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What Is a Dummy Leg? | Piping Engineering Glossary
What Is a Dummy Leg?
In piping engineering and process engineering, a dummy leg is a short length of pipe welded to an elbow so the piping can rest on a support that the elbow itself cannot reach. It is a common way to support a line where it changes direction, especially on a vertical riser turning horizontal near grade or on a pipe rack. The dummy leg carries no process fluid; it acts purely as a structural extension.
How a Dummy Leg Works
The dummy leg is usually one or two pipe sizes smaller than the run pipe and is welded to the outside of the elbow, sometimes with a reinforcing pad. It is a type of trunnion support. Despite the similar name, it is unrelated to a dead leg, which is a stagnant section of process piping. A small drain or vent hole is often drilled in the dummy leg so trapped water or gas cannot build pressure during heat treatment or operation.
Dummy legs are attached to elbows rather than straight pipe because the change of direction is often exactly where the line needs support, yet there is no straight run nearby for a conventional shoe or clamp. Placing the support at the elbow also helps control loads on nearby equipment nozzles.
Applications
Supporting Elbows at Grade
A vertical dummy leg lets a riser elbow rest on a concrete pedestal or base plate near ground level.
Horizontal Supports on Racks
Horizontal dummy legs extend an elbow sideways to reach a rack beam or structural member.
Pump and Equipment Piping
Dummy legs help support heavy lines near pumps and vessels, reducing loads on nozzles as part of the overall pipe supports arrangement.
Benefits
Simple and Economical
A dummy leg uses standard pipe and fittings, so it is easy to fabricate in the shop or field.
Supports Awkward Locations
It provides support exactly where the line changes direction, without adding a separate structure.
Flexible Design Options
Its length, size, and orientation can be adjusted to suit the support elevation and available steel.
Limitations
Local Stresses on the Elbow
The weld attachment concentrates loads on the elbow wall, which must be checked for pressure, weight, and thermal effects.
Can Restrict Movement
A dummy leg resting on a fixed point may add unwanted restraint if thermal movement is not considered.
Inspection Access
The attachment weld and hidden faces can trap moisture and corrosion, so they need periodic inspection.
FAQ
How is a dummy leg sized?
Designers check its bending and buckling capacity and the local stress at the elbow attachment, often within a broader pipe stress analysis that includes the sustained loads the support must carry.
What sits under a dummy leg?
It typically rests on a base plate, a structural beam, or a pipe shoe-style bearing that allows sliding where thermal movement is expected.
Can any elbow have a dummy leg?
Not always. Thin-walled, high-temperature, or alloy elbows may need reinforcement, a different support type, or detailed analysis before a dummy leg is attached.
Does a dummy leg need a drain hole?
Usually, yes. A small hole near the attachment lets condensation drain and prevents pressure buildup if the closed dummy leg is heated during post-weld heat treatment or hot service. Some specifications instead require the dummy leg to be left open at the bottom end.
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