Tech Tips July 2005
July 26, 2005
TECH TIP OF THE WEEK:
Preventing or containing potential explosions.
Unless process media are completely inert, the potential exists for fire, explosion, corrosion, and/or environmental damage in case of an alarm situation. Of these disasters, explosion and fire are often the deadliest to plant personnel.
Explosions can be prevented in several ways. One way is by limiting the amount of electrical energy available in hazardous areas. Controlling electrical parameters such as voltage and current requires the use of energy limiting devices known as intrinsically safe (IS) barriers. IS barriers limit the levels of power available in a protected barrier. If a spark or excess electrical heat cannot occur, neither can a fire or explosion. Although used in Europe for many years, intrinsic safety was not adopted as part of the U.S. National Electrical Code until 1990.
An intrinsically safe circuit contains three components: the target device, IS barrier, and wiring. Devices within the protected area can be categorized as simple (contacts, resistors, thermocouples, RTDs, etc.) or complex (transmitters, relays, solenoids, etc.). Complex devices often have complicated circuitry that can store excess electrical energy and are normally certified ‘intrinsically safe’ by safety testing and certification organizations, such as Underwriters Laboratories (Northbrook, IL.) or Factory Mutual (Norwood, MA).
Selection of proper IS barriers requires calculation of both the open-circuit voltage and short-circuit current of simple devices. For complex devices, both allowed capacitance and inductance values must be calculated. Results are then compared to ignition curves that have been calculated for a wide variety of flammable/explosive media (gases, vapors, airborne dusts or fibers, etc.) to determine if the available energy is below the amount needed for ignition.
Brute force approach
Explosion-proof enclosures provide a brute-force method of preventing or controlling potentially explosive situations. These heavy, cast-usually but not always-devices feature sealed and securely fastened access doors. They protect the normal power level devices within them from coming into contact with an explosive atmosphere. Even under fault conditions, an explosion or fire usually cannot occur because of limited air for combustion within the sealed container. If an explosion does occur, the housing is strong enough to contain it.
Although there have been many refinements in explosion-proof enclosure design, the fact remains that they are bulky, can be difficult to mount because of their weight, and are not the handiest of housings to access. Additionally, seals, gasketing, and purging systems require inspection and maintenance if their integrity is to be trusted. The fact remains that in industries where high voltages and currents are routinely encountered and process systems are rarely reconfigured, explosion-proof enclosures remain a practical method of preventing an industrial tragedy.
Dick Johnson, consulting editor
Source: Johnson, Dick, ‘Keeping the Explosion Genie in the Bottle,’ Control Engineering , July ’00, p. 76.
JULY 19, 2005
TECH TIP OF THE WEEK:
Deciding if an automation project should be done in-house or outsourced?
Here are things to think about when deciding whether to execute an automation project in-house or outsource it to a system integrator.
Should we go it alone?&o:p>&/o:p>
Is the required technology mature and well understood? Has it been applied to the automation of similar processes before?
Will this project have a higher return on investment (ROI) than other jobs that the in-house engineers could be working on?
Do the in-house engineers have the experience and skills required to complete the project?
Would it be otherwise advantageous to develop more industrial automation expertise in-house?
Will the amount of work required fit into the in-house engineers’ existing workload?
Will the in-house engineers be able to dedicate entire days to the project without interruptions?
Can the in-house engineers finish the project within the timeframe dictated by business considerations?
Do the in-house engineers have access to the necessary technical tools, such as design software?
Does plant management need to maintain tight controls over project design and execution?
Will the project involve proprietary processes that must remain trade secrets?
Should we get help?&o:p>&/o:p>
Will the proposed automation system be coped from another facility?
Will this system be reproduced at other facilities?
Will this system be expanded in the future?
Would an outside viewpoint help overcome internal disagreements and political issues?
Is the ROI high enough to make the cost of quick delivery worth the extra expense of outside assistance?
Is there a system integrator available who is willing to commit to a fixed price bid for the work?
Is there a system integrator available who has done this kind of work before?
Are the in-house engineers (due to time constraints or lack of specific knowledge) more likely to provide an ad hoc solution , rather than a well-designed and documented system?
Is there a hard-and-fast deadline for finishing the project?
Source: Control Engineering with data from Nol-Tec Systems; NEEDAM Software Technologies; American Standard/Trane; TransAmerican Automation; and AIA Automation.
Source: VanDoren, Vance, “In-House/Outsource Debate,” Control Engineering, July 2004, p. 32.
JULY 12, 2005
TECH TIP OF THE WEEK:
Turbine flowmeter installation rules of thumb.
|Installation rules of thumb|
|Installation scenario||Straight pipe lengths upstream||Straight pipe lengths downstream|
|Wide-open pipe fittings, or valves upstream and downstream||10 pipe diameters||5 pipe diameters|
|Torturous flow path produced by sharp angles or valve design||20 pipe diameters||5 pipe diameters|
|Use of straightening vanes upstream of meter||3 pipe diameters prior to straightening vanes, 2 pipe diameters of straightening vanes; and 2 pipe diameters after the straightening vanes.||5 pipe diameters|
Similar to most flowmeters, the accuracy of turbine flowmeters is highly dependant on the ability of the installation to ensure non-swirling conditions.
