Piping practice: Normal screwed (threaded) fittings are typically used up to what nominal pipe size (inches)?

Introduction / Context:Choosing a jointing method (threaded, flanged, welded) depends on pipe size, pressure, and service. Threaded (screwed) fittings are convenient for small diameters but become impractical and unsafe as size and pressure increase due to leakage and make-up torque limitations.

Given Data / Assumptions:

• Industrial service with standard carbon steel piping.
• “Normal” threaded fittings—i.e., routine plant practice.
• Moderate pressures and temperatures.

Concept / Approach:Threaded joints are common in small-bore piping because assembly is simpler and cheaper. As diameter grows, the risk of leakage, thread galling, and low mechanical rigidity rises. Many design handbooks recommend limiting routine use to small sizes; typical conservative limits are ≤ 1.5 inch in many plants (with some practices extending to 2 inch for benign service). Beyond that, flanged or welded connections are preferred.

Step-by-Step Solution:

Verification / Alternative check:Company standards and codes of construction commonly cap threaded joints at small nominal sizes for pressure-rated service.

Why Other Options Are Wrong:

• 3, 5, 7 inches: too large for “normal” threaded practice; flanged or welded joints are preferred.

Common Pitfalls:Applying small-bore rules to high-pressure/high-temperature or hazardous service—threaded joints may be prohibited regardless of size in such cases.

Final Answer:1.5