VARIOUS TYPES of multivariable (MV) transmitters have been around for quite some time, but their value to skid builders and ultimately to process manufacturers sometimes has been mixed, especially in terms of their effect on faster, more economical, and more reliable installations.
Multivariable transmitters increasingly are specified to save space and reduce costs, says Dudley Harrison, senior technical specialist and 30-year veteran with Invensys Process Systems Measurement and Instrumentation division. These versatile systems enable customers to replace separate static pressure, differential pressure, and temperature transmitters with one transmitter that communicates all three measurements to the customers system. This also saves the cost and maintenance issues associated with separate transmitter installationespecially wiring and I/O terminations.
Dudley reports that Bechtel Corp. is contracted by the U.S. government to manage and clean up wastewater contaminates around the nuclear storage facility at Hanford, Wash. Unlike what many skid builders use in their process controls, Bechtel implemented a distributed control system (DCS) instead of PLC technology. With a DCS, more than 20,000 I/O can be tied into the system and then monitored. Invensys MV transmitters are implemented in many areas of this project using HART, Foundation fieldbus, and 4-20 mA standard connectivity.
Invensys offers three models of multivariable transmitters to cover a spectrum of applications, says Dudley. One model is widely used in the natural gas industry for production, processing, transmission, and distribution. Other models provide variable data collection, ranging from compressor/expander applications to compressible fluid processing. All communicate via HART and FoxCom, as well as configurable 4-20 mA.
Moore Industries-Intl. provides another piece to the remote, variable-transmitting puzzle with its HART Loop Interface module (HIM). The HIM interface works with multivariable transmitters to provide users with process information that would be unavailable otherwise, says Steve Todd, Moores corporate marketing director.
The HIM module monitors non-primary variables. Smart HART MV transmitters sense multiple process variables. They also perform an internal calculation to derive a measurement such as mass flow. This primary measurement information is transmitted as a 4-20 mA signal to the control system.
Unless the user has a HART-based control system, theres no way to continuously monitor the non-primary variables used to make the calculation, continues Todd. Monitoring non-primary variables might be desirable if one or more of the process variables is especially important to the quality or safety of the process. For example, a typical, multivariable, mass-flow transmitter measures differential pressure, pressure, and temperature. From these three non-primary measurements, it calculates mass flow and sends it via 4-20 mA back to the control system. The problem is that the non-primary measurements, and the data they provide, never make it out of the field. HIM breaks out non-primary measurements, and sends them as 4-20 mA signals to the control system.
Siemens offers a not-so-optimistic opinion on MV transmitters. We recognize the need for a multivariable transmitter and are presently developing one, says Lou DiNapoli, transmitter product manager for Siemens Energy and Automation. One large transmitter customer wont use MV transmitters because they dont want to put all their eggs in one basket. They feel that making three different independent measurements, and using a small PLC in their RTU to do the calculations is very straightforward and more cost-effective. Their experience shows that multivariable transmitters are very difficult to set up and maintain.
DiNapoli adds that another company pulled more than a dozen MV transmitters, replaced them with three separate transmitters, and then did the calculations in its control system. The companies technicians couldnt verify the results from even simple changes in variable set-points, such as the full-scale differential-pressure value, he says.
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