A complete valve specification for which an automated package may be specified should include:
•Valve Size
•Valve Type
•Valve Manufacturer
•Model #
•Valve Torque (In-Lb)
•Line pressure (Psi)
•Media Temperature (°F)
•Media (Slurry, Gas, etc.)
The most important step in developing an automated valve specification is to determine a sizing criteria. If a valve is to operate in a process handling clean liquids at moderate pressures and temperatures, the manufacturer’s published operating torque is usually adequate for actuator sizing. Under certain conditions, however, the torque required to operate a valve may increase. In this case a sizing safety factor may need to be applied based on the following guidelines:
Media Factors Multiplier
Clean, particle free, non - lubricating (water, alcohol, ect) 1.00
Clean, particle free, lubricating (oils, hydraulic fluid, ect) 0.80
Slurries or heavily corroded and contaminated systems 2.00
Gas or saturated steam, clean and wet 1.00
Gas or superheated steam, clean and dry 1.30
Gas, dirty unfiltered e.g natural gas, chlorine 1.50
Liquid, Black Liquor, Lime Slurry 1.80
Liquid, Viscous, Molasses 1.30
Service Factors Multiplier
Simple On and Off Operations 1.00
Manual Throttling Service 1.25
Positioner Control 1.50
One per day operations on/off 1.20
Once every two days or critical operation on/off 1.50
Once per month or less frequently on/off 2.00
Applications below -20°F 1.25
Note: Consult the valve manufacturer for specific Safety Factor recommendations.
SELECTION DATA:
•Sizing Safety Factor (%)
•Valve Torque (In-Lb)
•Sizing Safety Factor (%)
•Double Acting or Spring Return
•Fail Open or Close or Last
•Operating Temperature Range (°F)
•Min/Max Operating Speed (sec/90°) to open & to close
•Mounting Kit
•Paint Specification
Also important to consider in pneumatic actuator sizing is the minimum air pressure available to power the actuator. If a range of pressure is available (60 - 90 psig for example) the minimum should always be specified for actuator sizing.
•Minimum air pressure (psig)
CONTROL SYSTEM COMPONENTS
FILTERS
Pneumatic/Hydraulic supplies are often contaminated by particles, failure to provide clean pneumatic or hydraulic supplies will reduce operational life of the actuator and control its control system.
PRESSURE REGULATOR
Pressure regulators control downstream pressure, most designs incorporate a filter, drain and pressure gauge into a single assembly.
RELIEF VALVE
Pressure relief valves open and vent the supply source at a specific (know as its set-point) system pressure. Once the systems pressure has fallen below the pressure relief’s set point pressure, it will close and stop venting the supply source. They are used in preventing downstream control components and actuators becoming over pressurised. In the event that a pressure regulator fails in its open position.
SPEED CONTROL VALVES
Speed control valves restrict the flow rates of pneumatic and hydraulic sources. They are most commonly used the control the speed at which actuators stroke valves.
SOLENOID / PILOT VALVES
Pneumatic/Hydraulic actuators are usually piloted by a solenoid/pilot valve. The actuator manufacturer will size the valve flow rate to provide adequate speed of operation and economy. If faster or slower speeds are critical to a process this must be specified for more accurate valve sizing. The following valve types are generally used for piloting actuators:
•Double Acting Actuator 4-way, 5 ported, single coil
•Spring Return Actuator 3-way, 2 ported, single coil
•Remote, local or direct mounted
Direct: Some actuator manufacturers offer an integral manifold to which a solenoid valve may be direct mounted to eliminate tubing and fittings.
Materials of construction (aluminium, bronze, 316 stainless steel)
LIMIT SWITCHES
Limit switches are used with actuators to indicate the open, closed, or intermediate position(s) of the process valve.
· # of switches for Open, Close & other positions
· Electrical Rating (Amps & Volts)
· Visual Indicator
· Switch designs: Snap acting (mechanical switches/dry contact), Hermetically sealed reed switches, Non-contact (proximity type) or Others-consult actuator supplier
POSITIONERS
Positioners are used with piston actuators and some diaphragm actuators to convert a control signal to a valve position in modulating service. There are two basic styles of positioners: pneumatic and hydraulic.
Pneumatic Positioners
· Input control signal (psig)
· Signal/position (psig) for valve open & valve closed
· Cam Style*: linear / quick opening / other
· Visual Indicator
Hydraulic Positioners
· Input control signal (maDC)
· Signal/position (maDC) for valve open & valve closed
· Cam Style*: linear / quick opening / other
· Options: Gauge set, Internal limit switches
*Most off-the-shelf quarter turn valves have equal percentage flow characteristics. This is generally acceptable for most applications.
Position Transmitter
A position transmitter provides continuous feedback of the valve position with a resistive or current output signal.
Resistive Feedback is achieved through a potentiometer but may not be accurate over long distances due to the inherent resistance of the wire
Specify resistive range/90° of rotation (Ohms)
Current Feedback is achieved through a resistance-to-current conversion circuit (RI) and is generally preferred for transmission over long distances. Output is 4-20 maDC over 90° of rotation.
SELF-CONTAINED ELECTRO HYDRULIC ACTUATORS
Self-contained electro hydraulic actuators operated solely by electrical power. SCEHAs eliminate the need for separate hydraulic pumps and tubing, simplifying system architectures and improving safety and reliability.
This technology originally was developed for the aerospace industry but has since expanded into many other industries where hydraulic power is commonly used.
The following guide is for hydraulic actuators and controls only. It is assumed that the valve to be automated has already been selected based on its own unique ability to handle the service for which it is intended.
A complete valve specification for which an automated package may be specified can be found in previously in this guide.
SIZING DATA
Valve Type?
•Control
•Choke
•Globe
•Ball
Valve Size?
Valve Torque /Thrust?
Fail Position?
•Fail in Place (double acting actuator - least expensive option)
•Fail Closed or open on Loss of Signal (double acting actuator with spring return directional control valve)
•Fail Closed or open on Loss of Supply SR (spring return actuator)
Power Supply?
•Hydraulic
•Electric AC/DC
Sizing Safety Factor (%)?
Operating Temperature Range? (°F)
Min/Max Operating Speed (to open & to close)?
Additional Controls?
•Limit switches
•Positioners
•Partial Stroking
•Pressure Transmitters
•Flow Control
A position transmitter provides continuous feedback of valve position.
•Slide wire (potentiometer)
•2-wire 4-20.output (integral power supply)
•4-wire 4-20.output (requires a power supply)
OTHER CONSIDERATIONS
•Battery Backup for Fail-Safe Operation
•Solar Panels
•Bluetooth Technology for PST (Partial Stroking Testing)
