Suitability of P91 material for Low temperature. 2

[Kudroli]

I am looking for the suitable material for Superheated steam (110 Barg @550 Deg C). After analyzing between P22 & P91, it is observed that P91 is best suited in terms of Rating and wall thickness point of view.

However the plant been built in a cold country, where minimum design temparature is -40 deg C, which is lower than the DBTT.

I have recieved the charpy test values for P91 @ -40C and are as under.

Standard: ASTM A-335 Gr.P91

OD X WT: 426.0mm X 32.0mm

Heat treatment: normalize and temper

Tensile strength: YS=499MPa, TS=664MPa, El=29%

(Specimen=round bar, L-direction)

Impact values: 98, 115, 66 (average 93) J/cm2 (at - 40 degC,
L-direction)

Ductility fraction: 34, 40, 29 (average 34) % (at - 40degC, L-direction).

Can I dare to use P91 material at -40C (Shutdown condition)?

 

[strider6]

If the applicable standard is B 31.3 then you need to look into the table and graph that specify the minimum temperature for P91 materials. If the minimum temp is reached during shutdown then there will be no internal pressure, i suppose, so you can apply a reduction in the minimum temperature.

S.

Corrosion Prevention & Corrosion Control

 

[metengr]

The applicable standard should be B31.1 if this is piping carrying superheated steam. Also, are these pressure retaining components for boiler pressure parts or external piping or what?

Even in cold shutdown conditions, the pipe or pressure retaining item should have adequate notch toughness to prevent brittle fracture from residual stress (unpressurized and pressurized).


What is the code of construction for the plant?

 

[stanweld]

You problem will be to qualify a welding procedure with impact testing at -40C. Why do you need a -40 C MDMT? Your water feed must be above 0 C. Just because a plant is in a cold location, does not mean that you are required to set a MDMT at that temperature.

 

[davefitz]

stanweld:
as meteng indicated, a drained + dry P91 transfer pipe at -40C still has "residual stress", pipeline stresses due to thermal expansion coupled with creep relief at full operating temp.

For example , if erected at 0 C without cold springing, the thermal shrinkage down to -40 C will imply pipeline stresses, which will act at stress raisers such as notches at butt welds ( internal surface, root pass, which is not ground internally after weld).

 

[stanweld]

davefitz,
I did not mean to imply that the low temperature need not addressed in the piping design or startup procedures to avoid thermal shock, only that the MDMT impact requirements of ASME B31.3 or similar Codes need not be. P91 and other high temperature Chrome-Moly alloys that were not subject to impact testing have been used in Power Plants located in similar low temperature environments without incident.

 

[vesselguy]

Kudroli,

My experience with P91 material was when I had to write the proceedure for pneumatic testing of the reformer furnace coils and piping all made from P91. So, I can share that with you.

For high temperature service, T/P91 material has excellent strength at high temperature and good creep strength. However, this 9Cr-1Mo-V material has very low toughness and it is susceptible to brittle fracture at something like 10°C. That's why when hydrotesting this material, the water temperature should be 19°C or higher. Hence, if you think you can get 20ft-lb absorbed energy at your MDMT of -40°C, forget it, you'll never get it. However, this does not stop anyone from using this material. There is no way any half-ass decent operating procedure will permit a cold start up at -40°C. What you have to do is to do a slow warm up of the piping with low pressure hot gas to warm up the G91 tube and piping past 25°C before you slowly ramp up the pressure to full pressure.

I hope this helped. You should goggle this subject on the net. There's a few very useful paper floating around to help you on this subject.