Even at constant flow rates, swirl can change the angle of attack between the fluid and the rotor blades, causing varying rotor speeds and thus varying flow rate indications.
Effects of swirl can be reduced or eliminated by ensuring sufficient lengths of straight pipe—a combination of straight pipe and straightening vanes, or specialized devices, such as Vortab’s flow conditioner—are installed upstream and downstream of the turbine flowmeter.
Turbine flowmeters for liquid applications perform equally well in horizontal and vertical orientations, while gas applications require horizontal flowmeter orientation to achieve accurate performance.
When installing turbine flowmeters in intermittent liquid applications, it’s recommended that the flowmeter be mounted at a low point in the piping.
Turbine flowmeters are designed for use in clean fluid applications. Where solids may be present, installation of a strainer/filter is recommended. Also, because the strainer/filter can introduce swirl, it needs to be located beyond the recommended upstream straight pipe lengths.
Source: Harrold, Dave, ‘Turbine Flowmeters: Simple Elegance,’ Control Engineering, Oct. ’03, p. 41.
JULY 5, 2005
TECH TIP OF THE WEEK:
Connectors tie networks together
|Basic Network Connector Methodologies
The majority of industrial buses use one or more of the following connectors or applicable variants:
9DB , which consist of nine-pin, subminiature, D-shell connectors. Daily use of approximately 250,000 connectors.
Micro (dc) or euro, which are two- through six-pin connectors based on M12 threads (approximately 14 mm out-side diameter coupling unit). Daily use of approximately 150,000 connectors.
Mini , which include two- through seven-pin connectors based on a 7/8-in-diameter barrel with a 16 pitch. Daily use of approximately 50,000 connectors.
Network connectors have several tough jobs. They must make positive connections to transmit data; keep out unfriendly substances; withstand their environments for long periods, and often hold cables together under mechanical stress. And, as network data rates continue to increase from 9,600 bps to 12 Mbps and beyond, connectors are also being required to shield conducting pins and sleeves from electrical noise and interference.
To select and implement the most appropriate connectors, you must take into account physical requirements, application needs, layout/topology, and even different philosophies of each network protocol. Although all connectors must satisfy the basic physical layer and cabling needs, these different philosophies have resulted in different connector specifications.
Since some protocol specifications define connectors while others do not, users are often forced to seek out de facto industry standards. It’s usually up to individual users to determine the appropriate connectors for their network (see sidebar). For example, typically the connector used in semiconductor manufacturing is the M8 pico, which is considered too small by users in the process industries. Process applications often use the 7/8-in. mini connectors for their FOUNDATION fieldbus, DeviceNet, Profibus, AS-interface and SDS networks installations.
Evolving industrial technologies and trends also influence network cabling and connector requirements. For instance, the process field has long relied on screw terminations, and has not traditionally used many network connectors found in other applications. However, many process users are starting to use 7/8-in. mini connectors rated at 9 amps, with three, four or five pins, as well as M12 micro connectors rated at 4 amps, with four, five or eight pins.
Secure plug-in and snap-on versions of many traditional threaded connectors, as well as adapters, are also increasingly prevalent in many applications. Meanwhile, in industrial factory applications, M12 micro connectors have become the ‘tradition’ because of the strict requirements for installation of I/O points and devices. The 7/8-in. mini connector is almost exclusively used to supply power, which has been fueled by the recent growth in PLC and related networks.
End-users need to take care that the connectors they are using have the right thickness of gold plating on their contacts. This is critical because networks need a low contact resistance when going through each connection. Generally, you want that contact resistance to be less then 5 milliohms (mV) over the life of those contacts. Most traditional connectors do not work as well.
Low contact resistance is extremely important. On a point-to-point DeviceNet network with 64 nodes, the signal on your last device could potentially go through 128 connections before returning. High contact resistance could spell real trouble for a bus system like this.
Current-carrying capacity is usually not a big concern since industrial networks generally do not draw much power. For example, FOUNDATION fieldbus typically draws about 20 milliamps (mA). Likewise, DeviceNet is designed to handle 4 amps. A mini connector can handle 9 amps.
It is equally crucial to use quality connectors because they are also subject to the same heat, vibration, abrasives and corrosives as the rest of the network. Good quality connectors mean your network installations will not experience the drifts in voltage that occur in systems with connections that decay more quickly over time.
Beyond these essential basic requirements, your choice of connectors is also a matter of your applications needs and individual preference. Technicians who are installing an overhead network using gloves will want large connectors for easier handling, while a small panel network system works better with smaller connectors.
With the emergence of Industrial Ethernet, RJ-45 connectors are being used, but much effort has been made to help them survive and serve well in plant-floor applications. Although the traditional RJ-45 connector can be industrially hardened, its 1-in. diameter and straight connections are still more subject to vibration and other forces in manufacturing applications. A better industrial alternative is the M12 micro connector, which has a 0.5-in. diameter and has better contacts.
Ann Feitel, senior product manager for physical media
InterlinkBT, Minneapolis, MN
Source: Feitel, Ann, ‘Connectors tie networks together,’ Back to Basics, Control Engineering, Nov. ’02, p. 62.