CERN (Conseil Européen pour la Recherche Nucléaire) houses the world’s largest complex of particle accelerators and detectors. These scientific instruments probe the fundamental structure of the particles that make up everything around us. In his presentation at the Emerson Exchange EMEA 2024, Juan Casas Cubillos, engineer at CERN, outlined the use of Coriolis flow meters in very challenging cryogenic applications.
CERN particle accelerators are fed with beams of charged particles that are accelerated, guided and focused using electric and magnetic fields. Particle collision energy is increased by increasing the diameter of the particle accelerators and the field of the bending magnets. The Large Hadron Collider (LHC) tunnel, for example has a circumference of 27 km, and uses 1232 main dipoles and 392 main quadrupoles operating in 1.9 K liquid helium. High magnetic fields and high energetic particles require the use of superconducting materials and cryogenics.
Refrigeration below 4.5 K requires the use of helium. Helium is a fluid with a comparatively low density and the lowest liquefaction temperature. Differential pressure flow meters are the most widely used technology for low temperatures, but within refrigerators, orifice flow meters can operate in temperatures from 4.5 K. For superconducting magnet test benches, V-Cone flow meters are used and calibrated at 83 K. Venturi flow meters are suitable for both liquid and gas, with a cold pressure sensor needed for exotic applications.
Coriolis mass flow meters provide another option and when used in cryogenic applications are calibrated with water as surrogate fluid. Coriolis technology does require sufficient liquid density to provide accurate measurements, and therefore pressure drop is the main concern. When Coriolis technology is used for cryogenic service, the transmitter (e.g. electronics) needs to be kept warm, so is installed remotely.
Other special considerations when deploying flow meters in cryogenic applications include appropriate selection of ancillary equipment (e.g. mechanical supports, cables, etc.). Also, electrical cables within the vacuum and cold envelope have a low thermal conductance and thus wires with a smaller cross section should be used.
Coriolis flow meters are used in a number of (non-CERN) cryogenic facilities around the world. Emerson’s Micro Motion™ Coriolis CMF010 and CMF025 sensors were put through the CERN cryogenic qualification program in early 2000. This used metrological apparatus (e.g. venturi) to determine the performance of the devices in cryogenic conditions when measuring the flow of 4.2 K liquid helium, 1.8 K superfluid liquid helium and supercritical helium (5 K, 3 bar).
Micro Motion CMF200 Flow Meters installed inside the insulation vacuum
Radiation is present in the LHC accelerator and therefore the flow meter electronics need to be placed in protected areas. The distance between the sensor and electronic unit can be up to 1 km. To see if the readout was affected by such long cables, tests were performed in supercritical helium at 3 bar and 4.7 K. The tests concluded that measurements were not affected by cable lengths of up to 1 km.
Experience with Coriolis mass flow meters
Micro Motion Coriolis mass flow meters from Emerson have subsequently been used to qualify cryogenic equipment at CERN. They are currently being deployed to better understand the LHC accelerator perturbances observed on the cryogenic control system. An experimental set-up has enabled the characteristics of the cryogenic centrifugal pump, currently part of the ATLAS magnets cooling system, to be measured. In particular, the efficiency of the pump as a function of the mass flow rate was determined. Emerson Micro Motion Elite CMF200 Flow Meters were used, along with a venturi device, in case vibration was an issue.
The Micro Motion Coriolis meters have also been used to understand beams-induced heat loads. For some cooling circuits (typical length ca 50 m) the combined measurement of temperature and mass flow enables better assessment of the heat deposited by the circulating beams in each of the two cooling lines. Twenty Micro Motion CMF010 Coriolis Flow Meters were installed in pairs. The flow meters helped to corroborate the correct estimation of heat load and identify a flow restriction issue. Another area in which the Micro Motion Coriolis meters are helping is with the optimization of the refrigeration plant process. Reliable flow measurements allow the maximum cooling capacity in relation to cost. The Coriolis mass flow meters have operated reliably. The measurements provided have permitted the operations team to look for “missing flows” and revealed a few problems related to the cryogenic equipment.
In 2025 the LHC accelerator will be upgraded with higher field superconducting magnets. This will require a study on the transient effects when a magnet loses its superconducting state. Large amounts of heat will be generated and deposited in the liquid helium, causing an outflow of evaporated helium. Flow meters deployed will need to cope with temperatures of between 5 and 20 K, flow rates of 1 to 2.5 kg/s, and transmit data in 25 Hz bandwidth. Emerson’s Micro Motion 2700 Transmitter with a Profibus interface and analog transmitters are being tested.
