Application Corner: Measuring under vacuum, part 1

Three situations come to mind that most technical people should carefully consider: operation under vacuum, measuring downward liquid flow and gravity flow. These situations may not be routinely encountered by the majority of practitioners, but they can present significant issues that are not readily apparent. Of course, if you deal with these situations on a regular basis, you will likely be comfortable with proceeding at normal speed.

The vapor pressure of a liquid is the pressure at which that liquid will boil. Operating under vacuum conditions — below atmospheric pressure — can cause ordinary liquids such as water to boil at room temperature. Processes often use this phenomenon to an advantage when drying heat-sensitive products by removing, say, water under a vacuum at room temperature instead of potentially degrading the product by heating it to over 100°C under atmospheric conditions. 

Systems operating under a vacuum are more likely than a pressurized system to inadvertently operate below the vapor pressure of the liquid. This is because liquids in many process vessels in vacuum processes operate at or near their vapor pressure such that a small pressure reduction, such as piping losses, or a small temperature increase can cause the liquid to boil. 

The installation of every instrument in vacuum service should be analyzed in detail to determine if a problem could occur. For example, measuring the level of a tank operating below atmospheric pressure using a level probe installed in an external stilling well might seem normal. However, ambient conditions may make the stilling well slightly warmer than the liquid in the tank — operating at the vapor pressure of the liquid — and cause the liquid in the stilling well to boil, adversely affecting the level measurement. 

Read more on this topic next month.