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Wednesday, October 1, 2008

4.FLOW MEASUREMENT






HIGH PRESSURE VENTURI TUBE














Flow Detectors

To measure the rate of flow by the differential pressure method, some form
of restriction is placed in the pipeline to create a pressure drop. Since flow in
the pipe must pass through a reduced area, the pressure before the restriction
is higher than after or downstream. Such a reduction in pressure will cause
an increase in the fluid velocity because the same amount of flow must take
place before the restriction as after it. Velocity will vary directly with the
flow and as the flow increases a greater pressure differential will occur
across the restriction. So by measuring the differential pressure across a
restriction, one can measure the rate of flow.

Orifice Plate
The orifice plate is the most common form of restriction that is used in flow
measurement. An orifice plate is basically a thin metal plate with a hole
bored in the center. It has a tab on one side where the specification of the
plate is stamped. The upstream side of the orifice plate usually has a sharp,
edge.

When an orifice plate is installed in a flow line (usually clamped between a
pair of flanges), increase of fluid flow velocity through the reduced area at
the orifice develops a differential pressure across the orifice. This pressure is
a function of flow rate.
With an orifice plate in the pipe work, static pressure increases slightly
upstream of the orifice (due to back pressure effect) and then decreases
sharply as the flow passes through the orifice, reaching a minimum at a
point called the vena contracta where the velocity of the flow is at a
maximum. Beyond this point, static pressure starts to recover as the flow
slows down. However, with an orifice plate, static pressure downstream is
always considerably lower than the upstream pressure. In addition some
pressure energy is converted to sound and heat due to friction and
turbulence at the orifice plate. The measured differential pressuredeveloped
by an orifice plate also depends on the location of the pressure sensing points
or pressure taps.


Flange Taps
Flange taps are the most widely used pressure tapping location for orifices.
They are holes bored through the flanges, located one inch upstream and one
inch downstream from the respective faces of the orifice plate.
The upstream and downstream sides of the orifice plate are connected to the
high pressure and low-pressure sides of a DP transmitter. A pressure transmitter,
when installed to measure flow, can be called a flow transmitter. As in the case of level measurement,the static pressure in the pipe-work could be many times higher than the
differential pressure created by the orifice plate.

Vena Contracta Taps
Vena contracta taps are located one pipe inner diameter upstream and at the
point of minimum pressure, usually one half pipe inner diameter

Pipe Taps
Pipe taps are located two and a half pipe inner diameters upstream and eight
pipe inner diameters downstream.
When an orifice plate is used with one of the standardized pressure tap
locations, an on-location calibration of the flow transmitter is not necessary.
Once the ratio and the kind of pressure tap to be used are decided, there are
empirically derived charts and tables available to facilitate calibration.

Advantages and Disadvantages of Orifice Plates
Advantages of orifice plates include:

• High differential pressure generated
• Exhaustive data available
• Low purchase price and installation cost
• Easy replacement

Disadvantages include:

• High permanent pressure loss implies higher pumping cost.
• Cannot be used on dirty fluids, slurries or wet steam as erosion will
alter the differential pressure generated by the orifice plate.

Venturi Tubes
For applications where high permanent pressure loss is not tolerable, a
venturi tube can be used. Because of its gradually curved inlet
and outlet cones, almost no permanent pressure drop occurs. This design
also minimizes wear and plugging by allowing the flow to sweep suspended
solids through without obstruction.

Venturi tube disadvantages:

• Calculated calibration figures are less accurate than for orifice plates.
For greater accuracy, each individual Venturi tube has to be flow
calibrated by passing known flows through the Venturi and
recording the resulting differential pressures.
• The differential pressure generated by a venturi tube is lower than
for an orifice plate and, therefore, a high sensitivity flow transmitter
is needed.
• It is more bulky and more expensive.

One application of the Venturi tube is the measurement of
flow in the primary heat transport system. Together with the temperature
change across these fuel channels, thermal power of the reactor can be
calculated.

Flow Nozzle
A flow nozzle is also called a half venturi.
The flow nozzle has properties between an orifice plate and a venturi.
Because of its streamlined contour, the flow nozzle has a lower permanent
pressure loss than an orifice plate (but higher than a venturi). The
differential it generates is also lower than an orifice plate (but again higher
than the venturi tube). They are also less expensive than the venturi tubes.
Flow nozzles are widely used for flow measurements at high velocities.
They are more rugged and more resistant to erosion than the sharp-edged
orifice plate. An example use of flow nozzles are the measurement of flow
in the feed and bleed lines of the PHT system.

