Control, signals and introduction to valves and switches Flashcards
region over which a quantity may be received (input range) or transmitted (output range)
Defined by stating lower and upper range values
Range
Also known as full scale deflection (FSD)
Magnitude of the range of the instrument
Span
Ratio of maximum capacity to lowest capacity
turndown ratio
ratio of change in magnitude of output signal corresponding to change in magnitude of input signal
sensitivity
minimum difference in values of a quantity that can be measured by a device
Resolution
Signal is from the control unit
Output
signal is to the control unit
Input signal
Signal over a range
Analogue
Signal is discreet (on or off)
Digital
Sensor output is often a ____which is converted to a proportional DC-current
voltage
Sensor output is a voltage and this is converted to
a proportional dc-current
Why is voltage converted to current?
Cannot transmit sensor output voltage over long distances
allows for self monitoring
Live zero
Inherent error detection
Current outside range indicates a fault: zero mA means line is broken, without this zero mA could mean empty tank or broken sensor
What does a valve do?
- Turns flow ON or OFF
- Diverts flow or non-return
- Regulates flow of a manipulated variable
- Safety functions such as pressure relief
Often the “final control element”
Control valve
Components of a control valve
- Valve plug, disc etc.
- Actuator
- Housing and transducers etc.
A pneumatic, electrically or hydraulic, powered device that supplies force and motion to open or close a valve
Actuator
Types of valves
- Linear motion
- Rotary motion
- Pinch and diaphragm valves
Example of linear motion valve
Globe valves
Example of rotary motion valves
Ball valves
Butterfly valves
Air pressure causes a stem to move up or down
Plug moves in and out of a “seat” to prevent or allow flow respectively
Commonly used for control
Flow path is changed by valve event when open
Associated pressure drop
Globe valves
Opens by lifting a plate or wedge out of the fluid path
Typically limited to ON/OFF applications
Gate Valve
Ball with hole through it rotates around vertical axis
When bore is aligned in the pipe flow direction the fluid flows
Quartner turn 90 degrees bore is perpendiculaar to flow path and fluid cannot pass.
The valve actuator is designed so that an axial movement of a piston is transformed into a 90 degrees rotation of a haft. The actuator is bracketed to the valve.
Mainly used for ON/OFF (imprecise flow control)
Ball valves
Axial movement of the piston is transformed into a rotation of a shaft. The actuator is bracketed to the valve
Valve actuator
Operation similar to ball valve
Disc rotates around vertical axis
When disc is aligned perpendicular to pipe flow direction the fluid flows
Quarter turn (90o) bore is parallel to flow path and fluid cannot pass
Mainly used for ON/OFF (imprecise flow control)
Butterfly valves
Advantages of butterfly valve over ball valve
- Butterfly valve is cheaper and lighter weight than ball valve
- Butterfly valve is hygienic
- Diaphragm is pushed into contact with the valve body to close off the flow
- Straight pipe or sear
- Material chosen for chemical resistance
- Traditionally used for ON/OFF but newer models are used for control
- Good for low pressure applications
Diaphragm valve
Opening or closing of valves is usually performed using …
pneumatic or electrical actuators
Pneumatic (air driven) actuators are most common due to simplicity and low cost
For ON/OFF actuators a digital 24Vdc signal can be sent to a solenoid to either send or not send air to the valve
Valves require air pressure to stay closed
If air pressure is removed, the valve will naturally open
Under a power outage, the source of the air pressure would be lost and the valve would “fail” in the open position
Designed to allow flow in an emergency
Normally open/fail open
Valves require air pressure to stay open
If air pressure is removed, the valve will naturally close
Under a power outage, the source of the air pressure would be lost and the valve would “fail” in the closed position
Designed to stop flow in an emergency
Normally closed/fail closed
This applies to a control valve such as ball or butterfly that does not react to loss of power
This would be useful when the process cannot be shut down or where it is unnecessary to halt the process
Fail in place
Operate either in fully open or fully closed states
To operate this valve and force it to its driven position compressed air (or current) must be supplied to the actuator
ON/OFF Valves
Use analogue outputs from the control system to allow a certain degree of valve opening raher than ON/OFF
Control valves
Sometimes higher pressures are required to actuate the valve: An additional penumatic positioner can be used. This has its own higher pressure air supply.
A device which will open or close a circuit when a parameter reaches a certain level
Switch
A device which sends a signal of the magnitude of the parameter
Transmitter
allow a 24Vdc signal input to the control system, typically confirming the valve is in its driven position.
valves with single feedback
allow a 24V dc signal indicating that the valve is open or indicating that the valve is closed
valves with double feedback
digital input to the control system
feedback
What feedback is provided using and are fitted to the brackets that connect the valve to the actuator.
Proximity switches
output 24V
What feedback is provided using and are fitted on top of the actuator
Micro switches
output 24V
phenomenon where a fluctuating current which by defition has a magnetic component induced an electromotive force in a target object.
Inductance
required so that the control system has verification of the valve status
valve feedback
type of signal feedback is
digital
a form of safety override to prevent dangerous or damaging situations.
Software and hardware interlocks
uses the control system wiring/signals. For example, a discahrge pump cannot empty a vessel if the tank outlet valve upstream of the pump inlet is closed
Software interlock
does not use the software or control system wiring
hardware interlock
For example a pressure switch is fitted
downstream of a pump. The switch output would be
linked to the pump electrical supply breaker and the
pump would cut off when the switch was activated.
