BURIED PIPE STRESS ANALYSIS 1

[PIPING_DESIGNER]

How to reduce stress on elbow of a buried GRE/GRP pipeline ?

 

[1503-44]

Add another 2 elbows with a lot of pipe between them.

A black swan to a turkey is a white swan to the butcher ... and to Boeing.

 

[LittleInch]

Use a bend instead of an elbow.

GRE normally ends up in tension so it is trying to pull the elbow towards you.

Or insert some flexible fill around the elbow and let the pipe move a bit - though GRE is poor at abrasion so that's often not a good move.

Don't bury it?

Any more details? - Size, pressure, temperature, that sort of thing.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[HTURKAK]

You may consider to add thrust block for the bend..

 

[1503-44]

Remembering that adding any restraint tends to increases stresses.

A black swan to a turkey is a white swan to the butcher ... and to Boeing.

 

[LittleInch]

Thrust blocks are generally bad news and need to be a significant size to do anything. Usually lead to greater stresses elsewhere.

In GRE especially they create high stress points and cracking.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[1503-44]

Agreed. Every "alternate material" crack Ive ever seen have been at supports and restraints.

A black swan to a turkey is a white swan to the butcher ... and to Boeing.

 

[HTURKAK]

I would like to share my past experience for the GRP PIPE;

-The use of thrust blocks is a must for UG installation if the couplings are REKA coupling without straining..
-If the joints locked ( axial thrust restraining ) and if in case of high sustained internal pressure , still the use of TB is the best option
- The bends normally encased in thrust blocks and if necessary , provide short pieces at both sides,

The following figure is the concept for the TB for bends,

- AWWA Manual M5 Fiberglass Pipe Design adresses the use of TB at section 7.

- the following GRP installation guide is useful..

 

[1503-44]

Yes. Those full encasement blocks are good. I would avoid one-sided point bearing block type.

A black swan to a turkey is a white swan to the butcher ... and to Boeing.

 

[bimr]

HTURKAK,

You are making the case not to buy Flowtite pipe, instead of the need for thrust blocks. The practicality of installing encased thrust blocks will make the use of Flowtite pipe uneconomical.

Ameron, another supplier of FRP pipe states:

"No thrust blocks are required at rated system pressure for most buried piping configurations and most soil conditions."

As stated by LI, "Any more details? - Size, pressure, temperature, pipe thickness, water hammer, soil loading, that sort of thing."

 

[HTURKAK]

Dear BIMR,

I am retired engineer for several years after 40++ active working life and do not have any relation with any manufacturer..If you are still active, try GRP pipe loop ( Bondstrand, AMeron ,etc ..) with mechanical locked joints , without TB's and the bends fabricated miter bend , the sustained pressure is more than say 10-15 bars, dia 20 in or greater..when the bends start to fail , excavate bend locations, change the bends with new ones with TB ,

Then remember this post ..

 

[MJCronin]

First, I believe that the OP has some sort of thermal bending stress problem as per his piping stress analysis program. We really have no way of knowing what program he is using or if his model is correct. We do not know if and how the piping is restrained by expanding against the soil (which I suspect) or if the piping has an insulating jacket, which can mitigate the calculated bending stresses. As is typical from NEWBIE piping stress analysts, we just do not know....

Second, the use of thrust blocks on underground GRE piping is not appropriate for systems subject to thermal expansion. They may be needed to resist pressure loadings on the joints and keep the system together, but they do not solve thermal expansion issues

Thirdly, I suggest that the origin of the problem lies with MBA selection of low-cost GRP piping without Management consideration of the total installed cost.

I have seen this time and time again ...

MBAs/PMPs and Project accountants consider ONLY the cost of GRP piping and ignore/forget the cost of thrust blocks.

Then, either the field forces screw up the blocks, do not want to put them in, or the engineers add many more underground piping systems.

The proper and lowest cost solution is probably a Ductile Iron Piping System with retrained joints (Such as a MEGALUG system ... there are others)

https://ebaa.com/files/pdf/brochures/Brochure.1100.pdf

Made in the USA

(But the Chinese are rapidly developing a cheap, defective knock off)

MJCronin
Sr. Process Engineer

 

[HTURKAK]

Mr CRONIN in general nails the subject with excellent English and a BPS for this respond..

 

[Alex Matveeff]

Just add expansion cushions. Usually it reduce stresses a lot. See paragraph 4.1 here

https://www.passuite.com/kbase/doc/start/WebHelp_en/index.htm#t=gruntmodel.htm

If not help, then add U-shape expansion loop
I think thrust blocks is not good idea. Expensive and unnecessary.

I'm the PASS/START-PROF Pipe Stress Analysis Software Developer

 

[Smith55]

In my experience, for fully constrained buried GRE/GRP systems using TB/TS or lamination joints, thrust blocks are not required. The high stresses at the changes in direction are dealt with by providing additional thickness by way of overlay lamination to provide reinforcement.