Allowable Pipe Sag

For one thing, it may depend on how long a span between supports. For another, it may depend on the slope of the pipe (it may be desirable to have no uphill portions). A third consideration is how much of the pipe's strength goes to supporting the pipe and its contents, versus containing the pressure of the contents. A fourth question is how critical a failure would be.

I think the basis for the need to have a limit on the pipe sag is the pipe strength. The allowable stress in the material when deflecting is the real limit.

There might be just a different safety factor applied by the different companies.

Are the limits all for the same pipe dimension, application and material?

• 20mm sag on a 15mm ID pipe means the fluid will be going uphill at some point in a nominally horizontally-installed pipe. Is this acceptable (rhetorical question - NNTR)?
• It is possible to introduce those levels of sag on some pipes by standing on top of the pipe between pipe supports, given diameters, wall strengths and distance between supports. Is this acceptable (rhetorical question - NNTR)?
• Where would one use a hose in preference to a pipe and maintain the sag criterion(rhetorical question - NNTR)?
• Aesthetics - is drooping pipe acceptable (rhetorical question - NNTR)?
• Etc.

Deviating from Client Standards has the following possible consequences:

• Dismantle and re-install to a higher standard at the Contractor's expense.
• Invoke of penalty clauses for delays incurred by the above.
• Delays to leaving site.
• Delays to arrival at the next site.
• Loss of business.
• Loss of reputation.
• Reduced risk of being approached for further contracts.
• Etc.

just keep the 1/4 inch / foot slope maintained while the pipe is loaded and forget minutiae

Shafft:

"What is the basis of limiting pipe sag?"

Competent Piping Engineers know that the piping deadload stresses must be limited. These stresses can be limited by establishing piping span tables that are appropriate for the specific piping system. Concentrated loads (such as valves) must be supported separately. The goal here is to make sure that the span lengths are not exceeded, not to limit "pipe sag". On some selected systems, such as steam systems the pipe span must not be exceeded AND the design pipe slope must also be maintained. This can be a challenge.

In B31.1 it is limited to 2.5mm (0.1 inch).

NO, B31.1 makes recommendations for span lengths, and, as stated above span lengths are related to deflection.... This is only a suggestion, NOT A MANDATORY REQUIREMENT. !! B31.1 is quite clear that different span lengths are needed for water filled vs. gas filled systems. Also, the pipe materials and pipe schedule has an affect ( Fiberglass & schedule 10S systems have very short span lengths)

Span lengths and defections are very simple to calculate.