TECHNICAL AND MARKETING SERVICES
FOR INSTRUMENTATION SUPPLIERS AND END-USERS
Differential Pressure Transmitter Performance (Part 3 of 3) by David W Spitzer
Differential pressure flow transmitter performance is affected by process pressure,
ambient temperature, and drift (stability).
Process pressure effects are expressed as a percentage of set span per unit
pressure change. As such, the pressure effects for all of the (hypothetical) differential pressure
transmitters are the same.
It should be noted that the ambient temperature effects can contribute more error to the
measurement than accuracy. For example, according to The
Consumer Guide to Differential Pressure Flow Transmitters, the contributions of accuracy and a 25 degC
ambient temperature variation for the best transmitter with a set span of 250 mbar (100 inch WC) are 0.1250
and 0.1002 mbar respectively. The temperature effect of almost all of the other transmitters is greater
than the error associated with their respective accuracy specification. Further, these seemingly small
fixed errors tend to produce relatively large flow errors in the lower portion of the flow range due
to the nonlinear nature of the Venturi flow element.
Ambient temperature effect specifications are generally expressed as a percentage of
URL per unit temperature plus a percentage of set span per unit temperature. The ambient temperature
effect of the 1000 inch WC (hypothetical) transmitter is larger than that of the URL of the 250 inch WC
(hypothetical) transmitter because its URL is larger. Therefore, when temperature effects are considered
for a 150 inch WC range, the performance of the 250 inch WC transmitter is superior to that of the 1000
inch WC transmitter --- even though their accuracy specifications are the same.
Stability specifications are generally expressed as a percentage of URL per unit time.
The error associated with the stability of the 1000 inch WC (hypothetical) transmitter is four (4) times
larger than that of the URL of the 250 inch WC (hypothetical) transmitter because its URL is four (4) times
larger. Therefore, when stability is considered for a 150 inch WC range, the performance of the 250 inch
WC transmitter is superior to that of the 1000 inch WC transmitter --- even though their stability
specifications are expressed identically (for example, 0.2% URL for 12 months) and their accuracy
specifications are the same.
The overall result of this analysis shows that a "one-size fits all" strategy based
solely on accuracy specifications is flawed due to the existence of other influences on differential
pressure transmitter performance.
Part 3: Measuring Downward Flow: Considerations for Accuracy and Vibration by David W Spitzer
We continue the discussion of three important situations to carefully consider: operation under vacuum, measuring downward liquid flow and gravity flow. Operating under vacuum was discussed in the previous two columns, so let's move on to measuring downward flow.
When selecting a flowmeter for liquid service, it is common to consider its upstream pressure, downstream pressure and the pressure drop to ensure that the flowmeter will pass the design flow in the process piping system. Interestingly, under no flow conditions in downward flow, the pressure actually increases through the flowmeter as a result of static pressure due to decreasing elevation. This pressure increase gets smaller as flow is increased and may eventually become a pressure drop at higher flow rates when the pressure drop across the flowmeter becomes larger than the static pressure increase due to elevation.
It is not clear how this pressure rise might affect flowmeters --- if at all. However, some years ago, I was charged with fixing a particular vortex shedding flowmeter installed in downward flow that consistently differed from process calculations by approximately 10 percent since its installation approximately a decade prior. This particular flowmeter design had exhibited problems in other plant locations subject to high vibration, but vibration was not an issue in this location and similar vortex shedding flowmeters of the same design not subject to vibration performed accurately. Being pragmatic, we purchased a new vortex shedding flowmeter of different design, but installed it with upward flow, and it performed accurately. Given the potential vibration problem, it is not clear whether this measurement issue can definitively be assigned to downward flow. Reinstalling the existing flowmeter in upward flow would have reduced this doubt but might have compromised plant operation.
Which of the following flowmeters can measure 0 to 100 kg/min of liquid flow?
Answer A is the straightforward answer to the literal question because kg/min is a mass flow, and Coriolis mass is the only flowmeter technology in the list that measures mass flow.
A less literal interpretation of the question would focus answers on what the flowmeter displays --- not on what the flowmeter actually measures. In this interpretation, all of the flowmeters listed can display the flow in kg/min. However, with the exception of Coriolis mass, displaying the flow will typically be the result of a calculation based upon a fixed density of the liquid.
Additional Complicating Factors
The measurement of some fluids may be traditionally displayed in certain units. For example, reactor feed flows are often expressed in kg/min or lb/min --- independent of what the actual flowmeter measures. This can be confusing, so we displayed mass units in our plant when the flowmeter actually measured mass flow, except where a strong precedent applied, such as steam flow.
In addition to over 40 years of experience as an instrument user, consultant and expert witness, David W Spitzer has written over 10 books and 450 articles about flow measurement, level measurement, instrumentation and process control. David teaches his flow measurement seminars in both English and Portuguese.
Spitzer and Boyes, LLC provides engineering, technical writing, training seminars, strategic marketing consulting and expert witness services worldwide.
Copyright 2020 Spitzer and Boyes, LLC
The content of this message is protected by copyright and trademark laws under U.S. and international law. All rights reserved.