In places like Faraday cages, medical MRI suites, and SCIFs, it can be hard to maintain the integrity of an RF-shielded enclosure. Every cable, door frame, and air duct has the potential to become a leakage point.
When mechanical systems like metal pipes and fluid-filled items cross an RF-shielded boundary, they create paths for electric energy that ''couples'' or is ''conducted''. To prevent this and keep the fluid flow for building systems like heating, cooling, and fire protection systems, engineers allow hydronic waveguide or other approved penetration treatments.
A hydronic waveguide permits fluid to pass through a shielded enclosure while helping reduce RF and EMI transmission through the penetration.
Why Water Pipes Can Create RF Leakage Paths
Any penetration through an RF-shielded wall creates a potential weak point that must be designed and installed to preserve the required shielding performance. Plumbing specifically has two notable problems:
1. Conductive Pathways
If a metal pipe is inside a shielded enclosure, it becomes a great conduit for electrical energy and will carry it to the other side of the enclosure. Although plastic piping is nonmetallic, the fluid in the piping can carry electrical energy, especially if the fluid has electrolytes.
2. Aperture and Slot Antennas
A gap in a shielded wall can allow energy to leak through a shield if the size of the gap is large in comparison to the wavelengths being insulated.
TEMPEST and security threats
In facilities undergoing a TEMPEST inspection, metallic utility lines and penetrations may be assessed by the CTTA to determine if other isolation, grounding, or shielding will be necessary.
What Is a Hydronic Waveguide?
Hydronic waveguides are engineered piping penetrations that permit fluid flow while helping maintain the specified shielding performance of an enclosure.
This uses a plumbing and electromagnetic physics principle called waveguide below cutoff. Whether an electromagnetic mode can propagate through a passage depends on the passage geometry, dimensions, material properties, and signal frequency.
The internal structure and effective path length are designed to provide the required attenuation for a defined frequency range.
How Hydronic Waveguides Work
Allowing fluid flow while attenuating electromagnetic energy requires coordination of RF performance, fluid dynamics, materials, and installation details.
Cutoff Frequency Calculation
Each type of waveguide can only support certain frequencies of wave propagation. Above the cut-off frequency, the waveguide can propagate the signal, whereas below cut-off the signal decays along the length of the waveguide.
Attenuating electromagnetic energy while allowing the flow of fluid requires an understanding and integration of RF engineering, fluid mechanics, and materials, coupled with precision in the construction.
The Dielectric Constant Shift
The normal methods of calculating hydronic piping penetrations work similarly for air or gas, but with the introduction of liquids, the electromagnetic behavior changes.
Filling a passage with water or a water-glycol solution changes its electromagnetic behavior compared with an air-filled passage. Fluid permittivity and conductivity vary with frequency, temperature, concentration, and dissolved substances.
For this reason, engineers should not rely on air-filled waveguide calculations alone when predicting hydronic-waveguide performance. Manufacturer test data should be used for final selection.
Depending on the manufacturer and design, a hydronic waveguide may use multiple smaller fluid passages or another engineered internal geometry to achieve the required combination of flow and RF attenuation.
Grounding and Perimeter Bonding
Even with excellent internal designs, if a shielded assembly is not bonded to the shielded boundary, it can lose its shielding integrity. To prevent leaks on the perimeter, the assembly must be affixed to the shield boundary with the flange, gasket, bonding, fastening method, and gasket as allowed by the manufacturer.
Where Hydronic Waveguides Are Used
Hydronic waveguides may be used where liquid services need to cross an RF shielded boundary and a project calls for high containment of electromagnetic fields.
SCIFs and Secure Government Facilities: For SCIF projects, pipe penetrations, waveguides, grounding mitigations, and non-conductive sections must be identified and assessed. The countermeasures employed are dictated by the facility design, CTTA, and the accrediting authority.
MRI Suites and Medical Facilities: In MRI suites where water and/or glycol services penetrate the RF shield, an engineered penetration may be provided to facilitate flow while maintaining the RF performance of the suite.
