| Introduction
There seems to be a trend of different types
of flow meters not performing correctly in Water
and Wastewater plants. This paper will try to
explain some of the causes and ways of correcting
certain problems. In my previous ISA article
titled Understanding Flow Metering Specifications,
the installation, turndown, repeatability, and
velocity were all important components for a
magnetic flow meter’s performance and
accuracy. For this particular article I will
address some misconceptions, truths, and poor
standard practices associated with venturi flow
meters.
Piping Configuration
A common problem which can cause errors in
accuracy is improper piping installations. Many
consultants will design a treatment plant or
a reclaim water application utilizing a venturi
tube. In most cases, due to the constraints
of the area in which a water treatment or a
water reclamation plants are generally designed,
piping laying length considerations normally
seem less than ideal. This is especially true
with regards to older plants when expanding
the plant with new process tanks, mechanical
equipment, and piping is required. The consultant
will generally try to locate the flow meter
where it is best possibly suited. The consultants
are generally at an understanding that the venturi
flow meters are very accurate. One of the misconceptions
is the consultants and end users also believe
that the flow certification document from the
flow laboratory is directly related to their
flow meter installed design. Warning, the flow
certificate document can unintentionally be
misleading.
A venturi tube is sensitive to flow profiles
due to the actual design of the tube. In many
cases venturi tubes are always surrounded with
piping elbows, tees, piping increasers, piping
reducers, and valves. All of these have significant
impacts depending on what distance they are
located upstream and downstream of the flow
meter in regards to accuracy.
Accuracy Statement
A venturi manufacturer accuracy statement
is usually generated from a tested result provided
from a flow laboratory. The misconception is
that the end user doesn’t realize that
the flow laboratory will usually install the
flow tube in a smooth pipe at an ideal condition
where upstream and downstream flow constraints
will not impede a proper flow profile. The truth
is that the flow meter that was installed and
tested at the flow laboratory may not mimic
the actual installed condition required at the
plant.
Another truth is the flow laboratory’s
test procedure is not documented well enough
for the consultant or end user to verify if
the flow meter had matched the required installed
condition for the plant. This, in my opinion,
leaves in doubt the test result and the certification
of that flow tube’s flow coefficient from
the manufacturer. This problem can easily be
avoided if the consultant when designing the
system gives the manufacturer actual laying
lengths of upstream and downstream conditions,
pipe size, pipe material, minimum and maximum
design flow rates, and the type of solution
flowing in the pipe.
Another recommendation to consultants and
end users, while the plant is in the design
phase, ask the manufacturer to review the contract
specifications for the flow meter.
The venturi tube material and installation
may not necessarily be correct for that design
application. Remember that the accuracy statement
along with the coefficient number will change
on that tube depending on its location and how
the flow profile looks.
Let’s look at a misconception in how
using the Reynolds’s Number alone, without
adding to the equation the relative roughness
of the pipe, can give you an ideal Reynolds
Number.
You have been asked to provide by a project
manager an 18”influent venturi flow meter
for a wastewater treatment plant. The minimum
flow rate that needs to be monitored is 500gpm
and the maximum is 5000gpm. In addition, the
flow meter is installed on ductile pipe.
After crunching out the Reynolds number for
minimum and maximum flows you find at minimum
flow your number yields approximately 88,000
and your maximum flow produces a number of 887,000.
The problem is that although at minimum flow
the Reynolds number indicates greater than 75,000,
you also find that the velocity is only a little
greater than .6ft/sec. The use of the Reynolds
Number alone did not give a proper evaluation
for the ability of reading and measuring with
accuracy the minimum flow. As earlier stated
the flow laboratory may be using smooth pipe
in its testing procedure and in your design,
you may be using ductile pipe. The truth is
that piping material matters greatly when determining
turbulent conditions in pipes. This is very
well depicted in hydraulic books that show the
Moody diagram which actually shows the relationship
of relative roughness to flow conditions. My
recommendation is to use the relative roughness
equation in your calculations when using the
Reynolds Number. In my experience this gets
over looked as being insignificant and so consultants
and end users must be careful. These types of
meters are used for various flow control applications
like chemical dosing, where accuracy does become
critical.
Turndown Capability
This also brings up a misconception issue
with regards to the differential pressure transmitter
with stated 200:1 turn downs. The truth is that
you can not assume that you can measure differential
at 1/200th “H2O. The transmitter may be
accurate on a test stand but the physical hydraulic
constraints of the installed venturi flow meter
with respect to velocity may not always let
you measure that low of a differential pressure
with high accuracy. Typically a venturi tube
will have at best an 8:1 turndown. The manufacturer
who publishes a 200:1 turndown on differential
pressure transmitters are not distinguishing
that the transmitter is on a venturi tube and
neither should the consultant or end user. The
manufacturer’s accuracy statement on these
type of transmitters is also not based on actual
field conditions of any given venturi tube.
The 200:1 turndown does not have any real relationship
with the 8: 1 turn down capability of the venturi
flow meter. In addition, if any manufacturer
suggests that two transmitters on a venturi
tube will give the end user a higher accuracy
performance, the consultant or end user may
what to review actual installation and flow
requirements.
Field Calibration and Inspection Ports
In some cases there are calibration procedures
for venturi tubes being misapplied. The misconception
that the end user will get a high accuracy performance
from a digital manometer as a calibration device
for venturi tubes is erroneous. The digital
manometer is not a proper calibration device
because it only looks at the differential pressure
transmitter diaphragms and not the flow tube.
