by David W. Spitzer
Most differential pressure (DP) primary flow elements have straight run requirements, so they’re usually not applied where limited straight run is available. In addition, DP primary flow element technology has Reynolds number constraints, so it may find limited application in low flow applications, and where the liquid exhibits high or varying viscosity.
This number is used to identify flow patterns, such as laminar or turbulent flow, with laminar flow occurring at low Reynolds numbers, where viscous forces are characterized by smooth, fluid motion; while turbulent flow occurs at high Reynolds numbers and is dominated by inertial forces, producing random eddies, vortices and other fluctuations.
DP flowmeters measure velocity head, from which the fluid velocity is inferred, after which the volumetric flow rate is inferred. The differential pressure produced is a function of the square of the velocity, so this technology exhibits a relatively small flow turndown as compared with other flowmeter technologies. But within Reynolds number constraints, the range of accurate flow measurement is relatively easy to change after installation.
DP flowmeter measurements are inherently affected by fluid density. Density changes in liquid applications are usually small because of its non-compressible nature and because (in many applications) process temperature has a relatively small affect on density. In gas and vapor applications, both temperature and pressure can affect density and significantly degrade the quality of the flow measurement. Notwithstanding the above, note that changes in fluid composition can affect the density of the fluid.
Flow computers can be used to compensate for density (and other operating parameters) in applications where degradation of the flow measurement produces unacceptable flow measurement performance. A flow computer can be implemented as a separate hardware device that calculates the compensated flow measurement from field devices, such as differential pressure, pressure, and/or temperature instruments. These calculations can also be performed in the process control system. In addition to measuring flow, temperature and pressure, some multivariable differential pressure flow transmitters can perform these calculations internally.
Multivariable flowmeters, such as multivariable DP flow transmitters and other multivariable flowmeter technologies are outside the scope of this report because even though there is some overlap with the information contained in this report, additional parameters are used to evaluate the relative performance of the different technologies.
About the Author: David Spitzer is a principal at Spitzer & Boyes LLC, an engineering, market research and consulting services firm to instrumentation users, manufacturers and their representatives based in Chestnut Ridge, NY. Serving on several ASME committees for fluid flow measurement, he’s also authored several related textbooks and has taught numerous training seminars over 20 years. This article is excerpted from “The Consumer Guide to Differential Pressure Flow Transmitters.” Contact: 845-623-1830 or www.spitzerandboyes.com
Measuring Design Performance
Differential pressure (DP) flow transmitters generate an electrical signal that represents differential pressure at its ports. Of importance is how well the flow transmitter performs this function. Because performance is the prime concern in many applications, the technology used to effect the measurement is typically a secondary or tertiary matter. The following sections provide brief explanations of principles used in DP flow transmitter designs:
Capacitance: In a capacitance design, DP at the ports causes the wetted diaphragm to move an internal diaphragm located between two fixed plates. The movement of the internal diaphragm causes a capacitance change that can be converted into a signal that’s proportional to the applied DP.
Differential Transformer: In a differential transformer design, DP at the ports causes the wetted diaphragm (or bellows) to move the magnetic core in a transformer. Movement of the core causes an electrical imbalance that can be converted into a signal that’s proportional to the applied DP.
Force Balance: In a force balance design, DP at the ports causes the wetted bellows to create a force that’s counteracted by a force generated by an electromagnet (or perhaps a servomotor). A measurement of the generated counteractive force can be converted into a signal that’s proportional to the applied DP.
Piezoelectric: In a piezoelectric design, DP at the ports causes the wetted diaphragm to apply force on a crystal. This force generates an electric signal that can be converted into a signal proportional to the applied DP.
Potentiometer: In a potentiometer design, DP at the ports causes the wetted diaphragm (or bellows) to move the wiper of a variable resistor (potentiometer). This movement causes a resistance change that can be converted into a signal proportional to the applied DP.
Silicon Resonance: A silicon resonance sensor is a micro-machined semiconductor structure fabricated on a silicon crystal. The structure is shaped such that it can oscillate and resonate at high frequencies. When a DP is applied, part of the structure is under compression while another part of is in tension. Compression and tension forces change the resonant frequency of the structure in a manner proportional to the applied DP.
Strain Gage: In a strain gage design, DP at the ports causes the wetted diaphragm to apply a force on a strain gage. This force stretches the gage and causes resistance of the gage to change. The resistance change generates an electric signal that can be converted into a signal proportional to the applied DP.
