What is Flow?


Flow -The rate or quantity of a moving fluid either in an open channel or closed conduit.

The force that causes liquid to flow is created by a change in pressure. Flow measurement is primarily 

concerned with static, dynamic and differential pressures.


Static Pressure – Pressure exerted by fluid at rest.


Dynamic Pressure – is the increase in pressure above static pressure that results from the transformation

of the fluid’s kinetic energy into potential energy.

Differential Pressure – Pressure difference between two related pressures.


Differential Pressure Transmitter


A d/P transmitter measures differential press pipe, the pressure drop across the restriction not

the 100psi static pressure.

In the tank, the transmitter senses the pressure equivalent of liquid height.

The 30psi static pressure is not sensed since it is applied to both sides.


What is Flowrate?

Types of Flow

1. Volumetric Flowrate – indicates volume of fluid that passes a point over a period of time. 

2. Mass Flowrate – indicates the amount of mass that passes a point over a period off time.

Common Types of Flow meters

1. Head or Differential Pressure Meters

2 Variable Area or Rotameter

3. Turbine Meters

4 Electromagnetic

5. Coriolis


Head Pressure or Differential Pressure Meters 

The Bernoulli equation states that for a non-viscous,  incompressible fluid in steady flow,

the sum of pressure, potential and kinetic energies per unit volume is constant at any point.


Principle of Measurement

This type of meters works by introducing introducing restriction into the flow line and then pressure taps

on each side of the restriction measure the pressure differential caused by the fluid flow. The resulting

pressure is proportional to flow rate in accordance with flow rate in with the formula:


                                                                    Head Meter

Head Meter Characteristics

1. Square root relationship between flowrate and differential pressure.

2. Density of fluid must be taken into account both for volume and for mass flow measurements. 

3. Measurement deppendent on Beta Ratio


Most differential pressure meters depend on a restriction on the flow path to produce a change in velocity.

For the usual circular pipe and circular restriction. Beta ratio is the ratio between the diameter of the

restriction and the diameter of the pipe.


Because the relationship between flow and pressure involves a square rootthe differential drops off

quickly as flow decreases For this reason, the dynamic range for these sensors is limited to about 4:1.

Accuracy also varies with flow rate, from 1 to 3%.


Common Methods of Producing Differential Pressure (d/p) 

1. OrificePlate

2. Venturi Meter

3. Pitot Tube


The Orifice Plate 

The orifice meter when installed in small to moderate sized pipes is probably the cheapest and

simplest flow measuring device at present available for metering liquids, gases and vapors.

It is not suitable for high viscous liquids or fluids in a pulsating or extremely turbulent condition.


When the fluid flows through the orifice, its velocity increases and the diameter of the jet decreases 

to minimum at a point v,known as the vena contractaThe jet expands until it again occupies the full

bore of the pipe.


The static static pressure pressure profile first shows a gradual decrease over the distance L to M

due to friction losses in the pipe. 

From M to P, as light rise occurs due to the resistance of the orifice plate. A sharp drop in the pressure

occurs from P to R due to the fluid velocity through the orifice. Finally, there is a partial recovery

of pressure from R to S. The net pressure loss due to friction and turbulence across the orifice is,

typically, about 65% of the pressure difference measured by d/p diaphragm. The fluid flow is

proportional to the square root of the pressure difference.


                                                                         Orifice Plate Installation

Typical Components of an Orifice Metering Assembly



Orifice Plate Designs

1. Concentric – commonly used for general applications (gas, liquid & vapor).

2. Eccentric – recommended for fluids with extraneous matter to a degree that would clog up concentric type.

3. Segmental – recommended for fluids combine with vapor or vapor with fluids.


Concentric Orifice PlatePlate


Eccentric Orifice Plate


Segmental Orifice Plate


Types of Orifice Plate Entrance

1. Square Edge – applicable for higher pipe Reynolds Number; typical Re 500 to 10,000

2. Quadrant – for lower pipe Reynolds Number; typicallyranges from Re 250 to 3300.

3. Conical – for Reynolds Number typically range from Re 25 to 75.


Square Edge Orifice Plate


Quadrant Edge Orifice Plate


Conical Entrance Orifice Plate



Pipe Bends and Valves


Pipe bends, elbows, valves and surface rougghness distort the fluid flow pattern in a pipe

and can affect meter accuracy.

