I'll give an example to explain better what I'm trying to say. In Australia you've long abandoned the British measures, so I'll use metric units.
Let's suppose I've got to size a piping carrying 100 cubic meters an hour of water at ambient temperature. I'll adopt a velocity of 1,8 meters per second of water within the pipe.
100 cubic meters per hour is equal to 0,0278 cubic meters per second. 0,0278 cubic meters per second divided by 1,8 meters per second gives a cross sectional area of 0,0154 square meters. 0,0154 square meters corresponds to a diameter of 0,140 meters or 140 millimeters. This is of course the inside diameter.
There's no pipe whose inside diameter is 140 millimeters, so I'll choose the next upper inside diameter, 154 millimeters, which is the inside diameter of a 6 inches, Schedule 40, nominal size pipe.
Now, I'll recalculate the velocity within the piping to be able to calculate the loss of head of the water flow.
The cross sectional area of the selected pipe (6 inches, Sch 40) is 0,0186 square meters, based of course on the inside diameter.
0,0278 cubic meters per second divided by 0,0186 square meters gives a velocity of 1,49 meters per second, which I'll use to calculate the loss of head.
All of the calculations have been made using the inside diameter.
Cordially
Giovanni S. Crisi
Shane/Giovanni,
In a sense you both are right. One is talking predominantly of manufacturing pipe the other design considerations.
And especially in power the manufacturing specifications for pipe diameter is often irrelevent. Heavy walls for steam service is often designed by ID regardless of ASME manufacturing specifications.
The design engineer will function just as Giovanni mentioned determining ID and minimum wall.
Min wall pipe is quite common in power. And it is always determined by ID since you cannot find manufactured wall thicknesses exactly as designed. Otherwise the number would be theoretically infinite.
ID is based upon the desired flow velocity, thickness upon temp and pressure. The OD will be whatever happens to be available at the time, within limits of course due to weight, restraint, cost, and support issues.
Just to keep things going, not all tube is exact OD,
Have a look at:
Type K copper (ASTM B88) 2" type K is 1.959 ID, 2.125 OD, .083 wall
2" SS tubing (ASTM 213/269), 1.624 ID, 2" OD, .188 wall
ACR tubing (ASTM B280), happens to go by exact OD
DWV copper tube has the same OD as the Type K, but very thin wall
I think Pipe has been well covered above