1 August 2002
Wireless valve monitoring? Can do.
By M. E. Peters
Communications, software advances make it possible.
There are many occasions when it would be advantageous to monitor valves wirelessly. To name just a few, remote-mounted manual valves; meter proving valves; well head valves; off-shore platform valves, where low energy is a plus; eye wash station and safety shower valves; tank farms; natural gas distribution valves; any automated valves; blind-mounted valves; tank valves; relief valves; stack valves; and contaminated area valves.
Let's start with a short history lesson. The rotary valve position indicating limit switch has evolved greatly during the past 20 years. Prior to the mid-1980s, it was common practice to mount any commercial limit switch to a quarter-turn valve device (90° rotation). Most of the switches available were typically lever type, with very expensive complex linkage and mounting hardware that was very difficult to adjust and wire.
In the mid-'80s, limit switches combined into a single box with a single output shaft. Adding a top-mounted visual indicator reduced the uncertainty about whether a valve was open or closed. This design offers switches in a rotary shaft–operated box with independently adjustable cams, providing one or more conduit connections to a single box for all valve assembly wiring. This eliminated multiple switch housings and junction boxes.
When combined with a top-mounted visual indicator, this set a new standard for the valve industry and revolutionized the valve automation market.
During the next 15 years, many other domestic and international manufacturers developed similar technologies. Some incorporated a solenoid pilot valve as an integral part of the box, making field installation and wiring even easier.
Today, buses standard
Let's look at the present situation, where bus technology is becoming the control standard.
Many bus standards exist, such as Foundation fieldbus, Profibus, AS-I, Modbus, and DeviceNet, to name some of the more common ones. Bus technology reduces the amount of wiring required to signal back to the control room.
Bus control typically works on a two-wire bus (hence the name) that allows for digital communication based on an industry standard and, in some cases, proprietary protocols. The newest technology embeds the interface (in the past, this was the I/O) to interface a field device with a programmable logic controller or distributed control system (DCS), which allows direct bus communications.
Because most bus systems can talk to multiple devices over a two-wire system, this was a major step, as it reduced the number of wires required in the field.
While the control industry advances into the future, it is continuously looking for ways to do the following:
- Reduce installation time
- Eliminate wiring
- Increase functionality
- Increase reliability
- Predict maintenance
- Reduce cost
- Prevent unscheduled breakdowns
- Reduce equipment size
Considering all of the above as design inputs for a new product, the best way to accomplish them would be through a microprocessor-based, digitally operated wireless solution.
The answer? Go wireless . . .
A product developed with wireless control offers the following advantages:
- Reduces or eliminates the need for additional wires at a plant
- Makes wiring an option
- Speeds installation time
- Allows use use in remote areas without electric power
- Provides intrinsic safety, due to low power consumption
- Operates on low energy
- Works during power failure
- Adapts easily to manual and automated valves and dampers
- Permits data to be posted to a Web site
- Enables equipment locating
- Offers local access to critical information
- Overcomes blocked line-of-sight indication of valve status
- Communicates to specific groups of equipment via software filters
- Offers multiple sensing ranges (radio to receiver)
- Uses standard PC operating systems such as Palm OS and Windows
- Uses standard industry buses such as Ethernet
- Comes with easy-to-use software
Until recently, the biggest problems with wireless were it could basically offer only an analog solution and was too slow. Today, it is possible to encode a digital message on a radio frequency.
The usual way this works is through packets of information. To protect the packets of information from outside signal corruption, it is possible to use security such as CRC16, an error check algorithm that verifies a message packet to ensure it is from a certified source and is an authorized message. Speeds of up to 1 megabits per second are becoming more common.
Bluetooth pros and cons
The buzz among those talking wireless today is Bluetooth, a relatively new wireless protocol originally developed for the telephone and PC industry. Objectives Bluetooth enables include creating a wireless interconnection of peripherals such as head sets, cordless phones, handheld computers, and personal digital assistants, such as the Palm Pilot.
Bluetooth's most common uses are to operate as a wireless wide-area network, local-area network, or personal-area network. Bluetooth doesn't seem as well suited to the limit switch because of its limited range of about 30 feet.
There have been some setbacks with Bluetooth. It operates at 2.4 gigahertz, a very common, easily accessed frequency allowed for commercial use by the Federal Communications Commission. This frequency is heavily used, and collisions and interference could be a problem.
Bluetooth technology will continue to move ahead, but it is still more of a developmental technology. After a communication problem at an important demonstration, Microsoft announced it is not going to support Bluetooth any time soon.
Lower band frequencies are better for wireless field communications and operator device interface. They are less susceptible to obstruction and have a wider scatter, as opposed to higher-frequency microwave signals that can, with the appropriate power supply, travel long distances but require a straight-line, unobstructed signal path.
A wireless system would have the local devices communicate via a lower frequency to a repeater. Long-range transmission could be via a higher-frequency microwave to the receiver in the control room. Users would strategically place the repeaters throughout the plant to give full coverage of transmitting devices and a straight-line path to the control room.
Money saver when system down
Wireless technology's largest single advantage is, of course, no wires. However, another significant advantage is the ability to address a piece of equipment and go online in a matter of minutes. This can be a powerful tool when a system is down.
Today, wireless is not as fast as some control formats but is great for data acquisition and monitoring.
Wireless technology can do simple control in applications where high-speed reactions are not required. Similar to two-wire control buses, technology advances will let wireless devices play a key role in the control scheme.
Will wireless technology replace bus technology? Maybe the wires will, in some cases. However, both wireless and bus will probably work together on the same or similar control platforms.
Wireless technology is here today, solving real-world problems in the industry, while also creating some new opportunities never before available. IC
Behind the byline
Mark E. Peters is president of ControLink LLC in West Chester, Ohio. ControLink specializes in valve control products, limit switches, positioners, and proximity switches. Peters previously worked 23 years for Flowserve, most recently as director of engineering for the Automax/Accord Group.
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