Cryogenic flow measurement explained

When we talk about cryogenic flow measurements, we are actually always talking about measuring a gas, which has been brought to a very low temperature and has therefore become liquid. A common application of this phenomenon is the thermal separation of air gases. Air consists of 80% nitrogen, which liquefies at -196 ºC. Another air gas is oxygen, of which air consists of approximately 20%. Oxygen condenses under atmospheric conditions at around -183ºC. This phenomenon allows for the thermal separation of these 2 air gases.

Not every flow meter can be used to measure cryogenic liquid flows. The instrument must of course be able to withstand these low temperatures. And not only inside the pipe, but also outside. After all, a cold instrument leads to ice formation on the outside through condensation of water vapour in the ambient air.

Photo: Ice formation on parts in a cryogenic flow application.

Flow meter selection for cryogenic applications

The most commonly used flowmeters are differential pressure flowmeters, turbine flowmeters, ultrasonic flowmeters and coriolis flowmeters.

Some typical cryogenic applications are:

• Cargo measurements of cryogenic liquids transported and delivered by trucks
• Measurements during the air separation process
• LNG (Liquified Natural Gas)
• Liquid fuel engine rocket installations

Because the supply of cryogenic liquids falls under the MID (because the MID legally applies to the settlement measurements of liquids other than water, download the MID here), certain requirements are imposed on the instruments used for this application. These are described in an OIML report, the R81-1. OIML stands for Organisation Internationale de Métrologie Légale (download the OIML R117 here).

LNG: cryogene flow measurement

LNG plants are being built in various places around the world (particularly where natural gas is produced but the market is limited). In other places LNG terminals are being built where there is a high demand for natural gas. In these applications dP measurements are sometimes used. The turbine flow meter is another option. Ultrasonic flow meters are increasingly being used for very large LNG pipelines.

Specific issues in cryogenic applications are the risks of flashing (sudden formation of gas in the liquid) and the need for cooling and (vacuum) insulation.

Photo: In LNG tankers ultrasonic flowmeters in large diameter stainless steel are used.

Space: cryogenic flow measurement

Some rockets use hydrogen as fuel, others use methane or kerosene. And of course oxygen is needed. In the rocket motor the liquid hydrogen (or other fuel) and liquid oxygen is injected at pressures of a few 100-den bars. Such a test only takes about 15 seconds. Of course there are special requirements for the flowmeters which must function at such high pressures and low temperatures. Usually special turbine flowmeters are used for such applications.

Photo: Cryogenic flow measurement in liquid fuel spacecraft engines. On this picture: the engine of the by Teesing sponsored DARE student team of TU Delft.