PSI - Issue 3

J.L. González et al. / Procedia Structural Integrity 3 (2017) 48–56 Author name / Structural Integrity Procedia 00 (2017) 000–000

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this result has some degree of conservativism, but it is not possible to know how much, so a reasonable recommendation is to shut down the reactor in less than one month in order to extend the service as much as possible, but previous to a catastrophic failure. It is worth to mention that the operator ran the reactor for a little more than 30 days, carefully controlling the operating temperatures, removing the thermal insulation of the overheated ring and using cooling fans to maintain the shell temperature below 1000  F. By doing this the overheated ring actually was deformed by creep, but endured to withstand the operation window, ending in the condition described in the previous section of this paper. 3. Rehabilitation recommendation The damages caused by the creep deformation basically affected the ability of the cylindrical shell in the stripper section to withstand the vertical loads, because of the material softening and the loss of straightness of the shell, therefore the proposed rehabilitation method should compensate this loss. At first, the replacement of the deformed sections may be the logical and most straight forward form of rehabilitation, but the time constraints, the difficult to perform the post weld heat treatment and the risk of collapse due to the enormous weight and volume of the vessel sections above the damaged ring and the lateral loads under strong winds (the refinery is located near the shore and strong wind were expected), made the operator to reject this option and to look for an external reinforcement that obviated there limitations and risks. According to Secc. 7.2.7 of the API 510 code [API (2008)], the rehabilitation of a pressure vessel by the addition of welded reinforcements is allowed if the repair is not made over cracked areas and the repaired section is able to withstand the internal pressure and the vertical loads. Therefore the design of an external reinforcement was done based on the API BULL 2U [API (2004)] with the following methodology: 1. Proposal of the geometry of the reinforcement. 2. Verification of the ability to withstand the internal pressure of the deformed ring, considering the reduction in tensile strength and the expansion of the diameter, according to the ASME Secc. VIII code. 3. Estimation of the bucking stress of the unrepaired damaged ring. 4. Estimation of the strength of the reinforced ring to demonstrate its effectiveness. The conceptual design of the proposed external reinforcement is shown in Figure 4. It consists of an orthogonally stiffened set of four circumferential rings and twelve longitudinal (vertical) stringers. The steel plates that form the orthogonally stiffened ring may be of A 516 Gr 70 steel or similar, since the important variables are the plate thickness and the Young Modulus. All rings and stringers are fillet welded to the reactor´s shell. The significant dimensions of the orthogonally stiffened ring are given in Table 2.

Table 2. Significant dimensions of the orthogonally stiffened ring. Dimension

Term

Value

Total length of the stiffened ring

Lr Lb

2870 mm(113”)

Stringer length

Lb = Lr

Stringer thickness

tr

25.4 mm ( 1”) 203.2 mm (8”)

Stringer width

hs

Number of stringers

Ns

12

Horizontal separation between stringers

b

1888.5 mm (74.35”)

Material yield strength Material Young Modulus

Fy

38 ksi

E

29000 ksi

To verify if the thickness and strength of the damaged ring are enough to withstand the internal operating pressure, the minimum required thickness t as for section UG-27 of the ASME Secc. VIII Division 1 [ASME (2001)] is calculated.

PR

(1 )

t



0.6 SE P 