Liquid flow measurement is critical to meet requirements in industrial settings. In some scenarios, the capability of conducting precise flow measurements is so critical that it could spell the difference between facing a loss or taking a profit. Failing to take accurate measurements or inaccurate liquid flow measurements can lead to disastrous results.
Liquid flow measurement devices rely on the velocity of the liquid being measured. The velocity is the pressure differential which forces the liquid down the conduit or pipe. Since the cross-sectional area of the pipe remains constant (and is known) the typical velocity indicates flow rate.
There are many types of liquid flow measurement devices. These are broadly categorized under four categories: differential pressure, velocity, positive displacement, open channel and mass meters.
The following is a brief discussion of these five types of liquid flow measurement devices:
These are the most common types of liquid flow measurement devices used today. According to some estimates, more than 50% of all liquid flow meter applications consist of this type.
Their basic operating principle is based on the fact that a pressure drop in the meter is comparative to the square of the velocity or flow rate. This flow rate can be calculated by computing the pressure differential and taking the square root.
Differential flow meters have a primary and secondary element. The former causes changes in kinetic energy, thereby creating differential pressure within the pipe. The latter measure this differential pressure and sends a signal or read-out which is then computed to determine flow rate.
Examples of differential flow meters include orifices, flow tubes, venturi tubes, pilot tubes, target meters and flow nozzles.
This type of liquid measurement device works by separating liquids into precisely measured increments and then furthering it on. Each increment is linked to a connecting register. Since each increment indicates a particular volume, positive displacement measurement devices are often using in accounting and automatic batching applications. This type of liquid flow measurement device is great for measuring the flow of a viscous fluid.
Examples of positive displacement flow meters include rotary-vane, oval gear and piston.
This type of liquid flow measurement device operates linearly with regards to flow rate. Since a square root is not required, velocity meters have greater range ability. Velocity meters are usually directly fitted into pipelines. They have meter-type housings that are fitted with special fittings or flanges which permit them to be placed this way. This type of flow meter has minimum sensitivity to viscosity fluctuations, especially when Reynolds numbers are more than 10,000.
Examples of velocity meters include electromagnetic, vortex shredders and turbine.
Mass Flow Meters
The need for accurate measurements in heat transfer and chemical reaction-type applications has led to the development of mass flow meters. There are many types of mass meters but the most common ones are thermal and Coriolis.
Coriolis meters are special because they measure mass rate directly without the need to calculate volumetric flow. Since mass never changes, the meter remains linear without requiring adjustments for variations in fluid properties. There is no need for compensation for changes in pressure and temperature conditions. Coriolis meters are helpful for measuring liquids whose thickness varies at given pressures and temperatures. These meters are named after the natural phenomenon on which they operate: Coriolis force.
Open Channel Meters
An open channel meter is any waterway or channel where the liquid flows on a free surface such as in canals, rivers, streams, tunnels and non-pressurized sewers. The most common types of open channel meters include flumes and weirs. They rely on depth-related methods. Instantaneous flow rate is measured by knowing the water depth. Flumes and weirs are the oldest and most commonly used types of open channel meters.