Preparation Before Installation
Flange and Gasket Inspection: Confirm that the pipe flange specifications (DN, PN, sealing surface type such as RF/MF) completely match the probe flange; clean the flange sealing surface, ensuring it is free of scratches, oil stains, or corrosion. The gasket should be of a type suitable for the operating conditions:
High temperature and high pressure: Flexible graphite spiral wound gasket (temperature resistance ≤450℃, pressure resistance ≥10MPa)
Sanitary grade: PTFE coated gasket or EPDM gasket, compliant with USP Class VI
Explosion-proof environment: Metal serrated gasket or stainless steel coated gasket, ensuring no static electricity accumulation
Probe Status Confirmation: Check that the protective tube is not deformed, the wire insulation layer is not damaged, and the four-wire leads are clearly marked (usually: W1, W2 are excitation lines, W3, W4 are detection lines).
Tool Preparation: Torque wrench (accuracy ±5%), vernier caliper, megohmmeter (500V DC), lint-free cloth, explosion-proof thread sealant (only for threaded connections, not required for flanges).
Mechanical Installation Procedure
Alignment and Positioning: Insert the probe into the flange hole, ensuring that the probe axis coincides with the pipeline centerline, and the insertion depth is ≥100mm to avoid the "wall temperature effect".
Gasket Placement: Center the gasket on the flange sealing surface; do not use adhesives to fix it.
Bolt Pre-assembly: Insert the bolts and nuts, manually tighten them until "slightly snug", and use a steel ruler to measure the gap between the four corners of the flange, with a deviation ≤ 1.5‰ of the flange outer diameter (≤2mm).
Sequential Tightening:
First round: Apply 10% of the target torque in a diagonal sequence.
Second round: Apply 20%.
Third round: Apply 60%.
Fourth round: Apply 100% of the target torque (typical value: approximately 25–35 N·m for DN25 flanges, according to standard GB/T 38343-2019).
Multi-bolt systems (≥12 bolts): Multiply the target torque by a 1.1 compensation factor.
5.Retightening and Additional Tightening:
Retightening: 4–8 hours after initial tightening, repeat the fourth round operation 1–2 times to compensate for gasket creep.
Additional tightening: Perform a full torque retightening once after heating to operating temperature and once after cooling to ambient temperature.
Electrical Wiring Specifications
Four-wire connection (recommended for Class A accuracy):
Excitation circuit: W1 and W2 are connected to a constant current source (such as I+ and I– of a temperature transmitter).
Detection circuit: W3 and W4 are connected to the voltage measurement terminals (V+ and V–), and sharing conductors with the excitation lines is strictly prohibited.
Shielding: Use a twisted-pair shielded cable, with the shield grounded at one end in the control room and left open at the field end to avoid ground loop interference.
Junction box sealing: Use explosion-proof sealing connectors (Ex d type) or IP67 rubber plugs at the cable entry points to ensure no gas penetration.
Grounding requirements:
Normal environment: The probe protection tube is connected to the flange grounding terminal, and the grounding resistance is ≤10Ω.
Explosion-proof environment: Must be connected to the explosion-proof grounding grid through a dedicated copper grounding bar, and the resistance between flanges is ≤0.03Ω (according to GB standard). (50235-2010)
Special Installation Requirements
Hygienic Grade (Pharmaceutical/Food):
Flange uses Tri-Clamp quick-connect structure (compliant with 3A, EHEDG)
Probe insertion angle is tilted 45° to prevent media residue
All contact surfaces are mirror polished to Ra≤0.8μm, supporting CIP/SIP (135℃ steam sterilization)
Explosion-proof Environment (Ex d / Ex ia):
Must use cast aluminum or 316L stainless steel explosion-proof junction box (compliant with GB 3836.1-2010)
Cable entry uses explosion-proof flexible conduit, and the sealing ring is made of fluororubber material
No exposed terminals are allowed in the junction box; all wiring terminals must have terminal caps
Post-Installation Testing and Acceptance
Insulation Resistance Test: Use a 500V DC megohmmeter to measure the resistance between the lead wires and the protective tube. Acceptance standard ≥100 MΩ
Grounding Continuity Test: Use a low-resistance tester to measure the resistance between the flange and the grounding terminal, ≤0.03 Ω
Function Verification: After power-on, use a standard resistance box to simulate Pt100 (e.g., 100Ω corresponds to 0℃) to verify that the transmitter output is within ±0.15℃ error.
Leakage Test: Pressurize the system to 1.25 times the working pressure, maintain pressure for 15 minutes, and visually inspect the flange surface for leaks.
The installation of fixed flange-type platinum resistance thermometers is a system engineering project involving mechanical sealing, electrical isolation, and industry compliance. The core is not "tightening bolts," but ensuring that temperature data can be obtained safely, stably, and audibly even under extreme working conditions. Any oversight can lead to leaks, miscontrol, or safety accidents.
