Lecture 3 - communications

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CP405 – Process Control
Lecture 3 – process communications and control valves
Process control systems process information
Signals from measurement instruments on the process, process outputs, need to be connected to controllers as controller inputs.
The controller outputs need to be connected to the final control elements, which are usually control valves to become process inputs.
What needs to be communicated?
With analogue communications the transmitted signal is a direct analogue to the measurement. If the measurement doubles, then the transmitted signal will double.
Analogue communications provide continuous values of the transmitted signal
Each instrument requires its own communication line
Until recently, analogue communications was the most common method used.
Analogue communications
Used since the 19th century
Air pressure, 3-15 psig, transmitted down small diameter (1/8-1/4”) metal tubes
Tube length limited due to pressure drop
Instrument air supply required
Still used in some older plants, particularly in hazardous areas
Most control valves are still pneumatically operated
Analogue communications - pneumatics
VC is not equal to Vm due to voltage drop in wire
Voltage drop will depend on length of wire
Voltage is NEVER used for communications in industrial plant
Analogue communications – Electrical signals - voltage
Measuring instrument
Controller
Vm
VC
Voltage drop
Transmitters adjust output voltage to drive a current through a loop to the controller.
Increasing the length of the line will cause the transmitter to increase voltage, but will maintain current signal
Current transmitters need to be powered
Standard transmission signal is 4-20 mA
Current signals are widely used in the process industries
Analogue communications – Electrical signals - current
Analogue communications – Engineering to transmitted units
Consider a temperature instrument with a measurement span of -10oC to 300oC and a 4-20mA output. What is the output of the instrument when the temperature is 200oC?
The signal that is transmitted is a number that represents the measurement
Digital communications are not continuous – values need to be scanned
A digital communications line can serve many instruments
Digital communications are becoming very widely used
Digital communications
Real-world analogue quantities need to be converted to digital values (numbers) before transmission
A/D convertors convert the values
The number of bits (binary digits) on an A/D convertor gives its resolution, e.g. a 16-bit convertor has a 1 in 65,536 (216) resolution (values 0 to 65,535)
High resolution convertors are slower, but this isn’t a problem in chemical plant
Digital communications – analogue to digital conversion
Digital communications – engineering to transmitted units
Consider a temperature instrument with a measurement span of -10oC to 300oC and digital output from an 8-bit A/D convertor. What is the output of the instrument when the temperature is 200oC?
Digital values can’t be communicated continuously - they need to be sampled
A sampled data control system (usually called a discrete time system) builds up a picture of a process like a film camera
Sampling is usually driven by the control system
Inadequate sampling can cause aliasing
Digital communications – sampling
Parallel communications send all the bits of a number at once
Parallel comms need a signal line for each bit, plus a few extra for comms control.
Serial comms send the bits one after the other (with some comms control bits at the start and end)
Serial comms can be transmitted on as little as two wires (or even one, if a common earth is present)
Digital communications – Parallel or serial
Each device is given a network address, which precedes other commands
As well as scan commands, devices can be ordered to do other things
Digital communications – Communicating with multiple devices
Controller
Multiple competing ‘standards’ for digital communications
Most common in process industries are Fieldbus systems such as Foundation Fieldbus and Profibus.
Ethernet based systems are growing in popularity
Digital communications – Standards
Wireless communications are relatively new, but are becoming more widely adopted
No cabling, other than power supplies
Care needs to be taken to ensure good reception
Digital communications – Wireless
Uses 4-20mA loops and superimposes digital signals
Allows retro-fitting
Can support multi-drop applications – multiple devices can share the same 4-20mA line
Mixed analogue/digital communications – HART
Instruments provide information on the process, but controllers need some way of adjusting process behaviour
Control valves are by far the most common final control element in the process industries
Others are things like variable speed belt drives, electrical heaters, etc.
Final control elements
Almost all control valves use a globe valve
Nearly all control valves are pneumatically operated
Final control elements – control valves
Valves can be setup to be normally open or closed
The choice is based on safety – if the air fails what way should the valve go?
Valve setup has important control effects
Control valves – air-to-open or air-to-close?
A positioner is a device that can be fitted to a control valve
It acts as a valve controller – driving the stem position to match the controller signal
Useful in some circumstances, but very overused
Control valves – Valve positioners
Valve manufacturers quote valve coefficients
Kvs is the flow of water in m3/h through a fully opened valve at a stated temperature (usually 5-40oC) at a pressure drop of 1 bar
Cv is the flow of water in gal/min through a fully opened valve at a stated temperature (usually 40oF-100oF) at a pressure drop of 1 psi
AV is the flow of water in m3/s at a pressure drop of 1 Pa
Control valves – sizing
Control valves – sizing – incompressible flow
Control valves – sizing – compressible flow
More complicated!
Control valves – characteristics
Thanks to Spirax Sarco for these illustrations
Control valves – Equal percentage characteristic
Control valves – installed characteristics
When the majority of the pressure drop in normal operation is over the valve, the installed characteristic is similar to the valve characteristic
Control valves – installed characteristics
When the majority of the pressure drop in normal operation is over the pipework
What’s the valve to do? What’s the fluid, nominal flowrate, pipe size?
What pressure drop is available? (Usually allow 25-40%, but can go down to 15%) Is flashing or cavitation likely to be a problem? Remember to allow for stop valve.
Look for a valve that will provide the required KVS at 60-70% open
Calculate installed characteristic – is it OK. Is the maximum flow around 1.4 times the nominal?
Control valves – sizing method
Any questions?