In the ISA Mentor Program, I am providing guidance for extremely talented individuals from countries such as Argentina, Brazil, Malaysia, Mexico, Saudi Arabia, and the USA. This question comes from Jeff Downen.

Jeff Downen’s Question

Our multivariable flow transmitters on new construction sites fail a lot. If the transmitter loses the RTD, the whole 4-20 loop goes bad quality along with the HART variables. I like the three devices being separate and their signals joined in the DCS logic much more.  I understand that it is more expensive. I want to see if there was any other reasoning behind it on the engineering side and how I can help get a better up front design.

How can we avoid the increasing use of multivariable flow transmitters as an industry standard despite a significant loss in reliability, accuracy, and diagnostic and computational capability from not having individual separate pressure, temperature and flow sensors and transmitters?

Greg Brietzke’s Answer

I like Jeff’s question on multivariable flow transmitters, as it would be relevant to control engineers, maintenance/reliability engineers, as well as maintenance personnel. What is the application? What are the accuracy requirements? Can you bring the individual variables back to the DCS/PLC through additional variable assignment? Would the increased cost of infrastructure justify the increased expense of a true mass flowmeter? This could be addressed from so many different viewpoints it could be a great discussion topic.

Greg McMillan’s Answer

I suggest you explain to plant and project personnel the advantages of separate measurements and true mass flowmeters. Separate flow, temperature and pressure measurements offer better diagnostics, reliability, sensors, and installation location that is particularly important for temperature (e.g., RTD in tapered thermowell with tip centered in pipe with good velocity profile). They can provide faster and perhaps more accurate and maintainable measurements that could be used for personalized performance monitoring calculations and safety instrumented systems.

Coriolis meters provide the only true mass flow measurements offering an incredibly accurate density measurement as well. Most people don’t realize that pressure and temperature compensation of volumetric flow meters to get a mass flow measurement only works if the concentration is constant and known. The Coriolis mass flow is not affected by component concentrations or physical properties in the same phase. Density can provide an inferential measurement of concentration for a two component process fluid. The Coriolis meter accuracy and rangeability is the best by far as noted in the Control Talk column Knowing the best is the best.

David De Sousa’s Answer

Using dedicated and separated measurements also allows for the use of hybrid virtual flowmeters in complex process applications where, for example, the technology for inline multiphase flow metering is not yet mature enough, or where physical units will greatly increase the cost of the associated facilities.

With the digital transformation initiatives associated with Industry 4.0, the use of distributed instrumentation, data-driven learning algorithms, and physical flow models, are being tested and explored more and more in the process industries, especially in upstream oil & gas wellsite applications.