Often the key difficulties to accurately meter the flow of liquids at elevated temperatures are more heavily based on the resultant changes in fluid properties than to the attributes of the measuring device being used.
Effect of Fluid Property Changes on Accurate Metering of Fluid Flow
In a broad variety of industrial applications, aqueous solutions are some of the most common fluid types which need to be metered and dosed. The changes in the water’s density and viscosity at varying temperatures are shown in the graph below.
The water’s density changes by ~3% from 1 to 90 °C, as shown above. Even though this seems like a relatively minor alteration, if an application needs an accurate mass of aqueous solution to be metered or dosed, this change could be critical.
However, over the same temperature range, the change in viscosity is not at all modest. Rather, between 1 and 90 °C, the aqueous solutions’ viscosity changes by a factor of 8.
This could be a disaster for a flowmeter which is Reynolds number (Rn) sensitive, as there would be a change in Rn of almost 8:1. If the meter’s flow range is only 10:1, and it is operating near to its extremes, there is a chance that the meter may drop out of the linear range.
The aqueous density and viscosity changes have different effects on different meter types when operating at different temperatures. Most flowmeters require Rn to exceed ~2200 (turbulent flow), however some can perform in the laminar region below this value. Any temperature increase may take the meter into the turbulent zone.
Steps to be Followed for Measuring Liquid Flow at High Temperatures
Consequently, it is recommended that you follow the easy key considerations checklist below, if you are thinking of measuring liquid flow at high temperatures:
- See whether your liquids’ physical properties, viscosity, or density changes at varying temperatures.
- Check your flowmeters principle of operation and operating envelope, always considering number 1 above.
- Bear in mind if it is necessary to maintain your flow line temperature for simple fluid transport (for example, heavy oil or liquid chocolate). If the answer is yes, is it necessary to insulate, heat trace, or jacket heat the flowmeter?
- Is it necessary for your process flowmeter to be accurate when operating at low temperatures?
- During the “warm-up period” is the flowmeter’s performance important? Further, is it possible for this flow measurement device to be ranged for the actual process running conditions?
This information has been sourced, reviewed and adapted from materials provided by Titan Enterprises Ltd.
For more information on this source, please visit Titan Enterprises Ltd.