Purchasing a 1 1/2" threaded flange might seem straightforward, but in high-precision industrial environments, the smallest deviation from the ASME B16.5 standard can lead to catastrophic leaks or installation failures.
Whether you are sourcing for a new project or replacing old components, ensuring the quality of your 1.5 inch NPT flange is paramount. Use this professional checklist to evaluate your suppliers and products.
1. Verify Thread Precision (NPT Standards)
The "Threaded" part of a 1.5" threaded pipe flange is its most critical feature.
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The Standard: Threads must comply with ASME B1.20.1.
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The Test: A high-quality flange should be checked using calibrated plug gauges. If the thread is too loose, it won't seal; if it's too tight, it will gall (seize up) before fully seating. Always ask your supplier if they perform 100% thread gauging.
2. Inspect the Flange Face Finish
Even though it is a threaded connection, the ASME B16.5 1.5 inch flange typically features a Raised Face (RF).
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Surface Texture: The face should have a specific serrated finish (often referred to as a "stock finish" or "smooth finish" depending on the gasket used).
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Why it matters: These concentric grooves grip the gasket to prevent lateral movement under pressure. Ensure there are no radial scratches that could create a "leak path."
3. Material Traceability and Marking
A legitimate 1 1/2 threaded flange must be traceable back to its original heat batch. Check the outer rim of the flange for mandatory stamps, which should include:
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Manufacturer’s Logo
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Material Grade: (e.g., ASTM A105 for Carbon Steel or F316 for Stainless Steel)
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Pressure Class: (e.g., 150# or CL150)
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Size: 1 1/2" (or DN40)
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Heat Number: A unique code linked to the Material Test Report (MTR).
4. Dimensional Accuracy Beyond the Threads
Beyond the bore, the external dimensions of the 1.5" threaded pipe flange must be precise to ensure it fits with other Class 150 components:
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Bolt Hole Alignment: The four bolt holes must be perfectly spaced at 3.88 inches (98.5 mm) center-to-center.
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Thickness: The minimum flange thickness for a 1.5" Class 150 flange is 0.69 inches (17.5 mm). Reducing this thickness is a common way for low-quality manufacturers to save costs, but it compromises the pressure rating.
5. Coating and Corrosion Protection
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Carbon Steel (A105): Should be supplied with a light oil coating, black paint, or electro-galvanization to prevent rust during transit.
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Stainless Steel (SS316): Should be "pickled and passivated" to restore corrosion resistance after the machining process.
Conclusion: Don't Compromise on Safety
When it comes to 1.5 inch NPT flanges, the cheapest option is often the most expensive in the long run due to downtime and repairs. Always prioritize suppliers who provide full MTRs (Material Test Reports) and adhere strictly to the ASME B16.5 dimensional tables.
