To make the best technical and economical choice, an engineer must understand the factors that are most essential for plant-wide valve automation selection of actuators.
Basic functions a valve actuator should have:
The actuator should move and hold the valve closure member in the desired position correctly.
Seat the valve closure member with sufficient torque to provide the desired shutoff specification.
Provide a failure mode in the event of system failure.
Provide the rotational travel required (90 °, 180 °, etc.). Multi-ported valves that require more than 90 ° of rotation.
Actuator should provide desired operational speed.
Fast cycle speeds (less than one-half the standard actuator cycle time) require careful valve selection.
Important considerations for choosing valve:
Power source availability:
There are two types of power source used to operate actuators pneumatic and Electrical.Both styles have benefits, and having information accessible to create the best choice is important.
Compatibility (Power Source) First and foremost in the selection of an actuator type (pneumatic or electric) is to determine the most effective power source for the actuator.
The most practical pneumatic actuators use 40 to 120 psi (3 to 8 bar) of air pressure. Electrical actuators are often used with a power supply of 110 VAC, but are accessible in single phase and three phase with a broad range of AC and DC motors.
Points to consider are:
• Power source availability
• Torque at the valve stem
• Failure mode
• Control accessories
• Speed of operation
• Frequency of operation
• Plant environment
• Size of valve
• System component costs
• System maintenance
The National Electrical Manufacturers Association (NEMA)has established rules for the building and installation of electrical actuators (and other electrical equipment) for use in hazardous fields. The guideline for NEMA VII reads.
Practically all manufacturers of electric actuators have an option for a NEMA VII version of their standard product line.
On the other hand, pneumatic actuators are explosion-proof. The inherent safety of pneumatic actuators in hazardous areas makes them a practical choice in hazardous applications. When using pneumatic actuators in hazardous areas with electrical controls, they are generally more cost-effective than electrical actuators.
Safety should be always the first priority of consideration while choosing a valve actuator. Guidelines such as NEMA VII aim to enhance safety in certain applications, such as dangerous environments, by requiring certain precautions for different kinds of actuators.
It is always good to install pneumatic actuators in hazardous conditions, electrical actuators are preferred only if pneumatic actuator is out of option.
Both kind of actuators (Pneumatic & Electrical) can employ at a large emperature range. A pneumatic actuator’s normal temperature range is from -4 to 174°F (-20 to 80°C) but can be expanded to -40 to 250°F (-40 to 121°C) with optional seals, bearing, and grease.
The quality of the supply air in relation to the dew point should be considered in low temperature applications.
A temperature range of -40 to 150° F (-40 to 65°C) is available for electrical actuators. An electrical actuator should be sealed from the environment when used in outdoors to prevent the entry of moisture into the internal workings.
The spring return (fail-safe) option is another safety accessory widely specified in the process industries on valve actuators. A spring-return actuator drives the valve to a pre-determined safe position when power or signal failure occurs.
Where springs are not practical due to the size or weight of the actuator or where a dual-acting unit is already installed, an accumulator tank can be installed to store air pressure.
A spring return version of electric actuators is not commonly accessible, but a battery backup scheme is an elegant option. To accomplish the spring-return function an electro-hydraulic actuator is often a good choice. Electro-hydraulic actuation is achieved by energizing a hydraulic pump, which pressurizes a spring-return cylinder.
Without overheating, pneumatic actuators can be stalled indefinitely. There should be no stoppage of electrical actuators. Stalling an electrical actuator draws excessive current, which can generate heat in the engine and cause damage.
The ability to control the speed of a pneumatic actuator is an important advantage of the design. The simplest way to control the speed is to fit the actuator with a variable orifice (needle valve) at the exhaust port of the air pilot.
An electrical actuator interfaces well with existing electronic control systems in the modulation of service and eliminates the need for electro-pneumatic controls. With pneumatic actuators, a pneumatic or electro-pneumatic positioner is used to control the valve position