Advantages and Disadvantages of Valve Actuators

A valve actuator is the mechanism for opening and closing a valve. Manually operated valves require someone in attendance to adjust them using a direct or geared mechanism attached to the valve stem. Power-operated actuators, using air pressure, hydraulic pressure, or electricity, allow a valve to be adjusted remotely or allow rapid operation of large valves.

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These valve actuators may be the final elements of an automatic control loop which automatically regulates some flow, level, or other processes. Actuators may be only to open and close the valve, or may allow intermediate positioning; some valve actuators include switches or other ways to remotely indicate the position of the valve.

Valve Actuators

Advantages and Disadvantages of Valve Actuators like Spring and diaphragm Actuators, Pneumatic piston Actuators, Electric Actuators, Hydraulic Actuators.

Spring and diaphragm Actuators

Spring and diaphragm pneumatic actuators can be “direct-acting”, meaning that air to the diaphragm casing pushes the actuator stem downward. This “air-to-close” action compresses the spring, which in turn pushes the actuator stem back up when the supply pressure is decreased or lost.

Additionally, spring and diaphragm pneumatic actuators can be “reverse-acting”, meaning that air to the diaphragm casing causes the actuator stem to move upward. This “air-to-open” action compresses the spring, which in turn causes the actuator stem to move downward when supply pressure is decreased.

Advantages

Lowest cost
Ability to throttle without positioner Simplicity
Inherent failure-mode action
Low supply-pressure requirement
Adjustability to varying conditions
Ease of maintenance

Disadvantages

Limited output capability
Large size and weight

High-pressure spring and diaphragm Actuators

Spring-based actuators hold back spring. Once the command was sent to the valve or power is lost, the spring is released, then it will operate the valve.

Advantages

Compact, lightweight
No spring adjustment needed
Costly cast components not needed
Inherent fail-safe action
No dynamic stem seals or traditional stem connector block needed
Design can include integral accessories

Disadvantages

Requires high supply pressure 40 psig (2.8 bars) or higher
Positioner required for throttling

Pneumatic piston Actuators

A pneumatic control valve actuator converts energy (typically in the form of compressed air) into mechanical motion. The motion can be rotary or linear, depending on the type of actuator.

A Pneumatic actuator mainly consists of a piston that develops the motive power. It keeps the air in the upper portion of the cylinder, allowing air pressure to force the diaphragm or piston to move the valve stem or rotate the valve control element.

Advantages

High force or torque capability
Compact, lightweight
Adaptable to high ambient temperatures
Fast stroking speed
Relatively high actuator stiffness

Disadvantages

Fall-safe requires accessories or the addition of a spring
Positioner required for throttling
Higher cost
High supply-pressure requirement

Electric motor Actuators

An electric actuator is powered by a motor that converts electrical energy into mechanical torque. The electrical energy is used to actuate equipment such as multi-turn valves.

Advantages

Compact
Very high stiffness
High output capability
Supply pressure piping not required

Disadvantages

High cost
Lack of fail-safe action
Limited duty cycle
Slow stroking speed

Electro-Hydraulic Actuators

Hydraulic systems work because of Pascal’s law, which states that an increase of pressure in any part of a confined fluid causes an equal increase of pressure throughout the container. If force is applied to one part of a hydraulic system, it travels through the hydraulic fluid to the rest of the system.