In-line Instrumentation

Established technology remains relevant as refinements keep traditional measurement devices competitive in the face of non-intrusive technology advances.

In-line measuring, monitoring, and analysis is found in all process industries. Whether involving liquids or gases, in-line methodologies provide a variety of benefits over sporadic grab samples, including greater data reliability, accuracy, and consistency (primarily because of automatic calibration and verification); more, faster, repeatable measurements; fewer manhours required; and tighter control of the process. Extending these benefits are current networking and reporting functions, which add greater consistency and precision through electronic record keeping and performance validation.

“You want sensor technology in-line as much as possible,” says Daryl Belock, commercial programs manager for GE Sensing. “The method offers the best representative sample. The installation is usually simple. Unlike grab samples, there is no extraneous hardware pulling a sample out of the line. And you get fast response having the sensor directly in the flow path.”

The primary issues revolve around whether the sensing device is sufficiently rugged and insensitive enough to all that’s going on in the process stream with respect to temperature, pressure, flow, contaminants, and corrosion.

Why in-line?

So what makes in-line monitoring and measuring technology so popular and prevalent? In-line devices are used to measure a multitude of parameters (pressure, temperature, flow), though pH, conductivity, and dissolved oxygen are probably among the more common. The methodology is used primarily for control, points out Mel Thweatt, applications manager, analytical instrumentation at ABB Instrumentation.

“In the wastewater industry, for example, you need to monitor the addition of disinfectant into the water to know how much disinfectant to add. Realtime data are needed to determine how much chemical is required to control the process. Grab samples analyzed by a lab will not suffice. There are too many variables. The composition changes too fast.”

Adds Tom Griffiths, product manager for Honeywell,“People want to get away from time-consuming grab samples. The trend today is ‘give me more information, faster, in a form that will make my job easier so that I can react faster and smarter.’ The user is looking for more, better information to make a smarter and faster decision about the process.”

In-line analysis provides just that. But other factors influence its use. In the opinion of ABB’s Thweatt, one reason for in-line analysis is downsizing.

“Analysis that used to be done by people in labs isn’t done that way anymore,” he observes. “Automated methods are taking precedence.”

Honeywell’s Griffiths agrees. “Fewer people in the plant make reliable and accurate in-line analysis and monitoring more important. In-line devices allow a reduction in the number of measurements and analyses that need to be done in the lab. But,” he warns, “some samples must be done in the lab. The lab isn’t going away. Some measurements require laboratory verification.”

Stanzi Prell, marketing director, industrial, at Hach Co. sees the situation similarly. “When we compare in-line and portable laboratory instrumentation at our company, growth rate on the in-line side is definitely higher. We attribute this to personnel cutbacks. There are fewer people to conduct these tests, but tests still need to be conducted. In addition, plants want and need faster response times. They don’t want to take three grab samples a day. They want to know pH values constantly in real time. This information is a great indicator about the process. It can help avoid shutting down a line or making a poor quality product.”

Refinements, advancements

The process of performing in-line analyses has progressed steadily, albeit slowly, continuing to respond to user demands. Sensor designs “have made incremental advancements in terms of reliability and dependability,” notes Andy Szeto, product manager, analytical instrumentation, ABB Instrumentation.

“However, they are based on the same principles developed in the past. It’s the surrounding peripherals such as analyzers and transmitters that have changed the most. Users are looking for analyzers with more features, such as easier-to-use interfaces, diagnostics, and digital communications. Although many facilities still operate with analog transmitters, there is increasing interest in fieldbus-enabled devices, such as FOUDATION fieldbus and Profibus.

These communication protocols allow access to data that traditionally were not available with the older analog instrumentation.”

GE Sensing’s Belock cites wireless applications as an area that will increase, but issues remain. He notes, “Can the distance be accommodated? Will storms interfere? Is the application sensitive to electromagnetic or radio frequency interference (EMI/ RFI)? Is the sensor susceptible to radio transmissions and vice versa? A lot of industrial installations aren’t ‘wireless-friendly.’ Networking is good for reporting data but not necessarily for controlling the process, yet.”

Many once-troublesome problems, however, have been overcome. Fragile glass electrodes prone to breakage can be replaced with non-glass varieties, such as Honeywell’s Durafet line. The non-breakable, solid-state in-line device performs pH measurement in real time. Problems with leaks, as well, are now almost a thing of the past. Cutting into a line admittedly creates another potential leak path or point of contamination, but technology today has made leak problems minimal.

Other analyses issues, contamination for one, have been alleviated through ex-situ approaches and sample-handling systems. In-line (or in-situ) analyses can lead to problems if a device fails. There must be a way to pull the device out of service without shutting down the process. In applications where shutdown is difficult or impossible, such as moisture analysis in a refinery, a sample panel may make more sense.

In these arrangements, explains GE Sensing’s Belock, a small amount of flow is tapped off. “It’s run through a sample panel—often a very simple configuration of isolation valve, filter, probe, pressure gauge, and flowmeter. The system protects the sensor and makes it more serviceable and accessible for calibration.”

In ex-situ designs, the sensor resides outside the probe. Belock illustrates with a flue gas analyzer example: “We move a small amount of flow coming out of the flue gas past the sensor using a thermal convection flow loop. Because we can control the flow to a very low level, we can reduce the particulate level at the sensor, reduce plugging, and not have as many problems with sensor contamination. If the sensor does need replacement, it’s not complicated because the sensor is not in the probe or probe sleeve.

These systems are somewhat more expensive and the response time can be slower because you’re not in-situ, but the other performance benefits outweigh the desire for an in-situ solution and the results are just as accurate.”

Sometimes a sample has to be run through a cooler, or filtered to lower the temperature, before measuring, adds ABB’s Szeto. It’s still a continuous, online/inline measurement, but putting the sample through treatment or conditioning does add time and slow down the reading. The preferred method is still to measure directly in-line in the process to get the quickest, fastest, most reliable measurement with the least delay.”

From a distance

There are lots of in-line devices and systems in use today, and that’s not likely to change significantly any time soon, most agree. But, as with every technology, change will come.

Refinements for in-line devices are helping to keep them competitive even as new technologies enter the market. End-users are asking for sensors that are smarter about when they need to be cleaned, calibrated, or changed. They want diagnostic capabilities that will bring information to the operator directly from the instrument. And those demands are being met.

However, non-intrusive methods are advancing; some—ultrasonic flowmeters and optical-based instrumentation, for example—are already in use. Several vendors offer clamp-on gas analysis using ultrasonic flowmeters, but it is still relatively expensive. Hach Co.’s LDO, luminescent dissolved-oxygen probe uses optical-based technology (see photo); it has no membranes and requires virtually no end-user maintenance.

End-users today need reliable instruments that generate data they can trust to accurately reflect what’s going on in a process. Technology will continue to evolve. New designs and better performance will become available, and be adopted. But the transition will be slow and steady. In-line devices will be around for a long time to come.

For more information, visit:

www.abb.com/instrumentation

www.gesensing.com

www.hach.com

www.honeywell.com/acs