EMC and RF Test Chambers: In cases where chilled water or other liquid services are needed in a shielded test chamber, an engineered hydronic penetration may be employed to mitigate the RF boundary while providing no uncontrolled breach to the chamber's environment.
Hydronic Waveguides for Chilled Water and Glycol Systems
In order to cool high-density server racks or MRI systems that require the circulation of large volumes of chilled water or water-glycol solutions, cooling high-density server equipment or MRI systems with chilled water or water-glycol solutions requires coordination among thermal, hydraulic, mechanical, and shielding requirements.
Glycol, when added to water, alters the fluid density as well as its dielectric properties. Commercial HVAC systems require low pressure drops, and where an engineered multi-channel design is used, its pressure-drop and shielding performance should be verified against the manufacturer’s published data.
Key Selection Factors for MEP Engineers
Attenuation needs to be defined in terms of shielding for the project, which will include shielding specifications, applicable agency requirements, and the acceptance-test plan. Although IEEE 299 outlines measurement techniques, it does not identify a global standard for attenuation for every facility scenario.
- ICD 705 Compliance: For SCIF projects, the hydronic penetration system must be aligned with project requirements for physical security, acoustic and RF shielding, grounding, and security of the information, and TEMPEST considerations.
- Pressure Loss and Flow: For some designs of RF shielding tubes, liquid flow pathways with internal flow passages may have increased flow resistance when compared with a standard flow passage RF shielding tube of the same internal cross-sectional flow passage area. The manufacturer’s data showing Cv, flow rate, and pressure drop should be consulted, as well as the fluid and its fittings.
- Dielectric Isolation: Where electrical isolation is required by the project design, the isolation component must be selected separately and coordinated with the piping, grounding, fire-protection, and shielding requirements.
Common Installation Problems
The following describes some installation practices that can greatly reduce or damage the performance of an RF wave guide.
Over Reliance on Mechanical Fasteners: Insufficient mechanical fastening (e.g., poorly mounted flanges, missing RF gaskets, fasteners with insufficient pressure), surface contamination, or discontinuous shield-boundary connections can compromise shielding integrity.
Unapproved Alteration to Grounding/Isolation: Adding or removing isolating components to/from the design can change the approved grounding and shielding design. Follow the manufacturer’s instructions and the project’s requirements of electrical, mechanical, fire protection, and shielding.
Excessive Heat During Installation: Do not expose the assembly to installation temperatures, welding processes, or joining methods that are not permitted by the manufacturer’s instructions.
Selecting the Appropriate Hydronic Waveguide
When selecting hydronic waveguides for typical systems with standard attenuation, the selection should consider the published data sheet of the waveguides, the required flow, Cv, pipe size, and shielding performance as tested.
Where space, flow, and attenuation requirements conflict, consult the manufacturer to select an appropriately tested model and internal configuration.
Testing and Maintaining Shielded Pipe Penetrations
Mechanical damage, corrosion, fluid leakage, damaged gaskets, loose connections, and unauthorized modifications can affect the performance of a shielded penetration.
IEEE 299 testing: Shielding performance should be tested at commissioning and after modifications when required by the project specification, facility owner, shielding consultant, CTTA, or accrediting authority. Where IEEE 299 is specified, testing should follow its measurement procedures.
Corrosion and Moisture Testing: Examine the penetration perimeter, mechanical connections, approved RF gaskets, adjacent isolation components, and piping for corrosion and damage or fluid leakage.
Control Repairs and Modifications: Don’t remove or replace isolation components, RF gaskets, flanges, or the adjacent piping components unless you have confirmed compatibility with the official design and manufacturer requirements.
Protect Your Facility's Secure Penetrations
Plumbing is essential for building operations. Untreated piping penetrations can create potential paths for electromagnetic coupling or signal leakage through an RF-shielded boundary. Properly selected and installed hydronic waveguides allow essential fluid services to cross an RF-shielded boundary while supporting the enclosure’s specified shielding performance.
That performance relies on an integrated design for the penetrations, authorized installation by the manufacturer, bonding to the shielded boundary, and tailored acceptance testing for all shielded enclosure hydronic connections.
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