Mass Flow - BC or AD

By Edgar J. Laderoute

Historical Perspective

The technological buzzwords of the 80’s were Quality, Reliability and Statistical Process Control. The basis of this emphasis was the substantial loss of America’s global competitiveness.

Early indicators strongly suggest that the survivors of the 90’s would be the practitioners of these underlying disciplines. Statistical Process Control, with its promise of “reduced product variation“ continues to distinguish the progressive organizations of the 90’s.

Contemporary conventional wisdom suggests that if your measurement capability is no better than 0.5% then the minimum product variation will be no better than 0.5%. Net gain can only be realized when measurement performance is better than target requirements.

With typical instrumentation accuracy’s of 0.25%, it is not uncommon to have a loop measurement capability of no better than 1%. When average performance is not acceptable, alternative measurement solutions need to be considered.

Two specific process applications will be reviewed with an emphasis in identifying how to achieve at least a decade improvement in system accuracy over traditional measurement methods. Both solutions utilize a true mass measurement technology employed for over 40 years and recently being re-discovered by the Process Industries

Summary of Basic Technology

The measurement principle is based on Newton’s 2nd Law, which states that force is equal to mass times acceleration. The useful form of Newton’s equation for a body at rest is Weight equals Mass times Gravity or W = Mg. Since local gravity is a constant, gravity can readily and precisely be accounted for during calibration. Re-arranging, Mass = W/g or mass is linearly proportional to weight.

An examination of the force distribution in an isotropic and homogenous structure reveals that the shear force is uniform across the structure. A series of strain gauges suitably bonded at the neutral axis of such a structure will linearly have their resistance altered in direct proportion to the applied force. Shear beam structures may be of the single cantilever, double cantilever or double ended type.

Double Cantilevered Shear Beams are a high performance sub-set of a class of Force Transducers generically referred to as “load cells”. It is frequently convenient to view force transducers as “ideal linear springs”.

As a change in weight occurs, the shear force is sensed, a proportional resistance change is detected and an output signal in the order of 20 to 30 millivolt is generated. The signal is typically connected to a high performance programmable A/D converter specifically designed to convert low microvolt signal levels to engineering units and post-processed for indication, re-transmission or control.

Load Cells for Level Measurement

Tank level is a poor indicator of vessel contents since level indication will change with expansion or contraction of the vessel, or its contents. Level measurements are inherently in-accurate and should not be considered for high performance (better than 0.25%) process control or inventory management applications.

Hydrostatic Tank Gauging Systems have attempted to satisfy higher measurement accuracy requirements by managing a large list of correction factors. Though improvements over many of the traditional “level” measurement instrumentation have been achieved, they are incapable of breaking the 0.25% barrier for most applications.

The ideal measurement would be made with a non-intrusive true mass measurement transducer that is dependent only on a singular physical variable. Load Cell based systems satisfy these requirements.

Load Cells for Mass Flow

Temperature, Pressure and Flow Measurements have been the traditional focus of process measurements with increasing attention toward analytical measurements (pH, ORP, selective ion, etc.) . Historically, Mass flow data was obtained using “inferential measurements” for a first order calculated approximation of mass flow in the 1 to 2% range. The Coriolis Mass Flow meter’s marketing success provided legitimacy to a more accurate in-line, however, intrusive paradigm and quickly became the measurement of choice for many loops where performance expectations where high.

Unfortunately, the promises of the Coriolis Mass Flow meters have yet to be validated below the 0.25% threshold on a consistent and reliable basis. The limited rangeability, maintenance and long term stability are being challenged by the non-intrusive simplicity of the strain gauge based “load cell” (MTBF > 50 Years) and it’s performance capability of greater than 0.025% .

A broad range of transducers and companion instrumentation are available. Transducer selection criteria include consideration of total weigh in addition to vessel dynamics, prevailing wind and/or seismic forces. Instrumentation is available for local or remote control by any DCS, PLC or PC with all contemporary protocols and commercial network technologies.

Conventional High Performance Mass Flow Weigh System

The rangeability (1:1,000,000) and total independence of the weigh media (liquid or solid, benign or corrosive) coupled with it’s high performance capability are beacons of light to a more accurate material management program.

When performance, “Life Cycle Cost“ or a sound “Return on Investment“ are critical or essential requirements of your operation, upgrading to an Electronic Weighing System should be considered.

Mass measurement with connectivity (electrical and protocol) to any DCS, PLC or PC is available from BLH, a major supplier of Electronic Weighing Systems to the Process Industry.

Transducer Performance Considerations

Weigh Modules are robust structural elements uniquely designed to support and precisely detect small changes in applied vertical forces.

The “weigh module” is an integration of the force transducer with it’s mounting hardware.

Since the coupling hardware may have a significant effect on a weigh module’s performance when subjected to external axial, side or torsional forces, prudent engineering suggest establishing a strong technical relationship with a qualified weigh system supplier. In addition, for outdoor vessels or seismically classified areas, the ability of the weigh module to resist significant uplift or lateral forces are major considerations in selecting the appropriate module.

Once the proper weigh module is selected, the functional features of the instrumentation and the signal interface requirements are determined. The simplest requirement of a 4 - 20 mA signal can be satisfied by a blind transmitter with an integral summing circuit. Typically, these units are design to be located in the vessels environment.

Indicating Transmitters with a choice of enclosures for mounting in the control room or at the vessel are also available. Analog or digital connectivity with a broad choice of contemporary protocols complement the users selection criteria.

Expert Systems with background diagnostics capable of monitoring the health of each individual load cell or functioning in a degrade mode of operation, represent recent advances available for the most critical applications.

For multi-vessel applications, an RS-485 Multi-drop Gateway System Network Controller can provide high speed data interface to Allen Bradley Remote I/O, Modbus Plus or Modbus RTU, at measurable cost savings.

A return on investment (table 1A and a cost benefit analysis (table 1B) will clearly demonstrate that load cell based measurement systems offer superior performance at reduced life cycle cost. As a non-intrusive transducer, it has no media measurement boundaries and offers a simple reliable and maintainable technology.