P91 Steam Piping 3

[Ein-no-stein]

Dear All,
P91 is a common high temperature steam piping for power generation and nuclear industry, which contains 9%Cr and 1%Mo.
Is it reasonable to be used in marine environment as steam piping considering the chloride SCC (it is grey area since P91 is not recognized as stainless steel)?
Any candidate for the bolts and nuts considering 520°C operating temperature?
It seems that B16 and Grade 660 are not good based on the strength level as well as the marine environment.
Do I need to use 718?

Thanks,

 

[metengr]

At 520 deg C, your steam pipe material should be seamless P22. I would not use P91 creep strength enhanced ferriic steel because of the risk associated with fabrication and handling. Vendors and contractors who work with Grade 91 and understand what is required regarding heat treatment and welding are few and far in between.

 

[Ein-no-stein]

Thanks, metengr.
Our system design pressure is really high, so P91 has a lot of creep strength benefit rather than P22.
Any idea of the bolts and nuts for P91. Appreciate your input.

 

[weldstan]

Agree with metengr. B16 bolting is extensively used at 520 C. The only thing you gain by using P91 is decreased thickness of pipe. If you have little experience with P91, the potential upside will be negated by the downsides presented by metengr plus the higher costs of the P91 material.

 

[metengr]

Ein-no-stein
stanweld brings up some very good points. You are vague about your marine application and design pressure. Unless you are a supercritical or even ultra-supercritical power boiler application, stay away from the CSEF steel. Grade P22 can easily handle high pressure steam.

 

[weldstan]

In our construction of numerous power plants using P91, it was found that costs were considerably greater than any other alloy used. From purchase to completed installation, it was by far the most expensive piping system on a same diameter/lineal foot basis and this includes Super 304H and 347H stainless piping/tubing.

 

[Ein-no-stein]

Our system is operated at 60 bar and the design life is 25 years.

 

[metengr]

For 60 Bar = 870 psi, seamless SA 335 Grade P22 would be my first choice of MS material.

 

[MJCronin]

I agree with metengr ..... the well known P22 material seems to be an economic choice for this application.

Regarding bolting materials,I have seen many HT steam systems where bolted connections are discouraged and for those few where bolting is required B16 was specified,

MJCronin
Sr. Process Engineer

 

I concur with that.
In practical metallurgy, 'better' is not always better.

"Everyone is entitled to their own opinions, but they are not entitled to their own facts."