Why both restraints and thrust blocks?

[BernieH]

Why is it that some municipalities demand both joint restraint and thrust blocking? I guess thrust blocks can be altered or removed during subsequent work on other utilities. So backing up thrust blocks with restraints could make sense. But why should restrained joints be backed up by thrust blocks? Do they think the restraints may be removed or damaged? Or that they may corrode? Maybe they wish to guard against poor workmanship or design...
Thanks

 

[coloeng]

The other question is why do some contractors insist on using both even when they are not both required.

From my experience it is a lack of knowledge in why restraining the fitting, with either thrust blocks or mechanical restraints, is necessary. They feel that if one of them is good by itself, it must be even better if you use both. A big waste of money if you ask me.

 

[bimr]

Agree with coloeng, probably lack of knowledge as well as poor specifications by the Engineer.

 

[rconner]

In this regard however, some hypothetical or possible points of view were discussed a few years ago on another list (I’ve seen now posted at

http://www.pipingdesign.com/mailarc/0510/10869.html),

that could at least perhaps provide some understanding of alternative points of view.

Of course large concrete reaction blocks and walls are also used by some designers in locations on welded or fused steel and plastic pipelines, not necessarily out of fear that the welded or fused joints cannot handle the pressure thrust loads imposed, but instead most generally to strictly control deformations or movement, or in the cases where some of the multiple drivers for same are cyclical (e.g. temperature), rebating deformations or movements.

If you are running into folks wanting both for a particular thrust application, it might not hurt to ask them why. While I agree with some said, some such battles may not be worth fighting.

 

[BigInch]

Pipe restraints, you can find no better example anywhere of when too much of a good thing can be harmful.

 

[jgailla]

BigInch,
When are pipe restraints harmful? I deal with pipe restraints for <16" diameter water and sewer lines, and the design process is straightforward. Restraints are cheaper, quicker, and easier to install than thrust blocks. What is the downside?

 

[BigInch]

"When are pipe restraints harmful?"
Wherever you don't need one. Every time you put one in, deflection and strain is reduced, but stress increases.

 

[pappaus]

I work in Philadelphia. We use both thrust blocks and restraints at bends. While it is technically a redundancy, a factor for us is the potential cost in damage and lawsuits versus the cost of some extra concrete. Most of our water mains are in high-density areas of population. When I started here, they called it "belt and suspenders".

 

[greggy]

I agree with pappaus....belts and suspenders is the way to go to sleep well at night and minimize the liability even though the cost is higher. Here in my water system we have had instances where the pipe thrust restraint failed due to corrosion or others hitting our main, and we have had thrust blocks dug out when other utilities remove them for their underground work when our stuff is in the way. The cost of redundancy is still less than the cost of a damage lawsuit in the typical urban setting.

 

[BigInch]

Its true, more can be "safer", if you're operating at low stress levels. When pipe stress crosses the limit, the party's over.

 

[stanier]

Civil engineers just love pouring that grey stuff up against timber. You would think they had shares in a concrete supply company.

Seriously, I continue to argue with civil engineers who just dont get the fact that a fully welded pipe or restrained pipe does not need thrust blocks. There must be a problem in university teaching or text books, but try as I might civil engineers go on burying concrete blocks.

I point to refineries where the pipes are above ground with no thrust blocks to no avail.

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

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

 

[greggy]

I suspect the pipe pressures in refineries are far less than the maximum we have operating a water distribution system (our maximum is 180 psi in the valley). In the perfect world without corrosion and backhoes there will be no thrust blocks. In the meanwhile, we stick with installing them.

 

[cvg]

by the way, thrust blocks are never poured against any timber, that would be a big no no. And frankly, I believe contractors generally are more likely to use blocks than restrained joints. They have the option here, and it is generally cheaper to pour concrete and as we all know, public works contracts are awarded to low bidder, not best or most qualified.

above ground piping can be easily inspected, underground typically is buried and forgotten about and 100-year old buried lines are still in service without inspections. So providing sufficient thrust restraint is perhaps more critical.

Most water distribution systems do not operate at 180 psi. 120 psi is a more "normal" upper limit at least for the numerous systems I have been involved with over the last 25 years in the US. Since I have no experience with refineries, I can only imagine what kind of pressures are typical but would not be surprised to see them much higher than those in a municipal water system... Perhaps that is one reason for increased concern over pipe stresses.

 

[BigInch]

Natural Gas, petroleum product and crude oil cross country transmission pipeline common operating pressures could be most anything between 350 to 2160 psi.

If you bury them and forget them, it won't be long before they start reminding you.

 

[stanier]

Greggy,

I suggset you check out the pressure rating of class 300, 600, 900 and even 2500 flanges. All can be found in refineries. Pressures in process are far higher than in any water project.

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

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

 

[stanier]

CVg,

Underground lines may have been installed for 100yearsd. Look at the statistics of water leakage! Utilities spend a fortune trying to get leakage down below 15%. This is not tolerable for petrochem.

It wouldnt happen if the pipelines were fully welded.

Sewage treatment plants are invariably designed for 6 times average dry weather flow! Why leakage. If the pipelines were fully welded PE or sement lined steel the plant size can be reduced to 1.6 times.

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

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

 

[cvg]

stanier, I'm aware of the statistics for water leakage. I agree it is not tolerable for petrochem or for water utilities. However, petrochem are private operations, they sell the product in the pipeline and make a lot of money and on top of that they have environmental and safety regulations that they have to follow and so they have a different business model. I wish the water utilities thought that way also, but they are non-profit and operate on a shoestring so to speak. I am also not sure the reduction in treatment and pumping costs would make up for the much higher cost for the distribution system piping. The typical municipal water department has perhaps a hundred miles of waterline to install and maintain and the additional capital cost for welded pipe would be staggering. Since these types of projects are typically funded through taxes / bonds, it is unlikely that they would be approved.

Regarding treatment plants, they are designed for peak flows which are generally more like twice the average dry weather flow. This is done to handle peak flows, not so much to handle I & I. Unfortunately in many parts of the world, the sewers handle both sanitary and storm water runoff. It would be interesting to see if the additional cost for piping could be offset by a corresponging reduction in treatment costs.

 

[stanier]

If you do the numbers butt fusion welding PE is not that costly.

Eurpoe is way a head of the USA when it comes to using non metallic piping for sewerage systems.

Another challenge to be considered is the corrosion of the cement lining with H2S. PE/ABS/GRP/PVC resists this corrosion.




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

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

 

[QualityTime]

PE piping is not the panacea you might think. Time will tell. The reason why they are using PE piping is Europe is because it has an overall lower installation cost

 

[BigInch]

Sounds good....