Flow and Density Measurement of Liquid

Power generation, processing plants, energy facilities and industrial utilities depend on cost effective management of energy which directly depends upon accurate and reliable flow meter instrumentation. Whether you require precise measurements of air or steam or the flow and density measurement of liquid, an appropriate flow meter must be used.

The role of density in flow measurement

Density is a vital component of fluid property and consequently flow measurement. Density is mass divided into a volume. As a property of the fluid, density depends on the fluid type and the impact of environmental conditions on that particular fluid. For example, oil has a different density than water. Hot water will have a different density than cold water.

In flow measurement applications, density is obtained in three ways:

1. Direct measurement.

Using densitometers, the density of gas and liquid is measured directly.

2. Measurement of fluid components.

For example, when measuring the component of natural gas, a chromatograph can analyze the gas sample and determine amounts of various components within it. Using the density calculations of each component, the density of the entire sample is then obtained.

3. Inference.

The density of the fluid is inferred using measurements of temperature and pressure. This is perhaps the most popular method due to its simplicity. The cost of using temperature and pressure measurements is far less than relying on a chromatograph or densitometer.

The role of Pressure & Temperature on flow and density measurement

Most liquids are non-compressible; therefore pressure does not have a significant impact on them. As a result, only temperature calculations are necessary to determine density compensation.

In many applications, density constantly changes. This is due to fluctuations in temperature. However fluid composition remains more or less static.

Which flow meters are capable of measuring the flow and density of liquids?

Density affects different flow meters in many ways.

Volume flow meters such as turbine and magnetic meters are not impacted by density fluctuations. The (proportional to flow) signal output (generally pulse (Hz) or analog) from such meters is read out in liters per minute or gallons per minute.

Differential flow meters such as V-Cones and orifice plates depend on density because they are based on Bernoulli’s theorem. This is true irrespective of whether the flow meter is measuring mass or volume flow rates.

Mass flow meters such as thermal mass meters and Coriolis flow meters take direct flow rates. They produce signal outputs (see previous) read out in pounds per minute.

How does a Coriolis flow meter measure density?

Coriolis flow meters make direct mass and density flow measurements based on the Coriolis Effect; “It is the deflection of the moving objects when they are viewed in the reference of the rotating frame”. The Coriolis flow meter works by artificially introducing Coriolis acceleration into the flow stream. The fluid is deflected within. As a result, the forces that are generated lead to slight distortions or twisting of the measuring tube which are directly proportional to mass flow rate. Special sensors pick up on this distortion and convert the output into a signal.

The advantage of using Coriolis mass flow meters is that they are capable of providing flow, temperature and density measurements of gasses and liquids inside a single meter. Given that the measurement principle is not reliant on fluid properties, these meters have a high accuracy rate. The lack of moving parts and straight-pipe requirements make them a suitable alternative to other types of flow meters. The downside is that these meters cost thousands of dollars.

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