Measuring flow is one of the most important functions in water treatment processes. Flow is defined as the amount of fluid (water) going past a certain reference point in a specific period of time. It is measured in units of volume per unit of time (Continuity Equation):
Q = AV
|Where:||Q = Quantity of flow
A = Cross sectional area of the flow
v = Velocity of the flow
There are two basic types of flow in water systems: open channel and closed channel (closed pipe). Since each type of flow has different characteristics, each type of flow requires a different system of measurement. Open channel flow, such as streams, canals, distribution channels, gravity sewer lines, etc., are free flowing and open to the atmosphere. Closed channel flow, such as force mains, sludge lines, pump discharge lines, etc., is a conduit or pipe that is completely filled and under pressure.
Flow Measurement Devices
Flow measurement devices are discussed in terms of primary and secondary units. Primary devices are hydraulic structures that are placed in the flow streams to change the flow characteristics if the stream. In open channel flow, primary elements are usually weirs or flumes. These primary devices change the height of the water column by adding restrictions or obstacles. In closed channel flow, the primary elements are often either a flow tube or orifice plate. These primary devices change the pressure on either side of the device that is inserted into the pipe.
Open Channel Flow Measurement
Secondary devices in open channel flow measure the change of the liquid level in the primary device and convert this to flow according to the known liquid level/flow rate relationship of the primary device.
Ultrasonic Flow Meter
An ultrasonic device works similar to sonar. The device is installed above the water flow. It emits sound waves which are reflected from the surface of the water and bounce back to the receiver. The level of the water is then converted to a flow rate.
Bubbler Flow Meter
A bubbler flow meter provides a constant flow of air through a small tube that is submerged into the water stream. The height of the water column is measured by the change in pressure required to push the air bubbles out of the tube. This change in pressure equates to a change in the water depth which is converted to a flow measurement.
Submerged Pressure Transducer
This instrument is similar to the bubbler, but the pressure difference is measured by a sealed pressure transducer submerged into the water column.
Closed-Channel Flow Measurement
In a closed channel system, the secondary element measures the pressure change brought about by the liquid flowing through the restriction and converts this pressure difference to a rate of flow.
A Venturi nozzle has an entrance section, a throat section and an exit section. The velocity of the liquid increases as it passes through the restricted throat section. The change in velocity results in a pressure differential between the entrance and throat sections. This pressure difference is measured by a manometer or converted to a flow rate.
Dall Flow Tube
As a modified Venturi Nozzle, the inlet and outlet sections are significantly reduced in length making a much shorter unit. Because of its shorter length, there is a lower permanent head loss through the unit as compared to the Venture nozzle while providing a higher differential pressure through the same entrance and throat sizes. This difference in pressure is converted to flow rate.
The plate on this flow meter has a precisely bored hole. The pressures on both sides of the plate are measured to assess the pressure drop through the plate. This difference in pressure is converted to a rate of flow.
Magnetic Flow Meter
Two coils are located inside the devise that create a magnetic field. When the water (conductor) flows through the magnetic field, a voltage is generated. This induced voltage is proportional to the strength of the magnetic field, the distance apart of the two coils and the velocity of water. The measured voltage is used with the continuity flow equation (given above) to calculate flow.
A propeller is inserted into the flow stream. The force of the water turns the propeller on this mechanical meter and gives a flow.
The drag of a fluid around a submerged buoyant object inside a tube gives a direct reading on this type of meter. As the rate of flow around the object increases, the drag forces also increase which causes the float to rise. The height of the float is taken as a measurement of flowrate.
Acoustic Flow Meters
There are two types of acoustic flow meters: one works on transit time measurement and other works on the Doppler effect.
Doppler - When an emitted sound wave (ultrasonic beam) is reflected by a moving object, there is a change in frequency. The velocity of the object can be determined from this change in frequency. The continuity equation can then use the velocity to calculate flow.
Transit Time – The velocity of a sound wave moving though a liquid is also dependent on the velocity of the liquid. Two sensors are mounted on an angle to the flow. The difference in the velocity of the two sound waves gives the velocity of the water. With the area of the pipe, the continuity equation can be used to calculate the flow.