It's long been the central mission of instrumentation companies to reveal what’s hidden inside your process piping. And, whenever possible, they opt not to drill a hole, but to divine those conditions using non-invasive means. The upsides include improved safety, process integrity and lower installation costs. But for some process variables there’s simply no way around it. Impulse lines or integral taps are needed to allow pressure, pressure drop and inferred flow readings. And accurate, responsive temperature measurement requires insertion of a protective thermowell through the pipe wall. At least, it did until now.
With new technology guided by digital models, ABB is challenging industry’s de facto use of thermowells, arguing that for the majority of liquid and steam measurement applications, its new non-invasive sensors are just as accurate and responsive as those placed in thermowells. To learn more, Control recently caught up with Doug Greaves, North America product manager for temperature and pressure instrumentation, to discuss the company’s latest advances in non-invasive temperature sensing that just launched in the North American market with a remote transmitter option for use in CSA and FM hazardous areas.
U.S. Product Manager, Temperature & Pressure, ABB Measurement & Analytics
Q: Skin temperature sensors that strap on to the outside of a pipe have been around for a while, but they frankly don’t have a reputation for high accuracy or responsiveness, and are used primarily when a thermowell is impractical. What’s different about ABB's new approach?
A: First, we model the physics—the fluid dynamics and heat transfer equations that govern the relationship between the skin temperature of a metal pipe and the fluid contained inside (see figure). As it turns out, for most liquids and steam flowing in a turbulent regime, skin temperature is effectively identical to the temperature of the fluid inside a metal pipe. In addition, this turbulent regime sets in at a flow velocity on the order of three feet per second, and also includes the vast majority of industrial liquid and steam measurement applications.
The second model at play is the sensor device itself. Traditional skin temperature sensors are very sensitive to changing ambient conditions and installation specifics. However, in addition to the primary resistance temperature detector (RTD) that measures the pipe skin temperature, our non-invasive TSP341-N sensor includes a second, reference RTD that cancels out variations in ambient conditions at the pipe surface for tighter control. The result is a temperature reading proven to be every bit as accurate as a sensor in a thermowell—and equally responsive, too.
Q: How can I be sure that the flow conditions in my application qualify me to use this approach?
A: The process model I referenced earlier is embedded in an online tool called ABB’s "My Measurement Assistant “Performance Predictor." Users only need to enter their metal piping characteristics (wall composition, thickness) and nominal flow parameters (temperature, viscosity, density) to get a quick thumbs up or thumbs down. Performance with many liquids, steam and even high pressure gases has been predicted, proven and validated.
Q: So it maintains accuracy and responsiveness relative to a thermowell, but is easier and less expensive to procure and install. Are there other advantages?
A: Due to erosion, corrosion and other effects, thermowells are occasionally subject to catastrophic failure—avoiding their use in the first place reduces risks to workers, the environment and production continuity. A non-invasive approach eliminates the maintenance and inspection rounds that thermowells require. There are also implications when it comes to regulatory compliance—fugitive emission points are eliminated, measurement redundancy is simple to add, and management of material certs, drawings and weld inspection reports is ended.
Non-invasive sensors make it far easier and less expensive to add a new measurement point to a brownfield facility without interrupting operations. In addition, for a greenfield application, non-invasive sensors represent a savings in capital expenditures of up to 75% due to reductions in design and engineering, materials, and installation costs. On an ongoing basis, they also represent reduced stock requirements, since one sensor can be used for a range of pipe dimensions. Further, there’s no stocking of replacement thermowells or flanges of different materials and sizes.
We recently bid out an oil and gas project with 75 measurement points and found that more than 70% of the measurement points qualified to skip the thermowell and go non-invasive. This represented a more than 40% capex savings on temperature measurement. Plus, only four product variants covered all the various piping configurations, and total cost of ownership savings were estimated at 50-75%.
Q: To what extent has this new approach been tested and validated by process industry users?
A: The solution was first introduced in Europe in 2018, and in 2019, it was a finalist for the Hermes award at Germany’s Hannover Messe trade fair. It's been proven in a number of installations, and is being tested and qualified by a laundry list of global leaders in the process and energy industries. Now that it’s been launched in the U.S. and Canada, we expect to see broader use and acceptance.
Q: Let me see now. Improved safety and flexibility, simplified engineering and installation, plus significant reductions in capital spending and lifecycle costs. Is there any reason a non-invasive sensor wouldn’t be my first choice, and a thermowell relegated to option of last resort?
A: We believe that the benefits are overwhelming, and that non-invasive sensing should now be one’s first preference. Sure, entrenched engineering practices are sometimes difficult to dislodge, but look at what configurable I/O has done to the design and engineering of distributed control systems over the past 10 years. Today, no one seems to miss the marshalling cabinets or exhausting, last-minute change-orders that were standard engineering practice just a few years ago. Today, industrial temperature measurement is ready for its revolution.
