Steel Pipe Restraint - 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.

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?

 

[JedClampett]

In your case the fact that you don't need harnessing doesn't have anything to do with the pressure, but the fact that the pipe is straight. If you cut out a section of pipe and draw a free body diagram you'll see that the pressures are equal and opposite at each end. No pressure difference means no harnessing.
If you want the best illustration of this get AWWA Publication M11.

 

[BigInch]

How will thermal expansion affect your couplings?

"I am sure it can be done. I've seen it on the internet." BigInch's favorite client.

"Being GREEN isn't easy." Kermit

http://www.youtube.com/watch?v=hpiIWMWWVco

http://virtualpipeline.spaces.live.com

 

[QualityTime]

You have to work out the thrust of the pipe with a tank full of water. Is it enough to blow out the joint. In this case probably not but you have to do the calculations.

A few years ago I was asked to help on a problem at a new high lift pump installation in a brand new pump station. The working pressure in the pump was about 240 psi and the discharge pipe was about say 36" in diameter. If you work the numbers out the thrust is very high. The consultant had a rigid vic coupling between the butterfly valve and the check valve. The pump went for a walk and the pump discharge pipe went out of alignment. I advised the owner that a rigid vic coupling is not the same as a flanged joint. The internal pressure in the pipe will cause the pipe to move until the rigid vic coupling catches on the built up pipe shoulders and that is the reason why the pump went for a walk. I advised the owner to have the consultant install tie rods between the butterfly valve and the check valve across the vic coupling. Problem solved.

 

[rconner]

I was just curious about the incident mentioned in the last post. Do you know what style of 36" grooved end coupling was employed for this "working pressure...about 240 psi"? While I don't know if it had anything to do with the happening described, at least some past traditional designs of such couplings have not rated for quite this high working pressure for such large pipe.

 

[QualityTime]

Hi, I can't honestly remember what type of coupling that it was but it was rated for that pressure. My point is that the use of rigid vic couplings versus the use of flanged joints are two different animals. Rigid vic couplings will keep the pipe together but there will be movement. In the case I was talking about the 4000 HP pump moved. With a flanged pipe joint there will be no movement