Welded vs. Seamless API Pipe

API pipe and tubing is mostly used for conveying oil, gas, and water. Tubular products may be welded or seamless, and each type has advantages and disadvantages. Welded tubing starts with a hot or cold rolled coil. Cold-rolled steel provides tighter tolerances and good mechanical properties. The steel coil is cut to precise widths and formed into tubing by forming and welding, and also sizing and cutting processes. Different types of welding may be used, with laser welding having the best reputation for meeting purity requirements.

How Seamless Pipe Is Made

Seamless pipe starts with a metal bar. It is pierced, extruded, or gun-drilled to create pipe. Piercing is used with stainless steel alloys with less than 12% chromium. Extrusion, which forces the bar over a mandrel and through a die is the most common method of making seamless pipe. Gun-drilling is more expensive, but many believe it creates a higher quality pipe. With gun-drilling the bar is fed over a stationary flute drill and is rotated throughout, producing superior straightness and concentricity.

Availability and Cost

API 5L pipe is widely used, and may be either seamless or welded. Seamless API 5L pipe is more expensive. With seamless hot-rolled pipe, the number of available alloys is limited, whereas with welded pipe, a wider variety of materials may be used. There is generally a wider variety of welded pipe readily available, whereas seamless pipe may have to be ordered.

Surface Options

With seamless pipe, surface condition is determined by hot forming. An "orange skin" surface is produced with hot forming, and this texture must be grinded away, raising the cost. Welded pipes may be made from raw materials with many different surface types. Further refinement of the surface of welded pipes may be done with processes like grinding as well.

Tolerances

For hot rolled seamless pipe, wall thickness tolerance is 12.5%, as specified by the American Society for Testing and Materials (ASTM). However, with hot rolled seamless pipe, the cross-section may not be perfectly symmetrical. Generally, with steel plate or coil that is even, the seamless or welded pipe created will have consistent tolerance throughout the entire pipe.