Effect of residual stress on failure of tube-to-tubesheet weld in heat exchangers

In a shell and tube heat exchanger, the failure of tube-to-tubesheet welds results in high-pressure water jet which erodes the refractory in front of the tubesheet. Finite element method was employed to simulate the welding process and post weld heat treatment (PWHT) to find the factors affecting the failure in tube-to-tubesheet weldments. Residual stresses in two different geometries of tube-to-tubesheet weldment were calculated through uncoupled thermal-structural analysis. The results showed that the values of residual stresses are higher in heat exchanger of site 1 than site 2 due to more weld passes and geometry of connection. Also, the maximum stress in site 1 occurs at the shellside face of tubesheet while it is on the weld toe in site 2. High tensile residual stresses, especially in Site 1, reduce the tubesheet life. Therefore, performing an efficient PWHT is vital. The PWHT simulation indicated that the process designed is effective for both sites by reducing the residual stress significantly. In addition, the effect of stress concentration was examined on both sites. Moreover, the stress concentration factor in site 1 is as twice as in site 2 and it is the main reason for more failures in site 1

Effect of Cedar (Ziziphus spina-christi) topical solution in mild to moderate acne vulgaris: a randomized clinical study

Background: Acne is the most prevalent skin disease in the world and antibiotics as its standard treatments have limited and also adverse effects. Cedar (Ziziphus spina-christi) has medicinal properties like antibacterial activity and is used topically for treatment of some kinds of skin problems in Persian medicine. The aim of this study was to evaluation the efficacy of topical cedar solution of acne vulgaris. Methods: Eighty patients aged between 15�45 years with mild to moderate acne vulgaris were conducted in this randomized, double blind trial. The participants were allocated to receive the topical cedar solution plus clindamycin 1 or topical placebo plus 1 clindamycin solution for six weeks. Patients were evaluated at the beginning of the study, second, sixth and eighth weeks after intervention for the acne severity index (ASI) and total acne lesions counting (TLC). Data was analyzed by SPSS software with Mann�Whitney U test. Results: From 105 subjects 68 people completed the study (33 persons in cedar group and 35 persons in placebo group). The mean and standard deviation of the age was 26.1 ± 7.5 years and 22 subjects (32.4) were male. TLC and ASI in the sixth and eighth weeks in cedar group were significantly less than in placebo group (p < 0.001). Topical cedar solution had no serious side effects. Conclusion: The topical cedar solution plus clindamycin 1 was more effective and safe than placebo plus 1 clindamycin for the treatment of acne vulgaris. © 2019 Taylor & Francis Group, LLC