Total pipe length in a piping isometric drawing

[etsen]

In a piping isometric drawing, there are generally two values about total pipe length. And my question is that why the value in the upper right of the drawing can't be equal to the sum of each pipe length written in it. Thanks a lot.

 

[pennpiper]

etsen,
Your concept of what is contained on a piping isometric seems to be out of touch with the rest of the industry.

Where are you?
What kind of plant or project are you working on?
What design method/tool (manual, AutoCADF, PDS, PDMS,etc.) are you using?

Why do you say two valves?
Is the "upper right corner" you are talking about really the BOM (Bill of Material)?

 

[StressGuy]

I'll take a stab at it - Your dimensions on the face of the ISO are probably the total lengths between points, such as direction changes. That length consists usually of pipe and fittings such as elbows and valves.

In your bill of material (BOM), they want to know how many feet of actual pipe is needs to be bought for fabrication, so that total length is going to be sum of the dimensions LESS the dimensions of the various fittings.

This may explain why you adding up the dimensions doesn't equal the length shown in the BOM.

Edward L. Klein
Pipe Stress Engineer
Houston, Texas

"All the world is a Spring"

All opinions expressed here are my own and not my company's.

 

[tnboy82]

I'll take a stab as well.

I'm assuming that this is exposed piping. Possibly flanged? If so, then it is possible that pipe lengths in the BOM are shorter than length as dimensioned on the isometric. Two possible scenarios,

1) The same reason that StressGuy has pointed out.

2) There may be a gasket allowance for the flange connections. Usually 1/16 to 1/2 inch for gaskets, grounding rings, dielectric insulating kits, etc.

Now I'm going to go out on a limb and ask if this is bell & spigot piping? If so, then laying length (from spigot end to the seat inside the bell) of the pipe will be shorter than the overall length (spigot end to bell face) of each piece of pipe. This will result in a +/- situation on the dimension relative to the BOM, depending on how the pipe is called out (lay length versus overall length) in the BOM.

Hope that helps.

 

[ApC2Kp]

etsen,

As pointed out by pennpiper, StressGuy, and tnboy82, there are a number of factors that could result in the total pipe length to be different than the overall run dimensions.

One more factor could be some isos and shop fab drawings are configured to include the weld gap dimension. These gaps of 0.060 - 0.120 inch can add up when considered to the overall lengths.

The bill of material could also round up the partial length to the next stick of pipe - to order and purchase pipe by the full 20 ft random lengths, and not pay for a cut to get a fraction of a pipe length.

There could be a possible split of shop fab material and field material. A pipe spool with a number of fittings could be shop fabbed, while the straight run of several pipe lengths could have the pipe material shipped to field.

 

[unclesyd]

What is the differences in lengths noted?

What is the total length?

 

[grandnobi]

More wild guesses:

The material listed in the BOM might include same extra straight pipe length to fabricate
- wear pads for sliding supports,
- reinforcing pads for branches,
- trunnions for pipe supports,
- dummy supports,
- mitered elbows,
- field bends (small bore piping),
- spool pieces for pressure test and commsissioning.

 

[11echo]

...A basic concept here, as most here are thinking the quantity in the upper right is the Material Take-Off ...material list. That value is the "estimate" of required pipe PLUS construction extra. Contractors/welders aren't know for there ability to "save" material. If this was gold it might be a different issue, but this is common ASTM steel or similar material. Also when you order pipe, suppliers aren't going to sell you 3'-4 7/16" of pipe. Pipe comes in random (20'-0") and double-random (40'-0") lengths. So quantities need to be rounded up ...I normally shoot to the nearest 5'-0" mark in individual Iso-sheets. And once I totalize the lot, it's rounded up to the nearest 20th increment. My $0.02, Good

 

[gmax137]

I'm just trying to learn something - when you guys say "random" 20'-0" what does the "random" mean? Is it just that the lengths aren't exactly the 20'-0" ?

Thanks!

 

[ApC2Kp]

gmax137,

Example from ASTM A 312,
"13. Lengths
13.1 Pipe lengths shall be in accordance with the following regular practice:
13.1.1 Unless otherwise agreed upon, all sizes from NPS 1?8 to and including NPS 8 are available in a length up to 24ft with the permitted range of 15 to 24 ft. Short lengths are acceptable and the number and minimum length shall be agreed upon between the manufacturer and the purchaser."