Cost-Saving Applications
Appliance
Manufacturing with Multidrop
A consumer appliance manufacturer used the networking capability of
the HART protocol to measure level, flow, and pressure. HART multidrop
provided substantial wiring and installation savings as well as digital
accuracy with the elimination of the analog to digital (A/D) and digital
to analog (D/A) conversions of the instrument and PLC I/O. The following
figure shows pressure transmitters connected to a PLC via smart transmitter
interface multiplexers.
Multidrop Network Example
Remote Rezeroing in a Brewery
The benefits of remote monitoring and rezeroing of smart transmitters
using the HART protocol are dramatically illustrated in this example
of two smart transmitters that control the fluid level in lauter tubs
in a brewhouse application. Similar benefits would be realized in
any application involving a closed vessel.
Two smart transmitters are installed on each lauter tub--one on the bottom of the tank and the other about nine inches from the bottom. The bottom transmitter is ranged ±40 inH2O; the upper transmitter is ranged
0-30 inH2O. As the lauter tub is filled, the bottom transmitter senses level based on pressure. When the level reaches the upper transmitter, that point is marked as the new zero-level point, and the upper transmitter becomes the primary sensing instrument for the lauter-tub level. The nine-inch zero-level offset from the bottom of the tank is necessary to accommodate loose grain that settles in the bottom of the tank.
Transmitters that are coordinated and working together control fluid level in each lauter tub to within a few barrels. However, the upper transmitter requires periodic maintenance or replacement and rezeroing. An undetected false upper-transmitter level reading can cause a tank level error of up to 40 gallons.
The usual procedure for transmitter rezeroing takes about 95 minutes and has been required as frequently as twice a day. Rezeroing a transmitter using configuration software and PLC interface modules eliminates the need to locate and identify the problem at the site as well as the need for verification by control-room personnel and greatly reduces the chance for inadvertent errors. Estimated total time to rezero each transmitter is reduced to 15 minutes.
Through the configuration software's instrument-status and diagnostic capabilities, a false level indication can be automatically detected while a lauter tub fill is in progress. The affected transmitter can then be automatically rezeroed by programming logic in the programmable controller to issue the appropriate command to the instrument.
Water Treatment Facility UpgradeHART transmitters and a control system with HART capability were chosen to upgrade a water treatment facility. The completed installation reduced capital, engineering, and installation costs. The process dynamics of the water treatment facility allowed the HART instruments to be used in all-digital mode without compromising plant performance.
The water treatment plant is divided into two areas, each with 14 filters. Each area is controlled by a separate control system for complete autonomy. A HART network monitors each filter for filter level, filter bed differential, and filter outlet flow. The multidrop installation used a three-wire system in order to accommodate both the two-wire and the four-wire devices (magnetic flowmeters) in use.
Multidrop Networks with 2-Wire
and 4-Wire Devices
Because the water treatment facility had a modular design, the use of HART instruments allowed the configuration from the one filter network to be copied to the others, which reduced the implementation time. Engineering, system configuration, drafting, commissioning, maintenance, and documentation were simplified. A reduced I/O card count also saved money.
Improved DiagnosticsA cleaning materials supplier required periodic checkup of the instrument condition and configuration information as compared to the initial installation. The field transmitters provided a historical record of status changes along with current configuration information. Periodic download of this information was made possible using PLC ladder logic developed for HART instruments.