1. Enrico Da Riva (EN/CV/PJ)
Aerosols sampling in CERN radiation monitoring stations - Evaluation of sampling efficiencies -
Enrico Da Riva (EN/CV/PJ)
E. Da Riva

2. SUMMARY 1. Introduction 2. Results 3. Conclusions Problem definition
Deposition mechanisms
Efficiency definitions
CFD simulations
2. Results
Sampling probes
Sampling pipework (diameter, flow rate, bends)
ISOLDE modification
3. Conclusions
Summary of recommendations
E. Da Riva

3. CERN ventilation monitoring stations
Radioactive particles bound to aerosols are exposed to the environment by the stack releases of ventilation stations.
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4. Radioactive aerosol monitoring
EXHAUST DUCT
SAMPLING PIPEWORK
Figure 1: ISOLDE stack final section and sampling system, with particles trajectories as a result of CFD calculation.
TO THE INSTRUMENTATION
Radioactive aerosol particles smaller than 10 μm are released from the exhaust ducts of ventilation stations at CERN to the environment.
Sampling systems consisting in a sampling probe inserted in the duct and a pipework are installed downstream of the ventilation filters.
Since a fraction of particles sticks to the walls of both the probe and the sampling piping, the sampling might be not representative.
E. Da Riva

5. Deposition Mechanisms
Sedimentation: deposition of particles on the lower surface of a pipe, driven by gravity.
Inertial deposition: deposition of particles in bends, contractions and enlargements, due to the particles inertia which causes particles not to follow the flow. It occurs also along straight pipe section during turbulent flows.
The sampling pipe should be as short and as straight as possible.
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6. Efficiency definitions
The sampling (or global) efficiency is the product of the three efficiencies.
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7. Why CFD? CFD SIMULATIONS
STANDARDS: ISO 2889 “Sampling airborne radioactive materials from the stacks and ducts of nuclear facilities”. CERN sampling systems don’t follow yet standards especially for what concerns the sampling probe.
Empirical correlations*: empirical correlations are available but their range of applicability is outside typical CERN operating conditions (e.g. max sampling pipe diameter in correlations D = 10 mm, at CERN 100 mm < D < 160 mm).
CFD SIMULATIONS
Capable to evaluate every single particle trajectory.
Gravitational, inertial and turbulent depositions taken into account.
Entrainment (return of a stuck particle in the flow) neglected in the simulations.
ISOLDE geometry has been taken as the reference because of the length of the sampling line (~15 m with 8 bends) and unconventional sampling conditions.
Results can be used to evaluate the sampling efficiency at any CERN station.
* P. A. Baron, K. Willeke, Aerosol Measurement - Principles, Techniques and Applications, John Wiley & Sons, 2001.
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8. CERN monitoring stations
Operating conditions
CERN monitoring stations
CFD simulations
Sampling probe
45° cut
45° cut + coaxial
Sampling pipe diameter
D = 160 mm (for ISOLDE)
D = 100 mm (all other stations)
Sampling pipe flow rate
25 m3/h (for ISOLDE) → 0.35 m/s
20 m3/h (all other stations) → 0.8 m/s
10 m3/h < Q < 300 m3/h
Velocity in the main exhaust duct
2 m/s (run mode for ISOLDE)
4 m/s (flush mode for ISOLDE)
Particle aerodynamic diameter dp*
0.1 μm < dp < 10 μm
*Aerodynamic diameter: diameter of a sphere of density ρ0 = 1000 kg/m3 that has the same gravitational settling velocity as the particle in question.
with ρ0 = 1000 kg/m3
ISOLDE sampling pipe
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9. RESULTS 2. Results Sampling probes
Sampling pipework (diameter, flow rate, bends)
ISOLDE modification
E. Da Riva

10. Sampling probe 1/3 A) 45° Cut B) Coaxial
Currently used in all the radiation monitoring stations at CERN
B) Coaxial
45° cut probe is very inefficient.
Particles at the inlet are subjected to a strong deviation.
The velocity field in the probe is characterized by large eddies.
High velocity and eddies make easier for the particles to stick on the walls.
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11. Sampling probe 2/3
The coaxial sampling probe is heavily recommended for all the CERN stations.
Coaxial probe allows to heavily increase the sampling efficiency.
The influence of the main exhaust pipe velocity is also decreased.
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12. Sampling probe 3/3
Shrouded probe: slightly diverging coaxial probe on which a “shroud” (cylinder) is applied.
Recommended in ISO 2889.
This design is expected to increase even more the sampling efficiency, especially when operates in off design conditions and not isokinetically.
Not considered in the CFD simulations.
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13. Figure 6: Example of deposition of 0.1 μm particles.
Bends
ISO 2889 recommends to always use a curvature ratio r/D equal to 3.
Others* recommend using the largest possible curvature radius, without exceeding 8D.
CFD simulations have been run with curvature ratios r/D = 1 and r/D = 3, showing no relevant influence.
The transport efficiency of bends at typical CERN operating conditions is ~95%.
* J. Charuau, Optimisation de la detection des aerosol radioactifs dans un local, Seminaire SFRP, Pierrelatte, Mars 1982
dp = 0.1 μm
No particles stick on the wall
Figure 6: Example of deposition of 0.1 μm particles.
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14. Figure 6: Example of deposition of 0.1 μm particles.
Sampling pipework 1/2
Example of deposition of 0.1 μm particles
Example of deposition of 10 μm particles
Transport efficiency depends on: 1) particle size
2) air flow rate
3) sampling pipework diameter
Gravitational deposition is relevant only for “big particles” (dp = 10 μm) in horizontal pipes.
Inertial and turbulent depositions are relevant for all particle sizes.
dp = 0.1 μm
No particles stick on the wall
Figure 6: Example of deposition of 0.1 μm particles.
E. Da Riva

