Steel Pipe Restraint - Is It Necessary for This Application

[ohiowater]

We are replacing a a very long 42" welded steel pipe header at a water treatment plant with the same. The header is in a gallery (not buried) and carries water from flocculation to sedimentation tanks - this is gravity flow with no bends (probably about 6' driving head). Because of the sequencing necessary to keep portions of the line in service, couplings are being shown to connect new segments to existing welded steel segments.

The contractor is asking for harnessing (tie-rod) details, and client is asking why is that necessary? It seems harnessing is always just an assumption to put it in to be safe, but there is no thrust to speak of, pipe flows less than half-full, and the cost could be come extensive considering the length of the header. Would harnessing really be necessary in this application?

 

[bimr]

There does not seem to be any forces to restrain so why would you use a restraint? Thrust restaints are not necessary.

 

[StephenA]

If the pipe is straight then there is nowhere for it to go. The danger would be if a section is removed and a blank fitted without restraining. The 6' head could then push the pipe out of a coupling.

Stephen Argles
Land & Marine

http://www.landandmarine.com

 

[SteveWag]

You say driving head is 6-feet and then you mention “half-full”. Are there any valves or gates? Is there any way that the pipe could see the entire six feet of head? If the pipe is less than full you have little to worry about. BUT, 42-inch pipe gives 1385 sq. in. and 6 feet is 2.6 PSI or about 3600 pounds of thrust with no water hammer thoughts. Look at getting set-screw couplings.
Steve

 

[rconner]

There is of course nothing inherently wrong with unrestrained pipe joints and connections, and indeed much piping in even exposed applications is configured successfully with such connections. However, they must be recognized for what they are i.e. that they do not provide any dependable resistance to axial pulls on the joints (from whatever source, including unbalanced pressure thrust acting on such features as unblocked dead ends, bends, reducers, closed or partially closed or throttled valves, etc.) Many unrestrained connections, unlike welded or flanged connections, are also “flexible”, meaning they are in effect structural hinges in the system, and as such they have no mechanism in an of themselves to prevent deflection or lateral movement. While this is not a problem in normal buried systems of relatively straight piping, where the soil provides ample resistance to restrain the normally quite small lateral forces (even with quite high pressures or flows), it can obviously be more of an issue in exposed piping. Quite simply air alongside a flexible pipe joint simply doesn’t provide much resistance to that joint moving laterally, even though the lateral forces due to incidental joint deflections etc. might be quite small!

Also, while not enough detail has been provided to know if same might be issues in your application, there are also other forces or movements that could conceivably be brought to bear on/in an unrestrained pipe joint of at least a “very long” exposed even low pressure length of piping, specifically hydrodynamic forces as a result of high flow velocities at bends or through branches, as well as periodic thermal expansion and contraction of piping length on either side of the joint(s).

Finally, I believe there have also been at least a few cases where gravity or normally very low pressure exposed piping happens to be/is inadvertently subjected to much more pressure or thrust than in normal service or assumed (this could happen e.g. if a section of piping were flanged or capped off and filled with a fire hose for testing, even if the “bleed” hole in test plumbing is left wide open, at that point in time when all the air is exhausted from the pipe section, the pipe section becomes full, and water hammers the convergence of the bleed aperture – the test gravity piping section, even when not fully closed off, might well then see much of the pressure of the fire line!) While it can certainly be argued that many low pressure lines can be safely filled without such effects, or this restraint is not a designer’s responsibility, I guess anyone who happens to be standing in front of exposed piping provided with restraint (including perhaps/incidentally the designer or Owner) when the inadvertent pressure hits has whoever the provider was to thank when it doesn’t let loose! As Mr. SteveWag eruditely notes, it doesn’t take much pressure in big piping to develop sizeable loads.

Maybe for some experiential reasons such as these most exposed plant piping (unless provided with secure tie downs or external anchorages) is inherently restrained joint, in the fashion of flanged, grooved or shouldered, or welded etc., and for wherever utilized unrestrained connections such as mechanical joints, stuffing-box couplings like Smith-Blair or Dresser etc., flanged adaptors or flexible isolation couplings, and conventional expansion joints etc., are often provided with at least some modicum of supplementary tie-rodding or harnessed-type external or other restraints?