4.FLOW MEASUREMENT

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HIGH PRESSURE VENTURI TUBE

VENTURI

FLOW NOZZLE

FLOW NOZZLE

FLOW NOZZLE

PITOT TUBE

ORFICE PLATE

PITOT TUBE

VENTURI TUBE

ORIFICE INSTALATION

ORIFICE PLATE

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.

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