This problem is usually offset by providing a straight section of a smooth pipe both upstream

and downstream of the meter.

Correct Installation of Orifice Plate


In steam service, lines should be filled with water to prevent contact of the live steam with the transmitter.


 For Liquid service, placing the tapping points at the side prevents accumulation of dirt, gas and vapors in the impulse lines.


In Gas service the taps should be located on the top to keep out liquids from the transmitter.

Operational Sequences of Three-Valve Manifold -putting a d/P Transmitter Out Of Service

The starting operating state is with the equalizing valve close and both block valves open.

1. Close the low pressure valve to trap pressure in the low side – check for leaks and

ensure the indicated d/P does not change.

2. Open the equalizing valve to force d/P to zero.

3. Close the high pressure block valve to isolate the transmitter.

4. Bleed down (i.e. vent) the pressure trapped in the d/P cell body – should continue to read zero d/P

5. The d/P transmitter is now out-of-service, isolated and depressurized.

Operational Sequence of Three-Valve Manifold -putting a d/P Transmitter Into Service 

To put the DP transmitter into service, the following steps should be followed: 

1. Check all valves closed.

2. Open the equalizing valve-this ensures that the same pressure will be applied to

both sides of the transmitter, i.e. zero differential pressure.

3. Open the High Pressure block valve slowly, check for leakage from both the high pressure

and low pressure side of the transmitter – still zero d/P.

4. Close the equalizing valve -this locks the pressure on both sides of the transmitter –

now look for leaks, should still be zero d/P.

5. Open the low pressure block valve to apply the process pressure to the low pressure

side of the transmitter and establish the working differential pressure.

6. The transmitter is now in service


Impulse Piping

Impulse piping, which is the piping between the process and the transmitter,

must accurately transfer the  pressure in order to obtain accurate measurements.

In this pressure transfer, there are five possible sources of error: leaks, friction loss

(particularly if purging is used), trapped gas in a liquid line, liquid in a gas line,

and temperature-induced or other density variation between the impulse piping

between the process and the transmitter, must accurately transfer the pressure

in order to obtain accurate measurements.

Guidelines for Transmitter Location and Placement of Impulse Piping. 

• Keep impulse piping as short as possible

• Slope the impulse piping at least one inch per foot (8 centimeters per meter)

upward from the  transmitter toward the process connection for liquid.

•Slope the impulse piping at least one inch per foot (8 centimeters per meter)

downward from the transmitter toward the process connection for gas.

• Avoid high points in liquid lines and low points in gas lines.

• Make sure both impulse legs are the same temperature.

• Use impulse piping large enough to avoid friction effects and prevent blockage.

• Vent all gas from liquid piping legs

• When using a sealing fluid, fill both piping legs to the same level.

• When purging is necessary, make the purge connection close to the process

taps and purge through equal lengths of the same size pipe.

• Avoid purging through the transmitter.

• Keep corrosive or hot (above 250 °F [121 °C]) process material out of direct

contact with the sensor module and flanges.

• Prevent sediment deposits in the impulse piping.

• Keep the liquid head balanced on both legs of the impulse piping.

• Avoid conditions that might allow process fluid to freeze within the process flange.

For steam service, do not blow down impulse piping through the transmitter.

Flush the lines with the blockingvalves closed and refill the lines with water

before resuming measurement.




Differential Pressure Transmitter


Principle of the dP transmitter


Setup of dP transmitter


Calibration Setup


Linear & Square Root Scale Comparison


Venturi Meters

Venturi meter is used instead of an orifice plate in process systems where it is important

to minimize permanent pressure loss across the restriction device. 

In venturi, the restricting element is a tapered tube instead of sharp-edge orifice.

The tube gives a smoother velocity change which results in a small permanent

pressure loss of approximately 10% of the differential pressure measurement.


Short Form Venturi Tube



Venturi Pressure Loss Profile


Pressure Loss Compparison (Orifice & Venturi)

Comparison Orifice and Venturi Meters

1. Orifice reducing element is sharp edged while venturi is tapered tube.

2. Permanent pressure loss of orifice is 65% of measured d/p while venturi is only 10%.

3. Venturi tube is less sensitive to Reynolds Number and gives more accurate measurement

when the process flow varies over a wide range.

4. Venturi tube is less affected by dirty fluid which build up deposits at orifice plates

and pressure tap connections.

