NT217 C GB1 6 / 27
1-3 – Speed of sound in fluids – Relation with density – Influence of P and T:
The speed of sound in a fluid is a full-fledged physical characteristic which may be linked
with others, such as density (d), pressure (P), temperature (T) or the % of two or several
miscible products or with another magnitude having good analogy, like compressibility.
For most refined petroleum products or petrochemical bases, experience shows some
relationship between the speed of sound and the density of the product for constant P and
T, but there are many exceptions. For water, the reliability of the relation allows a number
of tests to be run on the installation. Gases have lower speeds of sound than liquids,
although some refrigerating fluids come close.
Case of water:
Water has a very reliable “speed of sound < > temperature” relationship which will be put to
good use whenever possible for dimensional checking: a simple temperature measurement
is a means of predicting the speed of sound to be compared with the result obtained by
MiniSonic on the basis of the measured times and the dimensions as set up: A deviation
may require these values to be reexamined, like L for intrusive probes or the diameter and
thickness of the pipe for external probes.
The following simple equation gives a good estimation of C between 0 and 100°C:
C = 1557 – 0.0245 * (74 – t)
2
The following table defines the customary values more accurately.
The water load has little influence: this table applies to crude or used water.
When compared with other fluids, the water temperature coefficient for C is positive up to
74°C. Beyond that, the water becomes a conventional product again.
The water pressure coefficient is relatively low, at around 0.15 m/s per bar.
The presence of salts solutions increases the value of C by approximately 1 m/s per g / liter
(seawater…)
The addition of glycol ethylene or a similar product also increases the speed of sound within
the proportions of (50 / 50 % > 1550 m / s)
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