While consumer and commercial wireless technology has grown rapidly in functionality and availability, the adoption of wireless for process control has been a little slower. Reliability and security remain primary concerns in critical production applications, including flow measurement. The industrial landscape is changing, however, as wireless control systems, networking products and field devices become more widely available and industry standards crystallize.
Some market analysts predict the demand for wireless communication in the automation industry to grow at more than 25 percent per year over the next three to five years. Many of the applications will revolve around monitoring, data collection and programming. Industry concerns about reliability and data security are barriers to wider adoption that wireless technology developers will doubtless overcome in due course.
Meanwhile, if you’ve been thinking that wireless technology for process control is still too far away, or not as reliable as you would like for critical or hazardous applications in your plant, there’s a new reason that you might still want to evaluate it. Now you can get the best of both the wired and wireless worlds, thanks to a flowmeter that lets you read and send flow data with the reliability of conventional hard-wired communications, while using wireless technology to simplify maintenance and other routine tasks (Figure 1).
This approach does not achieve the major cost advantage of wireless—eliminating the wire and reducing installation labor—but does provide the flexibility and portability advantages of wireless while maintaining high reliability. At the same time, you will be gaining experience with wireless technology for potential future application in your plant.
This dual-technology approach is likely to gain traction with many field devices, where technicians need to carry out local monitoring, troubleshooting and maintenance. It is particularly effective for devices installed in crowded or hard-to-reach areas, saving field technicians’ time and improving safety.
How wired/wireless flowmeters work
The newest generation of mass flowmeters, such as FCI’s Models ST50 and ST75, offer an optional built-in wireless infrared (IR) link as well as standard 4–20 mA analog outputs and RS-232C serial I/O. The hard-wired connections provide a highly reliable way to communicate process flow data to the control room, while the IR link provides both an auxiliary route for the same measured data, and convenient access for field set-up and diagnostics.
The IR link eliminates the need for an on-board keyboard, hard-wired PC or custom field device to gain access to the flowmeter. The IR link communicates with any Palm OS-based personal digital assistant (PDA) (Figure 2). With the wireless IR link, the PDA facilitates the performance of a wide range of common flowmeter tasks, such as:
- field readings of process variables;
- changing configuration settings;
- reviewing self-diagnostic codes; and
- downloading calibration updates.
With a PDA and a wireless flowmeter, you are free to view measurements independently in the field, at any time and without disrupting the flowmeter’s normal operation or hardwired 4–20 mA signal output. You may need to adjust your process, which might mean updating the configuration settings on your flowmeter. Any changes can be made easily, from a distance, without ladders, scaffolds, or even opening the enclosure, with a few clicks on the PDA via the wireless IR link.
Troubleshooting is another potential application of wireless flowmeter technology. To resolve some problems in a process, field technicians must have accurate flow data at their fingertips. Sometimes, it can be really handy for a maintenance technician to obtain local, instant flow measurement data. This also eliminates the need for remote displays, ancillary readout devices and additional long wiring runs to suitable viewing locations.
If the field technician wants to test and validate the flowmeter, the wireless PDA provides a list of instrument self-diagnostic codes that can simplify resolution of the issue. Routine factory calibration updates also can be downloaded directly from the factory and delivered to the flowmeter without removing it from service or opening the device’s enclosure.
Considerations in choosing a flowmeter
In selecting a wired or a wireless flowmeter for any application, the first step is choosing the appropriate flow technology. There are multiple flow sensing technologies available, and the major ones now include:
- differential pressure;
- thermal (mass); and
- Coriolis (mass).
All these technologies have their advantages and disadvantages, depending on the media (air, liquid, gas or steam) and your application’s requirements. For certain media or applications, only a single option may be practical; in other cases, the choice may be wider. By looking at these factors, as well as your plant’s layout, environmental conditions, maintenance schedules, energy cost and return on investment (ROI), you will quickly be able to narrow the field to one or two best choices.
When looking at any flow technology, after you consider the media, there are several other factors to think about:
- accuracy and repeatability;
- plant environment;
- installation requirements; and
- maintenance and lifetime.
You need to know the accuracy, repeatability and flow range of the flowmeter that you plan to use. Depending on the application, manufacturers will provide these figures for water, air or another specific gas. A typical air flowmeter is FCI’s insertion-style thermal mass ST50 Series, which operates at velocities of 0.46–46 NMPS with an accuracy of ±2% of reading and ±0.5% of full scale, with a repeatability of ±0.5% of reading.
Plant operating conditions can play a major role in accurate flow measurement. Some flow measurement technologies cope better than others with extremes of ambient temperature, moisture in process gases, low flowrates, and high turndowns.
Where a plastic housing may be fine for indoor applications, a rugged metal enclosure with an appropriate NEMA/IP rating will ensure longest service life in non-climate-controlled or outdoor applications.
Some flowmeters are more straightforward than others when it comes to installation. Be sure to ask if the device can be inserted directly into the process pipe or if it requires an inline configuration that will require you to cut and splice your pipes in multiple places. The more penetrations required into the pipeline or ductwork, the greater the risk of pressure drop and increase in the complexity and overall cost of the installation. Some flow measurement devices feature minimally invasive or non-intrusive sensing technology, which make them much easier to install and require the least amount of installation time and labor cost.
Gerardo Vargas is an Electronics Design Engineer with Fluid Components International (FCI), San Marcos, CA/USA.
This article is protected by copyright. You want to use it for your own purpose? Infos can be found under www.mycontentfactory.de (ID: 282316 / Control & Automation)