A plant operator calibrating the flow meter
generally needs to zero out the differential
pressure transmitter to produce (4madc). This
is generally done by allowing the 3 or 5-way
manifold to be equalized at both the high and
low side. If zero is not present than that person
will adjust the span until they get a zero reading.
Now what that person just did is to unknowingly
eliminate the error which could be caused by
the displacement of the diaphragms. The person
has no idea at that point if the diaphragm is
at a factory standard tolerance or if any of
the diaphragms should be replaced. What also
is a problem, there isn’t any way of knowing
the true accuracy of the flow system.
In my own documented experiences seeing actual
field conditions of venturi tubes, when a plant
is using a digital manometer, there are generally
other errors associated with that flow meter’s
installation. For instance, the proper manometer
to check the tube itself is not purchased or
used; there is no flushing or rodding out connections
for the high and low static pipes, no manometer
or flushing taps present. The static tubing
from the flow tube to the differential pressure
transmitter may not to be rigid tubing but flexible
polyethylene tubing. This flexible tubing can
allow vibration to occur allowing additional
errors to be created. At this time I do not
know of any published documentation from any
manufacturer of transmitters showing an end
user how to detect that the degree of displacement
in the diaphragm at rest is out of factory tolerances
for its stated accuracy. This problem needs
to be addressed by the manufacturers either
in their operation manuals or at their training
seminars.
I’m seeing more of these digital manometers
being used for calibration purposes in various
plants as an acceptable standard practice. In
my opinion the use of a digital manometer is
a poor standard practice as a recommended calibration
tool for a venturi flow meter. The flow meter
manufacturers should play a more pro-active
role in resolving this problem. The manufacturers
should remember that end users are relying on
their expertise to help in assuring that proper
procedures associated with their equipment are
in compliance with the end user’s needs.
The flow tube, if correctly tested for actual
installed condition, should have a proper flow
coefficient and a calibrated differential vs.
flow curve assigned to that tube. The truth
is without checking the actual differential
pressure using a manometer (mercury or oil filled)
against the documented flow curve; a plant operator
would have no idea if there is an actual problem
with the flow tube itself. Problems such as
the static pipes (high and low) could be clogged,
coating on the inside lining of the tube could
be present. Coating could be from buildup due
to tuberculation or sludge build up. Most pipe
buildups occur due to solids falling out of
suspension from low velocity flow conditions.
I would like to recommend more than one inspection
port for a venturi flow tube, one inspection
port at the inlet and the other inspection port
at the outlet of the flow meter. Typically the
highest scouring velocity is located at the
throat of the venturi tube (where the low tap
side is located) which would generally mean
that tuberculation or sludge buildup at the
throat would not be present. However this would
not be true for the inlet section where the
high tap is located. If an inspection port was
located upstream and downstream near the flow
meter, an operator could inspect and clean more
frequently the inside of the flow tube and just
as critical, the pipe.
Combined Electrical and Process Equipment
Panel with Pressurized Water Piping
In regards to good standard practices, I have
been alarmed at seeing dangerous installation
procedures. The one that stands out mostly is
the installation of high service pumps with
the use of a effluent distribution pressure
transmitter and a high and low pressure switch
mounted in a stainless steel panel with pressurized
tubing, a power supply to the pressure transmitter,
and remote power sources wired to terminal blocks
for the pressure switches.
There is a probable chance that the pressurized
tubing could leak in the panel. Water spraying
inside any panel which has circuit breakers,
power supplies, or remote power sources wired
to terminal blocks is extremely dangerous.
My recommendation is for end users to investigate
their plants to see if these types of panels
exist and to immediately take a proactive role
to correct these installations. The end users
should move all transmitters and pressure switches
that are attached to pressurized tubing out
of panels which have power and wired terminal
blocks. The Transmitter, pressure switches,
and pressure lines can be mounted on a field
rack. If the field rack is required to be located
outdoors, provide sunshields. A separate electrical
panel should be provided that allows water tight
conduit and wire to the process equipment. Now
if the pressurized tubing leaks it should not
have any hazardous effect.
Summary
I would like to recap the following critical
points:
- Installed piping constraints play a large
role in the accuracy of the flow meter.
- The Reynolds’s Number alone can not
be the final determiner for evaluating the
accuracies of minimum and maximum flow requirements.
- Use the relative roughness equation with
your Reynolds Number evaluation.
- Understand the actual relationship of a
turndown ratio.
- Good calibration techniques with periodic
maintenance equates to a high accuracy condition.
- Do not mix water and electricity together.
In my many years of dealing with venturi manufacturers
I found them very helpful during the early design
phases of my projects. They will generally review
drawings and contract specifications. As stated
before if you give them the minimum and maximum
flow requirements, a drawing showing the installed
flow meter with the upstream and downstream
conditions, piping material, pipe size, and
type of solution flowing in the pipe they will
give you a flow meter that will actually fit
your installed requirements. Generally if you
give the manufacturer too many unknowns, they
will return the favor and let you know that
the actual accuracy will be unknown. It is generally
not a good idea to wait for startup construction
service to begin before you find out that the
flow meter may not be suited for the application.
John Davis is an Senior Member of ISA who Operates
Davis Instrumentation Services.
His e-mail address is Nira47@aol.com
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