1 August 2002
Synergize software, electronics, and primary element
By Gregory Robert Strom and Gregory Livelli
A new generation of dP measurement instrumentation eliminates traditional limitations.
In the year 1724, Bernoulli's streamlined equation outlined the principles that form the basis of today's differential pressure (dP) flow measurement.
This equation defines the relationship between kinetic and potential energy in a flow stream. It allows the calculation of volumetric flow rate based on the measurement of dP across a restriction.
dP-based measurement has a history that reaches back to ancient times, as well as the largest installed base of any process flow technology. dP flow measurement continues to be the most frequently specified flow technology, not because of its tradition in flow measurement but because of the value it offers.
While new flow measurement technologies have emerged that have strength in certain applications, the dP flowmeter remains a favorite for process flow applications because of its application flexibility, excellent repeatability, proven reliability, direct process mounting, ease of calibration and troubleshooting, worldwide industry standardization, low installation and operating costs, and meter interchangeability, among other things.
Recent developments in dP flow measurement bring the primary element (differential producers such as the venturi, orifice, and nozzle) together with the secondary element (dP measurement instrument).
The synergy of software, electronics, and primary element allows the end user to save on installation costs and improve reliability. Enhanced dP flowmeters allow compensation for the dynamic variation of process conditions and primary element flow coefficients.
The reliability of a flow measurement point can directly correlate to the length of impulse tubing used. The dP generated by a primary element must effectively convey to the sensing apparatus of a secondary element for accurate measurement.
Following direct mounting practices can minimize errors due to plugging, process leaks, and inequalities in the hydrostatic head of impulse tubes while lowering the total installed cost of the measurement point.
Plugging often occurs in steam application impulse tubing as a result of flashing. Flashing is the phenomenon in which a fluid quickly changes from a liquid to a gaseous state. Precipitate, composed of suspended solids found in steam, can adhere to the walls of an enclosure when flashing occurs.
Consistent flashing causes enough precipitate accumulation to plug the impulse tubing. A plugged impulse tube cannot communicate the dP generated by a primary element and results in inaccurate measurement.
How the primary element connects to the secondary element influences the potential for flashing in a given application. Flashing is common in remote mount applications with long impulse tubes. Temperature fluctuations along the length of the impulse tubes cause the fluid to change state continuously, forming layers of precipitate that ultimately result in plugged tubing.
Direct mounting produces a fixed transition point from the gaseous state to the liquid state. The geometric and thermal transfer characteristics of direct mount assemblies result in the fixed transition. This fixed transition point eliminates the opportunity for flashing and the plugging commonly associated with it.
Solid accumulation is another common cause of impulse tube blockage. Solids in a flow stream, being heavier than the surrounding fluid, have a tendency to accumulate at the lowest point in an impulse tube arrangement.
Impulse tubing often plumbs around and over obstacles, resulting in many areas of low potential energy where solids can collect. Direct mounting reduces elbows and traps where solids could accumulate, eliminating the potential for precipitate blockage.
Direct mounting avoids issues
Process leaks in impulse tubing result in erroneous dP readings and can have a significant effect on flow measurement accuracy. The potential for a leak in a dP flow measurement system is directly proportional to the number of connections in the system.
The integration of the electronics and differential producer through direct mounting eliminates fittings, tubing, valves, manifolds, and adapters, resulting in a significant reduction in potential leak points.
Variation in the hydrostatic head in impulse tubes can introduce a bias to the dP generated by a primary element. These variations in hydrostatic head are the result of temperature/pressure variation and geometric inconsistencies in the impulse tubing.
One must install impulse tubes carefully and properly insulate them from the environment to prevent serious accuracy issues. These issues become acute at flow rates that generate low dP signals.
Direct mounting ensures consistent impulse tube geometry and reduces the potential for temperature variation due to the close proximity of high and low side impulse tubes.
Dump hardware and costs
Integrated flowmeters offer significant savings over traditional installation configurations. These integrated flowmeters are innovative products consisting of a primary element, a secondary element, and connection accessories. When ordering, the user specifies a single model code only.
Typically, the supplier preassembles, precalibrates, and leak tests these products. Cost savings from fully integrated solutions come from a number of areas, including specification, procurement, material, and installation.
