The Towa Seiden PRL-F7EB and PRL-F7ED Explosion-Proof Flexible Wire Rope Rotary Paddle Level Switches, manufactured by Towa Seiden Industrial Co., Ltd. (Japan), are heavy-duty industrial point level detection instruments specifically engineered for vertical top-mounting in deep storage silos located within hazardous areas. The series resolves two of the most formidable engineering challenges encountered in bulk solids handling: destructive lateral bending forces caused by massive bulk material subsidence, and strict explosive atmosphere safety compliance.

Distinctive specialization of the two models:
1. Model PRL-F7EB (Gas Explosion-Proof / Flameproof Type): Certified to Japanese industrial d2G4 standards (equivalent to international Ex d IIB T4 Gb ratings). The heavy-duty terminal enclosure is cast from high-pressure aluminum alloy (ADC12) featuring precision-toleranced flame-arresting joints (flameproof paths). This mechanical containment quenches internal electrical arcs and prevents internal ignition fronts from igniting external volatile vapors, solvents, or flammable gases across Zone 1 and Zone 2 hazardous areas.
2. Model PRL-F7ED (Dust Ignition-Proof Type): Purpose-engineered for combustible dust atmospheres (Zone 21, Zone 22). It features dust-tight sealing architecture that prevents the ingress of fine particulate matter while maintaining enclosure surface temperatures safely below the minimum ignition temperature of airborne dust clouds (such as pulverized coal, flour, wood dust, starch, and agricultural grains).

Both models incorporate a high-tensile flexible SUS304 stainless steel wire rope suspension stem (Flexible Wire Rope Type). In deep silos exceeding 2 to 4 meters, conventional rigid stems are susceptible to permanent bending or shearing under lateral avalanching loads. The PRL-F7EB/ED replaces rigid stems with a flexible cable coupled to a lower guide weight and sensing paddle. This allows the assembly to deflect dynamically with bulk material flow without exerting destructive bending moments onto the internal drive shaft or bearings.

The operational mechanism utilizes an electromechanical torque-reaction principle with integrated motor overload protection: an internal synchronous motor turns the flexible assembly continuously at approximately 1 RPM. When rising bulk solids impede paddle rotation, the resulting reactive torque rotates the suspended motor assembly, tripping an internal SPDT microswitch (rated 250V AC, 5A) for alarm actuation or feeder shut-off while concurrently de-energizing the drive motor. This eliminates motor coil burnout even during prolonged paddle burial. Mounted securely via a standard JIS 5K65A flange and sealed to an IP65 protection rating, the Towa Seiden PRL-F7EB and PRL-F7ED deliver over a decade of maintenance-free performance in hazardous deep silo operations.

• Manufacturer: Towa Seiden Industrial Co., Ltd.
• Country of Origin: Japan
• Product Models Covered:
  - PRL-F7EB (Gas Explosion-Proof / Flameproof model Ex d IIB T4 / d2G4 for Zone 1, Zone 2)
  - PRL-F7ED (Dust Ignition-Proof model for combustible dust atmospheres Zone 21, Zone 22)
• Product Classification: Explosion-Proof Flexible Wire Rope Rotary Paddle Level Switch
• Explosion Protection Ratings:
  - Model PRL-F7EB: Flameproof Ex d IIB T4 (Japanese d2G4 standard)
  - Model PRL-F7ED: Dust ignition-proof (Dust-tight enclosure with controlled surface temperature)
• Suspension Stem Construction: High-tensile SUS304 stainless steel flexible wire rope absorbing lateral avalanching forces
• Operating Principle: Electromechanical torque-reaction with automatic motor cut-off
• Supply Voltage Ratings: 100V AC, 110V AC, 200V AC, 220V AC (50/60 Hz) or optional wide-range 120-250V AC / 24V DC
• Power Consumption: Approximately 2.5 W to 3.5 W
• Paddle Rotational Speed: Approximately 1.0 RPM (at 50 Hz) / 1.2 RPM (at 60 Hz)
• Electrical Output Contact: 1 SPDT microswitch (Single-Pole Double-Throw)
• Contact Current Rating: 250V AC, 5A (Resistive load)
• Detectable Bulk Density: From 0.2 g/cm³ or greater (Suitable for plastic pellets, cement, fly ash, coal, grains, and chemical powders)
• Process Mounting Connection: Standard Flange JIS 5K65A (Outer diameter 155 mm, 4 bolt holes on 130 mm pitch circle)
• Standard Flexible Rope Length (L): 500 mm to 4000 mm (Custom engineered to silo depth specifications)
• Standard Paddle Configuration: W-1 broad-vane paddle (80 x 80 mm) or folding paddle in SUS304
• Wetted Materials:
  - Flexible Rope, Guide Weight, and Paddle: Stainless Steel SUS304 (AISI 304) or optional SUS316
  - Shaft Sealing: High-performance abrasion-resistant mechanical packing seal / oil seal
• Terminal Housing Material: Heavy-duty aluminum diecast (ADC12) with explosion-proof enclosure integrity
• Process Vessel Operating Temperature: -10°C to +70°C (High-temperature options up to 150°C)
• Ambient Enclosure Operating Temperature: -10°C to +55°C
• Maximum Operating Pressure: Up to 196 kPa (approx. 2.0 bar / atmospheric pressure)
• Ingress Protection Rating: IP65 (per IEC/EN 60529 standard)
• Net Product Weight: Approximately 5.5 kg to 8.8 kg (depending on flexible rope length and flange options)