Bends less than 11.25 deg in rigid pipe 2

[francesca]

What do you do for gentle bends (<<11.25 degrees) in a 3" rigid forcemain? I have a 1000 ft long arc that traverses 33 degrees total (radius is 1850 ft). If I use tangents and 11.25 degree elbows I'm going to blow my easement, but I have been advised that the pipe is too rigid to bend to match the curve.

 

[bimr]

Depending on pipe diameter, push-on joint Ductile Iron pipe has a joint deflection of up to 5º (Figure 2 and Table 1). This deflection enables the pipeline to be diverted from a straight line when following the curvature of streets and roads or when avoiding obstacles.

http://www.dipra.org/pdf/joints.pdf

 

[rconner]

Further to bimr's good advice, there is also an explanation and a geometric relationship that may be of help to you in the future with variously specific joints and sizes of products of manufacturers as well (e.g. see the bottom of page 2-9 at

http://www.acipco.com/adip/products/Sect2.pdf).

 

[francesca]

The pipe is actually rigid PVC and was constructed using the worst plan set I've ever seen, possibly laid without being wrapped in steel tape and was not located by the surveyor. We are doing relocation plans for a portion of this pipe and I wanted to just draft it on the arc through the middle of the easement on the premise that it was "something they work out in the field." (In other words, those tangents shown on the plans are completely meaningless.) It seems you've confirmed what I thought, but the senior guy says to draw the tangents, so completely meaningless tangents it is! (Not that meaningless tangents are any worse than a meaningless arc.)

 

[jgailla]

The UniBell PVC Design Manual has the information you need. There are two ways to deflect PVC pipe described in the handbook. The first, and the only way I have seen used, is to deflect the pipe at the joints just like DIP as described above. The deflection can be up to 5 degrees, but in my specs I limit it to 2.5 degrees.
You should be able to get your radius in this manner.
The other way to deflect is to actually bend the pipe using blocking. 3" is a fairly small diameter pipe.
What is "rigid" PVC? I have not seen this terminology. 3" PVC pipe should be able to be bent to well under an 1850' radius without deflecting the joints, If I remember right.

 

[stanier]

Leave it in the sun. Tne upper layer will have a higher temperature than the lower because of solar radiation and will expand differentially. PVC then bends like a banana. Install it before it has a chance to straighten.

"Sharing knowledge is the way to immortality"
His Holiness the Dalai Lama.

http://waterhammer.hopout.com.au/

 

[rconner]

As you have clarified the piping you are dealing with is pvc, I have noticed that over many years there has been some apparent sort of flip-flopping by some authorities and manufacturers around the world as to how deviations from line and grade should be accomplished with this piping. It therefore may be advisable to get the specific manufacturer(s) on record as to exactly how this should be accomplished on your project. The latest from allegedly a very large pvc pipe manufacturer(I have noticed that has been in the news much over the last year)appears to be now at

http://www.jmeagle.com/plastic-pipe/faq.html#joining-recommendations

. What is said on this particular site, I accessed today, includes the following, much of which may be interesting for readers in light of some expert advice on this thread, and with regard to probability of accomplishment in practical underground construction environments:

“A joint that is assembled per our recommendations will exhibit a gap, in which the beveled end of the spigot is stopped short of being fully bottomed out in the bell.”

“…’s standard installation recommendations call for no axial deflection at the joint. This recommendation emphasizes the fact that any intentional change of direction should be accomplished through a fitting connection or by bending the barrel of the pipe rather than offsetting the pipe joint. “and later again in multiple locations:

“… does not recommend any deflection at the joint for any of our products.”

“The pipe should be assembled above ground, in a straight line, then curved and laid in the trench. All curvature results from the bending of the pipe lengths. There is no deflection at the joint.”

“Brace the bell while the spigot end is pushed under the gasket, so that previously completed joints will not be further inserted.” And repeated:

“Lastly, there should be no axial deflection at the joint. Bending the pipe, rather than deflecting the joints, should serve to accomplish small changes in direction.”

“… pipe does not meet AWWA C-111; this standard is for Ductile Iron Pipe.”

“The joint design meets ASTM D 3212 performance testing requirements, thereby assuring a watertight joint that does not exceed an infiltration / exfiltration allowance of 25 gallons / inch diameter / mile / day.”

With all this being said, it is my understanding field splitting failures of pvc pipelines, with and without eventual tapping involved, have incidentally been blamed (or explained away?) on the Contractor (or other installer) bending the barrel of the pipe. Go figure! [While I guess it could be argued a pvc sewer force main would not have as many required drilled holes or taps in the pipe wall(that are obvious stress concentrators)as a water distribution piping, pvc force mains can (as a practical matter) also can have some stress concentrations and also are possibly more subject to cyclical stress/fatigue issues, due to pumps cutting on and off etc.]

 

[stanier]

What rconner has posted has merit. In addition I would add that the stopping and starting of pumps creates waterhammer. The PVC manufacturers argue that the celerity is reduced in their material because of the low modulus. This is so, but the increase in pressure following column separation pays scant regard to this fact. The transient pressure is amplified and may exceed the design pressure of the PVC. Cyclic loading from pumps stop and start needs to be considered. See POP101 at

http://www.pipa.com.au/index.php?option=com_content&view=article&id=54&Itemid=72

It must be considered though that the physical properties, in response to a rapid pressure event, for a thermo plastic pipe are much higher than the publised steady state figures. So pipe burst is unlikely. However damage to the pipe will have occurred. The criteria to be most considered with PVC are surface defects when it comes to failures from cyclic events. Defects a are crack initiators. That is why the C ring test is so important.

"Sharing knowledge is the way to immortality"
His Holiness the Dalai Lama.

http://waterhammer.hopout.com.au/

 

[francesca]

I think I should restate that the pipe was put into the ground in 2003 and we are designing a relocation. The relocation will not have the curve; it was merely a question as to whether it was "something worked out in the field" or if I had to try and interpret the worst plan set ever to try and guess at where the angle fittings were when drafting in the existing line.

It is an interesting point about bending the pipe versus the joints and the potential for failure, because the relocation will likely go through Limited Access ROW. In a LAROW the utility owner (a church with a low pressure pump station, ~ 25 gpm @ 30 ft) will have to take the risk that the forcemain will never fail or they will have to reroute around an Interstate/US highway interchange. (The existing FM is along the US highway and it's a new-build Interstate.) We may as a precaution "relocate" everything within the LAROW to ensure the construction quality (as existing plans give no confidence) and we will be adding ARVs, etc. as the 4,000+ ft line currently has one, about 400 ft from the lift station, and the manhole its in is full of water!