Last Updated: April 7, 2026 | Author: Wu Peng, Senior Instrumentation Engineer

Stilling wells isolate radar antennas from surface turbulence, foam, and obstacles. When tanks have agitation or low-dielectric liquids, a properly sized well dampens noise and stabilizes signal. This guide covers sizing, materials, hole specifications, and installation practices based on API 2350 and field experience.

Quick Reference: Typical improvement: 20-40% reduction in measurement noise across foam applications, agitated vessels, and liquids with dielectric constants below 2.5.
Table of Contents

What Is a Stilling Well?

A stilling well is a cylindrical pipe inserted into a tank. It creates a low-velocity zone for the radar antenna, isolating it from surface agitation. Small holes in the pipe sidewall allow liquid inside to reach equilibrium with tank level while damping rapid fluctuations from foaming or mixing action.

The mechanism is simple: liquid enters through calibrated holes (typically 4-5mm, spaced 5-10cm apart) on the lower pipe sections. Above those holes, the wall is solid, creating a damping volume. This arrangement allows step changes in tank level to propagate through within 1-2 seconds while filtering high-frequency noise.

Straight Pipe vs. Bypass Chamber

Straight Pipe: Suspended from a tank nozzle. Fast installation, low cost, typical response time 1-2 seconds. Suitable for temperatures up to ~200°C (SS316L). Requires tank drain-down for removal.

Bypass Chamber: External vessel with inlet and outlet connections. Higher cost and larger footprint but operates to 400°C+. Better for high-pressure vessels and where tank shutdown is impractical. Response time ~5-10 seconds due to external chamber volume.

When Do You Need One?

Foam or surface agitation: Radar cannot distinguish between liquid surface and foam layer. A stilling well separates the antenna from the foam zone—typical error without one is 5-15%.

Mixing or agitation: Moving surface creates noise in the radar signal. The well’s damping removes these fluctuations, stabilizing PID loops and reducing false alarms.

Low-dielectric liquids (under 2.5): Weak reflections demand a stable, calm interface. Even small turbulence can cause signal loss. The quiet environment inside a well improves signal return by 30-50%.

Obstacles or internals: Baffles, heating coils, or feed pipes block direct antenna view. A stilling well provides a clear measurement path independent of tank internals.

Floating roof tanks: Roof movement produces false echoes. The stilling well separates the antenna from structural reflections.

High-viscosity liquids: Slow to equilibrate. A stilling well with appropriate hole sizing dampens surface disturbance while allowing level changes to propagate through in 1-2 seconds.

Design Specifications

Material Selection

SS316L: Standard choice for corrosive or food/pharma contact. Resists acids, salts, and most organics. Typically 20-30% cost premium over carbon steel.

Carbon Steel: Adequate for water, non-corrosive oils, and neutral liquids. Requires paint or internal coating—bare steel will scale and affect hole diameter and signal reflectivity. Spherical tanks often mandate SS due to stress concentration at nozzles.

Alternatives: Inconel or Hastelloy for extreme chemical exposure. PVC for very low temperature. Duplex stainless for high-pressure, high-corrosion environments.

Stilling Well Diameter & Sizing Specifications

Nominal Diameter Tank Nozzle Size Antenna Type Typical Length Wall Thickness
DN125 (125mm/5″) 5″ NPT or Flanged Horn antenna, Small horn 1.5 – 3m 2.0-2.5mm SS, 2.5-3mm CS
DN150 (150mm/6″) 6″ NPT or Flanged Standard horn, Drop antenna 2.0 – 4m 2.0-2.5mm SS, 2.5-3mm CS
DN200 (200mm/8″) 8″ NPT or Flanged Large horn, 26GHz/80GHz 2.0 – 5m 2.5-3.0mm SS, 3.0-3.5mm CS
DN250 (250mm/10″) 10″ Flanged Large horn, Multi-antenna 2.5 – 6m 3.0-3.5mm SS, 3.5-4.0mm CS
DN300 (300mm/12″) 12″ Flanged Extra-large applications 3.0 – 7m 3.5-4.0mm SS, 4.0-4.5mm CS

Hole Design

Diameter: 4-5mm is standard. Smaller holes (<3mm) restrict flow and slow level response. Larger holes (>6mm) pass turbulence and foam. Typical tolerance ±0.3mm due to drilling.

Spacing: 5-10cm vertical distance between hole centers. This determines damping time constant. Closer spacing (5cm) increases damping. Wider spacing (10cm) allows faster response but less noise suppression.

Stagger pattern: Alternate holes on opposite sides to prevent preferential flow direction. For a DN200 pipe (200mm OD), alternate left and right every 5-7cm.

Bottom section: Keep the lowest 100-150mm without holes to prevent siphoning. This is critical—if all holes go to the pipe bottom, liquid can siphon out when tank drains, holding a false low reading.

Top vent: Two 6-8mm ports (or one 8mm port with 6mm check valve) near the pipe top. These equalize pressure and prevent vacuum lock when level drops rapidly. Essential for stable response.

Bottom reflector plate: A flat SS plate welded or bolted to the pipe bottom. Improves radar echo return. Thickness 2-3mm. Must be perpendicular to pipe axis (within 1-2°) to avoid side echoes.

Installation Practices

Gap tolerance (OD to nozzle ID): 1-3mm maximum. Larger gaps allow turbulence and foam to bypass the well. Spec a gasket of appropriate thickness. Measure with feeler gauges during assembly.

Connection method: Welded flange for permanent installations on pressurized tanks. Bolted flanges for modular or removable designs. Threaded NPT works for small wells (DN125) at pressures below 10 bar.

