Effect of surge pressures on CML steel pipes

[joe72]

Good day,

Are there any articles available on the effect of surge pressures on cement mortar lined steel pipes? For instance, when a cml pipe is subjected to a positive surge event, the steel will undergo a certain strain, which, depending on the pressure can be higher than the allowable strain in the cml. This will surely lead to cracking of the lining. As the lining has self healing characteristics, will the surge events be problematic?

best regards

 

[BigInch]

I'd guess that too many would lead to corrosion under the lining, which defeats the purpose of the lining. For the lining to remain effective it would seem that there should be a limit on surge events. Should not the allowable pressure be based on limiting the liner's strain limit?

Independent events are seldomly independent.

 

[joe72]

The problem is that the liner has a very low tensile strength, in the order of 4 MPa. If an allowable pressure is used based on 50% yield of X42 steel (Young's modulus=207 GPa) I arrive at the following:

Pipe OD: 1524 mm
Pipe wall thickness: 9.5 mm
Allowable operating head: 185 m (based on 145 MPa, which is 50% of yield)
Strain in steel 0.0007

Allowable stain in liner based on 4 MPa tensile strength (Young's modulus of 26 GPa) = 0.0001538, which is much lower than the strain in the steel. Am I correct in saying that if the pipeline is operated at 185 m head the liner will definitely crack?

 

[LittleInch]

Surge events should be limited to no more than a few percent over the design pressure of the pipe. The design pressure of the pipe should be set such that the cml can withstand this pressure without an issue, otherwise you have a pipe which cannot meet it's design purpose. Therefore the circumstances you describe should not occur if you've designed your system properly. I've never had much experience of cml so i will listen to those who have, but flexing of the pipe during transport or instalation has a much greater impact on the lining than circumfrential expansion.

Is this just a general query or do you have a specific problem?

My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way

 

[joe72]

Hi LittleInch,

Yes the problem is rather specific as im not accustomed to work with cml lining (see my previous post). We usually limit surge pressures to within 75% of the pipeline yield. As cml is not very flexible I was wondering on the effect of surge pressures on the lining. But just looking at normal operating pressure the strain seems to be too excessive for the lining. But clearly i'm missing something..

 

[LittleInch]

I think you're approaching this the wrong way round. Forget what the steel pipe can do, at 9.5mm it's already incredibly thin for its diameter so thickness of the steel pipe is driven by handling and construction more than it's pressure capacity. What do you need the pipe to do is the real question and then figure out the surge pressures based on the design requirement. If the liner is actually sprayed on as opposed to a "liner" then yes, small cracks might appear, but i think you're looking at the basics here and not the actual circumferential expansion of the pipe. Cml pipes have their disadvantages like many things and this may be one of them.

Surge doesn't only affect the pipe, but also lots of other bits of equipment. I think what you're missing is a look at the whole system design and concentrating on a small part of it inthe middle.

I always like using electrical anlogies here - what this is is saying is my cable can withstand 660V. The rest of the system is only rated / runs at 220V. If I get a power spike of 500V, my cable is Ok, but everything else has caught fire....

My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way

 

[BigInch]

You've got to start using fuses in your analogies.

Independent events are seldomly independent.

 

[joe72]

The system will operate at around 900 kPa with maximum surge in the order of 1400 kPa. This is well below the 50% and 75% yield of the pipe. So yes, handling and installation factors are sometimes the driving factor. But even with these relative low pressures, the strain in the pipe still exceeds the allowable strain in the lining. I was tasked to do the surge analysis on the pipe and was thinking about the effect on the lining.

Talking about analogies... a fuse could be analog to a rupture disc..lol

 

[LittleInch]

Fuse - more like a excess flow or high high pressure trip, rupture disc more like a grounding to earth. Anyway - looking around a bit more it would appear yes, there could be a problem at these sorts of pressures. I found one reference which quoted max steel YS to use of 21kpsi for cement lined pipes, which would give your pipe a max pressure of 18bar , but this wans't referenced or backed up and may just be a rule of thumb.

The cement lining does expand a bit in contact with water and has some self sealing / healing properties and there are some application techniques which pressure the pipe during application and curing to apply a compressive stress to the liner so especially operating mainly at 9 bar, you should be OK, but at that size of pipe, what ins't an issue at smaller pipes could be if the expansion gets concentrated one location

You could always make the steel a bit thicker.... or reduce your surge pressure. I would also question the pipe supplier a bit more to find out what he recomends / will certify or if indeed you are operating outside of the normal pressure range for this type of pipe.

For info, how did they construct the internal field joints? They could easily be your weakest point.

My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way