How to choose an Actuator?


#1

The selection of actuators when using an opposed pneumatic piston and diaphragm actuators involve the unique combination of many variables. The analysis of many combinations is done by showing the direction resulting from the action of the force of the variables considered. The combinations are also grouped in terms of the stability of the control valve and process operations.

Selection criteria:

There are different types of actuators classified based on the operating force, hydraulic, pneumatic and electrical actuators. The purpose of pressure in the actuator is to position the valve closure member for proper process control. An electrical actuator is the fastest responsive actuator and the hydraulic actuator can be used for heavy actions.

1. Controller output:

Engineers and buyers of process control systems have for many years been specifying systems such that the actuator pressures would match controller output ranges of 3-15 and 3-27 (6-30) psig. The real objective then is to select system variables such that the ideal 3-15 and 3-27 (6-30) psig controller output ranges can be closely realized. These must be compatible with the valve in service, process conditions, and/or leak class requirements.

2. Force consideration:

Stem Force: The stem force is simply the area of the stem times the pressure in the valve body acting on the stem. Normally, valve open and valve closed positions result in different values of stem force.

Packing Friction Force: The packing friction force acts to impede stem movement and is considered to be a constant value, consistent with packing type and pressure class. Therefore, the packing friction force equals the stem diameter times a packing friction factor. The packing friction force is recognized to be a function of many variables.

Spring Force: The spring creates a force which is a function of its initial compression or preload, valve travel, and spring rate Spring preload and valve travel are in units of distance. The valve travel factor accounts for the point of analysis and will vary from zero to one.

Packing Friction Force: The friction force of the packing acts to prevent the movement of the shank and is considered to be a constant value, consistent with the type of packing and the kind of pressure. Therefore, the frictional force of the packing is equal to the diameter of the shank by a packing friction factor. It is recognized that the frictional force of the package is a function of many variables.

Actuator Force: The actuator force is the actuator effective area times the actuator effective area factor, times the actuator pressure. The actuator effective area factor is used to account for area variation.

Leak Class Force: The closure member must contact the seat with some force to establish an initial seal, consistent with the leak class used. The unit force (unit circumference) may be represented in the general form of an equation for a straight line (i.e., y = mx + b). A specified leak class often requires other than ideal output pressure ranges be used.

Closure Member Force: Closure member force is that force created by the process acting on the closure member. The closure member force equals the differential pressure times the closure member unbalanced area, times a closure member unbalanced area factor. This force magnitude can be extremely complex, considering flow direction, valve type, closure member flow characteristic, and valve travel.

Packing friction factor:

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3. Direction Of Force Action

The actuator air pressure, or air mass, is the controlled variable which is the result or answer calculated from the force summation. The actuators are in different types mainly, pressure to open and pressure close. The valve and actuator types and flow direction are identified by a series of letters. Flow direction considers actuator vertical up orientation.

4. Flow medium:

Lubrication property of operating fluid whether fluid is slurry or contain soild particles erode the sealing surface. Wear and tear depends on operating fluid. Actuator output decreases with time which needs additional capacity for actuator.

5. Operating temperature:

Provided with metallic bearing higher temperature. Higher friction coefficient is needed for elevated temperature. Cryogenic application needs more torque due to high friction of plastic seat and bearing.

6. Cyclic speed and rate:

Fast cyclic speed need special actuators

7. Response time:

Major factor in determine the correct actuator is it response time, how fast it responds to the controller signal. Pneumatic actuators are fast responsive.

Equations:

Where:

DS = Stem diameter
P1 = Upstream pressure
P2 = Downstream pressure
FFP = Packing friction factor
M = Slope factor
Y = Intercept factor
DP = Port diameter
CMU = Closure member unbalanced area factor
ACMU = Closure member unbalanced area
LS = Spring preload
XVT = Valve travel
VT = Valve travel factor
RS = Spring rate
AEA = Actuator effective area
AEA = Actuator effective area factor
PA = Actuator pressure
t = Tare factor