Separating process conditions from product attributes

What is your instrumentation telling you? Can the information help you learn about the product?

06/24/2011


Dear Control Engineering: The article Beyond the PV discusses the differences of measuring process conditions versus measuring the product directly. How do we separate those two?

In a given process there will be instruments designed to measure all sorts of things. We have what we call the “big four process variables,” which are pressure, temperature, flow, and level. These are all process conditions, in that they tell you, for the most part, about what’s happening with the process, but no specific attributes of the product.

Let’s say you have a tank holding a given amount of liquid. Measuring the level, pressure, or temperature of the liquid will tell you what the conditions are in the tank, but they will not help you determine if it is water, ethanol, or gasoline. If you’re clever, you can determine the weight of the liquid using a pressure sensor which will help you determine its density. That is a specific attribute of the product.

In a given manufacturing process, you will normally need both kinds of measurements at appropriate stages. As the article points out, the number of options available is huge, but often there are gaps. There are relatively few process conditions that can’t be measured one way or another. (Another article in the June issue describes using infrared to measure difficult temperature applications.) However if you want to find out product attributes, things get much more complicated. The more specific the attribute, the fewer choices you have in most cases.

In his article, Scheele points out that the sophisticated signal processing capabilities available now allow users to combine readings from a group of instruments to determine characteristics that cannot be measured directly. Back to the tank example, if you measure the physical level (volume) of liquid in the tank with a radar sensor, the temperature of the liquid with an RTD, and the weight of the liquid with a pressure sensor, you can get a very accurate picture of its density by performing the appropriate calculations. From process conditions, you can learn about specific product characteristics. Admittedly, that’s a pretty basic example, but the same concept applies in far more sophisticated applications. A Coroiolis flowmeter can tell you the density of what is flowing through it, but it does that function by calculating the value from other attributes, just like the tank example. It is not some type of native densitometer.

Ultimately the key is figuring out how to select and place instrumentation in a process to get the information you really need. As Scheele says, there are possibilities of “building a new knowledge base in which data from conventional field devices is combined with applications expertise, modeling, and advanced process control software to enable measurements that could not have even been imagined as little as 20 years ago.”

Peter Welander, pwelander@cfemedia.com



No comments
The Top Plant program honors outstanding manufacturing facilities in North America. View the 2015 Top Plant.
The Product of the Year program recognizes products newly released in the manufacturing industries.
The Engineering Leaders Under 40 program identifies and gives recognition to young engineers who...
2016 Product of the Year; Diagnose bearing failures; Asset performance management; Testing dust collector performance measures
Safety for 18 years, warehouse maintenance tips, Ethernet and the IIoT, GAMS 2016 recap
2016 Engineering Leaders Under 40; Future vision: Where is manufacturing headed?; Electrical distribution, redefined
SCADA at the junction, Managing risk through maintenance, Moving at the speed of data
Safety at every angle, Big Data's impact on operations, bridging the skills gap
The digital oilfield: Utilizing Big Data can yield big savings; Virtualization a real solution; Tracking SIS performance
Applying network redundancy; Overcoming loop tuning challenges; PID control and networks
Driving motor efficiency; Preventing arc flash in mission critical facilities; Integrating alternative power and existing electrical systems
Package boilers; Natural gas infrared heating; Thermal treasure; Standby generation; Natural gas supports green efforts

Annual Salary Survey

Before the calendar turned, 2016 already had the makings of a pivotal year for manufacturing, and for the world.

There were the big events for the year, including the United States as Partner Country at Hannover Messe in April and the 2016 International Manufacturing Technology Show in Chicago in September. There's also the matter of the U.S. presidential elections in November, which promise to shape policy in manufacturing for years to come.

But the year started with global economic turmoil, as a slowdown in Chinese manufacturing triggered a worldwide stock hiccup that sent values plummeting. The continued plunge in world oil prices has resulted in a slowdown in exploration and, by extension, the manufacture of exploration equipment.

Read more: 2015 Salary Survey

Maintenance and reliability tips and best practices from the maintenance and reliability coaches at Allied Reliability Group.
The One Voice for Manufacturing blog reports on federal public policy issues impacting the manufacturing sector. One Voice is a joint effort by the National Tooling and Machining...
The Society for Maintenance and Reliability Professionals an organization devoted...
Join this ongoing discussion of machine guarding topics, including solutions assessments, regulatory compliance, gap analysis...
IMS Research, recently acquired by IHS Inc., is a leading independent supplier of market research and consultancy to the global electronics industry.
Maintenance is not optional in manufacturing. It’s a profit center, driving productivity and uptime while reducing overall repair costs.
The Lachance on CMMS blog is about current maintenance topics. Blogger Paul Lachance is president and chief technology officer for Smartware Group.
This article collection contains several articles on the vital role of plant safety and offers advice on best practices.
This article collection contains several articles on the Industrial Internet of Things (IIoT) and how it is transforming manufacturing.
This article collection contains several articles on strategic maintenance and understanding all the parts of your plant.
click me