Updated: Dec 7, 2020
In the previous lesson we introduced you to process control loops and gave a sample of how they work in a home heating system.
In today's lesson, we are going to expand on this and talk about how process variables are used in a typical modern control system within industry, we are then going to talk about how process variables are measured and processed.
We previously learnt that process variables are all the conditions of the process plant that are critical to its operation, both in terms of operation of the plant, and to the safety of those working on the site or in neighbouring areas.
To recap, the main types of process variables are:
These process variables are used in industry in a few distinct ways:
Process Control: Just like our home heating example some process variables are used an input to a process control loop to maintain a desired set-point.
Shutdown Systems: These systems will shut down the plant if there are unsafe conditions.
A typical example would be a pressure transmitter acting as an input into a HIPPS or high integrity pressure protection system, that protects pipework or vessels from becoming over pressured past there safe design limit.
Indication: Process variables can be displayed to operators of the plant, this is often to a control room operator who will see a visual representation of the plant through a HMI (Human machine interface) that displays live plant information and generates alarms to inform the operator there is a condition that an operator must respond to.
Process variables can also be displayed to an operator through local indicators such as pressure gauges or annunciator panels.
This brings us onto the question of how we detect these process variables? Devices called transmitters do this job.
Transmitters are devices that are designed to detect the various process conditions and turn them into a signal our process control system can understand.
Transmitters are at the core of what Instrumentation is about, engineered devices that process and transmit information or signals from one protocol or format to another.
Transmitters usually send their signal to a PLC or local controller in a format they will be able to understand and interpret.
Because industry has been evolving and changing for many, many years you are likely to find a wide range of different signal types that Transmitters output, this will be dependant on the age of the site the equipment is installed in.
There are broadly 3 categories of signal types seen in industry.
Pneumatic –These transmitte
rs are fully pressurised air powered. Pneumatic transmitters work to regulate the pressure of there output mechanically and usually give a signal of 3 – 15 PSI or 0.2-1 bar in response to changing input detection.
Pneumatic transmitters are useful when there might not be local infrastructure in place to carry and receive electrical signals.
These transmitters also have application in hazardous areas where controlling source of ignition is of high importance, in this instance pneumatic transmitters can be used in place of ATEX / IEC hazardous area rated equipment.
Raw electrical signals – For electrically powered transmitters they work by manipulating an output voltage or current.
The most common form of raw electrical signals are:
4-20mA – This is the most common form of signal you will find even in modern equipment.
COMMS signals - As well as raw signals, data can be transferred using multiple different COMMS signal standards. Common signal standards you will find are:
me Instrumentation works with both raw signals and a comms protocol. HART or highway addressable remote transducer works to transmit a comms protocol over an existing DC current signal.
The HART protocol is often used by Instrument technicians to interrogate and setup transmitters with a hand held communicator that uses HART while the input card the transmitter is connected to uses a 4-20mA raw signal.
Example Signal Transmission
To make this a bit clearer lets go over an example of how a typical pressure transmitter works with a 4-20mA signal type.
Lets say we have a vessel we are interested in knowing the pressure of. The pressure of this vessel is usually at 5barG but could potentially get up to 8 bar under certain process conditions.
In this circumstance, we might see a 0-10 bar transmitter installed on one of the nozzles of the vessel.
The transmitter is connected to a PLC input card that is setup to send out a voltage (usually 24VDC) and read a varying current input. The transmitter is powered by the input card voltage, it detects the pressure in the vessel and then manipulates the current in the loop by changing its own internal impedance.
So at 0 BAR in the vessel, the transmitter gives 4 mA.
At 10BAR in the vessel the transmitter outputs 20mA.
The PLC can then be interrogated by a SCADA system or HMI to display the information.
Notice how the zero of the transmitter is 4 ma and not 0 mA. This gives the benefit of being able to detect if there is an issue with the transmitter, let’s say a broken cable, the input card will detect this as an out of range condition that will alert an operator there is a problem. This would not happen if 0 Bar was represented by 0mA.
So now we have a basic understanding of what transmitters do and the role they play in control systems. In future lessons we are going to look at specific transmitter types and talk about the principles of their operation. If you are interested in receiving updates on new posts and please subscribe to our mailing list at the bottom of this page.
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