Summary:

FEA is performed for the desander pressure vessel of the 10 Mpa pressure rating. Other loads including dead weight, fluid weight, nozzle loads are considered in these calculations. Membrane stress, bending stress, and membrane plus bending stresses are calculated at the critical locations, and compared with acceptable criteria as per ASME Section VIII, Division 2, Part 5.2.1.1. From the FEA analysis, it was observed that high stresses   at the nozzle and shell interfaces are higher than the allowable stresses of the material. Hence, fatigue analysis is performed to determine adequate life. The total stress intensity range and cumulative fatigue usage factor are investigated to determine the adequacy of the design as per  ASME Sec.VIII, DIV.2.

Scope:

The aim of this analysis is to find fatigue life at the critical locations as per ASME Sec.VIII and Div.2

Methodology:

The finite element analysis is made based on the thickness and dimensions indicated in the fabricated drawings. The full corrosion allowance is considered in the FEA. According to design data, for cyclic Pressure Service of 0 MPa to 10 MPa, design number of cycles is < 4000.

As per ASME Sec. VIII Div. 2 , Annex 3-F.1.2, following formulae are specified to calculate the "design number of design cycles" (N),

·         N=Number of the allowable design cycle

·         X=Exponent used to compute the permissible number of cycles

·         ET = modulus of elasticity of the material under evaluation at the average temperature of the cycle being evaluated.

·         Cus = conversion factor, 1.0 for ksi & 6.894757 for MPa stress units

·         EFC = modulus of elasticity used to establish the design fatigue curve

·         Salt= Alternative equivalent stress from smooth bar curve for carbon steel for desired design cycles. It depends on fatigue strength reduction factor, fatigue penalty factor, poison correction factor, local thermal equivalent stress and Range of primary plus secondary plus peak equivalent stress.

·         The coefficients C1, C2... are Coefficient of fatigue curves (ASME Section VIII, Division 2, Part3).

The fatigue damage is calculated using the below equation,

Nk is the required design cycle. And Nk is the design number of design cycles.

Fatigue calculations carried out at critical locations including; Through Shell & Saddle, Shell to Nozzle , Ellipsoid to Nozzle Pipes, Ellipsoidal head to Boot Junction, Shell to Boot Junction, Shell to Boot Pad Junction, nozzles to transition, and shell to transition. From the fatigue calculations, it is concluded that all evaluation points for fatigue are within allowable limits specified by code.