Elbow Taps
Centrifugal force generated by a fluid flowing through an elbow can be used
to measure fluid flow. As fluid goes around an elbow, a high-pressure area
appears on the outer face of the elbow. If a flow transmitter is used to sense
this high pressure and the lower pressure at the inner face of the elbow, flow
rate can be measured.
One use of elbow taps is the measurement of steam flow from the boilers,
where the large volume of saturated steam at high pressure and temperature
could cause an erosion problem for other primary devices.
Another advantage is that the elbows are often already in the regular piping
configuration so no additional pressure loss is introduced.

Pitot Tubes
Pitot tubes also utilize the principles captured in Bernoulli‘s equation, to
measure flow. Most pitot tubes actually consist of two tubes. One, the low-
pressure tube measures the static pressure in the pipe. The second, the high-
pressure tube is inserted in the pipe in such a way that the flowing fluid is
stopped in the tube. The pressure in the high-pressure tube will be the static
pressure in the system plus a pressure dependant on the force required
stopping the flow.
Pitot tubes are more common measuring gas flows that liquid flows. They
suffer from a couple of problems.The pressure differential is usually small
and hard to measure.
The differing flow velocities across the pipe make the accuracy dependent
on the flow profile of the fluid and the position of the pitot in the pipe.

Annubar
An annubar is very similar to a pitot tube. The difference is that there is
more than one hole into the pressure measuring chambers. The pressure in
the high-pressure chamber represents an average of the velocity across the
pipe. Annubars are more accurate than pitots as they are not as position
sensitive or as sensitive to the velocity profile of the fluid.

Flow Measurement Errors

We have already discussed the pros and cons of each type of flow detector
commonly found in a generating station. Some, such as the orifice, are more
prone to damage by particulate or saturated steam then others. However,
there are common areas where the flow readings can be inaccurate or
invalid.

Erosion
Particulate, suspended solids or debris in the piping will not only plug up the
sensing lines, it will erode the sensing device. The orifice, by its design with
a thin, sharp edge is most affected, but the flow nozzle and even venturi can
also be damaged. As the material wears away, the differential pressure
between the high and low sides of the sensor will drop and the flow reading
will decrease.

Over ranging Damage to the D/P Cell
Again, as previously described, the system pressures are usually much
greater than the differential pressure and three valve manifolds must be
correctly used.

Vapour Formation in the Throat
D/P flow sensors operate on the relation between velocity and pressure. As
gas requires less pressure to compress, there is a greater pressure differential
across the D/P cell when the gas expands on the LP side of the sensor. The
flow sensor will indicate a higher flow rate than there actually is. The
turbulence created at the LP side of the sensor will also make the reading
somewhat unstable. A small amount of gas or vapour will make a large
difference in the indicated flow rate.
The opposite can occur if the vapour forms in the HP side of the sensor due
to cavitation or gas pockets when the fluid approaches the boiling point. In
such an instance there will be a fluctuating pressure drop across the D/P cell
that will give an erroneously low (or even negative) D/P reading.

Clogging of Throat
Particulate or suspended solids can damage the flow sensor by the high
velocities wearing at the flow sensor surfaces. Also, the build-up of material
in the throat of the sensor increases the differential pressure across the cell.
The error in flow measurement will increase as the flow increases.
Plugged or Leaking Sensing Lines
The effects of plugged or leaking D/P sensing lines is the same as described
in previous modules, however the effects are more pronounced with the
possible low differential pressures. Periodic maintenance and bleeding of
the sensing lines is a must. The instrument error will depend on where the
plug/leak is:
On the HP side a plugged or leaking sensing line will cause a lower reading.
The reading will become irrational if the LP pressure equals or exceeds the
HP sensing pressure.
On the LP side a plugged or leaking sensing line will cause a higher reading.

8 comments:

harshita said...

Hi,
It was very different language. I don’t like your blog.

===================================
manish
nitishrocks

Jacob Holly1 said...

your given information is great about the differential pressures measurement method it has great importance and having a lot of benefits......thanks

Intrumentation Student Kingston said...
This comment has been removed by the author.
Sijo Joy said...

its a good post. TO know more about flow measurement check out the blog www.aboutinstrumentation.blogspot.com

LJ said...

Nice article - flow meters are becoming more and more important in modern day industries, and I'm sure they will become even more important in the future. I recently went to Micronics for an ultrasonic flow meter - as it didn't have to intrude and halt the process.

Unknown said...

Hi,

You can find more information on instrumentation.

http://www.instrumentationtools.com/

Thanks

Unknown said...

One of the most difficult flow measurements for the automotive engineer is to measure the engine oil flow rate under operating conditions. With the engine mounted on a test bench and coupled to a dynamometer, the job is slightly easier due to the increased space availability, but none the less, careful selection and installation of the flow measurement system is required. Flow measurements required in the engine bay of a vehicle become further restricted due to instrument power requirement, installation space required and the harsh under bonnet conditions.

Micheal Alexander said...

Thanks a lot for sharing this amazing knowledge with us. This site is fantastic. I always find great knowledge from it. Orifice Plate

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