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| David W. Spitzer, P.E.
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To verify the
approximate operation of a flowmeter, it may be desirable to calculate
the amount of fluid in a vessel before and after fluid flows into (or
out of) the vessel. Comparing the calculations with the totalized flow
can provide a check on the performance of the flowmeter.
Consider checking a flowmeter that is piped to fill a sump
measuring six feet long by nine feet wide and 12 feet deep. The liquid
level is three feet before water is pumped into the sump. When pumping
is complete, the liquid level is nine feet high. Approximately what
flow should be totalized on the flowmeter?
A. 1,212
B. 1,818
C. 2,424
D. 3,635
Commentary
It should be
understood that the validity of this checking technique is dependent
upon liquid being only added to the sump. Any liquid that leaves the
sump during this procedure will result in error because a
lower-than-actual volume will be calculated. To ensure there is no flow
out of the sump, the sump should be checked for leaks and the sump pump
should not be allowed to operate during the test period.
Before the water is
added, the sump contains 6x9x3, or 162 cubic feet of water. After the
water is added, the sump contains 6x9x9, or 486 cubic feet of water.
The difference of 486-162, or 324 cubic feet of water was added to the
sump. Because each cubic foot contains 7.48 gallons, a volume of 324
cubic feet contains 324x7.48, or 2,424 gallons (Answer C).
If this type of calculation is needed, you might consider using
metric measurements. Consider a similar size sump measured to be two
meters long by three meters wide and four meters high, with sump levels
of one meter and three meters before and after filling, respectively.
Before the water is added, the sump contains 2x3x1, or six cubic meters
of water. After the water is added, the sump contains 2x3x3, or 18
cubic meters of water. The difference of 18-6, or 12 cubic meters of
water was added to the sump. Because each cubic meter contains 1,000
liters, a volume of 18 cubic meters contains 18x1,000, or 18,000
liters.
The point of using metric measurements is not a preference of one
over the other, but rather the ease with which some of the calculations
are preformed — in particular, the conversion from volume to gallons or
liters. And how much does the water in each of these examples weigh? To
convert gallons to pounds, multiply by 8.34. To convert liters to
kilograms, multiply by one. The latter is less taxing on my mind.
Additional Complicating Factors
Additional measurement and calculations would be needed if water were flowing out of the sump while the sump was being filled.
David W. Spitzer, P.E.,
is a regular contributor to Flow Control. He has more than 30 years of
experience in specifying, building, installing, startup, and
troubleshooting process control instrumentation. He has developed and
taught seminars for over 20 years and is a member of ISA and belongs to
the ASME MFC and ISO TC30 committees. Mr. Spitzer has written a number
of books concerning the application and use of fluid handling
technology, including the popular "Consumer Guide" series, which
compares flowmeters by supplier. Mr. Spitzer is currently a principal
in Spitzer and Boyes LLC, offering engineering, product development,
marketing, and distribution consulting for manufacturing and automation
companies. He can be reached 845 623-1830.
www.spitzerandboyes.com
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