Tutorials

Insertion Technology Main Features

All the insertion technology flow sensors are velocity-based flow measurement devices.
The installation typically requires only a small hole in the pipe for sensor perpendicular mounting.
Sensors� dimensions are not pipe size specific: almost independent from pipe cross section.

Insertion Technology Theory

Insertion technology is based on fluid speed meters, properly installed in a cylindrical straight pipe, and used to measure the local flow velocity Vm to calculate the average velocity Va and the volumetric flow rate Qv.
These flow sensors are theoretically supported by fluid-dynamic laws applicable to any circular cross section pipe when some physical conditions (fully developed turbulent flow) are respected.
Those laws state the relationship between the measured local flow velocity and the average flow velocity (UNI 10727; ISO 7145).
The relationship between average velocity Va and measured velocity is usually expressed through the �Profile Factor�:

Fp = Va / Vm

Using the above mentioned factor:

Qv = Va * ID� / 4 = Fp * Vm * ID� / 4

ID = pipe inside diameter

Two different positions are suitable for the flow velocity measuring point:
1. Critical position: the velocity sensor is inserted in a peculiar point where the local velocity correspond to the average velocity:

Va = Vm >>> Fp = 1.

2. Central position: the velocity sensor is placed exactly in the centre of the pipe cross section. The local velocity correspond to the maximum velocity:

Vm = Vmax >>> Fp < 1.

Fully Developed Turbulent Flow

Reynolds number [Reynolds number calculator]

All velocity based flow sensors provide an accurate and reliable indication only when they are measuring a fully developed turbulent flow.
Fully developed turbulent flow occurs in every Newtonian fluid when the Reynolds Number is greater than 4500.
Fully developed turbulent flow can be more difficult to achieve with high viscosity liquids, low flow rates or large pipes. Quite often a reduction of the pipe size to increase the local flow velocity is enough to produce a proper Reynolds Number:

Re = V x ID x Sg / �

where:
V = flow velocity in m/s
ID = pipe inside diameter in meter
Sg = Specific Gravity in Kg/m3
� = Dynamic Viscosity in Pa*s
(1 Pa*s = 10� cP)

or, converting flow velocity in flow rate:

Re = 1.2732 x Qv x Sg / � x ID

where:
Qv = flow rate in l/s
Sg = Specific Gravity in Kg/m3
� = Dynamic Viscosity in Pa*s
(1 Pa*s = 10� cP)
ID = pipe inside diameter in meter

Re = 3162.76 x Qv x Sg / � x ID

where:
Qv = flow rate in gpm
Sg = Specific Gravity in Kg/m3
� = Dynamic Viscosity in centipoises
(1 Pa*s = 10� cP)
ID = pipe inside diameter in inches

Flow Sensor Installation

The placement of a flow meter is critical to get an accurate and reliable reading. For a flow meter proper performance it is necessary to check:

full pipe at every time
uniform flow velocity into the pipe.

Full Pipe Condition

If the pipe is not full the flow meter will give inaccurate reading even if the sensor is always completely submerged. The meter will make the flow rate calculation on the assumption that the pipe is full, leading to overestimation of the flow. A pump intake or an outlet on the bottom of a tank does not necessary ensure the pipe always running full; air can be sucked by pumps or it could remain entrapped when the pipe was empty.
Anyway the flowmeter should be always situated in the lowest point of the pipe and there should be downstream the flowmeter a part of the pipe placed 1 x ID higher than where the flow meter is located.

Mounting Positions

The reading accuracy of paddlewheel flow sensors can be also affected by sediments present into the liquid and by mounting angle due to the effect of gravity increasing the friction between shaft and bearings.

Horizontal pipe runs: the mounting position to get the best performances is vertical or at a maximum 45� angle to avoid air bubbles. Do not mount the sensor on the bottom of the pipe if sediments are likely.
Vertical pipe runs: install sensor in any orientation. Upward flow is preferred to ensure full pipe.

Uniform Flow Velocity

Insertion flow meters measure the velocity of the liquid. It is important the velocity is uniform across the entire cross section of the pipe in the location of the sensor. Flow patterns are distorted both downstream and upstream of any disturbance.
In a pipe, liquid at the edge of the pipe moves slower than towards the center because of friction along the walls.
In a straight run of pipe, area with similar velocities can be depicted as concentric rings.

The next diagram depicts the distortion in the velocity profile that occurs just after the flow has gone through an elbow or a tee junction.
The liquid on one side of the profile is moving much faster than in the rest of the pipe.

Typical distorted flow velocity patterns related to different disturbances present in the pipeline:

Installation Guidelines

The six most common installation configurations are shown to help in selecting the best location in the pipeline for paddlewheel flow sensor.
For more information, please refer to EN ISO 5167-1.
Always maximize distance between flow sensors and pumps.

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