Piping - Large dia pipes Hydrostatic test. 1

[castbasalt]

Four pipe lines of 4.000 m diameter and about 400 m long each is being installed for conveying sea water from NDCT to Condenser and back in a generating station. 12.0 m pipe lengths are field welded, encased in 250 mm thick RCC and buried underground. The contract requires hydro-static test at 1.5 times the working pressure. NDE requirement is 10% RT for butt joints and 100% RT for T-joints. Now the contractor requests for exemption from hydrotest and proposes 100% UT for all butt welds and 100% RT for T-joints. He agrees for the final service leak test also.

My question to the members is this:
Keeping the contractual obligations aside, Can we consider contractor's request from purely design / engineering point of view? If the proposal is agreeable on design and engineering aspect, the owner is ready to amend the contractual requirement.

Kindly offer your comments. Thanks in advance.

 

[1503-44]

I wouldn't sign off on that. To even think about it, there should at least be some overpowering reason for why hydrotest is impossible, like there is no water, but that's not a likely excuse if these are for sea water. One break and your plant gets washed off the face of the earth.

Reality used to affect the way we thought. Now we somehow believe that what we think affects reality.

 

[MJCronin]

What is the official design pressure for the piping system ?

Since this is a pumped system, is it possible for the pump to operate at the "dead head" condition ?

What peak pressure will the piping system see under that "dead head" condition ?

What is RCC ?

What is the piping material and are there any coatings ?

Sea water is, VERY VERY corrosive ..... What happens to your system many years from now when the piping system is degraded, wall are thinned and a pressure transient comes along ?

Do contractural obligations mean anything in the Mid-East ?

MJCronin
Sr. Process Engineer

 

[XL83NL]

Four pipe lines of 4.000 m diameter ..

4 mtrs or 4000 mtrs = 4 km diameter? I assume the first but watch out with using dots and comma's if the unit system isnt defined.

Huub
- You never get what you expect, you only get what you inspect.

 

[LittleInch]

You need to ask why this very basic and very common test of the final installed pipeline cannot be undertaken.

Also your statement "He agrees for the final service leak test also" is not clear. Does that mean that he can do the final leak test but can't do the hydrotest??

This is a test of the entire installed system. It's pretty basic, but demonstrates structural integrity of the ENTIRE system, not just the welds of the pipeline.

Any stresses from badly installed pipe, pipe which is thinner than it should be etc etc is being tested.

Now you can agree to waive this requirement, but for me, I would need to be convinced that there really is no practical way to achieve it before even considering an alternative.

Some pressures, wall thickness and why the contractor wants to do tis would help us a lot here.

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

 

[MJCronin]

This paper may be helpful ...

https://www.penspen.com/wp-content/uploads/2014/09/hydrotest-eliminated.pdf

MJCronin
Sr. Process Engineer

 

[weldstan]

One assumes that hydrotesting can be performed prior to RCC (reinforced concrete cover?) and prior to burial. Normally the pressure in these sea water intake systems is quite low so the only impediment to hydrostatic testing is the volume of water to be used and one assumes sea water could be used for the test. While you may consider eliminating the hydro, only the Owner can amend the contract hydro requirements provided the jurisdictional requirements are not broken.

 

[weldstan]

One more question, will the pipe be internally coated and when will it be coated, assuming that it will be?.

 

[1503-44]

I don't see any logic to not hydro. You obviously have water available. The pipe will be filled with water anyway. You don't say why does the contractor not want to do it? He doesn't have test pumps? I might even suspect the contractor has something to fear about doing a hydro.

If I didn't have water available, I might agree, but only if it was 100% RT. You have water, so no way. I don't see why this merits further discussion, unless the contractor parks a new Mercedes in your garage. Just kidding.

The Penspen article has been around for awhile now, but I have not seen anyone seriously considering it when water is available. If water is not available, the artical's conclusions are not needed to make a decision. Neither have I seen any movement to forego hydrotesting requirements for other options it mentions. Can anyone name one case?



Reality used to affect the way we thought. Now we somehow believe that what we think affects reality.

 

[HTURKAK]

It is strange that ,the cooling system NDCT (natural draught cooling tower ) and the cooling water is saline water. Is the pipe PCCP ( prestressed concrete cylindirical pipe ) ? manufactured at precast plant and each 12 m length pipe factory tested ? As far as I know, RCC ( reinforced cement concrete ) , Are the steel pipes installed in the trench and then encased with RCC ?

You did not define the interior coating .( cement ? PU ?..)..

If the each pipe installed without testing and the welded joints interior and exterior repair coated, moreover backfill is completed, IMO, hydrostatic test will be meaningless..

Please provide more info. to get better responds..