5. Venturi tube meter is more costly compared to orifice plate costly compared

to orifice plate and requires greater length of pipeline.

6. Orifice plate is relatively easy to change for new range.


Pitot Tube

The pitot tube is a tube with an open end facing the incoming open end facing the incoming fluid stream.

The difference between the pitot tube (impact pressure) pressure) and the static

and the static pressure in the line is a measure of flow rate.



Pitot Tube


Elbow-Tap Flowmeter

Elbow-Tap flowmeter operates on the principle that when a fluid moves moves around a curved

path, the acceleration of the fluid creates centrifugal force.

In operation, the centrifugal force results in a higher pressure on the outside of the elbow than on the inside.

Thus, a d/p is produced which is proportional to the square of the flow through the elbow.


                                                                             Flow Nozzles

Flow nozzle is another type of differential pressure flowmeter.

Flow nozzle is a restriction consisting of an elliptical contoured inlet and a cylindrical throat section.

Pressure taps used to measure the difference instatic pressure created by flow nozzle are commonly

located one pipe diameter (1D) upstream and ½ pipe diameter (1/2D) downstream from the inlet face of

the nozzle.


Flow Nozzle Construction


 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 pitot tubes

as they are not as position sensitive to the velocity to the velocity profile of the fluid.

Variable Area or Rotameter



Rotameter is the most common type of flowmeter. It consists ofa vertitical glass tube, tapered

down from the top end, and contains a “float” which is heavier than the fluid being metered.

                                                             Principle of Operation

The general operating principle of a rotameter is similar to an orifice plate but instead of measuring

the variable pressure diffference across a fixed size restriction, the size of the restriction is varied

by a moving restriction and the pressure drop is kept constant.



Positive-displacement (PD) flowmeters measure flow directly in quantity (volume) terms instead

of indirectly or inferentially as rate meters do.

PD meter separates the flow to be measured into different discrete portions or volumes (not weight units) and,

in essence, counts these discrete volumes to arrive at a summation of total flow.


Nutating-disk Flowmeter.

Each cycle (complete movement) of the measuring disk displaces a fixed volume of liquid. 


A precise volume of liquid is captured and transferred from inlet to outlet


A big advantage of positive displacement types is their ability to discern extremely low flow,

down to a few cc pper minute. They are also highly accurate (typically 0.5 to 1% of flow rate),

have a dynamic range  up to 400:1,, and are bidirectional.

Hydraulic pulsation has no effect on these sensors, and they can be

placed almost anywhere in the system.


Electromagnetic Flow Meter

The operation of magnetic flow meter is based on Faraday’s well-known law of electromagnetic induction.

The voltage (E) induced in a conductor moving in a magnetic field at a right angle to the field is directly

proportional to the number of conductors, or, as in this case, the distance between the probes (D),

the intensity of magnetic field (B) and the velocity of the motion of the conductor


Schematic Representation Magnetic Flowmeter


E = Constant x D x B x v


Typical Installation


Advantages of Magnetic Flow Meter

1. No obstruction to fluid flow.

2. Pressure drop is minimal.

3. Good for measurement of slurries and other corrosive or abrasive fluid.

4. Easy and bi-directional installation.

5. Not affected by viscosity, density, temperature and pressure.


Important Notes

1 Fluid Conductivity should be higher than the minimum value of 5 uS/cm2.

2. The meter must be full all the time.

3. Gas bubbles should be avoided.

4. Electrode fouling should be considered.

5. Hazardous area application should be certified.


Ultrasonic Flowmeters

Ultrasonic flow instruments measure the velocity of sound as it passes through the fluid in a pipe.

Some designs allow measurements to be made external to the pipe, while others require that sensor

be in contact with the fluid. The measurement is based on the time of flight of the sound waves.

Pulses are transmitted along and against the fluid flow. The time of transit of the ultrasonic beam

is measured and used to calculate the flow through the pipe. Some designs allow measurements

to be made external to the pipe, while others require that sensor be in contact with the fluid.

The measurement is based on the time of flight of the sound waves. Pulses are transmitted along

and against the fluid flow. The time of transit of the ultrasonic beam is measured and used to

calculate the flow through the pipe.


External Transducer (Clamp-on)

Transit-time Flowmeter



                                           Transit-time Flowmeter (Bouncing)