Mechanical Strength Allowance

[inertia1]

Background:

The tender document/specification asked for piping with a pressure class of NP16. There are various services that shall use stainless steel 316 as the material. The spec asked for pipe/wall thicknesses for corrosion resistance and to consider the working environment of a shipyard (i.e. lots of potential impact damage)

During the detailed design phase we have now gone onto specify Schedule 40 pipe. We chose this wall thickness/schedule to ensure a very robust pipe was used, suitable for the operating environment.

The contractor is classing this as a change. They have provided calculations to show a schedule 10 pipe will satisfy the additional thickness required to resist corrosion under ASME 31.1.

We are in the situation of having to justify that schedule 40 is required, and their calculations need to be revised to consider additional factors.

So what additional factors could these be? And can we quantify them?

We have done some research and found one factor could be mechanical strength allowance, giving additional thickness. However, after looking in ASME B31.3 there is little guidance on this. Can someone guide me to how to quantify mechanical strength allowance?

As I am sure you all understand this has big commercial implications.

 

[BigInch]

A dropped object (12.75" OD Sch40 pipe, or whatever else is commonly carried around about the yard) from a customary carying height impacting an exposed service pipe, a common maintenance load as a heat exchanger bundle pull, or even an anchor drop load could be used offshore. The criteria should be something entirely reasonable for your expected environment, and need not necessarily be something specifically included in the code. The code implies that the pipe and the installation of the pipe, must be designed for ALL loads. In other words, if you expect a dropped anchor from a large ship, you might want to bury the pipe beyond the reach of anchor penetration, rather than increase the pipe wall thickness to 3". If you can reasonably expect a particular load to occur from any situation you can fairly envision during the pipe's service life, its fair to call it mechanical strength criteria.

"If everything seems under control, you're just not moving fast enough."
- Mario Andretti- When asked about transient hydraulics

http://virtualpipeline.spaces.msn.com

 

[rconner]

What exactly is the "operating environment" (you mentioned a "shipyard" -- is the pipe exposed, and/or buried, and is it e.g. exposed to saltwater and/or chlorides etc. inside or whatever soil outside etc.?) Perhaps more informaqtion might get you better answers.

 

[JohnBreen]

Inertia1,

What Code did THE OWNER specify for the facility?

May I ask why the contractor is quoting from ASME B31.1? Did you specify that ASME B31.1 be used? If you are not including a power plant as part of the dock facilities, ASME B31.1 is not the appropriate Code. However, if calculations performed in compliance with the B31.1 Code show that schedule 10 is OK then it would likely be OK for other piping Codes. But check the calculations against the Code of record.

Look over the calculations that were provided by the contractor. What allowances were used for erosion, corrosion, mill tolerances, grooving, etc? If you want to go with ASME B31.3 as the Code of record it might be good to look at B31.3 Appendix G, Safeguarding. Specifically look at paragraph G300.3(a)(2). Since your tender DID implicitly mention "potential damage" due to operations, you may be able to make the point that the contractor must design "armor, guards, barricades or other protection from mechanical abuse". The contractor will likely find that heavier wall thickness in the pipe will be cheaper than providing protection per B31.3.

Regards, John.

 

[MJCronin]

To add to the remarks above...

Diameter also makes a difference when specifying thin walled systems.

Repeatedly, I have seen piping line specifications with Schedule 10S used down to 2" NPS, then Schedule 40S piping used for anything smaller.

The reason...?

Field wear and tear and the fact that a 200 lb pipe fitter can bend the smaller pipe sizes by simply standing on them.

Smaller field run conduit is schedule 40 for the same reason.

-MJC

 

[proinwv2]

Don't forget piping supports. Less may be required for a heavier wall, if supports are involved.

Also, fork lifts and the like could also cause damage.

I have never liked the thinner walls on pipe or pressure vessels for all the reasons above, unless the lighter wall is definitely appropriate.

Paul

http://www.ostand.com

 

[JohnBreen]

Don't forget piping supports.

Excellent point proinwv2!

If your pipe is a larger diameter you might need saddle type supports to keep the pipe from deforming under the loading - alternately, the pipe wall would be increased to prevent deformation at the supports.

John