TECHNICAL AND MARKETING SERVICES
FOR INSTRUMENTATION SUPPLIERS AND END-USERS
Differential Pressure Transmitter Performance (Part 1 of 3) by David W Spitzer
Differential pressure flow transmitters are used in conjunction with various
primary flow elements to measure the amount of fluid transferred in pipes. In general, the literature
can leave one with the impression that most of the flow measurement errors result from the primary flow
element. Which device introduces more error --- a Venturi flow element with an accuracy of 0.50% or a
differential pressure flow transmitter with an accuracy of 0.10%? If you answered the question, your
answer is likely incorrect.
For starters, you may have noticed that the question itself is incomplete. The
percentage associated with the accuracy of the Venturi is a percentage of the actual flow rate while
making the assumption that the Venturi is hydraulically operated in the properly manner. The percentage
associated with the differential pressure flow transmitter can be a combination of a percentage of set
span (typically full scale), calibrated span, and upper range limit. Therefore, these percentages are
percentages of different quantities, so they cannot be directly compared. In other words, 0.10% is not
necessarily better than 0.50%.
Because the differential pressure flow transmitter error is a combination of fixed
percentages of fixed values, the transmitter error can be calculated and is also fixed. This transmitter
error can be expressed as a percentage of the flow rate that is not fixed, but rather increases as
the flow rate decreases. Note that the flow error calculation is complicated by the nonlinear
relationship between flow rate and the differential pressure produced by the Venturi, so the
transmitter error expressed as a percentage of flow rate is different at each flow rate.
Therefore, at a given flow rate, the calculated transmitter error can be
compared with the Venturi error. In general, the error associated with the Venturi tends
to dominate high in the flow range while the transmitter error tends to dominate lower in the range.
Further, the contribution of the transmitter can be surprising large. For example, according to The Consumer Guide to Differential Pressure Flow Transmitters, the flow error associated with the best
differential pressure transmitter with a 0-250 mbar (0-100 inch WC) span is tabulated below that
illustrates the increasing domination of the transmitter error as flow decreases.
50 % of full scale flow
0.23 % of flow rate
30 % of full scale flow
0.64 % of flow rate
10 % of full scale flow
5.90 % of flow rate
Part 2 will discuss transmitter accuracy in more detail.
Part 1: Measuring Under Vacuum: Feeling the Pressure by David W Spitzer
Three situations come to mind that most technical people should carefully consider: operation under vacuum, measuring downward liquid flow and gravity flow. These situations may not be routinely encountered by the majority of practitioners, but they can present significant issues that are not readily apparent. Of course, if you deal with these situations on a regular basis, you will likely be comfortable with proceeding at normal speed.
The vapor pressure of a liquid is the pressure at which that liquid will boil. Operating under vacuum conditions --- below atmospheric pressure --- can cause ordinary liquids such as water to boil at room temperature. Processes often use this phenomenon to an advantage when drying heat-sensitive products by removing, say, water under a vacuum at room temperature instead of potentially degrading the product by heating it to over 100 degC under atmospheric conditions.
Systems operating under a vacuum are more likely than a pressurized system to inadvertently operate below the vapor pressure of the liquid. This is because liquids in many process vessels in vacuum processes operate at or near their vapor pressure such that a small pressure reduction, such as piping losses, or a small temperature increase can cause the liquid to boil.
The installation of every instrument in vacuum service should be analyzed in detail to determine if a problem could occur. For example, measuring the level of a tank operating below atmospheric pressure using a level probe installed in an external stilling well might seem normal. However, ambient conditions may make the stilling well slightly warmer than the liquid in the tank --- operating at the vapor pressure of the liquid --- and cause the liquid in the stilling well to boil, adversely affecting the level measurement.
What is the rule of thumb for the amount of straight run required upstream and downstream of a flowmeter?
0 diameters upstream and 0 diameters downstream
5 diameters upstream and 3 diameters downstream
10 diameters upstream and 5 diameters downstream
40 diameters upstream and 5 diameters downstream
Responses from students in my Industrial Flow Measurement seminar (and many others) would indicate that the rule of thumb for installation is 10 diameters of straight run upstream of the flowmeter and 5 diameters of straight run downstream of the flowmeter so it would seem that Answer C would be correct.
However, there is no valid rule of thumb for installation because every flowmeter has straight-run requirements that result from multiple considerations, including the flowmeter technology, flowmeter design and the geometry of the piping in which the flowmeter is installed.
All of the answers can apply to one or more flow technologies and/or flowmeter designs.
Additional Complicating Factors
The ramifications associated with not having one rule of thumb are many. Consider that many flowmeters are installed in locations where limited space does not allow for the required amount of straight run. In such instances, flowmeter performance would be degraded so another technology and/or flowmeter with shorter straight-run requirements should be applied.
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.