate valve by turning the spindle several times.
Common voltages in Europe are 24V AC/DC, 230 VAC and 400 VAC. The running time from open to closed and vice versa is generally slow compared to pneumatically operated drives. It is also relatively difficult to achieve a safe position in the event of a power failure with an electric drive. This is only possible by means of an internal spring (very expensive) or a power capacitor or battery.
The pneumatic actuator is the most common actuator and opens the valve with the force of the available compressed air. Here, too, the quarter turn actuator is very common, but the linear version is also simple and common (e.g. pneumatically operated diaphragm valves). The pneumatically operated actuators are explained taking into account the available compressed air pressure as this determines the force. In contrast to the electric drive, it is more difficult to adjust the angle of rotation in the case of pneumatic drives. For example, for valves with 180 degrees. However, it is easy to create a safe pneumatic condition in case of compressed air failure by means of a spring in the actuator. The additional costs for a spring in a pneumatic actuator are almost nil.
What do we need to think about when selecting a drive for valves?
Actuator on ball valve
The concept of ball valve construction is mainly based on a polished ball (including hole in the ball) between two seats (upstream and downstream). Turning the ball with the hole makes it possible to let a flow of liquid through or stop it. The differential pressure over the ball valve pushes the (floating) ball against the seat. This causes friction when turning the ball. The required torque will be highest when the ball valve is closed and needs to be opened (breakaway moment).
Actuator on butterfly valve
A butterfly valve is essentially based on the concept of a disc (butterfly) attached to a shaft which, when closed, is completely surrounded by the seal. When open, the butterfly is parallel to the flow and the shaft and the thickness of the butterfly are the only obstructions. When closed, the butterfly is at right angles to the flow and is pressed into the seal. The required torque is therefore greatest in the closed position due to the friction on the seal. The pressure difference over the butterfly valve also influences the torque required for operation. Once the butterfly is out of the seat, the torque is considerably less.
Actuation on plug valve
The construction principle of the plug valve is basically a conical plug that rotates in a conical hole. In the plug, a channel is made through which the liquid can flow in an open position. The required torque depends on the friction between the plug and the seal and less on the pressure difference. The required torque is highest when closing the plug valve and will have to be explained on it.