Joining perpendicular piping as "pipe in pipe"

[Pederator]

Hello all,

Currently the company I work for services as technical advisor for the construction of CHP plant. The contractor, instead of joining perpendicular pipes by tees (T-elements), is going to join pipes as "pipe in pipe". This is regarding for example steam bleeds / extraction pipings from the turbine (DN150 - DN 350). They are probably going to weld this joints on site. We are afraid of quality of the welds, because this is kinda tricky to obtain and finish suitable shapes on site, and then weld it. Besides, this kind of joints have lower strength, and there is graeter turbulence in this places, which implies greater pressure drop and heat exchange. From the contractor point of view it is obviously advantageous, because there is only one weld instead of three, and they do not spend money for the tees. But I am of the opinion that it is not "state of the art". I would like to ask you, what is your opinion about this - is this kind of joining pipes / pipings acceptable? Do you know if there is any standard (European prefered) that treats this issue?

Many thanks for your help.
Pederator

 

[if7005]

I think you talk about stub-in for pipe branching. Yes, this is one of methode accepted such as by ASME 31.3 (Appendix H), ASME 31.8 ( App-F) for pressure strength whether reinforcement pad is required. Practically you can use FEA or pipe stress software to assess and include thermal influence.

 

[weldstan]

Pipe to pipe branches are permitted. As are set-in sweep o'lets and set-on o'let type fittings. Tees are more expensive.

 

[pennpiper]

Pederator,
I would like to pass on some information that normally covers this type of issue.

The Piping Material Engineer for the original Engineer/Designer of record will create/write a set of Piping Material Line Class Specifications (PMLCS). This document includes the specific material, fittings, flanges, valves and connection methods for each of the commodities by pressure/temperature ratings.

Branch connection methods are included in each of the individual line classes. A Stub-in branch connection may be allowed in some low to medium pressure line classes. You, as the technical adviser for the construction of CHP plant should have access to an 'Approved for Construction" copy. The Constructor should also have a copy and should be following it without deviation.

Why is this important? Consider this, say five years from now one of these joints fail. Someone is killed. The original PMLCS called for a TEE fitting. Based on your approval of the "cheaper" justification, the Stub-In was used. The family of the victim sues the construction company.

The construction company says "the technical adviser for the construction of CHP plant" said it is alright to use the cheaper method.

Now, you have a real problem.

Sometimes its possible to do all the right things and still get bad results

 

[Pederator]

Thank you guys for your reply. Pennpiper, the problem is that I do not know if I (or we, as a company) have a "tool" to force the contractor to use tees. Technical terms of reference do not talk about this issue (at least not directly - there are mentioned standards such as EN 13480 etc.). So that is why I was asking about any standard. If the standards allow joining pipes as stub-in (thanks for proper terminology), then I/we am/are clear in case of any accident. But, I would like to be in line with standards, "engineering art", and my conscience.

Pederator

 

[pennpiper]

Pederator,
Where are you located (USA or other)?
Is the Construction Company working under a 'Sub-Contract' agreement?
if so,
Do you have a 'Contracts Administrator'(CA)?
if so,
What does the CA have to say about 'Deviations'?
Does the CA have a complete set of the Contract documents?
Do the Contract Documents include a set of Piping Material Line Class Specifications (or the equivalent)?
What Piping Code is this plant designed to(ASME B31.1 or ASME B31.3)?

Sorry, the questions could go on and on, don't approve deviations without a sound basis.

Sometimes its possible to do all the right things and still get bad results

 

[Pederator]

In fact we are the Contract Engineer for this project (I think this means the same as your "Contract Administrator"). The plant is located in eastern Europe. The construction company works under contract agreement, not "sub-contract". The obligatory piping design code is EN 13480-3 (probably the European equivalent of ASME B31.1 or ASME B31.3). In every detailed design (ie. live steam piping; turbine extraction piping; feedwater piping with heat exchangers; etc.) that we receive from the contractor for approval are listed materials (valves, pipes, elbows, flanges, etc.) - maybe this is a kind of PMLCS.

Thanks for your effort, Pennpiper.

Pederator

 

[weldstan]

As the Engineer, you can mandate the use of tees for branches. If you do it after award of Contract, it is a Change and you will pay dearly. One should have a legitimate engineering reason for specifying forged or extruded tees; e.g., severe cyclic service, severe seismic service, etc. to justify the added costs to your customer.

 

[Pederator]

Tees are not mentioned in Technical terms of reference - this can be a flaw. The system is not going to operate in high cycling regime, and there are no seismic shocks on the location. I do not care about the money - this is not my money, the contract is already awarded, and remuneration is fixed. I am the "policeman", who ensures that execution of the project is in line with the Terms of reference, technical as well (or rather minly). If there are no guidelines in the standards (like EN 13480) in which aplications tees must be used, it will be hard to force the contractor to use tees arguing that it is "state of the art".

Thanks for your involvement.
Pederator

 

[weldstan]

As previously stated, you will not be able to require a Change to the Cntract without paying for it. As long as the Contractor is performing its work per the Contract, it should be considered acceptable. You only need to assure that the branch calculations are correct, the proper reinforcement is provided, components, welding and the inspections thereof meet Contract/Code requirements.

 

[racookpe1978]

If the cheaper joint is going to be "inside" a condenser below the turbine (not exposed directly to plant personnel during operation at pressure) the risks are lowered of personnel injury.

the risks of failure are increased because the joint is more severely eroded (steam flows and flow erosion on both inside and outside of the pipe joints) and because the joints are invisible for years at a time between outages - if they are opened and inspected at all.

 

[BigInch]

So in effect you are the QA/QC inspector-engineer, not the design engineer and have no authority to make changes. Unfortunately your hands are tied. I would say that you have to raise a TQ concerning your alleged observed design inadequacy and let the design engineers and contract manager decide how to handle it.

 

[Pederator]

BigInch, yes, I am a kind of design QA/QC, among others.