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Vortex Shedding and Fluidic Flowmeter Applications (Part 3 of 4) by David W Spitzer and Walt Boyes
All-Welded Vortex Shedding and Fluidic Flowmeters Many vortex shedding and fluidic flowmeters are assembled using seals, O-ring or gaskets to prevent the fluid from leaking out of the flowmeter. In contrast, all-welded flowmeter designs reduce the number of potential leak paths by not using any seals, O-rings or gaskets. In doing so, leak paths are generally limited to the piping connections and welds. All-welded designs are generally preferred when a potential hazard can be created by fluid leakage.
Low Flow Vortex Shedding and Fluidic Flowmeters
At low flow rates, achieving sufficient Reynolds number to operate vortex shedding and fluidic flowmeters can be an issue. Some vortex shedding and fluidic flowmeters are designed can measure these relatively low flows in pipes that are generally under 15 mm (0.5 inch).
High Flow Vortex Shedding and Fluidic Flowmeters
At high flow rates, the frequency of oscillation of vortex shedding and fluidic flowmeters becomes so low that it can reduce response time and become an issue for the flowmeter electronics. Some vortex shedding and fluidic flowmeters are designed to measure flow in pipes that are 250 mm (10 inch) and larger.
Metal (Other Than CS/316SS) Vortex Shedding and Fluidic Flowmeters
Due to the cost and general applicability of carbon steel and 316SS, most metal vortex shedding and fluidic flowmeters were designed and are constructed using these materials of construction. However, to increase the applicability of the technology, vortex shedding and fluidic flowmeters can be constructed using other metals, most notably Hastelloy. Note that some vortex shedding and fluidic flowmeters have seals that are constructed from non-metallic materials. Therefore, the availability of a vortex shedding and fluidic flowmeter made of materials other than carbon steel and 316SS does not necessarily imply that the seals are (or can be) constructed of a material that is compatible with the process.
Excepted from The Consumer Guide to Vortex Shedding and Fluidic Flowmeters.
Qualifications Are Questioned by David W Spitzer
Previous articles described the sewage collection systems for two adjacent sewage districts where the flow measurements used to allocate expenses and bill one of the sewage districts flowmeters was questioned in court.
The first sewage district went to court to contest invoices from the second sewage district as not being accurate. Based upon a previous court proceeding, the performance of four flowmeters measuring sewage from the first district and the total plant flowmeter were considered by the court to be accurate and could not be discussed or questioned.
Remember that this was my first time testifying in court. However, I was not prepared for the reality presented to me - even after having seen my share of courtroom scenes in films over the years where witnesses swore to tell the "truth, the whole truth and nothing but the truth" and justice prevailed in the end. In this case, it seemed that my testimony would be futile given the legal restrictions placed on my testimony - even though there were valid technical points to be made.
An omen of what was to come occurred as I approached the witness stand. The judge said something to the effect that you could pay an expert to say anything. The judge's words were duly reported by the court reporter before we proceeded to the next part of the case where the second district tried to exclude my testimony on the grounds that I was not qualified to testify. This is a normal procedure that is unpleasant at best and generally not shown in films, so I was somewhat taken aback. The challenge failed, so I could proceed to testify.
More next month about my testimony.
This article originally appeared in P. I. Process Instrumentation magazine.
Calibrating a Pressure Transmitter for Steam Header by David W Spitzer
A pressure transmitter with a span of 300 psi will be mounted on a pipe stand for convenient access to measure the pressure of a nominal 200# steam header that is located approximately 25 feet above grade. What is the approximate bench calibration of the pressure transmitter?
0 to 300 psig
0 to 300 psia
10 to 310 psig
10 to 310 psia
Steam header pressures in this range are traditionally measured and expressed as gauge pressures, so Answer B and Answer D would likely not be correct.
In operation, steam will condense and form a condensate leg in the impulse tube. The transmitter should be calibrated to take this leg of liquid into account to accurately measure the steam header pressure - not the pressure at the transmitter tap.
The steam header is approximately 25 feet above grade. Transmitters are typically mounted approximately 4.5 feet above grade. Therefore, the transmitter calibration at zero steam header pressure should account for a fixed condensate leg of approximately 20 feet, or approximately 10 psi where 2 feet of water column roughly corresponds to 1 psi. Answer C is correct.
Additional Complicating Factors
Mechanical factors can come into play when the condensate leg is not approximately 20 feet, such as can occur when the transmitter is installed at a different height above grade and/or when the pressure tap is located on the top of the pipe that would increase the height of the condensate leg.
In addition, the density of the condensate leg can vary with temperature and affect the accuracy of the pressure measurement.
This article originally appeared in P. I. Process Instrumentation magazine.
ABOUT SPITZER AND BOYES, LLC
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 500 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.
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