In this session we are gonna discuss about how to select a control valve actuators and we will briefly talk about it’s criteria
What is a control valve actuator?
The control valve regulates the rate of fluid flow as the position of the valve plug or disk is changed by force from the actuator.
How to select a control valve Actuator ?
The actuator is the distinguishing element that differentiates control valves from other types of valves.The control valve industry has evolved to fill a variety of needs and desires. Actuators are available with an array of designs, power sources and capabilities. Proper selection involves process knowledge, valve knowledge and actuator knowledge. A control valve can perform its function only as well as the actuator can handle the static and dynamic loads placed on it by the valve. Therefore, proper selection and sizing are very important. Since the actuator can represent a significant portion of the total control valve price, careful selection of actuator and accessory options can lead to significant dollar savings.
The range of actuator types and sizes on the market today is so great that it seems the selection process might be highly complex. It is not! With a few rules in mind and knowledge of fundamental needs, the selection process can be very simple. The following parameters are key as they quickly narrow the actuator choices:
- Power Source Availability
- Fail-safe Requirements
- Torque or Thrust Requirements
- Control Functions
- Economics
- saftey
Note : There is an actuator tool selection check the link below
Actuator selection tool:
http://www.belimo.ch/CH/EN/Product/Actuators/ActuatorSelection.cfm
Power Source Availability
The power source available at the location of a valve often can point directly to what type of actuator to choose. Typically, valve actuators are powered either by compressed air or by electricity. However, pipeline pressure is often used in the natural gas transmission industry.
Fail-safe Requirements
The overall reliability of power sources is quite high. However, many loops demand specific valve action should the power source ever fail. Desired action upon a signal failure may be required for safety reasons or for protection of equipment. Fail-safe systems store energy, either mechanically in springs, pneumatically in volume tanks, or in hydraulic accumulators. When power fails, the fail safe systems are triggered to drive the valves to the required position and to then maintain this position until returned to normal operation. In many cases the process pressure is used to ensure or enhance this action. Actuator designs are available which allow a choice of failure mode between failing open, failing closed, or holding in the last position. Many actuator systems incorporate failure modes at no extra cost. For example, spring-and-diaphragm types are inherently fail open or closed, while electric operators typically hold their last position.
Torque or Thrust Requirements
An actuator must have sufficient thrust or torque for the prescribed application. In some cases this requirement can dictate actuator type as well as power supply requirements. For instance, large valves requiring a high thrust may be limited to only electric or electro-hydraulic actuators due to a lack of pneumatic actuators with sufficient thrust capability. Conversely, electro-hydraulic actuators would be a poor choice for valves with very low thrust requirements. The matching of actuator capability with valve body requirements is best left to the control valve manufacturer as there are considerable differences in frictional and fluid forces from valve to valve.
Control Functions
Knowledge of the required actuator functions will most clearly define the options available for selection. These functions include the actuator signal (pneumatic, electric, etc.), signal range, ambient temperatures, vibration levels, operating speed, frequency, and quality of control that is required. Generally, signal types are grouped as being either: Two-position (on-off) or Analog (throttling). Digital Two-position electric, electro-pneumatic, or pneumatic switches control on-off actuators. This is the simplest type of automatic control and the least restrictive in terms of selection. Throttling actuators have considerably higher demands put on them from both a compatibility and performance standpoint. A throttling actuator receives its input from an electronic or pneumatic instrument that measures the controlled process variable. The actuator must then move the final control element in response to the instrument signal in an accurate and timely fashion to ensure effective control. The two primary additional requirements for throttling actuators are:
- Compatibility with instrument signal
- Better static and dynamic performance to ensure loop stability
Economics
Evaluation of economics in actuator selection is a combination of the following:
- Cost
- Maintenance
- Reliability
A simple actuator, such as a spring-and-diaphragm, has few moving parts and is easy to service. Initial cost is low. Maintenance personnel understand and are comfortable working with them. An actuator made specifically for a control valve eliminates the chance for a costly performance mismatch. An actuator manufactured by the valve vendor and shipped with the valve will eliminate separate mounting charges and ensure easier coordination of spare parts procurement. Interchangeable parts among varied actuators are also important to minimize spare-parts inventory.
Safety
In certain applications, such as hazardous environments, guidelines such as NEMA VII aim to improve safety by requiring certain precautions for different types of actuators. Safety should always be a consideration when selecting a valve actuator and, where possible, the safest and most reliable option based on the application’s specifications should be used. An actuator should be able to maintain the safety position if a fire occurs. To know more about fail safe options
However, in some situations, the ideal actuator can not be used due to extenuating circumstances. When a different type of actuator should be used than the most ideal solution, there are often additional components that can be added to improve safety and comply with regulatory specifications.
Temperature ranges of actuators
- Pneumatic actuators can operate at temperatures ranging from -4 to 150 ° F (-20 to 70 ° C), or in some cases in the range of -40 to 121 ° C (-40 to 250 ° F), provided that The proper seals, grease and bearings are in place.
- Electric actuators can operate at temperatures ranging from -40 to 150 ° F (-40 to 65 ° C).
What are the different types of Actuator Designs ?
There are many types of actuators on the market, most of which fall into four general categories:
Each actuator design has weaknesses, strong points and optimum uses. Most actuator designs are available for either sliding stem or rotary valve bodies. They differ only by linkages or motion translators; the basic power sources are identical.
Advantages and disadvantages of different type actuators
What are the parameters and conditions are noted for actuator selecting?
Type of movement (linear or rotational).
Transient response (proportional or integral).
Action of movement (direct or reverse).
Failure position in case of auxiliary power loss: (Open, Closed, Hold).
Actuator signal (electrical or pneumatic).
Input signal range (e.g. 4 to 20 mA).
Nominal travel or nominal angle of rotation.
Mounting interface between valve and actuator.
Required actuator thrust with closed valve.
Required actuator thrust with open valve.
Required travel rigidity (stiffness) for a stable control service.
Permissible accuracy deviation from the characteristic curve.
Hysteresis, reversal error and sensitivity.
Regulating time for full stroke travel or angle of rotation.
Switching frequency (on time in percent for of electric actuators).
Actuator volume (in case of pneumatic actuator).
Time constant and corner frequency (with positioner).
Resistance against shock and vibration (fatigue strength of actuator).
Permissible ambient temperatures.
Required protection class (against dust, water, moisture, e.g. IP65).
Requirements with regard to corrosion resistance.
Actuator and all accessories like positioners, boosters, solenoid valves and air supply pressure reduction device need proper air quality acc. to ISO 8573-1 Class 4 regarding micro-parts ( 1.0 < d < 5 μm; < 1000 parts/m3) and Class 3 (≤ 1 mg/m3) regarding oil contamination otherwise filter systems are needed to protect each device.
Required auxiliary energy for actuator (e.g. min. and max. pressure).
Typical characteristics (e.g. effective diaphragm area).
Actuator connections (e.g. G⅛ to G1 or ⅛ to 1 NPT).
Repeatability of positioning taking into account
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