Definition
Internal Diameter — Internal diameter (ID) is the bore diameter of a tubular — the diameter of the fluid passage — which together with the outside diameter defines the wall thickness and governs fluid flow area and hydraulic pressure loss.
Also known as: internal diameter, inner diameter, ID, bore
What internal diameter is
Internal diameter (ID) is the diameter of a tubular’s bore — the open fluid passage down the center of the pipe. For a given outside diameter, the ID is fixed by the wall thickness: a thicker wall gives a smaller bore. The ID sets the cross-sectional flow area, which in turn controls fluid velocity and frictional pressure loss as drilling mud is pumped down the string.
On drill pipe the tube ID differs from the bore through the tool joint; connections like Internal Flush are designed to keep the tool joint bore close to the tube ID to minimize turbulence.
Why it matters
- Hydraulics — a larger ID lowers standpipe pressure for a given flow rate.
- Wall relationship — ID plus wall thickness equals OD, so any two define the third.
- Pass-through — the drift diameter, always slightly less than ID, governs what tools will pass.
Renting the right bore
Balancing ID for hydraulics against wall thickness for strength is part of string design. We rent drill pipe and workstring tubing in the sizes and wall thicknesses that suit your flow and pressure program — contact us for guidance.