PACE 60-year Anniversary Series: Process Instrumentation
supported by Endress+Hauser Australia
Process control has come a long way since the early years when process variables were monitored manually by an operator who walked around the plant and adjusted valves to obtain the desired pressures, temperatures and flows.
As technology evolved, pneumatic controllers were invented and mounted in the field; later, these were moved to a control room and would monitor the process via signals from a valve or pump in the field.
“In the 1960s – before I started in the industry – there was a tussle between a number of companies about what would be the primary signal transmission standard,” McCrometer director of industrial markets – Asia, Africa and Middle East, David Splatt, told PACE.
Splatt began his career in 1974 as an instrument apprentice at instrument design and manufacturing company, Fischer & Porter in Melbourne – now owned by ABB.
“As I recall the story being told to me, Foxboro developed a technology based on 10-50 milliamps (10 milliamps being zero flow or level, and 50 milliamps being 100%). The company I worked for, Fischer & Porter, established a standard of 4-20 milliamps with 4 milliamps being 0% and 20 was 100%,” said Splatt.
“That was the first step towards electronics; at the time, and even in the 70s, signal transmissions used the pneumatic method, which we used to affectionately call ‘huff and puff’. You’d have pressurised lines travelling all around the plant, and 3PSI or 20kpa would be 0%, and 15psi (which is basically equivalent to 100kpa) would be 100%.”
Splatt’s first exposure to instrumentation in the control room was with individual process controls – what we would call a PID controller today.
“They sat in panels and you’d have rows and rows of them disappearing in to the distance, each one of them controlling a valve or a pump somewhere around the plant,” Splatt recalled.
“It was very complex: each of them had their own set of wires coming to them or in the earlier cases a set of plaits transmitting the pneumatic signal.”
In the early years of process control, process variables were monitored manually by an operator who walked around the plant and adjusted valves to obtain the desired pressures, temperatures and flows. This photograph of a control post for a steam turbine was probably taken pre-1950s. (Image courtesy of Wikimedia Commons.)
For Splatt, one of the biggest milestones affecting process instrumentation has been the invention of the distributed control system (DCS) in the 1970s.
“The initial DCSs weren’t able to handle ‘on’ and ‘off’; they were able to handle analogue signals, but they had fairly poor interaction with the ability to do anything else with pumps, like sequences of events. That type of sequencing tended to be the realm of the PLC manufacturer,” Splatt said.
“Then there were a number of companies that started to combine the PLC functionality in their DCSs, and likewise a number of PLC manufacturers started to combine the DCS functionality in to their PLCs.
"I think most of us at my age are quite surprised how long it’s taken – if it has actually happened yet – for DCSs and PLCs to become very close to one another. I think there’s still a distinct difference between them. We thought there’d be less of a difference between the two technologies 30 or 40 years down the track.”
Technology in-roads
Bürkert Fluid Control Systems managing director – Pacific Region, Chris Hoey, started his instrumentation career in 1979 at Amoco Oil Refinery in Brisbane, working with a broad range of process instrumentation from a variety of vendors.
“This was a time of pneumatic control, circular chart recorders, individual loop controllers and manual data acquisition,” Hoey told PACE.
“Instrumentation was back then, a truly specialised field. You had to deal with all forms of mechanical and electromechanical instruments. Whilst 3-15PSI and 4-20mA were the signals used, these were generated from predominantly mechanical instruments. Force balance relays, bellows driven recorders, and even pneumatic square root extractors and computers (yes GenYers, pneumatic computers).
“This mechanical background led to a far deeper understanding than you could ever get from the market today, where most of these elements are embedded and invisible. Back then you had to deal with every aspect of the control loop in a singular way.”
Siemens pressure transmitters from the current SITRANS P family offer temperature compensation to handle fluctuating temperatures, along with a separate static pressure sensor. (Image courtesy of Siemens.)
According to Hoey, while the fundamentals of process instrumentation remain the same, the technology has come in leaps and bounds.
“The major changes in the industry have been in technology, but this has led to a vast reduction in the cost of products and their implementation. Individual wiring, individual controllers and I/P converters gave way to DCS systems, but the first versions of those were extremely expensive ($50K plus for a single operator screen). Due to this, high-end process automation stayed with the bigger plants,” he explained.
“The PLC came into existence, but the early versions just replaced relay logic and were not suitable for continuous control. Even after they added scaling and PID blocks, they were still clumsy at modulating control and hopeless at complex batch applications. But they played a big part in forcing DCS to come down in price and opened vast new markets where instrumentation was only a small part of the process.”
Now, PLC and DCS have merged into advanced process automation controllers (PAC), which are able to handle all forms of complex process automation, explained Hoey.
“Sensors as well have undergone significant change and now fit firmly into the ‘commodity’ market rather than with specialist vendors. They also started to embed all of those individual components we once needed, like square root extractors, signal isolators, I/P converters and even the PID loops themselves as standard features. There are now so many more options when designing a system,” he said.
Intelligent communication
Siemens Australia product manager – PCS 7 Process Control System, Marek Lisik, has been working around Australia’s process industries for 24 years, in various sectors including food and beverage, automotive, chemicals and healthcare.
“My first major experience with instrumentation, around 1989, was on an animal feedstock plant, where there was a problem with the level sensors in bulk storage bins. As it turned out, the capacitive-type sensors were not really suitable for the type of material in the bins, causing a fair degree of havoc in the control program,” Lisik told PACE.
“Another early experience was driving and subsequently carrying half a tonne of test weights to calibrate the load cells on a weigh bin. A sobering experience with actuators came after spending hours tuning a particular PID loop, only to realise that the actuator had a stiction problem and hence exhibited a ‘highly unpredictable degree of non-linearity’.”
User-friendly and simple operation, push buttons and graphics-enabled local displays are just some of the features you will find on many of today’s modern instruments. (Image courtesy of Siemens.)
In Lisik’s opinion, there have been various significant developments in process instrumentation technology since he first began working in the industry, but none reaping as many benefits as the introduction of intelligent instruments and serial communication, for example HART, Profibus PA and Foundation Fieldbus (FF), and more recently, Ethernet-based communications, such as Profinet.
“To start with, the quality of measurement data has increased, including accuracy, reduction/elimination of noise in the transmission line, and elimination of encoding/decoding inaccuracies. Other benefits include multiple process variables from one transmitter, diagnostic data, simplicity of calibration, and signal simulation,” Lisik said.
“When I started my work in Australia, 20 odd years ago, I encountered a variety of current- and voltage-based signals: 0-20mA, 4-20mA, +/- 20 mA, 1-5V, 0-10V, etc. Then increasingly, HART became popular, with other fieldbus technologies in their perceived or actual infancy and with uptake pretty well limited to a few ‘Brave New World’ early adopters.
“In the last 10 years I have seen a huge shift of acceptance of fieldbus like Profibus DP and PA, with recent major DCS installations in the range of 30,000 plus signals, predominantly via fieldbus (Profibus). Latest figures from PI International show over 50 million installed Profibus (combined DP & PA) nodes worldwide.”