15. Figure 6: Example of deposition of 0.1 μm particles.
Sampling pipework 2/2
TRANSPORT EFFICIENCY OF ONE METER OF HORIZONTAL PIPE
At CERN operating conditions, the transport efficiency is expected to decrease when increasing the sampling flow rate.
D = 160 mm is better than D = 100 mm.
The current diameter and flow rate of ISOLDE (D = 160 mm, 25 m3/h corresponding to m/s and Re ~ 3000) is fine (but not the pipe length!).
The most typical CERN operating condition (D = 100 mm, 20 m3/h corresponding to 0.8 m/s) is only slightly worse.
dp = 0.1 μm
No particles stick on the wall
Figure 6: Example of deposition of 0.1 μm particles.
E. Da Riva

16. Figure 6: Example of deposition of 0.1 μm particles.
ISOLDE modification 1/2
Two modifications are proposed:
Sampling replacement with an coaxial probe
Installation of the aerosol samplers on a bungalow on the rooftop
(sampling pipe length 8 m with 3 bends instead of 15 m with 8 bends)
dp = 0.1 μm
No particles stick on the wall
Figure 6: Example of deposition of 0.1 μm particles.
E. Da Riva

17. Figure 6: Example of deposition of 0.1 μm particles.
ISOLDE modification 2/2
With the modifications to the inlet probe and the sampling pipework (green curve), the global efficiency is increased by 2~5 times (depending on ventilation mode and particle size).
No change for the present sampling pipe diameter is foreseen.
Isokinetic sampling (i.e. same velocity in the main and sampling duct) is not recommended, since it would imply a much bigger and cumbersome aspiration pump (25 m3/h → 300 m3/h!), without any relevant improvement.
dp = 0.1 μm
No particles stick on the wall
Figure 6: Example of deposition of 0.1 μm particles.
E. Da Riva

18. General transport efficiency correlation
The CFD results were reduced in order to obtain a simple algebraic equation able to provide a fast estimation of transport efficiency at CERN ventilation stations.
Provided that the technical standard ISO 2889 is complied with, such equation can be used as an additional aid for the design of future aerosol sampling stations at CERN.
dp = 0.1 μm
No particles stick on the wall
Figure 6: Example of deposition of 0.1 μm particles.
E. Da Riva

19. Figure 6: Example of deposition of 0.1 μm particles.
CONCLUSIONS
SAMPLING PROBES
The 45° cut sampling probe currently used at CERN is not taken into account in the ISO2889 and CFD simulations showed that it has very poor efficiency, especially when the sampling is not isokinetic (i.e. same air velocity in the sampling pipe and the stack).
A coaxial nozzle is heavily recommended.
SAMPLING PIPEWORK
The sampling pipe should be as short and as straight as possible.
CFD simulations show a negligible influence of the curvature ratio at typical CERN diameters. However, the adoption of curvature ratio 3 is suggested, as recommended by ISO 2889.
The current pipe diameter and air flow rate at CERN stations (i.e. D = 100 mm, Q = 20 m3/h corresponding to 0.8 m/s) is reasonable.
An only slightly better transport efficiency could be obtained using bigger pipes such as in ISOLDE (D = 160 mm, Q = 25 m3/h corresponding to 0.35 m/s).
Plastic pipes should be avoided since electric fields can cause higher particle losses.
ISOLDE
The installation of a coaxial probe and the transfer of aerosol samplers on a bungalow on the rooftop is highly recommended (without any change of pipe diameter and air flow rate).
ADDITIONAL INFORMATION IN EDMS
dp = 0.1 μm
No particles stick on the wall
Figure 6: Example of deposition of 0.1 μm particles.
E. Da Riva

20. REFERENCES
ISO 2889 “Sampling airborne radioactive materials from the stacks and ducts of nuclear facilities”.
EDMS : CFD Project report: Aerosols sampling in CERN radiation monitoring stations - Evaluation of sampling efficiencies.
P. A. Baron, K. Willeke, Aerosol Measurement - Principles, Techniques and Applications, John Wiley & Sons, 2001.
J. Charuau, Optimisation de la detection des aerosol radioactifs dans un local, Seminaire SFRP, Pierrelatte, Mars 1982.
E. Da Riva

21. THANK YOU
E. Da Riva