 

[robyengIT]

just to clarify : "PCCP = Prestressed Concrete Cylinder Pipe" where Cylinder means "steel Cylinder" according to AWWA C301 and/or EN 642 (lined or embedded type). For diam 4000 mm should be "embedded"

 

[castbasalt]

First of all, thank you all for your immediate response.
My bad, I didn’t give enough information in my post. Here I furnish the details:

Working pressure : 2.55 bar.
Design pressure : 5.7 bar.
Pump dead head : 5.2 bar.
Pipe Material: : Fabricated out of 16 mm thick plates of IS:2062
(Comparable / equivalent grade: ASTM SA36)
Pipe OD : 3.832 m
Internal coating : Glass flake for 1500 micron thick. (Pipes not coated for 200 mm on both ends to allow for welding; Coating to be done after hydrotest as per
original method statement)
Pipe centreline : - 4.0 m Finished floor level.
Encasing : 250 mm thick reinforced Cement concrete.
Laid on : RCC pedestal over 200 mm thick PCC.
Average length of each pipe line: 400 m
Being nearer to the sea ( about 1 km from the shore), water table is 6.3 m below FFL.
Availability of water is not an issue.

Contractor seeks exemption due to issues arising out of
• non-availability / delayed availability of excavated trenches and PCC works.
• Ensuing rainy season resulting in flooding / muck filling of trenches.
• Non-availability of access to other fronts as the trenches could be closed only after hydrotest of the entire stretch.
Owner would look into exemption if quality can be assured by thorough NDE.

I think I have provided sufficient information. As I have already requested, Can the owner be assured of quality with NDE examinations alone without hydro test?

Thanks in advance.

 

[1503-44]

Not buying that. It is usually not recommended to fabricate pipelines only to have installation delayed for what might amount to many months by events mostly under your control (except perhaps bad weather) that can easily be avoided by exercising good, simple project management practices. Usually seasonal bad weather periods can even be avoided entirely. Normally no pipeline pipe is ever fabricated prior to 30 days before actual installation. Neither are these pipelines so long that they will take an excessive amount of time to fabricate and install easily avoiding all of the potential delays you mention. They do not amount to even 1km. You also run risks of damage after NDT and before and during installation. You will have no testing afterwards to ensure adequate strength installed in place.

I'd spend some more time reviewing the contractor's planning and scheduling capability. Its weak at best.

Reality used to affect the way we thought. Now we somehow believe that what we think affects reality.

 

[HTURKAK]

castbasalt :

Let me explain what I understand , and correct me if I am wrong;...
The steel pipe material is 16 mm thick plates equivalent to grade ASTM SA36. The pipes delivered 12 m length and factory tested, internal coating Glass flake for 1500 micron thick and external coating bitumen coated ??.

The steel pipe designed for internal pressure and RCC 250 mm thick for external soil and traffic loading..
This type of piping is an old system in general used for penstocks of power plants.

I know that, the cooling system piping materials could be ; GRP , Steel pipe , PCCP... and all these pipes are (and could be ) with rubber gaskets. The nice detail provided by robyengIT (Mechanical), is for conn . detail of PCCP pipe with single rubber seal and there is another detail for double gasket similar to single .

I just want to remind that a limited leakage is is allowed for joints with rubber gaskets. This is copy and paste from AWWA M11 Steel Pipe
( Allowable Leakage ;........ There should be no significant leakage in an all-welded pipeline or one that has been joined with properly installed mechanical couplings. On pipe joined with O-ring rubber gaskets, a small amount of leakage should be allowed.
A leakage of 10 gal (37.85 L) per in. (25.4 mm) of diameter per mile per 24 hours is usually permitted.... ).

IMO, if the welding performed by certified welders, if the visual inspection is carried by certified experienced inspector, NDE proposed by the contractor is acceptable. I assume that the contractor knows ; if there is a pinhole in a welded joint, Glass flake repair at welded joints will stop the leakage so agrees for the final service leak test.


IMO, this is a contractual issue .. the relaxation of the specification requires Equitable price reduction...
If I were in your shoes, my concern will be CATHODIC PROTECTION of the steel pipe rather than pinholes which could be filled by glass flake ..

I hope this respond answers your question..

 

[robyengIT]

As pointed by Hturkak there is also a double rubber gasket joint for PCCP pipes. And this mainly because it can be tested (pneumatically) immediately after laying the pipe, before laying the next one. In such a case any leakage can be immediately shown and the last pipe replaced.

 

[LittleInch]

castbasalt

You are asking how the owner can be "assured" of quality without hydrotest. I don't think that's possible.

I'm not really understanding when in the process the hydrotest is taking place?

Normally I look at a hydrotest of a pipeline when it is, as far as possible, in it's final finished condition, fully buried and basically ready to go.

Then your hydrotest not only covers the welding, it covers the pipe itself, the construction, any defects or stress induced during the installation process, any settlement etc
The 100% NDE of the welds will increase the confidence that the welding is OK, but doesn't do anything about the other issues I mention.

So it is up to the Owner to determine if the reduction in confidence that a hydrotest gives you for the complete installed system is acceptable or not.

I think your contractor is whinging and feeling sorry for himself. Why is he waiting for the hydro before backfilling the pipe? Doesn't make sense to me.

I take it PCC is Plain Cement Concrete? i.e. like a blinding layer, presumably because your soil conditions are poor, But what are these reinforced cement concrete (RCC) pedastals for a buried pipe??



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