PRODUCTION • PROCESSING • HANDLING
PWHT is required by the codes and if required the codes provide lower temperatures with longer holding times. In general the best idea is to agree about the parameters of PWHT with the steel supplier at the time of the inquiry.
TMCP STEELS IN SOUR SERVICE Even if the stored gas or liquid causes the risk of sour service-related HIC or SSC, the application of TMCP plates offers the best solution. When for normalised plates a tempering of the plate or a post weld heat treatment of the whole vessel even under 38mm is necessary to achieve HIC resistance, verified in accordance with NACE TM 0284, this additional heat treatment is not required for TMCP plates.
Fig. 7. Comparison of impact values in the heat affected zone for TMCP and normalised plates
Referring to the ASME BPVC, the same
advantage of TMCP steels regarding preheating is included. Based on table 6.8 of ASME BPVC VIII Div. 2 for P-No.1, Group 1, 2, 3 steels, which are normalised steels, a PWHT for plate thicknesses between 32mm and 38mm becomes mandatory, unless a preheating of 95°C has been applied. For the TMCP grades SA841-A-1 and SA841-B-2, this preheating is not necessary, if their carbon equivalent according to S77 of ASTM A841 does not exceed 0.40 %. Concerning the welding process itself, there
are two main requirements relating to the plate material that have to be fulfilled. On the onehand, the impact values in the heat-affected zone have to exceed the required values from the codes. On the other hand, the hardness values have to stay under a defined value. Due to their lower contents of carbon, alloying
and microalloying elements, TMCP plates show superior toughness even with high heat inputs and reduced hardness values in the heat affected zones compared to normalized steels (Fig. 6 and Fig. 7). Subsequently, TMCP steels offer an increased
working range for the welding process, resulting in an increased safety to fulfil the requirements of the code, especially due to the fact that storage tanks are fabricated on site and not in a workshop.
TAKE GUIDANCE FROM THE STEEL SUPPLIER One matter of concern of TMCP steels is their limited resistance against severe PWHT requirements, in example with holding temperatures above 600°C. However, for most storage tanks no
TAILOR-MADE SOLUTION After investigating a large number of plate enquiries, studying the requirements of different construction codes and having in mind the benefits of TMCP steels, Dillinger developed a tailor-made TMCP steel branded Di-Tank for the construction of storage tanks. Di-Tank is available with 355 MPa and 415 MPa yield strength in accordance to different construction codes, offering improved toughness properties and a very low carbon equivalent compared to grades that are listed in Table 1. Di-Tank 355 is also available with improved resistance to hydrogen induced cracking, offering a CLR ≤ 10 %, CTR ≤ 3 % and CSR ≤ 1 %, verified in accordance with NACE TM 0284.
IN SUMMARY TMCP plates offer various advantages compared to the equivalent normalised plates and are furthermore in accordance with the construction codes for storage tanks. During recent years some projects using TMCP steels have been realised, achieving overall cost savings of approximately 10%. Te fact that TMCP steels are uncommon for the application of storage tanks derives from the experience-based approach by the specification writers regarding the selection of materials. Looking at the worldwide growing energy demand and the need for storage along the whole oil and gas value chain, there will be many future opportunities to profit from the use of thermo- mechanically rolled heavy plates. l
Dr Peter Flüß, Joerg Maffert and Philipp Schirra are with Dillinger.
www.dillinger.de
www.engineerlive.com 35
Page 1 |
Page 2 |
Page 3 |
Page 4 |
Page 5 |
Page 6 |
Page 7 |
Page 8 |
Page 9 |
Page 10 |
Page 11 |
Page 12 |
Page 13 |
Page 14 |
Page 15 |
Page 16 |
Page 17 |
Page 18 |
Page 19 |
Page 20 |
Page 21 |
Page 22 |
Page 23 |
Page 24 |
Page 25 |
Page 26 |
Page 27 |
Page 28 |
Page 29 |
Page 30 |
Page 31 |
Page 32 |
Page 33 |
Page 34 |
Page 35 |
Page 36 |
Page 37 |
Page 38 |
Page 39 |
Page 40 |
Page 41 |
Page 42 |
Page 43 |
Page 44 |
Page 45 |
Page 46 |
Page 47 |
Page 48 |
Page 49 |
Page 50 |
Page 51 |
Page 52 |
Page 53 |
Page 54 |
Page 55 |
Page 56
