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Differential Pressure Transmitter Performance (Part 2 of 3) by David W Spitzer
Microprocessor-based differential pressure flow transmitters are generally more accurate
and exhibit a larger turndown than analog differential pressure flow transmitters. As a result, fewer
models (capsule sizes) are needed to measure the applicable differential pressure ranges. For example,
2 or 3 microprocessor-based transmitter models may be needed to measure over the same differential
pressure ranges handled by 5 or 6 (older) analog transmitter models. As result, users could reduce their
spare parts inventory costs while suppliers benefited by the savings resulting from the design and support
of fewer models. From a performance perspective, transmitter accuracy specifications for each model
generally apply over a range of set spans.
For example, a (hypothetical) differential pressure transmitter may have an accuracy
specification of 0.10% of set span when the span is set between 10 and 100 percent of its upper range
limit (URL). Assume also that the (hypothetical) differential pressure transmitter is available with
URLs of 1000, 250 and 50 inches WC.
With these transmitter offerings, a 150 inch WC range can be measured accurately
using the 250 or 1000 inch WC models. The set span of the 150 inch WC range is 15 percent of the
URL of the 1000 inch WC transmitter and 60 percent of the URL of the 250 inch WC transmitter. The
span of both transmitters is between the stipulated 10 and 100 percent of URL over which the accuracy
statement applies, so both transmitters have the same accuracy specification.
As a result of this analysis, many suppliers and users applied microprocessor-based
differential pressure flow transmitters using a "one-size fits all" design philosophy. For example,
the following table illustrates the range selection strategy for this (hypothetical) transmitter
Differential Pressure Range
100 - 1000 inch WC
1000 inch WC
50 - 100 inch WC
250 inch WC
under 50 inch WC
50 inch WC
Sizing flow elements to avoid differential pressure ranges between 50 and 100 inch WC
will eliminate the need for 250 inch WC transmitters.
All may seem well, but Part 3 will discuss the effects of pressure,
temperature and stability on transmitter performance.
Part 2: Measuring Under Vacuum: The Phenomenon of Reverse Flow by David W Spitzer
Last month, I mentioned three situations that one should carefully consider: operation under vacuum, measuring downward liquid flow and gravity flow. Let's continue to discuss operating under vacuum.
Liquids exhibit a pressure loss and subsequent recovery as they flow through an obstruction in the piping, including many flowmeters. Given operation that is often at or near the vapor pressure, it is possible for the liquid to form bubbles when pressure is reduced, only to implode during pressure recovery. This cavitation can catastrophically damage flowmeters and other equipment, so flowmeter selection should be performed carefully. Selection, sizing and specification of control valves (and their internals) should likewise be carefully performed.
In general, flowmeters should be selected and sized to have a low pressure drop. However, selecting, for example, a full-bore magnetic flowmeter with no piping obstruction would comply with this preference, but particular models may not be acceptable because their liners may not suitable for vacuum service. Therefore, it is common to specify flowmeters and other instruments for vacuum service to operate between -1 to 6 bar --- even when the process does not operate under pressure --- to ensure that they have the mechanical integrity to operate in a full vacuum.
Reverse flow is another phenomenon that can occur in some processes operating under vacuum. Normally, liquid will flow downward by gravity. However, when a vacuum is applied to a vessel, liquid may flow upward from below (like drinking through a straw). This can cause strange occurrences, such as when a flowmeter measures reverse flow while a process is not operating and one would think that there should be no flow. Careful attention should be given to whether a flowmeter in a given application should measure reverse flow --- or not.
There are many other idiosyncrasies associated with measuring under vacuum, but we will discuss measuring downward flow next month.
Centrifugal Pump Speed Reduction by David W Spitzer
Various reference materials contain information describing the Affinity Laws associated with reducing the speed of a centrifugal pump. Mathematically, they show that the:
Pump flow is proportional to pump speed
Pump discharge pressure is proportional to the square of pump speed
Mechanical load on the pump is proportional to the cube of pump speed
What are the approximate pump speed, pressure and mechanical load at 50 percent flow?
Using the information given, 50 percent flow would correspond to 50 percent speed, 25 percent pressure and 12.5 percent mechanical load. Note that the mechanical load is closely related to electrical energy consumption.
Of significant interest is that the electrical energy consumption would be reduced by approximately 87.5 percent at half flow. Similar calculations show that electrical energy consumption would be approximately 99.9 percent less at 10 percent flow.
Additional Complicating Factors
Not so fast --- pumps almost always pump uphill, so the pump has to overcome the static head of the piping system in order for the liquid to start to flow. If the static pressure needed to start flow is 64 percent, the pump must operate at 80 percent speed just to start flow --- at which point its electrical energy consumption would be approximately 50 percent to produce zero 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.
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