Nozzle Inspection Minimum Required Thickness

[Eddy1988]

Hello,

I was having a discussion with an inspector regarding how to determine whether a nozzle would need repairs or not. He is aware of the ASME BPVC calculations for nozzles, but he can not perform calculations for every nozzle (budget constraints).

He has been running the calculations for thickness required from pressure in conjunction with a table in API 574 for determining the required thickness from nozzles (Table-6 Minimum Thicknesses for Carbon and Low-Alloy Steel Pipe); although API 574 is meant for piping rather than nozzles.

Could anyone share any procedures or tips for inspectors regarding how to determine whether a nozzle has corroded past its thickness required asides from running ASME BPVC calculations?

Thanks,

Eddy

 

[marty007]

Required nozzle thickness can be defined in many different ways in the code (VIII-1). The minimum thickness may have been purely defined by nozzle ID/pressure, or it could have been increased in thickness beyond this to provide for additional reinforcement area. Without looking at calcs, I don't think you can safely determine the required thickness. The cleanest way to know would be to have a copy of the original calcs at hand, but sadly this isn't kept around for many older pieces of equipment...

I'd vote for calcs...

 

[rzrbk]

For an ASME vessel. As a first pass, you should use nominal thickness minus corrosion allowance (C.A.). If the nozzle neck is made of pipe, you can typically take mill tolerance(12.5%) off the nominal thickness before subtracting the C.A., as it would have been allowed for during the design.

The pressure calc is useful information but should be done using ASME VIII or API 510 formulas since the result may differ slightly from piping code. In any case the pressure calc alone will not account for any nozzle thickness used for reinforcement; so if you go below the C.A. on the nozzle, both pressure and reinforcement calculations will be needed to ensure that any additional reinforcement thickness, or interaction with the shell thickness, is accounted for.

 

[Duwe6]

You tell the Inspector what the pipe stresses are and *then* ask him/her to establish Tmin. Since there id very little chance that the actual [say nothing of Emergency/Upset] loads are, establishing a reasonable Tmin is tough. Most nozzles are like pipe -- the Pressure Tmin ts trivial, and the structural loading governs. Two 'rules-of-thumb' are 50% wall loss, and using Sch10 wall as Tmin.

When the loadings are unknown, the nozz Tmin is a SWAG*

*scientific wild-ass guess.

 

[Swlamech]

I see this all of the time. There is no one magical method that can quickly determine the integrity or remaining life of a pressure vessel nozzle unless there are calculations present that indicate the maximum acceptable corrosion allowance. There are just too many factors that can lead to a code failure. This is why the evaluation calculation for this situation involves iterating to solve a minimum of 3 simultaneous equations.

 

[r6155]

See ASME VIII Div 1 table UG-45 (for new vessel)

Regards
r6155