Flow Rate Measurement of Cryogenic Fluids

Application
Flow rate measurement and totalization of cryogenic fluids provide the manufacturer with better product control and improved plant efficiency.

Background
Due to the extremely low temperature, most conventional flowmeters cannot operate reliably, especially in small diameter pipes. During start-up, if cold liquid enters a warm pipe, it can boil and generate gas velocities that are high enough to overspin and ruin a turbine meter. An orifice plate, on the other hand, may produce a pressure drop large enough to cause “flashing” of the cryogenic liquid, producing erratic readings.

In this application, the GE Sensing ultrasonic transit-time liquid flowmeter provides the cryogenic manufacturer with the ability to detect leaks of valuable liquid. By monitoring the output of the plant to storage vessels, one customer detected a failure in the insulating system of the storage vessels. Typically, cryogen storage vessels are double walled with a vacuum between the walls for thermal insulation. If the vacuum system fails, the stored cryogen will vaporize and be returned to the plant for reliquification. By measuring the liquid flow rate to the storage vessel, one can quickly detect deviations from desired plant operation. In this example, reliquification caused the measured flow to storage to double its normal value, indicating failure of the storage vessel insulation.

Installation
An offset flowcell with 90° tees was installed in the argon liquification plant as an integral part of the plant piping in the cold room. The electronic instrument was mounted in a remote analyzer house approximately 30 m (98 ft) away from the cold room.



The GE ultrasonic flowmeter provides direct reading in gallons per minute and total flow in gallons. The offset flowcell and wetted transducer sizes are dependent upon pipe size.

Advantages
Due to a unique GE transducer design, the transducers can be exposed to temperatures as low as -200°C (-328°F). Also, the flowmeter requires no maintenance because there are no moving parts. The particular ultrasonic flowcell selected for this application (axial offset, 90° tees) can operate with laminar, transitional and turbulent flow. At low (laminar) flow rates, the pipeline must be well insulated to prevent flashing.
(Note: For smaller pressure drops, other flowcell configurations are available.)

Equipment
• Transit-time ultrasonic liquid flowmeter
• Wetted transducers
• 12.7 mm (1/2 in) offset flowcell

Specifications
- Liquid argon, single phase
- 1.1 to 11.4 L/m (0.3 to 3 gal/min)
Temperature
-183°C (-298°F)
Viscosity
- 0.25 cp (0.6 lbm/ft-h)
Density
- 1.38 g/cm3 (86 lb/ft3)
Pressure
- 328 kPa (47.5 psi)


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http://www.gesensing.com/productservices/tech_flowmeters.htm