Antenna position: Mount on a rigid support to prevent drift or vibration. Center the antenna along the pipe axis. For horn antennas, maintain 5-10cm radial clearance from the pipe wall to avoid reflections. Position antenna 10-15cm below the nozzle top.

Sealing: Use rubber gasket with Teflon or PTFE reinforcement. Silicone gaskets above 150°C. Metal O-rings for high-pressure service (over 20 bar).

Installation height: Submerge the stilling well to about 80% of minimum operating level. This ensures holes remain in liquid for most operating range, preventing air entry that would degrade radar signal.

Radar Transmitter Compatibility

26GHz pulse radar: Preferred for low-dielectric liquids (0.5-20, e.g. refined oils, alcohols). The longer wavelength penetrates weak reflections. Stilling well response time ~1-2 seconds. Cost-effective and robust in industrial environments. See 26GHz pulse radar transmitters.

80GHz FMCW radar: Higher frequency, narrower beam, faster signal processing (0.5-1 second response). Better immune to external RF noise. Suitable for foam and high-agitation tanks. See 80GHz radar level transmitter guide.

Guided wave radar (GWR/TDR): Rod or cable probe conducting signal directly through the liquid. Works on any dielectric over 1.4 (including very low values). Requires only a small bypass nozzle, not a full stilling well. Excellent for complex tank geometry. See guided wave radar specifications.

Common Mistakes

Gap over 3mm: Foam and turbulence bypass the well. Seals fail from corrosion or mechanical stress. Measure with feeler gauges; use proper gasket thickness.

Holes under 3mm: Restrict flow; level lags tank changes. Drilling tolerances widen gaps. Minimum 4mm holes, staggered pattern.

Holes over 6mm: Turbulence enters; noise increases. Bottom holes allow siphoning. Limit to 5mm, place only in middle 70% of pipe.

No vent ports or blocked vents: Vacuum prevents level drop. Liquid gets trapped inside well. Install two 6-8mm ports at top; use ball valves for maintenance access.

Antenna too close to wall: Reflections from metal pipe create double echoes. Center antenna; maintain 5-10cm clearance. Check with feeler gauge during commissioning.

Well positioned too high: Holes above minimum operating level expose antenna to air. Tank bottom becomes unreachable. Keep lowest holes at least 10-15cm below minimum level.

Scale or corrosion buildup: Blocks holes; changes damping. Removes protective surface from stainless steel. Specify removable flange design; implement annual cleaning for corrosive service.

Standards & Compliance

API 2350: Section 5.4 covers stilling well requirements for petroleum tank gauging. Mandates vent ports to prevent siphoning, specifies hole sizing and spacing for equalization within 30-60 seconds, requires documentation of diameter, material, and installation method.

IEC 61511: Functional safety standard applicable to safety-critical level measurement (overflow protection, process interlocks). Stilling well material, pressure rating, and installation must support the required SIL rating of the overall instrumentation system. Material compatibility with the process fluid is mandatory.

ASME B31.1: Power Piping Code governs bypass chamber design and connections. Relevant for high-temperature or high-pressure installations. Specifies weld quality, hydrostatic testing, and pressure relief requirements.

FAQ

What is a stilling well?

A cylindrical pipe inserted into a tank that dampens surface turbulence and foam. Calibrated holes allow liquid inside to equilibrate with tank level while damping rapid fluctuations. Isolates radar antenna from agitation.

When do I need one?

Tanks with foam, agitation, or mixing. Low-dielectric liquids (under 2.5). Obstacles or internals blocking antenna view. Floating roof tanks. High-viscosity liquids. Calm, clear tanks with good dielectric do not require one.

What is the difference between a stilling well and a bypass chamber?

Straight pipe well: suspended from tank nozzle, fast response (1-2 sec), low cost, limited to ~200°C. Bypass chamber: external vessel, high cost, operates to 400°C, slower response (5-10 sec), no tank shutdown needed for maintenance.

What material should I specify?

SS316L for corrosive or food/pharma service. Carbon steel for water, non-corrosive oils (requires coating). Spherical tanks typically mandate stainless due to stress concentration at nozzles. Select based on liquid chemistry and temperature.

What hole size and spacing?

Diameter 4-5mm. Spacing 5-10cm vertical. Staggered on opposite sides. Keep bottom 100-150mm without holes to prevent siphoning. Two vent ports (6-8mm) at top to prevent vacuum lock.

What is the gap tolerance between well and nozzle?

Maximum 1-3mm. Measure with feeler gauges. Larger gaps allow bypass of foam and turbulence. Proper gasket thickness and surface finish are critical.

Which radar frequencies work best?

26GHz pulse radar: preferred for low-dielectric liquids (0.5-20). 80GHz FMCW: faster response, better for high-foam applications. Guided wave radar: smallest footprint, any dielectric over 1.4.

Do stilling wells meet API 2350 and IEC 61511?

Yes. API 2350 Section 5.4 specifies stilling well requirements for tank gauging: vent ports, siphon prevention, hole sizing. IEC 61511 requires material compatibility and pressure rating to support the SIL rating of the overall instrumentation system.

Related Resources

Technical references on radar level measurement:

About the Author

Wu Peng is a Senior Instrumentation Engineer with 18+ years in level measurement, process control, and tank gauging. Leads the technical team at Sino-Inst Engineering across oil & gas, chemical, water treatment, and power generation applications. Certified in IEC 61511 Functional Safety and API 2350 compliance.

Engineering consultation available for: Custom stilling well design, material selection, installation commissioning, API 2350 and IEC 61511 compliance review.

Contact: info@drurylandetheatre.com | +1-555-0123 | https://www.drurylandetheatre.com