Total installed cost comparison
Traditional measurement requires the separate specification of differential, gauge, and temperature transmitters and a primary element, impulse lines, mounting hardware, RTD, thermowell, electrical wiring and conduit, and flow computer.
When using integrated meters, a single model code defines the primary element, secondary element, and connection accessories.
Labor in procuring measurement instruments is a key cost commonly overlooked. Savings occur during requests for quote and purchase order generation, due to the single model code. Receiving costs are lower, due to reduction in paperwork and model codes to check against product received.
Using integrated solutions also reduces material costs. Suppliers will typically offer a price break on the discrete components of the system to ensure single source ordering. The cost of the transmitter and primary element often accounts for only a fraction of the total cost of the flow measurement system.
Depending on the primary element selected, additional cost for flanges, mounting hardware, isolation valves, and plumbing are necessary to complete the system.
The labor cost associated with installing an integrated meter can be significantly lower than that of a traditional installation. The transmitter, manifold, and primary element arrive as an assembly.
Impulse tubing, root valves, mounting brackets, and interconnections are all unnecessary. Preconfigured units apply directly to the process—an especially important fact in light of emerging multivariable technologies.
Perform complex real time
The new generation of multivariable flow transmitters allows simultaneous measurement of dP, static pressure, and temperature. Traditionally, a flow computer or a distributed control system (DCS) calculated the dP flow using a simplified mass flow equation.
This practice does not produce the most accurate flow measurement possible, due to the fact that it compensates for changes in fluid density only. State-of-the-art multivariable transmitters perform complex real-time calculations to compensate for coefficient variation in the flow equation.
A simplified flow equation cannot compensate for changes in all the terms that relate to mass flow; thus, there are uncaptured errors in the calculated flow rate. Compensating for changes in discharge coefficient, velocity of approach factor, gas expansion factor, differential producer bore, and density can minimize flow measurement uncertainty.
The discharge coefficient is the ratio of theoretical flow rate to actual flow rate. Many primary elements have discharge coefficients that vary depending on the velocity profile of the flow stream. The velocity profile relates to a unitless value called the Reynolds number, which can be calculated in real time with a multivariable transmitter allowing dynamic discharge coefficient compensation.
Discharge coefficients tend to change most at the low Reynolds numbers most often found in liquid applications. So despite the incompressible nature of liquids, multivariable technology allows significant accuracy improvement over traditional methods.
As the area available to a fluid traveling through a pipe changes, the velocity of the fluid must change. A primary element is a tightly toleranced restriction associated with a collection of empirical data that relates the dP generated across it to fluid velocity. Process temperature affects the restriction presented to the flow stream by the primary element. The velocity of approach factor compensates for geometric variation due to temperature effects.
When a compressible fluid flows past a primary element, a change in density accompanies the velocity change. The gas expansion factor applies to compensate for this change.
The expansion factor also compensates for small changes in the internal energy of the fluid due to the temperature difference between the upstream and downstream ports of a primary element.
Process temperature variation causes thermal expansion and contraction in the pipe and primary element. The change in geometry of the system impacts the blockage the primary element presents to the flow stream. Dynamic calculation of blockage eliminates any error changing geometry can introduce.
Multivariable technology improves measurement accuracy in liquids and gases by compensating for changes in flow equation coefficients. Performing compensation in the transmitter saves system resources and reduces the programming expertise required to configure a flow measurement point. IT
Gregory Strom works as an application engineer at Dieterich Standard. Gregory Livelli is the manager of worldwide pressure marketing at Rosemount, Inc.
Real-world accuracy can be significantly worse
|Accurate dP measurement is a prerequisite to accurate flow measurement. Many factors adversely impact the installed accuracy of a measurement point, including ambient temperature variation, high static pressures, and the frequency of calibration.
All transmitters are not equal with respect to resistance to error introduction from these factors. Reference accuracy is not the absolute measure of an instrument's ability to make a measurement.
Even with a well-installed, well-maintained transmitter, real-world accuracy can be worse than lab accuracy because real-world transmitters don't install or operate under lab conditions. Real-world effects may include the following:
Reputable suppliers publish specifications that let users calculate and predict the impact of "real world" effects on installed flow accuracy and repeatability. These flow errors are for three different transmitters taken at 100% flow. The errors are due to typical installed conditions. Reference accuracy contributes a trivial component of total installed error.
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