How to solve the allowable stress issue

Dear Expert,

As I am working with piping stress analysis as per design code ASME B31.3 2016 edition, the 72 inch sea water line (Internally cement lined) with pipe material of (305) APL 5L B, line D.T 75 degree C and found that the TEE point stresses are getting failed (allowable stress value - 206842 Kpa). Please note that TEE has been considered as Reinforced condition.

However we are changing material to qualify the TEE joint with higher grade (307) APL 5L X70 at TEE portion along and we have verified the results and found that the allowable (SA) are not changing as per design code (allowable stress value still - 206842 Kpa).

Kindly advice how to solve the allowable stress issue and how to qualify the TEE failure, advance response highly appreciated.

By
Rajivgandhi.R

Please send the job in to Tech Support via Smart Support. Make sure you indicate exactly what version (version and build) of c2 you are running.

Dear Sir,

With reference to above discussion and earlier TEE failure issue, I am working with 72 inch line sea water line and Thickness of 15.88 mm with below temperature parameters.

Operating Temperature : 35 ◦C - T1
Max. Design Temperature : 75 ◦C - T2
Min. Design Temperature : -10 ◦C - T3

Load cases given below

Hydro test Case (L1) WW+HP Hydrotest case
Operating Case (L2) W+T1+P1 Max. Operating Temp. Case
Operating Case (L3) W+T2+P1 Max. Design Temp. Case
Operating Case (L4) W+T3+P1 Min. Design Temp. Case
Sustained Case (L5) W+P1 Sustained case
Sustained Case (L6) WNC Sustained case
Expansion Case (L7) L2-L5 Expansion at Maximum Operating Temp.
Expansion Case (L8) L3-L5 Expansion at Maximum Design Temp
Expansion Case (L9) L4-L5 Expansion at Minimum Design Temp.
Expansion Case (L10) L8-L9 Maximum to Minimum Expansion Stress Range

Experts, kindly advice the expansion stress range case (L10) is really required. If required when this case will occur in the piping system. Also advice alternate options without routing change, since this piping is congested area.

The first issue to consider is whether or not you really need a flexibility analysis for your Max/Min temperatures? Theoretically your system will never see these temperatures. However, if you have a requirement to analyze these two temperatures I agree with your cases 1 to 9.

Second, I think your Case 10 is incorrect, it should be L8+L9. I also think it would be cleaner and more obvious if instead you setup L10 = L3 - L4.

Third, you need to evaluate if Case 10 is even possible. Can the system cycle between the max design state and the minimum design state?

I'll note that at 72" and 0.625" WT, your D/t ratio (115) exceeds what B31.3 recommends (<100) for purposes of estimating SIFs and Flexibilities of the fittings.

Dear Richard Ay,

Based on your feedback, I had checked load case L10 = L8+L9, L10= L8-L9 and L10 = L3-L4 both showing the same results. Kindly clarify the L10 = L8-L9 incorrect case
Since the stresses are failing in range case L10 = L8+L9 or L10 =L3-L4 only whether it is compulsory to consider Load case L10
Please advise the expansion stress range case (L10) is really needed.

Rajivgandhi,

Personally, I don't think your cases 8, 9, or 10 are needed. These cases are analyzing the "design conditions" of the system. Unless otherwise stipulated in the project requirements, a flexibility analysis addresses the operating conditions of the system (your cases 1, 2, 5, 6, 7).

As to (L8+L9) and (L8-L9) yielding the same results, what was the combination method: algebraic or scalar?

Dear Richard Ay,

With the reference to your feedback, I had checked load case L10 = L8+L9 with Scalar, stresses are higher side only. Previously for stress range case L10= L8-L9 considered as absolute

Kindly advice the expansion stress range case (L10) which one we should use for conservative design.

By
Rajivgandhi

L10=L3-L4 (EXP) is the one you should use. Combination type is Algebraic. In this way there is no doubt.
You do not need to be more conservative than required.

Regards

For your question regarding the allowable, this is because the Sc and Sh used to calculate the SA is limited to 138MPa as per B31.3 302.3.5. It is not a software error.
However, you may use the liberal stress. Pay note on your D/t ratio as Michael wrote.

Dear Fletcher,

With the reference to feedback, As we know pipe of 72 inch and thickness 0.625 inch D/t Ratio (<100) is exceeding, However we have calculated the SIF as per ASME B31.3 and FEA tool as follows.

As per ASME B 31.3 the SIF Out-plane - 6.7 & SIF In-plane - 5.3

As per FEA the SIF Out-plane - 5.3 & SIF In-plane - 4.7

We checked again by inputting FEA analysis SIF values and found that the stresses are still failure at Tee Connection.

Kindly Experts suggest us to overcome this critical condition.

By
Rajivgandhi

In addition to the SIFs from FEATools, you also used the flexibilities also - right?

If you did use the more accurate SIFs and flexibilities, and the model still fails, then you'll have to make adjustments to the system as you would with any other model that fails. Maybe:

- reroute part of the system
- use guides or limit stops to redirect thermal growth
- employ an expansion joint

One challenge i find with large bore fittings - tees in particular - is that the quality control is low, and you'll want to talk to your manufacturer to see if you can get an appropriate minimum crotch radius for purpose of estimating SIFs and flexibilities.

There is a "normal" practice of using a crotch radius equal to 130% of the wall thickness.

Using a bigger crotch radius results in a stiffer tee with lower SIFs. Higher loads, but usually a higher capacity to absorb those loads.

Crotch radius is difficult to measure or control.

You can also opt to move to a thicker wall for your fitting itself. This is often preferred to trying to garner flexibility from the pipe itself, unless you possess a large amount of real estate and the expansion loops and whatnot are more cost effective (often no).

Hi Rajiv,

can you show the caesar plot where its failing. its offshore piping.