What Is a Flat Flange?

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  • Written By: Lori Kilchermann
  • Edited By: Lauren Fritsky
  • Last Modified Date: 02 December 2019
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A flat flange is a device used to create connections for pipelines. There are many different sizes of flat flange designs that are used on different diameters of pipe. A flat flange can be manufactured from several types of steel, from mild steel to stainless steel. They have a common connection method of welding that is used to attach a flange to a length of pipe. The material that the flange is manufactured from is dependent on the type of pipe to which it is being attached.

Many pipelines are built onsite from bulk materials. When this occurs, the pipeline begins as a plain section of pipe with no connective devices on either end. In order to attach the pipeline together, the pipe is welded by qualified welders to prevent leaks or weak spots. At certain points, however, the pipeline may require a section to be removable for any number of reasons. In this event, the pipeline is connected by welding a flat flange to each end of the pipe as well as each end of the removable section, and the section is connected using bolts.


The flat flange is a thick, flat piece of steel that has both a center opening as well as perimeter bolt holes located around the flange. When the flange is welded onto the pipe, the flange forms a connective area that can be used to connect and seal two pieces of pipe together with bolts and a gasket. Typically, the flat flange incorporates a ribbed section around the center opening on one side to allow for a non-slip grip on the gasket material. The flange can have as few as four bolt holes and as many as 12, depending on the pressure rating of the pipeline. For most food-grade pipelines, stainless steel is the material of choice for both the pipeline as well as the flanges used.

On petroleum pipelines, mild or what is also called black pipe is commonly used for the pipeline as well as the flange material. This type of steel produces a pipeline that is safe for high pressure while offering corrosion resistance for both above- and below-ground pipeline applications. The flat flange is attached to the black pipe by welding the two components together after the pipe has been ground smooth to present the optimum fit. The welds are often X-rayed to make certain that no deficiencies are present in the weld that could cause potential danger of breakage in the future.


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